Perl pod documentation

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Perl pod documentation

This translation of the Perl documentation (http://perldoc.perl.org) from POD to Texinfo is not official, and not endorsed by the Perl developers (indeed, they haven’t seen it). It was created by the GNU Texinfo developers because they found it useful to have the Perl documentation available in Info and other formats, and thought they would share the results. Suggestions welcome.

This is created entirely by the Texinfo tools; see the contrib/perldoc-all directory in the Texinfo sources for the procedure used. The output is available at http://www.gnu.org/software/perl/manual.

• perl:  
• perlapio:  
• perlartistic:  
• perlbook:  
• perlboot:  
• perlbot:  
• perlcall:  
• perlcheat:  
• perlclib:  
• perlcommunity:  
• perldata:  
• perldbmfilter:  
• perldebguts:  
• perldebtut:  
• NAME debug debugger:  
• perldiag:  
• NAME data structure complex data structure struct:  
• perldtrace:  
• perlebcdic:  
• perlembed:  
• perlexperiment:  
• perlfilter:  
• perlfork:  
• NAME format report chart:  
• NAME function:  
• perlgit:  
• perlgpl:  
• perlguts:  
• perlhack:  
• perlhacktips:  
• perlhacktut:  
• perlhist:  
• perlinterp:  
• perlintro:  
• perliol:  
• perlipc:  
• NAME warning, lexical warnings warning:  
• perllocale:  
• perllol:  
• perlmod:  
• perlmodinstall:  
• perlmodstyle:  
• perlmroapi:  
• perlnewmod:  
• perlnumber:  
• NAME object OOP:  
• perlootut:  
• NAME operator:  
• perlopentut:  
• perlpacktut:  
• perlperf:  
• NAME POD plain old documentation:  
• perlpodspec:  
• perlpodstyle:  
• perlpolicy:  
• perlport:  
• perlpragma:  
• NAME regular expression regex regexp:  
• perlreapi:  
• perlrebackslash:  
• NAME character class:  
• NAME reference pointer data structure structure struct:  
• perlreftut:  
• perlreguts:  
• perlrequick:  
• perlreref:  
• perlretut:  
• perlrun:  
• perlsec:  
• perlsource:  
• perlstyle:  
• NAME subroutine function:  
• NAME syntax:  
• perlthrtut:  
• NAME tie:  
• perltodo:  
• perltooc:  
• perltoot:  
• perltrap:  
• perlunicode:  
• perlunifaq:  
• perluniintro:  
• perlunitut:  
• perlutil:  
• perlvar:  
• perlvms:   
 — The Detailed Node Listing —

perl


• perl NAME:  



• perl SYNOPSIS:  



• perl GETTING HELP:  



• perl DESCRIPTION:  



• perl AVAILABILITY:  



• perl ENVIRONMENT:  



• perl AUTHOR:  



• perl FILES:  



• perl SEE ALSO:  



• perl DIAGNOSTICS:  



• perl BUGS:  



• perl NOTES:  



GETTING HELP


• perl Overview:  



• perl Tutorials:  



• perl Reference Manual:  



• perl Internals and C Language Interface:  



• perl Miscellaneous:  



• perl Language-Specific:  



• perl Platform-Specific:  



• perl Stubs for Deleted Documents:  



perlapio


• perlapio NAME:  



• perlapio SYNOPSIS:  



• perlapio DESCRIPTION:  



DESCRIPTION


• perlapio Co-existence with stdio:  



• perlapio "Fast gets" Functions:  



• perlapio Other Functions:  



perlartistic


• perlartistic NAME:  



• perlartistic SYNOPSIS:  



• perlartistic DESCRIPTION:  



• perlartistic The "Artistic License":  



The "Artistic License"


• perlartistic Preamble:  



• perlartistic Definitions:  



• perlartistic Conditions:  



perlbook


• perlbook NAME:  



• perlbook DESCRIPTION:  



DESCRIPTION


• perlbook The most popular books:  



• perlbook References:  



• perlbook Tutorials:  



• perlbook Task-Oriented:  



• perlbook Special Topics:  



• perlbook Free (as in beer) books:  



• perlbook Other interesting, non-Perl books:  



• perlbook A note on freshness:  



• perlbook Get your book listed:  



perlboot


• perlboot NAME:  



• perlboot DESCRIPTION:  



perlbot


• perlbot NAME:  



• perlbot DESCRIPTION:  



perlcall


• perlcall NAME:  



• perlcall DESCRIPTION:  



• perlcall THE CALL_ FUNCTIONS:  



• perlcall FLAG VALUES:  



• perlcall EXAMPLES:  



• perlcall LIGHTWEIGHT CALLBACKS:  



• perlcall SEE ALSO:  



• perlcall AUTHOR:  



• perlcall DATE:  



FLAG VALUES


• perlcall G_VOID:  



• perlcall G_SCALAR:  



• perlcall G_ARRAY:  



• perlcall G_DISCARD:  



• perlcall G_NOARGS:  



• perlcall G_EVAL:  



• perlcall G_KEEPERR:  



• perlcall Determining the Context:  



EXAMPLES


• perlcall No Parameters, Nothing Returned:  



• perlcall Passing Parameters:  



• perlcall Returning a Scalar:  



• perlcall Returning a List of Values:  



• perlcall Returning a List in a Scalar Context:  



• perlcall Returning Data from Perl via the Parameter List:  



• perlcall Using G_EVAL:  



• perlcall Using G_KEEPERR:  



• perlcall Using call_sv:  



• perlcall Using call_argv:  



• perlcall Using call_method:  



• perlcall Using GIMME_V:  



• perlcall Using Perl to Dispose of Temporaries:  



• perlcall Strategies for Storing Callback Context Information:  



• perlcall Alternate Stack Manipulation:  



• perlcall Creating and Calling an Anonymous Subroutine in C:  



perlcheat


• perlcheat NAME:  



• perlcheat DESCRIPTION:  



• perlcheat ACKNOWLEDGEMENTS:  



• perlcheat AUTHOR:  



• perlcheat SEE ALSO:  



DESCRIPTION


• perlcheat The sheet:  



perlclib


• perlclib NAME:  



• perlclib DESCRIPTION:  



• perlclib SEE ALSO:  



DESCRIPTION


• perlclib Conventions:  



• perlclib File Operations:  



• perlclib File Input and Output:  



• perlclib File Positioning:  



• perlclib Memory Management and String Handling:  



• perlclib Character Class Tests:  



• perlclib stdlib.h functions:  



• perlclib Miscellaneous functions:  



perlcommunity


• perlcommunity NAME:  



• perlcommunity DESCRIPTION:  



• perlcommunity AUTHOR:  



DESCRIPTION


• perlcommunity Where to Find the Community:  



• perlcommunity Mailing Lists and Newsgroups:  



• perlcommunity IRC:  



• perlcommunity Websites:  



• perlcommunity User Groups:  



• perlcommunity Workshops:  



• perlcommunity Hackathons:  



• perlcommunity Conventions:  



• perlcommunity Calendar of Perl Events:  



Websites


• perlcommunity News sites:  



• perlcommunity Forums:  



perldata


• perldata NAME:  



• perldata DESCRIPTION:  



• perldata SEE ALSO:  



DESCRIPTION


• perldata Variable names:  



• perldata Context:  



• perldata Scalar values:  



• perldata Scalar value constructors:  



• perldata List value constructors:  



• perldata Subscripts:  



• perldata Multi-dimensional array emulation:  



• perldata Slices:  



• perldata Typeglobs and Filehandles:  



Scalar value constructors


• perldata Version Strings:  



• perldata Special Literals:  



• perldata Barewords:  



• perldata Array Interpolation:  



perldbmfilter


• perldbmfilter NAME:  



• perldbmfilter SYNOPSIS:  



• perldbmfilter DESCRIPTION:  



• perldbmfilter SEE ALSO:  



• perldbmfilter AUTHOR:  



DESCRIPTION


• perldbmfilter The Filter:  



• perldbmfilter An Example: the NULL termination problem.:  



• perldbmfilter Another Example: Key is a C int.:  



perldebguts


• perldebguts NAME:  



• perldebguts DESCRIPTION:  



• perldebguts Debugger Internals:  



• perldebguts Frame Listing Output Examples:  



• perldebguts Debugging Regular Expressions:  



• perldebguts Debugging Perl Memory Usage:  



• perldebguts SEE ALSO:  



Debugger Internals


• perldebguts Writing Your Own Debugger:  



Writing Your Own Debugger


• perldebguts Environment Variables:  



• perldebguts Debugger Internal Variables:  



• perldebguts Debugger Customization Functions:  



Debugging Regular Expressions


• perldebguts Compile-time Output:  



• perldebguts Types of Nodes:  



• perldebguts Run-time Output:  



Debugging Perl Memory Usage


• perldebguts Using $ENV{PERL_DEBUG_MSTATS}:  



perldebtut


• perldebtut NAME:  



• perldebtut DESCRIPTION:  



• perldebtut use strict:  



• perldebtut Looking at data and -w and v:  



• perldebtut help:  



• perldebtut Stepping through code:  



• perldebtut Placeholder for a, w, t, T:  



• perldebtut REGULAR EXPRESSIONS:  



• perldebtut OUTPUT TIPS:  



• perldebtut CGI:  



• perldebtut GUIs:  



• perldebtut SUMMARY:  



• perldebtut SEE ALSO:  



• perldebtut AUTHOR:  



• perldebtut CONTRIBUTORS:  



NAME debug debugger


• NAME debug debugger NAME:  



• NAME debug debugger DESCRIPTION:  



• NAME debug debugger The Perl Debugger:  



• NAME debug debugger Debugging Regular Expressions:  



• NAME debug debugger Debugging Memory Usage:  



• NAME debug debugger SEE ALSO:  



• NAME debug debugger BUGS:  



The Perl Debugger


• NAME debug debugger Calling the Debugger:  



• NAME debug debugger Debugger Commands:  



• NAME debug debugger Configurable Options:  



• NAME debug debugger Debugger Input/Output:  



• NAME debug debugger Debugging Compile-Time Statements:  



• NAME debug debugger Debugger Customization:  



• NAME debug debugger Readline Support / History in the Debugger:  



• NAME debug debugger Editor Support for Debugging:  



• NAME debug debugger The Perl Profiler:  



perldiag


• perldiag NAME:  



• perldiag DESCRIPTION:  



• perldiag SEE ALSO:  



NAME data structure complex data structure struct


• NAME data structure complex data structure struct NAME:  



• NAME data structure complex data structure struct DESCRIPTION:  



• NAME data structure complex data structure struct REFERENCES:  



• NAME data structure complex data structure struct COMMON MISTAKES:  



• NAME data structure complex data structure struct CAVEAT ON PRECEDENCE:  



• NAME data structure complex data structure struct WHY YOU SHOULD ALWAYS use strict:  



• NAME data structure complex data structure struct DEBUGGING:  



• NAME data structure complex data structure struct CODE EXAMPLES:  



• NAME data structure complex data structure struct ARRAYS OF ARRAYS:  



• NAME data structure complex data structure struct HASHES OF ARRAYS:  



• NAME data structure complex data structure struct ARRAYS OF HASHES:  



• NAME data structure complex data structure struct HASHES OF HASHES:  



• NAME data structure complex data structure struct MORE ELABORATE RECORDS:  



• NAME data structure complex data structure struct Database Ties:  



• NAME data structure complex data structure struct SEE ALSO:  



• NAME data structure complex data structure struct AUTHOR:  



ARRAYS OF ARRAYS


• NAME data structure complex data structure struct Declaration of an ARRAY OF ARRAYS:  



• NAME data structure complex data structure struct Generation of an ARRAY OF ARRAYS:  



• NAME data structure complex data structure struct Access and Printing of an ARRAY OF ARRAYS:  



HASHES OF ARRAYS


• NAME data structure complex data structure struct Declaration of a HASH OF ARRAYS:  



• NAME data structure complex data structure struct Generation of a HASH OF ARRAYS:  



• NAME data structure complex data structure struct Access and Printing of a HASH OF ARRAYS:  



ARRAYS OF HASHES


• NAME data structure complex data structure struct Declaration of an ARRAY OF HASHES:  



• NAME data structure complex data structure struct Generation of an ARRAY OF HASHES:  



• NAME data structure complex data structure struct Access and Printing of an ARRAY OF HASHES:  



HASHES OF HASHES


• NAME data structure complex data structure struct Declaration of a HASH OF HASHES:  



• NAME data structure complex data structure struct Generation of a HASH OF HASHES:  



• NAME data structure complex data structure struct Access and Printing of a HASH OF HASHES:  



MORE ELABORATE RECORDS


• NAME data structure complex data structure struct Declaration of MORE ELABORATE RECORDS:  



• NAME data structure complex data structure struct Declaration of a HASH OF COMPLEX RECORDS:  



• NAME data structure complex data structure struct Generation of a HASH OF COMPLEX RECORDS:  



perldtrace


• perldtrace NAME:  



• perldtrace SYNOPSIS:  



• perldtrace DESCRIPTION:  



• perldtrace HISTORY:  



• perldtrace PROBES:  



• perldtrace EXAMPLES:  



• perldtrace REFERENCES:  



• perldtrace AUTHORS:  



perlebcdic


• perlebcdic NAME:  



• perlebcdic DESCRIPTION:  



• perlebcdic COMMON CHARACTER CODE SETS:  



• perlebcdic SINGLE OCTET TABLES:  



• perlebcdic IDENTIFYING CHARACTER CODE SETS:  



• perlebcdic CONVERSIONS:  



• perlebcdic OPERATOR DIFFERENCES:  



• perlebcdic FUNCTION DIFFERENCES:  



• perlebcdic REGULAR EXPRESSION DIFFERENCES:  



• perlebcdic SOCKETS:  



• perlebcdic SORTING:  



• perlebcdic TRANSFORMATION FORMATS:  



• perlebcdic Hashing order and checksums:  



• perlebcdic I18N AND L10N:  



• perlebcdic MULTI-OCTET CHARACTER SETS:  



• perlebcdic OS ISSUES:  



• perlebcdic BUGS:  



• perlebcdic SEE ALSO:  



• perlebcdic REFERENCES:  



• perlebcdic HISTORY:  



• perlebcdic AUTHOR:  



COMMON CHARACTER CODE SETS


• perlebcdic ASCII:  



• perlebcdic ISO 8859:  



• perlebcdic Latin 1 (ISO 8859-1):  



• perlebcdic EBCDIC:  



• perlebcdic The 13 variant characters:  



• perlebcdic 0037:  



• perlebcdic 1047:  



• perlebcdic POSIX-BC:  



• perlebcdic Unicode code points versus EBCDIC code points:  



• perlebcdic Remaining Perl Unicode problems in EBCDIC:  



• perlebcdic Unicode and UTF:  



• perlebcdic Using Encode:  



CONVERSIONS


• perlebcdic tr///:  



• perlebcdic iconv:  



• perlebcdic C RTL:  



SORTING


• perlebcdic Ignore ASCII vs. EBCDIC sort differences.:  



• perlebcdic MONO CASE then sort data.:  



• perlebcdic Convert, sort data, then re convert.:  



• perlebcdic Perform sorting on one type of platform only.:  



TRANSFORMATION FORMATS


• perlebcdic URL decoding and encoding:  



• perlebcdic uu encoding and decoding:  



• perlebcdic Quoted-Printable encoding and decoding:  



• perlebcdic Caesarean ciphers:  



OS ISSUES


• perlebcdic OS/400:  



• perlebcdic OS/390, z/OS:  



• perlebcdic VM/ESA?:  



• perlebcdic POSIX-BC?:  



perlembed


• perlembed NAME:  



• perlembed DESCRIPTION:  



• perlembed Hiding Perl_:  



• perlembed MORAL:  



• perlembed AUTHOR:  



• perlembed COPYRIGHT:  



DESCRIPTION


• perlembed PREAMBLE:  



• perlembed ROADMAP:  



• perlembed Compiling your C program:  



• perlembed Adding a Perl interpreter to your C program:  



• perlembed Calling a Perl subroutine from your C program:  



• perlembed Evaluating a Perl statement from your C program:  



• perlembed Performing Perl pattern matches and substitutions from your C program:  



• perlembed Fiddling with the Perl stack from your C program:  



• perlembed Maintaining a persistent interpreter:  



• perlembed Execution of END blocks:  



• perlembed $0 assignments:  



• perlembed Maintaining multiple interpreter instances:  



• perlembed Using Perl modules, which themselves use C libraries, from your C program:  



perlexperiment


• perlexperiment NAME:  



• perlexperiment DESCRIPTION:  



• perlexperiment AUTHORS:  



• perlexperiment COPYRIGHT:  



• perlexperiment LICENSE:  



DESCRIPTION


• perlexperiment Current experiments:  



• perlexperiment Accepted features:  



• perlexperiment Removed features:  



perlfilter


• perlfilter NAME:  



• perlfilter DESCRIPTION:  



• perlfilter CONCEPTS:  



• perlfilter USING FILTERS:  



• perlfilter WRITING A SOURCE FILTER:  



• perlfilter WRITING A SOURCE FILTER IN C:  



• perlfilter CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE:  



• perlfilter WRITING A SOURCE FILTER IN PERL:  



• perlfilter USING CONTEXT: THE DEBUG FILTER:  



• perlfilter CONCLUSION:  



• perlfilter THINGS TO LOOK OUT FOR:  



• perlfilter REQUIREMENTS:  



• perlfilter AUTHOR:  



• perlfilter Copyrights:  



perlfork


• perlfork NAME:  



• perlfork SYNOPSIS:  



• perlfork DESCRIPTION:  



• perlfork CAVEATS AND LIMITATIONS:  



• perlfork PORTABILITY CAVEATS:  



• perlfork BUGS:  



• perlfork AUTHOR:  



• perlfork SEE ALSO:  



DESCRIPTION


• perlfork Behavior of other Perl features in forked pseudo-processes:  



• perlfork Resource limits:  



• perlfork Killing the parent process:  



• perlfork Lifetime of the parent process and pseudo-processes:  



NAME format report chart


• NAME format report chart NAME:  



• NAME format report chart DESCRIPTION:  



• NAME format report chart NOTES:  



• NAME format report chart WARNINGS:  



DESCRIPTION


• NAME format report chart Text Fields:  



• NAME format report chart Numeric Fields:  



• NAME format report chart The Field @* for Variable-Width Multi-Line Text:  



• NAME format report chart The Field ^* for Variable-Width One-line-at-a-time Text:  



• NAME format report chart Specifying Values:  



• NAME format report chart Using Fill Mode:  



• NAME format report chart Suppressing Lines Where All Fields Are Void:  



• NAME format report chart Repeating Format Lines:  



• NAME format report chart Top of Form Processing:  



• NAME format report chart Format Variables:  



NOTES


• NAME format report chart Footers:  



• NAME format report chart Accessing Formatting Internals:  



NAME function


• NAME function NAME:  



• NAME function DESCRIPTION:  



DESCRIPTION


• NAME function Perl Functions by Category:  



• NAME function Portability:  



• NAME function Alphabetical Listing of Perl Functions:  



• NAME function Non-function Keywords by Cross-reference:  



Non-function Keywords by Cross-reference


• NAME function perldata:  



• NAME function perlmod:  



• NAME function perlobj:  



• NAME function perlop:  



• NAME function perlsub:  



• NAME function perlsyn:  



perlgit


• perlgit NAME:  



• perlgit DESCRIPTION:  



• perlgit CLONING THE REPOSITORY:  



• perlgit WORKING WITH THE REPOSITORY:  



• perlgit Topic branches and rewriting history:  



• perlgit WRITE ACCESS TO THE GIT REPOSITORY:  



• perlgit Accepting a patch:  



WORKING WITH THE REPOSITORY


• perlgit Finding out your status:  



• perlgit Patch workflow:  



• perlgit Committing your changes:  



• perlgit Using git to send patch emails:  



• perlgit A note on derived files:  



• perlgit Cleaning a working directory:  



• perlgit Bisecting:  



Topic branches and rewriting history


• perlgit Grafts:  



Accepting a patch


• perlgit Committing to blead:  



• perlgit Committing to maintenance versions:  



• perlgit Merging from a branch via GitHub:  



• perlgit A note on camel and dromedary:  



Committing to blead


• perlgit On merging and rebasing:  



perlgpl


• perlgpl NAME:  



• perlgpl SYNOPSIS:  



• perlgpl DESCRIPTION:  



• perlgpl GNU GENERAL PUBLIC LICENSE:  



perlguts


• perlguts NAME:  



• perlguts DESCRIPTION:  



• perlguts Variables:  



• perlguts Subroutines:  



• perlguts Compiled code:  



• perlguts Examining internal data structures with the dump functions:  



• perlguts How multiple interpreters and concurrency are supported:  



• perlguts Internal Functions:  



• perlguts Unicode Support:  



• perlguts Custom Operators:  



• perlguts AUTHORS:  



• perlguts SEE ALSO:  



Variables


• perlguts Datatypes:  



• perlguts What is an "IV"?:  



• perlguts Working with SVs:  



• perlguts Offsets:  



• perlguts What's Really Stored in an SV?:  



• perlguts Working with AVs:  



• perlguts Working with HVs:  



• perlguts Hash API Extensions:  



• perlguts AVs, HVs and undefined values:  



• perlguts References:  



• perlguts Blessed References and Class Objects:  



• perlguts Creating New Variables:  



• perlguts Reference Counts and Mortality:  



• perlguts Stashes and Globs:  



• perlguts Double-Typed SVs:  



• perlguts Magic Variables:  



• perlguts Assigning Magic:  



• perlguts Magic Virtual Tables:  



• perlguts Finding Magic:  



• perlguts Understanding the Magic of Tied Hashes and Arrays:  



• perlguts Localizing changes:  



Subroutines


• perlguts XSUBs and the Argument Stack:  



• perlguts Autoloading with XSUBs:  



• perlguts Calling Perl Routines from within C Programs:  



• perlguts Memory Allocation:  



• perlguts PerlIO:  



• perlguts Putting a C value on Perl stack:  



• perlguts Scratchpads:  



• perlguts Scratchpads and recursion:  



Memory Allocation


• perlguts Allocation:  



• perlguts Reallocation:  



• perlguts Moving:  



Compiled code


• perlguts Code tree:  



• perlguts Examining the tree:  



• perlguts Compile pass 1: check routines:  



• perlguts Compile pass 1a: constant folding:  



• perlguts Compile pass 2: context propagation:  



• perlguts Compile pass 3: peephole optimization:  



• perlguts Pluggable runops:  



• perlguts Compile-time scope hooks:  



How multiple interpreters and concurrency are supported


• perlguts Background and PERL_IMPLICIT_CONTEXT:  



• perlguts So what happened to dTHR?:  



• perlguts How do I use all this in extensions?:  



• perlguts Should I do anything special if I call perl from multiple threads?:  



• perlguts Future Plans and PERL_IMPLICIT_SYS:  



Internal Functions


• perlguts Formatted Printing of IVs, UVs, and NVs:  



• perlguts Pointer-To-Integer and Integer-To-Pointer:  



• perlguts Exception Handling:  



• perlguts Source Documentation:  



• perlguts Backwards compatibility:  



Unicode Support


• perlguts What is Unicode, anyway?:  



• perlguts How can I recognise a UTF-8 string?:  



• perlguts How does UTF-8 represent Unicode characters?:  



• perlguts How does Perl store UTF-8 strings?:  



• perlguts How do I convert a string to UTF-8?:  



• perlguts Is there anything else I need to know?:  



perlhack


• perlhack NAME:  



• perlhack DESCRIPTION:  



• perlhack SUPER QUICK PATCH GUIDE:  



• perlhack BUG REPORTING:  



• perlhack PERL 5 PORTERS:  



• perlhack GETTING THE PERL SOURCE:  



• perlhack PATCHING PERL:  



• perlhack TESTING:  



• perlhack MORE READING FOR GUTS HACKERS:  



• perlhack CPAN TESTERS AND PERL SMOKERS:  



• perlhack WHAT NEXT?:  



• perlhack AUTHOR:  



PERL 5 PORTERS


• perlhack perl-changes mailing list:  



• perlhack #p5p on IRC:  



GETTING THE PERL SOURCE


• perlhack Read access via Git:  



• perlhack Read access via the web:  



• perlhack Read access via rsync:  



• perlhack Write access via git:  



PATCHING PERL


• perlhack Submitting patches:  



• perlhack Getting your patch accepted:  



• perlhack Patching a core module:  



• perlhack Updating perldelta:  



• perlhack What makes for a good patch?:  



Getting your patch accepted


• perlhack Patch style:  



• perlhack Commit message:  



• perlhack Comments, Comments, Comments:  



• perlhack Style:  



• perlhack Test suite:  



What makes for a good patch?


• perlhack Does the concept match the general goals of Perl?:  



• perlhack Where is the implementation?:  



• perlhack Backwards compatibility:  



• perlhack Could it be a module instead?:  



• perlhack Is the feature generic enough?:  



• perlhack Does it potentially introduce new bugs?:  



• perlhack How big is it?:  



• perlhack Does it preclude other desirable features?:  



• perlhack Is the implementation robust?:  



• perlhack Is the implementation generic enough to be portable?:  



• perlhack Is the implementation tested?:  



• perlhack Is there enough documentation?:  



• perlhack Is there another way to do it?:  



• perlhack Does it create too much work?:  



• perlhack Patches speak louder than words:  



TESTING


• perlhack Special make test targets:  



• perlhack Parallel tests:  



• perlhack Running tests by hand:  



• perlhack Using t/harness for testing:  



Using t/harness for testing


• perlhack Other environment variables that may influence tests:  



WHAT NEXT?


• perlhack "The Road goes ever on and on, down from the door where it began.":  



• perlhack Metaphoric Quotations:  



perlhacktips


• perlhacktips NAME:  



• perlhacktips DESCRIPTION:  



• perlhacktips COMMON PROBLEMS:  



• perlhacktips DEBUGGING:  



• perlhacktips SOURCE CODE STATIC ANALYSIS:  



• perlhacktips MEMORY DEBUGGERS:  



• perlhacktips PROFILING:  



• perlhacktips MISCELLANEOUS TRICKS:  



• perlhacktips AUTHOR:  



COMMON PROBLEMS


• perlhacktips Perl environment problems:  



• perlhacktips Portability problems:  



• perlhacktips Problematic System Interfaces:  



• perlhacktips Security problems:  



DEBUGGING


• perlhacktips Poking at Perl:  



• perlhacktips Using a source-level debugger:  



• perlhacktips gdb macro support:  



• perlhacktips Dumping Perl Data Structures:  



SOURCE CODE STATIC ANALYSIS


• perlhacktips lint, splint:  



• perlhacktips Coverity:  



• perlhacktips cpd (cut-and-paste detector):  



• perlhacktips gcc warnings:  



• perlhacktips Warnings of other C compilers:  



MEMORY DEBUGGERS


• perlhacktips Rational Software's Purify:  



• perlhacktips valgrind:  



Rational Software’s Purify


• perlhacktips Purify on Unix:  



• perlhacktips Purify on NT:  



PROFILING


• perlhacktips Gprof Profiling:  



• perlhacktips GCC gcov Profiling:  



MISCELLANEOUS TRICKS


• perlhacktips PERL_DESTRUCT_LEVEL:  



• perlhacktips PERL_MEM_LOG:  



• perlhacktips DDD over gdb:  



• perlhacktips Poison:  



• perlhacktips Read-only optrees:  



• perlhacktips The .i Targets:  



perlhacktut


• perlhacktut NAME:  



• perlhacktut DESCRIPTION:  



• perlhacktut EXAMPLE OF A SIMPLE PATCH:  



• perlhacktut AUTHOR:  



EXAMPLE OF A SIMPLE PATCH


• perlhacktut Writing the patch:  



• perlhacktut Testing the patch:  



• perlhacktut Documenting the patch:  



• perlhacktut Submit:  



perlhist


• perlhist NAME:  



• perlhist DESCRIPTION:  



• perlhist INTRODUCTION:  



• perlhist THE KEEPERS OF THE PUMPKIN:  



• perlhist THE RECORDS:  



• perlhist THE KEEPERS OF THE RECORDS:  



THE KEEPERS OF THE PUMPKIN


• perlhist PUMPKIN?:  



THE RECORDS


• perlhist SELECTED RELEASE SIZES:  



• perlhist SELECTED PATCH SIZES:  



SELECTED PATCH SIZES


• perlhist The patch-free era:  



perlinterp


• perlinterp NAME:  



• perlinterp DESCRIPTION:  



• perlinterp ELEMENTS OF THE INTERPRETER:  



• perlinterp OP TREES:  



• perlinterp STACKS:  



• perlinterp MILLIONS OF MACROS:  



• perlinterp FURTHER READING:  



ELEMENTS OF THE INTERPRETER


• perlinterp Startup:  



• perlinterp Parsing:  



• perlinterp Optimization:  



• perlinterp Running:  



• perlinterp Exception handing:  



• perlinterp INTERNAL VARIABLE TYPES:  



STACKS


• perlinterp Argument stack:  



• perlinterp Mark stack:  



• perlinterp Save stack:  



perlintro


• perlintro NAME:  



• perlintro DESCRIPTION:  



• perlintro AUTHOR:  



DESCRIPTION


• perlintro What is Perl?:  



• perlintro Running Perl programs:  



• perlintro Safety net:  



• perlintro Basic syntax overview:  



• perlintro Perl variable types:  



• perlintro Variable scoping:  



• perlintro Conditional and looping constructs:  



• perlintro Builtin operators and functions:  



• perlintro Files and I/O:  



• perlintro Regular expressions:  



• perlintro Writing subroutines:  



• perlintro OO Perl:  



• perlintro Using Perl modules:  



perliol


• perliol NAME:  



• perliol SYNOPSIS:  



• perliol DESCRIPTION:  



• perliol TODO:  



DESCRIPTION


• perliol History and Background:  



• perliol Basic Structure:  



• perliol Layers vs Disciplines:  



• perliol Data Structures:  



• perliol Functions and Attributes:  



• perliol Per-instance Data:  



• perliol Layers in action.:  



• perliol Per-instance flag bits:  



• perliol Methods in Detail:  



• perliol Utilities:  



• perliol Implementing PerlIO Layers:  



• perliol Core Layers:  



• perliol Extension Layers:  



perlipc


• perlipc NAME:  



• perlipc DESCRIPTION:  



• perlipc Signals:  



• perlipc Named Pipes:  



• perlipc Using open() for IPC:  



• perlipc Sockets: Client/Server Communication:  



• perlipc TCP Clients with IO::Socket:  



• perlipc TCP Servers with IO::Socket:  



• perlipc UDP: Message Passing:  



• perlipc SysV IPC:  



• perlipc NOTES:  



• perlipc BUGS:  



• perlipc AUTHOR:  



• perlipc SEE ALSO:  



Signals


• perlipc Handling the SIGHUP Signal in Daemons:  



• perlipc Deferred Signals (Safe Signals):  



Using open() for IPC


• perlipc Filehandles:  



• perlipc Background Processes:  



• perlipc Complete Dissociation of Child from Parent:  



• perlipc Safe Pipe Opens:  



• perlipc Avoiding Pipe Deadlocks:  



• perlipc Bidirectional Communication with Another Process:  



• perlipc Bidirectional Communication with Yourself:  



Sockets: Client/Server Communication


• perlipc Internet Line Terminators:  



• perlipc Internet TCP Clients and Servers:  



• perlipc Unix-Domain TCP Clients and Servers:  



TCP Clients with IO::Socket


• perlipc A Simple Client:  



• perlipc A Webget Client:  



• perlipc Interactive Client with IO::Socket:  



NAME warning, lexical warnings warning


• NAME warning, lexical warnings warning NAME:  



• NAME warning, lexical warnings warning DESCRIPTION:  



• NAME warning, lexical warnings warning SEE ALSO:  



• NAME warning, lexical warnings warning AUTHOR:  



DESCRIPTION


• NAME warning, lexical warnings warning Default Warnings and Optional Warnings:  



• NAME warning, lexical warnings warning What's wrong with -w and $^W:  



• NAME warning, lexical warnings warning Controlling Warnings from the Command Line:  



• NAME warning, lexical warnings warning Backward Compatibility:  



• NAME warning, lexical warnings warning Category Hierarchy:  



• NAME warning, lexical warnings warning Fatal Warnings:  



• NAME warning, lexical warnings warning Reporting Warnings from a Module:  



perllocale


• perllocale NAME:  



• perllocale DESCRIPTION:  



• perllocale WHAT IS A LOCALE:  



• perllocale PREPARING TO USE LOCALES:  



• perllocale USING LOCALES:  



• perllocale LOCALE CATEGORIES:  



• perllocale SECURITY:  



• perllocale ENVIRONMENT:  



• perllocale NOTES:  



• perllocale Unicode and UTF-8:  



• perllocale BUGS:  



• perllocale SEE ALSO:  



• perllocale HISTORY:  



USING LOCALES


• perllocale The use locale pragma:  



• perllocale The setlocale function:  



• perllocale Finding locales:  



• perllocale LOCALE PROBLEMS:  



• perllocale Temporarily fixing locale problems:  



• perllocale Permanently fixing locale problems:  



• perllocale Permanently fixing your system's locale configuration:  



• perllocale Fixing system locale configuration:  



• perllocale The localeconv function:  



• perllocale I18N::Langinfo:  



LOCALE CATEGORIES


• perllocale Category LC_COLLATE: Collation 1:  



• perllocale Category LC_CTYPE: Character Types 1:  



• perllocale Category LC_NUMERIC: Numeric Formatting:  



• perllocale Category LC_MONETARY: Formatting of monetary amounts 1:  



• perllocale LC_TIME:  



• perllocale Other categories:  



ENVIRONMENT


• perllocale Examples:  



NOTES


• perllocale Backward compatibility:  



• perllocale I18N:Collate obsolete:  



• perllocale Sort speed and memory use impacts:  



• perllocale write() and LC_NUMERIC:  



• perllocale Freely available locale definitions:  



• perllocale I18n and l10n:  



• perllocale An imperfect standard:  



BUGS


• perllocale Broken systems:  



perllol


• perllol NAME:  



• perllol DESCRIPTION:  



• perllol SEE ALSO:  



• perllol AUTHOR:  



DESCRIPTION


• perllol Declaration and Access of Arrays of Arrays:  



• perllol Growing Your Own:  



• perllol Access and Printing:  



• perllol Slices:  



perlmod


• perlmod NAME:  



• perlmod DESCRIPTION:  



• perlmod SEE ALSO:  



DESCRIPTION


• perlmod Packages:  



• perlmod Symbol Tables:  



• perlmod BEGIN, UNITCHECK, CHECK, INIT and END:  



• perlmod Perl Classes:  



• perlmod Perl Modules:  



• perlmod Making your module threadsafe:  



perlmodinstall


• perlmodinstall NAME:  



• perlmodinstall DESCRIPTION:  



• perlmodinstall PORTABILITY:  



• perlmodinstall HEY:  



• perlmodinstall AUTHOR:  



• perlmodinstall COPYRIGHT:  



DESCRIPTION


• perlmodinstall PREAMBLE:  



perlmodstyle


• perlmodstyle NAME:  



• perlmodstyle INTRODUCTION:  



• perlmodstyle QUICK CHECKLIST:  



• perlmodstyle BEFORE YOU START WRITING A MODULE:  



• perlmodstyle DESIGNING AND WRITING YOUR MODULE:  



• perlmodstyle DOCUMENTING YOUR MODULE:  



• perlmodstyle RELEASE CONSIDERATIONS:  



• perlmodstyle COMMON PITFALLS:  



• perlmodstyle SEE ALSO:  



• perlmodstyle AUTHOR:  



QUICK CHECKLIST


• perlmodstyle Before you start:  



• perlmodstyle The API:  



• perlmodstyle Stability:  



• perlmodstyle Documentation:  



• perlmodstyle Release considerations:  



BEFORE YOU START WRITING A MODULE


• perlmodstyle Has it been done before?:  



• perlmodstyle Do one thing and do it well:  



• perlmodstyle What's in a name?:  



DESIGNING AND WRITING YOUR MODULE


• perlmodstyle To OO or not to OO?:  



• perlmodstyle Designing your API:  



• perlmodstyle Strictness and warnings:  



• perlmodstyle Backwards compatibility:  



• perlmodstyle Error handling and messages:  



DOCUMENTING YOUR MODULE


• perlmodstyle POD:  



• perlmodstyle README, INSTALL, release notes, changelogs:  



RELEASE CONSIDERATIONS


• perlmodstyle Version numbering:  



• perlmodstyle Pre-requisites:  



• perlmodstyle Testing:  



• perlmodstyle Packaging:  



• perlmodstyle Licensing:  



COMMON PITFALLS


• perlmodstyle Reinventing the wheel:  



• perlmodstyle Trying to do too much:  



• perlmodstyle Inappropriate documentation:  



perlmroapi


• perlmroapi NAME:  



• perlmroapi DESCRIPTION:  



• perlmroapi Callbacks:  



• perlmroapi Caching:  



• perlmroapi Examples:  



• perlmroapi AUTHORS:  



perlnewmod


• perlnewmod NAME:  



• perlnewmod DESCRIPTION:  



• perlnewmod AUTHOR:  



• perlnewmod SEE ALSO:  



DESCRIPTION


• perlnewmod Warning:  



• perlnewmod What should I make into a module?:  



• perlnewmod Step-by-step: Preparing the ground:  



• perlnewmod Step-by-step: Making the module:  



• perlnewmod Step-by-step: Distributing your module:  



perlnumber


• perlnumber NAME:  



• perlnumber SYNOPSIS:  



• perlnumber DESCRIPTION:  



• perlnumber Storing numbers:  



• perlnumber Numeric operators and numeric conversions:  



• perlnumber Flavors of Perl numeric operations:  



• perlnumber AUTHOR:  



• perlnumber SEE ALSO:  



NAME object OOP


• NAME object OOP NAME:  



• NAME object OOP DESCRIPTION:  



• NAME object OOP SEE ALSO:  



DESCRIPTION


• NAME object OOP An Object is Simply a Data Structure:  



• NAME object OOP A Class is Simply a Package:  



• NAME object OOP A Method is Simply a Subroutine:  



• NAME object OOP Method Invocation:  



• NAME object OOP Inheritance:  



• NAME object OOP Writing Constructors:  



• NAME object OOP Attributes:  



• NAME object OOP An Aside About Smarter and Safer Code:  



• NAME object OOP Method Call Variations:  



• NAME object OOP Invoking Class Methods:  



• NAME object OOP bless, blessed, and ref:  



• NAME object OOP The UNIVERSAL Class:  



• NAME object OOP AUTOLOAD:  



• NAME object OOP Destructors:  



• NAME object OOP Non-Hash Objects:  



• NAME object OOP Inside-Out objects:  



• NAME object OOP Pseudo-hashes:  



An Object is Simply a Data Structure


• NAME object OOP Objects Are Blessed; Variables Are Not:  



Inheritance


• NAME object OOP How SUPER is Resolved:  



• NAME object OOP Multiple Inheritance:  



• NAME object OOP Method Resolution Order:  



• NAME object OOP Method Resolution Caching:  



Attributes


• NAME object OOP Writing Accessors:  



Method Call Variations


• NAME object OOP Method Names as Strings:  



• NAME object OOP Class Names as Strings:  



• NAME object OOP Subroutine References as Methods:  



• NAME object OOP Deferencing Method Call:  



• NAME object OOP Method Calls on Filehandles:  



Invoking Class Methods


• NAME object OOP Indirect Object Syntax:  



Destructors


• NAME object OOP Global Destruction:  



perlootut


• perlootut NAME:  



• perlootut DATE:  



• perlootut DESCRIPTION:  



• perlootut OBJECT-ORIENTED FUNDAMENTALS:  



• perlootut PERL OO SYSTEMS:  



• perlootut CONCLUSION:  



OBJECT-ORIENTED FUNDAMENTALS


• perlootut Object:  



• perlootut Class:  



• perlootut Methods:  



• perlootut Attributes:  



• perlootut Polymorphism:  



• perlootut Inheritance:  



• perlootut Encapsulation:  



• perlootut Composition:  



• perlootut Roles:  



• perlootut When to Use OO:  



Class


• perlootut Blessing:  



• perlootut Constructor:  



Inheritance


• perlootut Overriding methods and method resolution:  



PERL OO SYSTEMS


• perlootut Moose:  



• perlootut Class::Accessor:  



• perlootut Object::Tiny:  



• perlootut Role::Tiny:  



• perlootut OO System Summary:  



• perlootut Other OO Systems:  



Moose


• perlootut Mouse:  



NAME operator


• NAME operator NAME:  



• NAME operator DESCRIPTION:  



DESCRIPTION


• NAME operator Operator Precedence and Associativity:  



• NAME operator Terms and List Operators (Leftward):  



• NAME operator The Arrow Operator:  



• NAME operator Auto-increment and Auto-decrement:  



• NAME operator Exponentiation:  



• NAME operator Symbolic Unary Operators:  



• NAME operator Binding Operators:  



• NAME operator Multiplicative Operators:  



• NAME operator Additive Operators:  



• NAME operator Shift Operators:  



• NAME operator Named Unary Operators:  



• NAME operator Relational Operators:  



• NAME operator Equality Operators:  



• NAME operator Smartmatch Operator:  



• NAME operator Bitwise And:  



• NAME operator Bitwise Or and Exclusive Or:  



• NAME operator C-style Logical And:  



• NAME operator C-style Logical Or:  



• NAME operator Logical Defined-Or:  



• NAME operator Range Operators:  



• NAME operator Conditional Operator:  



• NAME operator Assignment Operators:  



• NAME operator Comma Operator:  



• NAME operator List Operators (Rightward):  



• NAME operator Logical Not:  



• NAME operator Logical And:  



• NAME operator Logical or and Exclusive Or:  



• NAME operator C Operators Missing From Perl:  



• NAME operator Quote and Quote-like Operators:  



• NAME operator Regexp Quote-Like Operators:  



• NAME operator Quote-Like Operators:  



• NAME operator Gory details of parsing quoted constructs:  



• NAME operator I/O Operators:  



• NAME operator Constant Folding:  



• NAME operator No-ops:  



• NAME operator Bitwise String Operators:  



• NAME operator Integer Arithmetic:  



• NAME operator Floating-point Arithmetic:  



• NAME operator Bigger Numbers:  



Smartmatch Operator


• NAME operator Smartmatching of Objects:  



perlopentut


• perlopentut NAME:  



• perlopentut DESCRIPTION:  



• perlopentut Open à la shell:  



• perlopentut Open à la C:  



• perlopentut Obscure Open Tricks:  



• perlopentut Other I/O Issues:  



• perlopentut SEE ALSO:  



• perlopentut AUTHOR and COPYRIGHT:  



• perlopentut HISTORY:  



Open à la shell


• perlopentut Simple Opens:  



• perlopentut Indirect Filehandles:  



• perlopentut Pipe Opens:  



• perlopentut The Minus File:  



• perlopentut Mixing Reads and Writes:  



• perlopentut Filters:  



Open à la C


• perlopentut Permissions à la mode:  



Obscure Open Tricks


• perlopentut Re-Opening Files (dups):  



• perlopentut Dispelling the Dweomer:  



• perlopentut Paths as Opens:  



• perlopentut Single Argument Open:  



• perlopentut Playing with STDIN and STDOUT:  



Other I/O Issues


• perlopentut Opening Non-File Files:  



• perlopentut Opening Named Pipes:  



• perlopentut Opening Sockets:  



• perlopentut Binary Files:  



• perlopentut File Locking:  



• perlopentut IO Layers:  



perlpacktut


• perlpacktut NAME:  



• perlpacktut DESCRIPTION:  



• perlpacktut The Basic Principle:  



• perlpacktut Packing Text:  



• perlpacktut Packing Numbers:  



• perlpacktut Exotic Templates:  



• perlpacktut Template Grouping:  



• perlpacktut Lengths and Widths:  



• perlpacktut Packing and Unpacking C Structures:  



• perlpacktut Pack Recipes:  



• perlpacktut Funnies Section:  



• perlpacktut Authors:  



Packing Numbers


• perlpacktut Integers:  



• perlpacktut Unpacking a Stack Frame:  



• perlpacktut How to Eat an Egg on a Net:  



• perlpacktut Byte-order modifiers:  



• perlpacktut Floating point Numbers:  



Exotic Templates


• perlpacktut Bit Strings:  



• perlpacktut Uuencoding:  



• perlpacktut Doing Sums:  



• perlpacktut Unicode:  



• perlpacktut Another Portable Binary Encoding:  



Lengths and Widths


• perlpacktut String Lengths:  



• perlpacktut Dynamic Templates:  



• perlpacktut Counting Repetitions:  



• perlpacktut Intel HEX:  



Packing and Unpacking C Structures


• perlpacktut The Alignment Pit:  



• perlpacktut Dealing with Endian-ness:  



• perlpacktut Alignment, Take 2:  



• perlpacktut Alignment, Take 3:  



• perlpacktut Pointers for How to Use Them:  



perlperf


• perlperf NAME:  



• perlperf DESCRIPTION:  



• perlperf OVERVIEW:  



• perlperf GENERAL GUIDELINES:  



• perlperf BENCHMARKS:  



• perlperf PROFILING TOOLS:  



• perlperf SORTING:  



• perlperf LOGGING:  



• perlperf POSTSCRIPT:  



• perlperf SEE ALSO:  



• perlperf AUTHOR:  



OVERVIEW


• perlperf ONE STEP SIDEWAYS:  



• perlperf ONE STEP FORWARD:  



• perlperf ANOTHER STEP SIDEWAYS:  



BENCHMARKS


• perlperf Assigning and Dereferencing Variables.:  



• perlperf Search and replace or tr:  



PROFILING TOOLS


• perlperf Devel::DProf:  



• perlperf Devel::Profiler:  



• perlperf Devel::SmallProf:  



• perlperf Devel::FastProf:  



• perlperf Devel::NYTProf:  



LOGGING


• perlperf Logging if DEBUG (constant):  



SEE ALSO


• perlperf PERLDOCS:  



• perlperf MAN PAGES:  



• perlperf MODULES:  



• perlperf URLS:  



NAME POD plain old documentation


• NAME POD plain old documentation NAME:  



• NAME POD plain old documentation DESCRIPTION:  



• NAME POD plain old documentation SEE ALSO:  



• NAME POD plain old documentation AUTHOR:  



DESCRIPTION


• NAME POD plain old documentation Ordinary Paragraph:  



• NAME POD plain old documentation Verbatim Paragraph:  



• NAME POD plain old documentation Command Paragraph:  



• NAME POD plain old documentation Formatting Codes:  



• NAME POD plain old documentation The Intent:  



• NAME POD plain old documentation Embedding Pods in Perl Modules:  



• NAME POD plain old documentation Hints for Writing Pod:  



perlpodspec


• perlpodspec NAME:  



• perlpodspec DESCRIPTION:  



• perlpodspec Pod Definitions:  



• perlpodspec Pod Commands:  



• perlpodspec Pod Formatting Codes:  



• perlpodspec Notes on Implementing Pod Processors:  



• perlpodspec About L<...> Codes:  



• perlpodspec About =over...=back Regions:  



• perlpodspec About Data Paragraphs and "=begin/=end" Regions:  



• perlpodspec SEE ALSO:  



• perlpodspec AUTHOR:  



perlpodstyle


• perlpodstyle NAME:  



• perlpodstyle DESCRIPTION:  



• perlpodstyle SEE ALSO 1:  



• perlpodstyle AUTHOR 1:  



• perlpodstyle COPYRIGHT AND LICENSE 1:  



perlpolicy


• perlpolicy NAME:  



• perlpolicy DESCRIPTION:  



• perlpolicy GOVERNANCE:  



• perlpolicy MAINTENANCE AND SUPPORT:  



• perlpolicy BACKWARD COMPATIBILITY AND DEPRECATION:  



• perlpolicy MAINTENANCE BRANCHES:  



• perlpolicy CONTRIBUTED MODULES:  



• perlpolicy DOCUMENTATION:  



• perlpolicy CREDITS:  



GOVERNANCE


• perlpolicy Perl 5 Porters:  



BACKWARD COMPATIBILITY AND DEPRECATION


• perlpolicy Terminology:  



MAINTENANCE BRANCHES


• perlpolicy Getting changes into a maint branch:  



CONTRIBUTED MODULES


• perlpolicy A Social Contract about Artistic Control:  



perlport


• perlport NAME:  



• perlport DESCRIPTION:  



• perlport ISSUES:  



• perlport CPAN Testers:  



• perlport PLATFORMS:  



• perlport FUNCTION IMPLEMENTATIONS:  



• perlport Supported Platforms:  



• perlport EOL Platforms (Perl 5.14):  



• perlport Supported Platforms (Perl 5.8):  



• perlport SEE ALSO:  



• perlport AUTHORS / CONTRIBUTORS:  



ISSUES


• perlport Newlines:  



• perlport Numbers endianness and Width:  



• perlport Files and Filesystems:  



• perlport System Interaction:  



• perlport Command names versus file pathnames:  



• perlport Networking:  



• perlport Interprocess Communication (IPC):  



• perlport External Subroutines (XS):  



• perlport Standard Modules:  



• perlport Time and Date:  



• perlport Character sets and character encoding:  



• perlport Internationalisation:  



• perlport System Resources:  



• perlport Security:  



• perlport Style:  



PLATFORMS


• perlport Unix:  



• perlport DOS and Derivatives:  



• perlport VMS:  



• perlport VOS:  



• perlport EBCDIC Platforms:  



• perlport Acorn RISC OS:  



• perlport Other perls:  



FUNCTION IMPLEMENTATIONS


• perlport Alphabetical Listing of Perl Functions:  



perlpragma


• perlpragma NAME:  



• perlpragma DESCRIPTION:  



• perlpragma A basic example:  



• perlpragma Key naming:  



• perlpragma Implementation details:  



NAME regular expression regex regexp


• NAME regular expression regex regexp NAME:  



• NAME regular expression regex regexp DESCRIPTION:  



• NAME regular expression regex regexp BUGS:  



• NAME regular expression regex regexp SEE ALSO:  



DESCRIPTION


• NAME regular expression regex regexp Modifiers:  



• NAME regular expression regex regexp Regular Expressions:  



• NAME regular expression regex regexp Quoting metacharacters:  



• NAME regular expression regex regexp Extended Patterns:  



• NAME regular expression regex regexp Special Backtracking Control Verbs:  



• NAME regular expression regex regexp Backtracking:  



• NAME regular expression regex regexp Version 8 Regular Expressions:  



• NAME regular expression regex regexp Warning on \1 Instead of $1:  



• NAME regular expression regex regexp Repeated Patterns Matching a Zero-length Substring:  



• NAME regular expression regex regexp Combining RE Pieces:  



• NAME regular expression regex regexp Creating Custom RE Engines:  



• NAME regular expression regex regexp PCRE/Python Support:  



Modifiers


• NAME regular expression regex regexp /x:  



• NAME regular expression regex regexp Character set modifiers:  



• NAME regular expression regex regexp /l:  



• NAME regular expression regex regexp /u:  



• NAME regular expression regex regexp /d:  



• NAME regular expression regex regexp /a (and /aa):  



• NAME regular expression regex regexp Which character set modifier is in effect?:  



• NAME regular expression regex regexp Character set modifier behavior prior to Perl 5.14:  



Regular Expressions


• NAME regular expression regex regexp Metacharacters:  



• NAME regular expression regex regexp Quantifiers:  



• NAME regular expression regex regexp Escape sequences:  



• NAME regular expression regex regexp Character Classes and other Special Escapes:  



• NAME regular expression regex regexp Assertions:  



• NAME regular expression regex regexp Capture groups:  



perlreapi


• perlreapi NAME:  



• perlreapi DESCRIPTION:  



• perlreapi Callbacks:  



• perlreapi The REGEXP structure:  



• perlreapi HISTORY:  



• perlreapi AUTHORS:  



• perlreapi LICENSE:  



Callbacks


• perlreapi comp:  



• perlreapi exec:  



• perlreapi intuit:  



• perlreapi checkstr:  



• perlreapi free:  



• perlreapi Numbered capture callbacks:  



• perlreapi Named capture callbacks:  



• perlreapi qr_package:  



• perlreapi dupe:  



Numbered capture callbacks


• perlreapi numbered_buff_FETCH:  



• perlreapi numbered_buff_STORE:  



• perlreapi numbered_buff_LENGTH:  



Named capture callbacks


• perlreapi named_buff:  



• perlreapi named_buff_iter:  



The REGEXP structure


• perlreapi engine:  



• perlreapi mother_re:  



• perlreapi extflags:  



• perlreapi minlen minlenret:  



• perlreapi gofs:  



• perlreapi substrs:  



• perlreapi nparens, lastparen, and lastcloseparen:  



• perlreapi intflags:  



• perlreapi pprivate:  



• perlreapi swap:  



• perlreapi offs:  



• perlreapi precomp prelen:  



• perlreapi paren_names:  



• perlreapi substrs 1:  



• perlreapi subbeg sublen saved_copy:  



• perlreapi wrapped wraplen:  



• perlreapi seen_evals:  



• perlreapi refcnt:  



perlrebackslash


• perlrebackslash NAME:  



• perlrebackslash DESCRIPTION:  



DESCRIPTION


• perlrebackslash The backslash:  



• perlrebackslash All the sequences and escapes:  



• perlrebackslash Character Escapes:  



• perlrebackslash Modifiers:  



• perlrebackslash Character classes:  



• perlrebackslash Referencing:  



• perlrebackslash Assertions:  



• perlrebackslash Misc:  



Character Escapes


• perlrebackslash Fixed characters:  



• perlrebackslash Example:  



• perlrebackslash Control characters:  



• perlrebackslash Example 1:  



• perlrebackslash Named or numbered characters and character sequences:  



• perlrebackslash Example 2:  



• perlrebackslash Octal escapes:  



• perlrebackslash Examples (assuming an ASCII platform):  



• perlrebackslash Disambiguation rules between old-style octal escapes and backreferences:  



• perlrebackslash Hexadecimal escapes:  



• perlrebackslash Examples (assuming an ASCII platform) 1:  



Modifiers


• perlrebackslash Examples:  



Character classes


• perlrebackslash Unicode classes:  



Referencing


• perlrebackslash Absolute referencing:  



• perlrebackslash Examples 1:  



• perlrebackslash Relative referencing:  



• perlrebackslash Examples 2:  



• perlrebackslash Named referencing:  



• perlrebackslash Examples 3:  



Assertions


• perlrebackslash Examples 4:  



Misc


• perlrebackslash Examples 5:  



NAME character class


• NAME character class NAME:  



• NAME character class DESCRIPTION:  



DESCRIPTION


• NAME character class The dot:  



• NAME character class Backslash sequences:  



• NAME character class Bracketed Character Classes:  



Backslash sequences


• NAME character class \N:  



• NAME character class Digits:  



• NAME character class Word characters:  



• NAME character class Whitespace:  



• NAME character class Unicode Properties:  



• NAME character class Examples:  



Bracketed Character Classes


• NAME character class Special Characters Inside a Bracketed Character Class:  



• NAME character class Character Ranges:  



• NAME character class Negation:  



• NAME character class Backslash Sequences:  



• NAME character class POSIX Character Classes:  



• NAME character class Negation of POSIX character classes:  



• NAME character class [= =] and [. .]:  



• NAME character class Examples 1:  



NAME reference pointer data structure structure struct


• NAME reference pointer data structure structure struct NAME:  



• NAME reference pointer data structure structure struct NOTE:  



• NAME reference pointer data structure structure struct DESCRIPTION:  



• NAME reference pointer data structure structure struct WARNING:  



• NAME reference pointer data structure structure struct SEE ALSO:  



DESCRIPTION


• NAME reference pointer data structure structure struct Making References:  



• NAME reference pointer data structure structure struct Using References:  



• NAME reference pointer data structure structure struct Circular References:  



• NAME reference pointer data structure structure struct Symbolic references:  



• NAME reference pointer data structure structure struct Not-so-symbolic references:  



• NAME reference pointer data structure structure struct Pseudo-hashes: Using an array as a hash:  



• NAME reference pointer data structure structure struct Function Templates:  



perlreftut


• perlreftut NAME:  



• perlreftut DESCRIPTION:  



• perlreftut Who Needs Complicated Data Structures?:  



• perlreftut The Solution:  



• perlreftut Syntax:  



• perlreftut Solution:  



• perlreftut The Rest:  



• perlreftut Summary:  



• perlreftut Credits:  



Syntax


• perlreftut Making References:  



• perlreftut Using References:  



• perlreftut An Example:  



• perlreftut Arrow Rule:  



Making References


• perlreftut Make Rule 1:  



Using References


• perlreftut Use Rule 1:  



• perlreftut Use Rule 2:  



Credits


• perlreftut Distribution Conditions:  



perlreguts


• perlreguts NAME:  



• perlreguts DESCRIPTION:  



• perlreguts OVERVIEW:  



• perlreguts Process Overview:  



• perlreguts MISCELLANEOUS:  



• perlreguts SEE ALSO:  



• perlreguts AUTHOR:  



• perlreguts LICENCE:  



• perlreguts REFERENCES:  



OVERVIEW


• perlreguts A quick note on terms:  



• perlreguts What is a regular expression engine?:  



• perlreguts Structure of a Regexp Program:  



Structure of a Regexp Program


• perlreguts High Level:  



• perlreguts Regops:  



• perlreguts What regop is next?:  



Process Overview


• perlreguts Compilation:  



• perlreguts Execution:  



Compilation


• perlreguts Parsing for size:  



• perlreguts Parsing for construction:  



• perlreguts Parse Call Graph and a Grammar:  



• perlreguts Debug Output:  



• perlreguts Peep-hole Optimisation and Analysis:  



Execution


• perlreguts Start position and no-match optimisations:  



• perlreguts Program execution:  



MISCELLANEOUS


• perlreguts Unicode and Localisation Support:  



• perlreguts Base Structures:  



Base Structures


• perlreguts Perl's pprivate structure:  



perlrequick


• perlrequick NAME:  



• perlrequick DESCRIPTION:  



• perlrequick The Guide:  



• perlrequick BUGS:  



• perlrequick SEE ALSO:  



• perlrequick AUTHOR AND COPYRIGHT:  



The Guide


• perlrequick Simple word matching:  



• perlrequick Using character classes:  



• perlrequick Matching this or that:  



• perlrequick Grouping things and hierarchical matching:  



• perlrequick Extracting matches:  



• perlrequick Matching repetitions:  



• perlrequick More matching:  



• perlrequick Search and replace:  



• perlrequick The split operator:  



AUTHOR AND COPYRIGHT


• perlrequick Acknowledgments:  



perlreref


• perlreref NAME:  



• perlreref DESCRIPTION:  



• perlreref AUTHOR:  



• perlreref SEE ALSO:  



• perlreref THANKS:  



DESCRIPTION


• perlreref OPERATORS:  



• perlreref SYNTAX:  



• perlreref ESCAPE SEQUENCES:  



• perlreref CHARACTER CLASSES:  



• perlreref ANCHORS:  



• perlreref QUANTIFIERS:  



• perlreref EXTENDED CONSTRUCTS:  



• perlreref VARIABLES:  



• perlreref FUNCTIONS:  



• perlreref TERMINOLOGY:  



TERMINOLOGY


• perlreref Titlecase:  



• perlreref Foldcase:  



perlretut


• perlretut NAME:  



• perlretut DESCRIPTION:  



• perlretut Part 1: The basics:  



• perlretut Part 2: Power tools:  



• perlretut BUGS:  



• perlretut SEE ALSO:  



• perlretut AUTHOR AND COPYRIGHT:  



Part 1: The basics


• perlretut Simple word matching:  



• perlretut Using character classes:  



• perlretut Matching this or that:  



• perlretut Grouping things and hierarchical matching:  



• perlretut Extracting matches:  



• perlretut Backreferences:  



• perlretut Relative backreferences:  



• perlretut Named backreferences:  



• perlretut Alternative capture group numbering:  



• perlretut Position information:  



• perlretut Non-capturing groupings:  



• perlretut Matching repetitions:  



• perlretut Possessive quantifiers:  



• perlretut Building a regexp:  



• perlretut Using regular expressions in Perl:  



Using regular expressions in Perl


• perlretut Prohibiting substitution:  



• perlretut Global matching:  



• perlretut Search and replace:  



• perlretut The split function:  



Part 2: Power tools


• perlretut More on characters, strings, and character classes:  



• perlretut Compiling and saving regular expressions:  



• perlretut Composing regular expressions at runtime:  



• perlretut Embedding comments and modifiers in a regular expression:  



• perlretut Looking ahead and looking behind:  



• perlretut Using independent subexpressions to prevent backtracking:  



• perlretut Conditional expressions:  



• perlretut Defining named patterns:  



• perlretut Recursive patterns:  



• perlretut A bit of magic: executing Perl code in a regular expression:  



• perlretut Backtracking control verbs:  



• perlretut Pragmas and debugging:  



AUTHOR AND COPYRIGHT


• perlretut Acknowledgments:  



perlrun


• perlrun NAME:  



• perlrun SYNOPSIS:  



• perlrun DESCRIPTION:  



• perlrun ENVIRONMENT:  



DESCRIPTION


• perlrun #! and quoting on non-Unix systems:  



• perlrun Location of Perl:  



• perlrun Command Switches:  



perlsec


• perlsec NAME:  



• perlsec DESCRIPTION:  



• perlsec SECURITY VULNERABILITY CONTACT INFORMATION:  



• perlsec SECURITY MECHANISMS AND CONCERNS:  



• perlsec SEE ALSO:  



SECURITY MECHANISMS AND CONCERNS


• perlsec Taint mode:  



• perlsec Laundering and Detecting Tainted Data:  



• perlsec Switches On the "#!" Line:  



• perlsec Taint mode and @INC:  



• perlsec Cleaning Up Your Path:  



• perlsec Security Bugs:  



• perlsec Protecting Your Programs:  



• perlsec Unicode:  



• perlsec Algorithmic Complexity Attacks:  



perlsource


• perlsource NAME:  



• perlsource DESCRIPTION:  



• perlsource FINDING YOUR WAY AROUND:  



FINDING YOUR WAY AROUND


• perlsource C code:  



• perlsource Core modules:  



• perlsource Tests:  



• perlsource Documentation:  



• perlsource Hacking tools and documentation:  



• perlsource Build system:  



• perlsource AUTHORS:  



• perlsource MANIFEST:  



perlstyle


• perlstyle NAME:  



• perlstyle DESCRIPTION:  



NAME subroutine function


• NAME subroutine function NAME:  



• NAME subroutine function SYNOPSIS:  



• NAME subroutine function DESCRIPTION:  



• NAME subroutine function SEE ALSO:  



DESCRIPTION


• NAME subroutine function Private Variables via my():  



• NAME subroutine function Persistent Private Variables:  



• NAME subroutine function Temporary Values via local():  



• NAME subroutine function Lvalue subroutines:  



• NAME subroutine function Passing Symbol Table Entries (typeglobs):  



• NAME subroutine function When to Still Use local():  



• NAME subroutine function Pass by Reference:  



• NAME subroutine function Prototypes:  



• NAME subroutine function Constant Functions:  



• NAME subroutine function Overriding Built-in Functions:  



• NAME subroutine function Autoloading:  



• NAME subroutine function Subroutine Attributes:  



Persistent Private Variables


• NAME subroutine function Persistent variables via state():  



• NAME subroutine function Persistent variables with closures:  



Temporary Values via local()


• NAME subroutine function Grammatical note on local():  



• NAME subroutine function Localization of special variables:  



• NAME subroutine function Localization of globs:  



• NAME subroutine function Localization of elements of composite types:  



• NAME subroutine function Localized deletion of elements of composite types:  



NAME syntax


• NAME syntax NAME:  



• NAME syntax DESCRIPTION:  



DESCRIPTION


• NAME syntax Declarations:  



• NAME syntax Comments:  



• NAME syntax Simple Statements:  



• NAME syntax Truth and Falsehood:  



• NAME syntax Statement Modifiers:  



• NAME syntax Compound Statements:  



• NAME syntax Loop Control:  



• NAME syntax For Loops:  



• NAME syntax Foreach Loops:  



• NAME syntax Basic BLOCKs:  



• NAME syntax Switch Statements:  



• NAME syntax Goto:  



• NAME syntax The Ellipsis Statement:  



• NAME syntax PODs: Embedded Documentation:  



• NAME syntax Plain Old Comments (Not!):  



• NAME syntax Experimental Details on given and when:  



Experimental Details on given and when


• NAME syntax Breaking out:  



• NAME syntax Fall-through:  



• NAME syntax Return value:  



• NAME syntax Switching in a loop:  



• NAME syntax Differences from Perl 6:  



perlthrtut


• perlthrtut NAME:  



• perlthrtut DESCRIPTION:  



• perlthrtut What Is A Thread Anyway?:  



• perlthrtut Threaded Program Models:  



• perlthrtut What kind of threads are Perl threads?:  



• perlthrtut Thread-Safe Modules:  



• perlthrtut Thread Basics:  



• perlthrtut Threads And Data:  



• perlthrtut Synchronization and control:  



• perlthrtut General Thread Utility Routines:  



• perlthrtut A Complete Example:  



• perlthrtut Different implementations of threads:  



• perlthrtut Performance considerations:  



• perlthrtut Process-scope Changes:  



• perlthrtut Thread-Safety of System Libraries:  



• perlthrtut Conclusion:  



• perlthrtut SEE ALSO:  



• perlthrtut Bibliography:  



• perlthrtut Acknowledgements:  



• perlthrtut AUTHOR:  



• perlthrtut Copyrights:  



Threaded Program Models


• perlthrtut Boss/Worker:  



• perlthrtut Work Crew:  



• perlthrtut Pipeline:  



Thread Basics


• perlthrtut Basic Thread Support:  



• perlthrtut A Note about the Examples:  



• perlthrtut Creating Threads:  



• perlthrtut Waiting For A Thread To Exit:  



• perlthrtut Ignoring A Thread:  



• perlthrtut Process and Thread Termination:  



Threads And Data


• perlthrtut Shared And Unshared Data:  



• perlthrtut Thread Pitfalls: Races:  



Synchronization and control


• perlthrtut Controlling access: lock():  



• perlthrtut A Thread Pitfall: Deadlocks:  



• perlthrtut Queues: Passing Data Around:  



• perlthrtut Semaphores: Synchronizing Data Access:  



• perlthrtut Basic semaphores:  



• perlthrtut Advanced Semaphores:  



• perlthrtut Waiting for a Condition:  



• perlthrtut Giving up control:  



General Thread Utility Routines


• perlthrtut What Thread Am I In?:  



• perlthrtut Thread IDs:  



• perlthrtut Are These Threads The Same?:  



• perlthrtut What Threads Are Running?:  



Bibliography


• perlthrtut Introductory Texts:  



• perlthrtut OS-Related References:  



• perlthrtut Other References:  



NAME tie


• NAME tie NAME:  



• NAME tie SYNOPSIS:  



• NAME tie DESCRIPTION:  



• NAME tie SEE ALSO:  



• NAME tie BUGS:  



• NAME tie AUTHOR:  



DESCRIPTION


• NAME tie Tying Scalars:  



• NAME tie Tying Arrays:  



• NAME tie Tying Hashes:  



• NAME tie Tying FileHandles:  



• NAME tie UNTIE this 4:  



• NAME tie The untie Gotcha:  



perltodo


• perltodo NAME:  



• perltodo DESCRIPTION:  



perltooc


• perltooc NAME:  



• perltooc DESCRIPTION:  



perltoot


• perltoot NAME:  



• perltoot DESCRIPTION:  



perltrap


• perltrap NAME:  



• perltrap DESCRIPTION:  



DESCRIPTION


• perltrap Awk Traps:  



• perltrap C/C++ Traps:  



• perltrap Sed Traps:  



• perltrap Shell Traps:  



• perltrap Perl Traps:  



• perltrap Perl4 to Perl5 Traps:  



• perltrap Discontinuance, Deprecation, and BugFix traps 1:  



• perltrap Parsing Traps 1:  



• perltrap Numerical Traps 1:  



• perltrap General data type traps 1:  



• perltrap Context Traps - scalar, list contexts 1:  



• perltrap Precedence Traps 1:  



• perltrap General Regular Expression Traps using s///, etc. 1:  



• perltrap Subroutine, Signal, Sorting Traps 1:  



• perltrap OS Traps 1:  



• perltrap Interpolation Traps:  



• perltrap DBM Traps 1:  



• perltrap Unclassified Traps 1:  



perlunicode


• perlunicode NAME:  



• perlunicode DESCRIPTION:  



• perlunicode BUGS:  



• perlunicode SEE ALSO:  



DESCRIPTION


• perlunicode Important Caveats:  



• perlunicode Byte and Character Semantics:  



• perlunicode Effects of Character Semantics:  



• perlunicode Unicode Character Properties:  



• perlunicode User-Defined Character Properties:  



• perlunicode User-Defined Case Mappings (for serious hackers only):  



• perlunicode Character Encodings for Input and Output:  



• perlunicode Unicode Regular Expression Support Level:  



• perlunicode Unicode Encodings:  



• perlunicode Non-character code points:  



• perlunicode Beyond Unicode code points:  



• perlunicode Security Implications of Unicode:  



• perlunicode Unicode in Perl on EBCDIC:  



• perlunicode Locales:  



• perlunicode When Unicode Does Not Happen:  



• perlunicode The "Unicode Bug":  



• perlunicode Forcing Unicode in Perl (Or Unforcing Unicode in Perl):  



• perlunicode Using Unicode in XS:  



• perlunicode Hacking Perl to work on earlier Unicode versions (for very serious hackers only):  



Unicode Character Properties


• perlunicode General_Category:  



• perlunicode Bidirectional Character Types:  



• perlunicode Scripts:  



• perlunicode Use of "Is" Prefix:  



• perlunicode Blocks:  



• perlunicode Other Properties:  



BUGS


• perlunicode Interaction with Locales:  



• perlunicode Problems with characters in the Latin-1 Supplement range:  



• perlunicode Interaction with Extensions:  



• perlunicode Speed:  



• perlunicode Problems on EBCDIC platforms:  



• perlunicode Porting code from perl-5.6.X:  



perlunifaq


• perlunifaq NAME:  



• perlunifaq Q and A:  



• perlunifaq INTERNALS:  



• perlunifaq AUTHOR:  



• perlunifaq SEE ALSO:  



Q and A


• perlunifaq perlunitut isn't really a Unicode tutorial, is it?:  



• perlunifaq What character encodings does Perl support?:  



• perlunifaq Which version of perl should I use?:  



• perlunifaq What about binary data, like images?:  



• perlunifaq When should I decode or encode?:  



• perlunifaq What if I don't decode?:  



• perlunifaq What if I don't encode?:  



• perlunifaq Is there a way to automatically decode or encode?:  



• perlunifaq What if I don't know which encoding was used?:  



• perlunifaq Can I use Unicode in my Perl sources?:  



• perlunifaq Data::Dumper doesn't restore the UTF8 flag; is it broken?:  



• perlunifaq Why do regex character classes sometimes match only in the ASCII range?:  



• perlunifaq Why do some characters not uppercase or lowercase correctly?:  



• perlunifaq How can I determine if a string is a text string or a binary string?:  



• perlunifaq How do I convert from encoding FOO to encoding BAR?:  



• perlunifaq What are decode_utf8 and encode_utf8?:  



• perlunifaq What is a "wide character"?:  



INTERNALS


• perlunifaq What is "the UTF8 flag"?:  



• perlunifaq What about the use bytes pragma?:  



• perlunifaq What about the use encoding pragma?:  



• perlunifaq What is the difference between :encoding and :utf8?:  



• perlunifaq What's the difference between UTF-8 and utf8?:  



• perlunifaq I lost track; what encoding is the internal format really?:  



perluniintro


• perluniintro NAME:  



• perluniintro DESCRIPTION:  



• perluniintro UNICODE IN OLDER PERLS:  



• perluniintro SEE ALSO:  



• perluniintro ACKNOWLEDGMENTS:  



• perluniintro AUTHOR, COPYRIGHT, AND LICENSE:  



DESCRIPTION


• perluniintro Unicode:  



• perluniintro Perl's Unicode Support:  



• perluniintro Perl's Unicode Model:  



• perluniintro Unicode and EBCDIC:  



• perluniintro Creating Unicode:  



• perluniintro Handling Unicode:  



• perluniintro Legacy Encodings:  



• perluniintro Unicode I/O:  



• perluniintro Displaying Unicode As Text:  



• perluniintro Special Cases:  



• perluniintro Advanced Topics:  



• perluniintro Miscellaneous:  



• perluniintro Questions With Answers:  



• perluniintro Hexadecimal Notation:  



• perluniintro Further Resources:  



perlunitut


• perlunitut NAME:  



• perlunitut DESCRIPTION:  



• perlunitut SUMMARY:  



• perlunitut Q and A (or FAQ):  



• perlunitut ACKNOWLEDGEMENTS:  



• perlunitut AUTHOR:  



• perlunitut SEE ALSO:  



DESCRIPTION


• perlunitut Definitions:  



• perlunitut Your new toolkit:  



• perlunitut I/O flow (the actual 5 minute tutorial):  



Definitions


• perlunitut Unicode:  



• perlunitut UTF-8:  



• perlunitut Text strings (character strings):  



• perlunitut Binary strings (byte strings):  



• perlunitut Encoding:  



• perlunitut Decoding:  



• perlunitut Internal format:  



perlutil


• perlutil NAME:  



• perlutil DESCRIPTION:  



• perlutil LIST OF UTILITIES:  



• perlutil SEE ALSO:  



LIST OF UTILITIES


• perlutil Documentation:  



• perlutil Converters:  



• perlutil Administration:  



• perlutil Development:  



• perlutil General tools:  



• perlutil Installation:  



perlvar


• perlvar NAME:  



• perlvar DESCRIPTION:  



• perlvar SPECIAL VARIABLES:  



DESCRIPTION


• perlvar The Syntax of Variable Names:  



SPECIAL VARIABLES


• perlvar General Variables:  



• perlvar Variables related to regular expressions:  



• perlvar Variables related to filehandles:  



• perlvar Error Variables:  



• perlvar Variables related to the interpreter state:  



• perlvar Deprecated and removed variables:  



Variables related to filehandles


• perlvar Variables related to formats:  



perlvms


• perlvms NAME:  



• perlvms DESCRIPTION:  



• perlvms Installation:  



• perlvms Organization of Perl Images:  



• perlvms File specifications:  



• perlvms PERL5LIB and PERLLIB:  



• perlvms The Perl Forked Debugger:  



• perlvms PERL_VMS_EXCEPTION_DEBUG:  



• perlvms Command line:  



• perlvms Perl functions:  



• perlvms Perl variables:  



• perlvms Standard modules with VMS-specific differences:  



• perlvms Revision date:  



• perlvms AUTHOR:  



Organization of Perl Images


• perlvms Core Images:  



• perlvms Perl Extensions:  



• perlvms Installing static extensions:  



• perlvms Installing dynamic extensions:  



File specifications


• perlvms Syntax:  



• perlvms Filename Case:  



• perlvms Symbolic Links:  



• perlvms Wildcard expansion:  



• perlvms Pipes:  



Command line


• perlvms I/O redirection and backgrounding:  



• perlvms Command line switches:  



Standard modules with VMS-specific differences


• perlvms SDBM_File:  












Next: , Previous: Top, Up: Top   [Contents]





1 perl





• perl NAME:  



• perl SYNOPSIS:  



• perl GETTING HELP:  



• perl DESCRIPTION:  



• perl AVAILABILITY:  



• perl ENVIRONMENT:  



• perl AUTHOR:  



• perl FILES:  



• perl SEE ALSO:  



• perl DIAGNOSTICS:  



• perl BUGS:  



• perl NOTES:  












Next: , Up: perl   [Contents]





1.1 NAME



perl - The Perl 5 language interpreter









Next: , Previous: perl NAME, Up: perl   [Contents]





1.2 SYNOPSIS



perl	[ -sTtuUWX ]
	[ -hv ] [ -V[:configvar] ]
	[ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
	[ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]
	[ -Idir ] [ -m[-]module ] [ -M[-]’module...’ ] [ -f ]
	[ -C [number/list] ]
	[ -S ]
	[ -x[dir] ]
	[ -i[extension] ]
	[ [-e|-E] ’command’ ] [ – ] [ programfile ] [ argument ]...









Next: , Previous: perl SYNOPSIS, Up: perl   [Contents]





1.3 GETTING HELP



The perldoc program gives you access to all the documentation that comes
with Perl.  You can get more documentation, tutorials and community support
online at http://www.perl.org/.



If you’re new to Perl, you should start by running perldoc perlintro,
which is a general intro for beginners and provides some background to help
you navigate the rest of Perl’s extensive documentation.  Run perldoc
perldoc to learn more things you can do with perldoc.



For ease of access, the Perl manual has been split up into several sections.





• perl Overview:  



• perl Tutorials:  



• perl Reference Manual:  



• perl Internals and C Language Interface:  



• perl Miscellaneous:  



• perl Language-Specific:  



• perl Platform-Specific:  



• perl Stubs for Deleted Documents:  












Next: , Up: perl GETTING HELP   [Contents]





1.3.1 Overview



    perl                Perl overview (this section)
    perlintro           Perl introduction for beginners
    perltoc             Perl documentation table of contents









Next: , Previous: perl Overview, Up: perl GETTING HELP   [Contents]





1.3.2 Tutorials



    perlreftut          Perl references short introduction
    perldsc             Perl data structures intro
    perllol             Perl data structures: arrays of arrays

    perlrequick         Perl regular expressions quick start
    perlretut           Perl regular expressions tutorial

    perlootut           Perl OO tutorial for beginners

    perlperf            Perl Performance and Optimization Techniques

    perlstyle           Perl style guide

    perlcheat           Perl cheat sheet
    perltrap            Perl traps for the unwary
    perldebtut          Perl debugging tutorial

    perlfaq             Perl frequently asked questions
      perlfaq1          General Questions About Perl
      perlfaq2          Obtaining and Learning about Perl
      perlfaq3          Programming Tools
      perlfaq4          Data Manipulation
      perlfaq5          Files and Formats
      perlfaq6          Regexes
      perlfaq7          Perl Language Issues
      perlfaq8          System Interaction
      perlfaq9          Networking









Next: , Previous: perl Tutorials, Up: perl GETTING HELP   [Contents]





1.3.3 Reference Manual



    perlsyn             Perl syntax
    perldata            Perl data structures
    perlop              Perl operators and precedence
    perlsub             Perl subroutines
    perlfunc            Perl built-in functions
      perlopentut       Perl open() tutorial
      perlpacktut       Perl pack() and unpack() tutorial
    perlpod             Perl plain old documentation
    perlpodspec         Perl plain old documentation format specification
    perlpodstyle        Perl POD style guide
    perlrun             Perl execution and options
    perldiag            Perl diagnostic messages
    perllexwarn         Perl warnings and their control
    perldebug           Perl debugging
    perlvar             Perl predefined variables
    perlre              Perl regular expressions, the rest of the story
    perlrebackslash     Perl regular expression backslash sequences
    perlrecharclass     Perl regular expression character classes
    perlreref           Perl regular expressions quick reference
    perlref             Perl references, the rest of the story
    perlform            Perl formats
    perlobj             Perl objects
    perltie             Perl objects hidden behind simple variables
      perldbmfilter     Perl DBM filters

    perlipc             Perl interprocess communication
    perlfork            Perl fork() information
    perlnumber          Perl number semantics

    perlthrtut          Perl threads tutorial

    perlport            Perl portability guide
    perllocale          Perl locale support
    perluniintro        Perl Unicode introduction
    perlunicode         Perl Unicode support
    perlunifaq          Perl Unicode FAQ
    perluniprops        Index of Unicode Version 6.0.0 properties in Perl
    perlunitut          Perl Unicode tutorial
    perlebcdic          Considerations for running Perl on EBCDIC platforms

    perlsec             Perl security

    perlmod             Perl modules: how they work
    perlmodlib          Perl modules: how to write and use
    perlmodstyle        Perl modules: how to write modules with style
    perlmodinstall      Perl modules: how to install from CPAN
    perlnewmod          Perl modules: preparing a new module for distribution
    perlpragma          Perl modules: writing a user pragma

    perlutil            utilities packaged with the Perl distribution

    perlfilter          Perl source filters

    perldtrace          Perl's support for DTrace

    perlglossary        Perl Glossary









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1.3.4 Internals and C Language Interface



    perlembed           Perl ways to embed perl in your C or C++ application
    perldebguts         Perl debugging guts and tips
    perlxstut           Perl XS tutorial
    perlxs              Perl XS application programming interface
    perlxstypemap       Perl XS C/Perl type conversion tools
    perlclib            Internal replacements for standard C library functions
    perlguts            Perl internal functions for those doing extensions
    perlcall            Perl calling conventions from C
    perlmroapi          Perl method resolution plugin interface
    perlreapi           Perl regular expression plugin interface
    perlreguts          Perl regular expression engine internals

    perlapi             Perl API listing (autogenerated)
    perlintern          Perl internal functions (autogenerated)
    perliol             C API for Perl's implementation of IO in Layers
    perlapio            Perl internal IO abstraction interface

    perlhack            Perl hackers guide
    perlsource          Guide to the Perl source tree
    perlinterp          Overview of the Perl interpreter source and how it works
    perlhacktut         Walk through the creation of a simple C code patch
    perlhacktips        Tips for Perl core C code hacking
    perlpolicy          Perl development policies
    perlgit             Using git with the Perl repository









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1.3.5 Miscellaneous



    perlbook            Perl book information
    perlcommunity       Perl community information

    perldoc             Look up Perl documentation in Pod format

    perlhist            Perl history records
    perldelta           Perl changes since previous version
    perl5162delta       Perl changes in version 5.16.2
    perl5161delta       Perl changes in version 5.16.1
    perl5160delta       Perl changes in version 5.16.0
    perl5160delta       Perl changes in version 5.16.0
    perl5143delta       Perl changes in version 5.14.3
    perl5142delta       Perl changes in version 5.14.2
    perl5141delta       Perl changes in version 5.14.1
    perl5140delta       Perl changes in version 5.14.0
    perl5124delta       Perl changes in version 5.12.4
    perl5123delta       Perl changes in version 5.12.3
    perl5122delta       Perl changes in version 5.12.2
    perl5121delta       Perl changes in version 5.12.1
    perl5120delta       Perl changes in version 5.12.0
    perl5101delta       Perl changes in version 5.10.1
    perl5100delta       Perl changes in version 5.10.0
    perl589delta        Perl changes in version 5.8.9
    perl588delta        Perl changes in version 5.8.8
    perl587delta        Perl changes in version 5.8.7
    perl586delta        Perl changes in version 5.8.6
    perl585delta        Perl changes in version 5.8.5
    perl584delta        Perl changes in version 5.8.4
    perl583delta        Perl changes in version 5.8.3
    perl582delta        Perl changes in version 5.8.2
    perl581delta        Perl changes in version 5.8.1
    perl58delta         Perl changes in version 5.8.0
    perl561delta        Perl changes in version 5.6.1
    perl56delta         Perl changes in version 5.6
    perl5005delta       Perl changes in version 5.005
    perl5004delta       Perl changes in version 5.004

    perlexperiment      A listing of experimental features in Perl

    perlartistic        Perl Artistic License
    perlgpl             GNU General Public License









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1.3.6 Language-Specific



    perlcn              Perl for Simplified Chinese (in EUC-CN)
    perljp              Perl for Japanese (in EUC-JP)
    perlko              Perl for Korean (in EUC-KR)
    perltw              Perl for Traditional Chinese (in Big5)









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1.3.7 Platform-Specific



    perlaix             Perl notes for AIX
    perlamiga           Perl notes for AmigaOS
    perlbeos            Perl notes for BeOS
    perlbs2000          Perl notes for POSIX-BC BS2000
    perlce              Perl notes for WinCE
    perlcygwin          Perl notes for Cygwin
    perldgux            Perl notes for DG/UX
    perldos             Perl notes for DOS
    perlepoc            Perl notes for EPOC
    perlfreebsd         Perl notes for FreeBSD
    perlhaiku           Perl notes for Haiku
    perlhpux            Perl notes for HP-UX
    perlhurd            Perl notes for Hurd
    perlirix            Perl notes for Irix
    perllinux           Perl notes for Linux
    perlmacos           Perl notes for Mac OS (Classic)
    perlmacosx          Perl notes for Mac OS X
    perlmpeix           Perl notes for MPE/iX
    perlnetware         Perl notes for NetWare
    perlopenbsd         Perl notes for OpenBSD
    perlos2             Perl notes for OS/2
    perlos390           Perl notes for OS/390
    perlos400           Perl notes for OS/400
    perlplan9           Perl notes for Plan 9
    perlqnx             Perl notes for QNX
    perlriscos          Perl notes for RISC OS
    perlsolaris         Perl notes for Solaris
    perlsymbian         Perl notes for Symbian
    perltru64           Perl notes for Tru64
    perluts             Perl notes for UTS
    perlvmesa           Perl notes for VM/ESA
    perlvms             Perl notes for VMS
    perlvos             Perl notes for Stratus VOS
    perlwin32           Perl notes for Windows









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1.3.8 Stubs for Deleted Documents



    perlboot            
    perlbot             
    perltodo
    perltooc            
    perltoot            



On a Unix-like system, these documentation files will usually also be
available as manpages for use with the man program.



In general, if something strange has gone wrong with your program and you’re
not sure where you should look for help, try the -w switch first.  It will
often point out exactly where the trouble is.









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1.4 DESCRIPTION



Perl officially stands for Practical Extraction and Report Language,
except when it doesn’t.



Perl was originally a language optimized for scanning arbitrary
text files, extracting information from those text files, and printing
reports based on that information.  It quickly became a good language
for many system management tasks. Over the years, Perl has grown into
a general-purpose programming language. It’s widely used for everything
from quick "one-liners" to full-scale application development.



The language is intended to be practical (easy to use, efficient,
complete) rather than beautiful (tiny, elegant, minimal).



Perl combines (in the author’s opinion, anyway) some of the best
features of C, sed, awk, and sh, so people familiar with
those languages should have little difficulty with it.  (Language
historians will also note some vestiges of csh, Pascal, and even
BASIC-PLUS.)  Expression syntax corresponds closely to C
expression syntax.  Unlike most Unix utilities, Perl does not
arbitrarily limit the size of your data–if you’ve got the memory,
Perl can slurp in your whole file as a single string.  Recursion is of
unlimited depth.  And the tables used by hashes (sometimes called
"associative arrays") grow as necessary to prevent degraded
performance.  Perl can use sophisticated pattern matching techniques to
scan large amounts of data quickly.  Although optimized for
scanning text, Perl also has many excellent tools for slicing
and dicing binary data.



But wait, there’s more...



Begun in 1993 (see perlhist NAME), Perl version 5 is nearly a complete
rewrite that provides the following additional benefits:





Okay, that’s definitely enough hype.









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1.5 AVAILABILITY



Perl is available for most operating systems, including virtually
all Unix-like platforms.  See perlport Supported Platforms
for a listing.









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1.6 ENVIRONMENT



See perlrun NAME.









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1.7 AUTHOR



Larry Wall <larry@wall.org>, with the help of oodles of other folks.



If your Perl success stories and testimonials may be of help to others 
who wish to advocate the use of Perl in their applications, 
or if you wish to simply express your gratitude to Larry and the 
Perl developers, please write to perl-thanks@perl.org .









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1.8 FILES



 "@INC"                 locations of perl libraries









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1.9 SEE ALSO



 http://www.perl.org/       the Perl homepage
 http://www.perl.com/       Perl articles (O'Reilly)
 http://www.cpan.org/       the Comprehensive Perl Archive
 http://www.pm.org/         the Perl Mongers









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1.10 DIAGNOSTICS



The use warnings pragma (and the -w switch) produces some 
lovely diagnostics.



See perldiag NAME for explanations of all Perl’s diagnostics.  The use
diagnostics pragma automatically turns Perl’s normally terse warnings
and errors into these longer forms.



Compilation errors will tell you the line number of the error, with an
indication of the next token or token type that was to be examined.
(In a script passed to Perl via -e switches, each
-e is counted as one line.)



Setuid scripts have additional constraints that can produce error
messages such as "Insecure dependency".  See perlsec NAME.



Did we mention that you should definitely consider using the -w
switch?









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1.11 BUGS



The -w switch is not mandatory.



Perl is at the mercy of your machine’s definitions of various
operations such as type casting, atof(), and floating-point
output with sprintf().



If your stdio requires a seek or eof between reads and writes on a
particular stream, so does Perl.  (This doesn’t apply to sysread()
and syswrite().)



While none of the built-in data types have any arbitrary size limits
(apart from memory size), there are still a few arbitrary limits:  a
given variable name may not be longer than 251 characters.  Line numbers
displayed by diagnostics are internally stored as short integers,
so they are limited to a maximum of 65535 (higher numbers usually being
affected by wraparound).



You may mail your bug reports (be sure to include full configuration
information as output by the myconfig program in the perl source
tree, or by perl -V) to perlbug@perl.org .  If you’ve succeeded
in compiling perl, the (perlbug) script in the utils/ subdirectory
can be used to help mail in a bug report.



Perl actually stands for Pathologically Eclectic Rubbish Lister, but
don’t tell anyone I said that.









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1.12 NOTES



The Perl motto is "There’s more than one way to do it."  Divining
how many more is left as an exercise to the reader.



The three principal virtues of a programmer are Laziness,
Impatience, and Hubris.  See the Camel Book for why.









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2 perlapio





• perlapio NAME:  



• perlapio SYNOPSIS:  



• perlapio DESCRIPTION:  












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2.1 NAME



perlapio - perl’s IO abstraction interface.









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2.2 SYNOPSIS



    #define PERLIO_NOT_STDIO 0    /* For co-existence with stdio only */
    #include <perlio.h>           /* Usually via #include <perl.h> */

    PerlIO *PerlIO_stdin(void);
    PerlIO *PerlIO_stdout(void);
    PerlIO *PerlIO_stderr(void);

    PerlIO *PerlIO_open(const char *path,const char *mode);
    PerlIO *PerlIO_fdopen(int fd, const char *mode);
    PerlIO *PerlIO_reopen(const char *path, const char *mode, PerlIO *old);  /* deprecated */
    int     PerlIO_close(PerlIO *f);

    int     PerlIO_stdoutf(const char *fmt,...)
    int     PerlIO_puts(PerlIO *f,const char *string);
    int     PerlIO_putc(PerlIO *f,int ch);
    int     PerlIO_write(PerlIO *f,const void *buf,size_t numbytes);
    int     PerlIO_printf(PerlIO *f, const char *fmt,...);
    int     PerlIO_vprintf(PerlIO *f, const char *fmt, va_list args);
    int     PerlIO_flush(PerlIO *f);

    int     PerlIO_eof(PerlIO *f);
    int     PerlIO_error(PerlIO *f);
    void    PerlIO_clearerr(PerlIO *f);

    int     PerlIO_getc(PerlIO *d);
    int     PerlIO_ungetc(PerlIO *f,int ch);
    int     PerlIO_read(PerlIO *f, void *buf, size_t numbytes);

    int     PerlIO_fileno(PerlIO *f);

    void    PerlIO_setlinebuf(PerlIO *f);

    Off_t   PerlIO_tell(PerlIO *f);
    int     PerlIO_seek(PerlIO *f, Off_t offset, int whence);
    void    PerlIO_rewind(PerlIO *f);

    int     PerlIO_getpos(PerlIO *f, SV *save);        /* prototype changed */
    int     PerlIO_setpos(PerlIO *f, SV *saved);       /* prototype changed */

    int     PerlIO_fast_gets(PerlIO *f);
    int     PerlIO_has_cntptr(PerlIO *f);
    int     PerlIO_get_cnt(PerlIO *f);
    char   *PerlIO_get_ptr(PerlIO *f);
    void    PerlIO_set_ptrcnt(PerlIO *f, char *ptr, int count);

    int     PerlIO_canset_cnt(PerlIO *f);              /* deprecated */
    void    PerlIO_set_cnt(PerlIO *f, int count);      /* deprecated */

    int     PerlIO_has_base(PerlIO *f);
    char   *PerlIO_get_base(PerlIO *f);
    int     PerlIO_get_bufsiz(PerlIO *f);

    PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
    FILE   *PerlIO_exportFILE(PerlIO *f, int flags);
    FILE   *PerlIO_findFILE(PerlIO *f);
    void    PerlIO_releaseFILE(PerlIO *f,FILE *stdio);

    int     PerlIO_apply_layers(PerlIO *f, const char *mode, const char *layers);
    int     PerlIO_binmode(PerlIO *f, int ptype, int imode, const char *layers);
    void    PerlIO_debug(const char *fmt,...)









Previous: perlapio SYNOPSIS, Up: perlapio   [Contents]





2.3 DESCRIPTION



Perl’s source code, and extensions that want maximum portability,
should use the above functions instead of those defined in ANSI C’s
stdio.h.  The perl headers (in particular "perlio.h") will
#define them to the I/O mechanism selected at Configure time.



The functions are modeled on those in stdio.h, but parameter order
has been "tidied up a little".



PerlIO * takes the place of FILE *. Like FILE * it should be
treated as opaque (it is probably safe to assume it is a pointer to
something).



There are currently three implementations:





1. USE_STDIO




All above are #define’d to stdio functions or are trivial wrapper
functions which call stdio. In this case only PerlIO * is a FILE *.
This has been the default implementation since the abstraction was
introduced in perl5.003_02.





2. USE_SFIO




A "legacy" implementation in terms of the "sfio" library. Used for
some specialist applications on Unix machines ("sfio" is not widely
ported away from Unix).  Most of above are #define’d to the sfio
functions. PerlIO * is in this case Sfio_t *.





3. USE_PERLIO




Introduced just after perl5.7.0, this is a re-implementation of the
above abstraction which allows perl more control over how IO is done
as it decouples IO from the way the operating system and C library
choose to do things. For USE_PERLIO PerlIO * has an extra layer of
indirection - it is a pointer-to-a-pointer.  This allows the PerlIO *
to remain with a known value while swapping the implementation around
underneath at run time. In this case all the above are true (but
very simple) functions which call the underlying implementation.



This is the only implementation for which PerlIO_apply_layers()
does anything "interesting".



The USE_PERLIO implementation is described in perliol NAME.








Because "perlio.h" is a thin layer (for efficiency) the semantics of
these functions are somewhat dependent on the underlying implementation.
Where these variations are understood they are noted below.



Unless otherwise noted, functions return 0 on success, or a negative
value (usually EOF which is usually -1) and set errno on error.





PerlIO_stdin(), PerlIO_stdout(), PerlIO_stderr()




Use these rather than stdin, stdout, stderr. They are written
to look like "function calls" rather than variables because this makes
it easier to make them function calls if platform cannot export data
to loaded modules, or if (say) different "threads" might have different
values.





PerlIO_open(path, mode), PerlIO_fdopen(fd,mode)




These correspond to fopen()/fdopen() and the arguments are the same.
Return NULL and set errno if there is an error.  There may be an
implementation limit on the number of open handles, which may be lower
than the limit on the number of open files - errno may not be set
when NULL is returned if this limit is exceeded.





PerlIO_reopen(path,mode,f)




While this currently exists in all three implementations perl itself
does not use it. As perl does not use it, it is not well tested.



Perl prefers to dup the new low-level descriptor to the descriptor
used by the existing PerlIO. This may become the behaviour of this
function in the future.





PerlIO_printf(f,fmt,...), PerlIO_vprintf(f,fmt,a)




These are fprintf()/vfprintf() equivalents.





PerlIO_stdoutf(fmt,...)




This is printf() equivalent. printf is #defined to this function,
so it is (currently) legal to use printf(fmt,...) in perl sources.





PerlIO_read(f,buf,count), PerlIO_write(f,buf,count)




These correspond functionally to fread() and fwrite() but the
arguments and return values are different.  The PerlIO_read() and
PerlIO_write() signatures have been modeled on the more sane low level
read() and write() functions instead: The "file" argument is passed
first, there is only one "count", and the return value can distinguish
between error and EOF.



Returns a byte count if successful (which may be zero or
positive), returns negative value and sets errno on error.
Depending on implementation errno may be EINTR if operation was
interrupted by a signal.





PerlIO_close(f)




Depending on implementation errno may be EINTR if operation was
interrupted by a signal.





PerlIO_puts(f,s), PerlIO_putc(f,c)




These correspond to fputs() and fputc().
Note that arguments have been revised to have "file" first.





PerlIO_ungetc(f,c)




This corresponds to ungetc().  Note that arguments have been revised
to have "file" first.  Arranges that next read operation will return
the byte c.  Despite the implied "character" in the name only
values in the range 0..0xFF are defined. Returns the byte c on
success or -1 (EOF) on error.  The number of bytes that can be
"pushed back" may vary, only 1 character is certain, and then only if
it is the last character that was read from the handle.





PerlIO_getc(f)




This corresponds to getc().
Despite the c in the name only byte range 0..0xFF is supported.
Returns the character read or -1 (EOF) on error.





PerlIO_eof(f)




This corresponds to feof().  Returns a true/false indication of
whether the handle is at end of file.  For terminal devices this may
or may not be "sticky" depending on the implementation.  The flag is
cleared by PerlIO_seek(), or PerlIO_rewind().





PerlIO_error(f)




This corresponds to ferror().  Returns a true/false indication of
whether there has been an IO error on the handle.





PerlIO_fileno(f)




This corresponds to fileno(), note that on some platforms, the meaning
of "fileno" may not match Unix. Returns -1 if the handle has no open
descriptor associated with it.





PerlIO_clearerr(f)




This corresponds to clearerr(), i.e., clears ’error’ and (usually)
’eof’ flags for the "stream". Does not return a value.





PerlIO_flush(f)




This corresponds to fflush().  Sends any buffered write data to the
underlying file.  If called with NULL this may flush all open
streams (or core dump with some USE_STDIO implementations).  Calling
on a handle open for read only, or on which last operation was a read
of some kind may lead to undefined behaviour on some USE_STDIO
implementations.  The USE_PERLIO (layers) implementation tries to
behave better: it flushes all open streams when passed NULL, and
attempts to retain data on read streams either in the buffer or by
seeking the handle to the current logical position.





PerlIO_seek(f,offset,whence)




This corresponds to fseek().  Sends buffered write data to the
underlying file, or discards any buffered read data, then positions
the file descriptor as specified by offset and whence (sic).
This is the correct thing to do when switching between read and write
on the same handle (see issues with PerlIO_flush() above).  Offset is
of type Off_t which is a perl Configure value which may not be same
as stdio’s off_t.





PerlIO_tell(f)




This corresponds to ftell().  Returns the current file position, or
(Off_t) -1 on error.  May just return value system "knows" without
making a system call or checking the underlying file descriptor (so
use on shared file descriptors is not safe without a
PerlIO_seek()). Return value is of type Off_t which is a perl
Configure value which may not be same as stdio’s off_t.





PerlIO_getpos(f,p), PerlIO_setpos(f,p)




These correspond (loosely) to fgetpos() and fsetpos(). Rather than
stdio’s Fpos_t they expect a "Perl Scalar Value" to be passed. What is
stored there should be considered opaque. The layout of the data may
vary from handle to handle.  When not using stdio or if platform does
not have the stdio calls then they are implemented in terms of
PerlIO_tell() and PerlIO_seek().





PerlIO_rewind(f)




This corresponds to rewind(). It is usually defined as being



    PerlIO_seek(f,(Off_t)0L, SEEK_SET);
    PerlIO_clearerr(f);





PerlIO_tmpfile()




This corresponds to tmpfile(), i.e., returns an anonymous PerlIO or
NULL on error.  The system will attempt to automatically delete the
file when closed.  On Unix the file is usually unlink-ed just after
it is created so it does not matter how it gets closed. On other
systems the file may only be deleted if closed via PerlIO_close()
and/or the program exits via exit.  Depending on the implementation
there may be "race conditions" which allow other processes access to
the file, though in general it will be safer in this regard than
ad. hoc. schemes.





PerlIO_setlinebuf(f)




This corresponds to setlinebuf().  Does not return a value. What
constitutes a "line" is implementation dependent but usually means
that writing "\n" flushes the buffer.  What happens with things like
"this\nthat" is uncertain.  (Perl core uses it only when "dumping";
it has nothing to do with $| auto-flush.)










• perlapio Co-existence with stdio:  



• perlapio "Fast gets" Functions:  



• perlapio Other Functions:  












Next: , Up: perlapio DESCRIPTION   [Contents]





2.3.1 Co-existence with stdio



There is outline support for co-existence of PerlIO with stdio.
Obviously if PerlIO is implemented in terms of stdio there is no
problem. However in other cases then mechanisms must exist to create a
FILE * which can be passed to library code which is going to use stdio
calls.



The first step is to add this line:



   #define PERLIO_NOT_STDIO 0



before including any perl header files. (This will probably become
the default at some point).  That prevents "perlio.h" from attempting
to #define stdio functions onto PerlIO functions.



XS code is probably better using "typemap" if it expects FILE *
arguments.  The standard typemap will be adjusted to comprehend any
changes in this area.





PerlIO_importFILE(f,mode)




Used to get a PerlIO * from a FILE *.



The mode argument should be a string as would be passed to
fopen/PerlIO_open.  If it is NULL then - for legacy support - the code
will (depending upon the platform and the implementation) either
attempt to empirically determine the mode in which f is open, or
use "r+" to indicate a read/write stream.



Once called the FILE * should ONLY be closed by calling
PerlIO_close() on the returned PerlIO *.



The PerlIO is set to textmode. Use PerlIO_binmode if this is
not the desired mode.



This is not the reverse of PerlIO_exportFILE().





PerlIO_exportFILE(f,mode)




Given a PerlIO * create a ’native’ FILE * suitable for passing to code
expecting to be compiled and linked with ANSI C stdio.h.  The mode
argument should be a string as would be passed to fopen/PerlIO_open.
If it is NULL then - for legacy support - the FILE * is opened in same
mode as the PerlIO *.



The fact that such a FILE * has been ’exported’ is recorded, (normally
by pushing a new :stdio "layer" onto the PerlIO *), which may affect
future PerlIO operations on the original PerlIO *.  You should not
call fclose() on the file unless you call PerlIO_releaseFILE()
to disassociate it from the PerlIO *.  (Do not use PerlIO_importFILE()
for doing the disassociation.)



Calling this function repeatedly will create a FILE * on each call
(and will push an :stdio layer each time as well).





PerlIO_releaseFILE(p,f)




Calling PerlIO_releaseFILE informs PerlIO that all use of FILE * is
complete. It is removed from the list of ’exported’ FILE *s, and the
associated PerlIO * should revert to its original behaviour.



Use this to disassociate a file from a PerlIO * that was associated
using PerlIO_exportFILE().





PerlIO_findFILE(f)




Returns a native FILE * used by a stdio layer. If there is none, it
will create one with PerlIO_exportFILE. In either case the FILE *
should be considered as belonging to PerlIO subsystem and should
only be closed by calling PerlIO_close().














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2.3.2 "Fast gets" Functions



In addition to standard-like API defined so far above there is an
"implementation" interface which allows perl to get at internals of
PerlIO.  The following calls correspond to the various FILE_xxx macros
determined by Configure - or their equivalent in other
implementations. This section is really of interest to only those
concerned with detailed perl-core behaviour, implementing a PerlIO
mapping or writing code which can make use of the "read ahead" that
has been done by the IO system in the same way perl does. Note that
any code that uses these interfaces must be prepared to do things the
traditional way if a handle does not support them.





PerlIO_fast_gets(f)




Returns true if implementation has all the interfaces required to
allow perl’s sv_gets to "bypass" normal IO mechanism.  This can
vary from handle to handle.



  PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
                        PerlIO_canset_cnt(f) && \
                        'Can set pointer into buffer'





PerlIO_has_cntptr(f)




Implementation can return pointer to current position in the "buffer"
and a count of bytes available in the buffer.  Do not use this - use
PerlIO_fast_gets.





PerlIO_get_cnt(f)




Return count of readable bytes in the buffer. Zero or negative return
means no more bytes available.





PerlIO_get_ptr(f)




Return pointer to next readable byte in buffer, accessing via the
pointer (dereferencing) is only safe if PerlIO_get_cnt() has returned
a positive value.  Only positive offsets up to value returned by
PerlIO_get_cnt() are allowed.





PerlIO_set_ptrcnt(f,p,c)




Set pointer into buffer, and a count of bytes still in the
buffer. Should be used only to set pointer to within range implied by
previous calls to PerlIO_get_ptr and PerlIO_get_cnt. The two
values must be consistent with each other (implementation may only
use one or the other or may require both).





PerlIO_canset_cnt(f)




Implementation can adjust its idea of number of bytes in the buffer.
Do not use this - use PerlIO_fast_gets.





PerlIO_set_cnt(f,c)




Obscure - set count of bytes in the buffer. Deprecated.  Only usable
if PerlIO_canset_cnt() returns true.  Currently used in only doio.c to
force count less than -1 to -1.  Perhaps should be PerlIO_set_empty or
similar.  This call may actually do nothing if "count" is deduced from
pointer and a "limit".  Do not use this - use PerlIO_set_ptrcnt().





PerlIO_has_base(f)




Returns true if implementation has a buffer, and can return pointer
to whole buffer and its size. Used by perl for -T / -B tests.
Other uses would be very obscure...





PerlIO_get_base(f)




Return start of buffer. Access only positive offsets in the buffer
up to the value returned by PerlIO_get_bufsiz().





PerlIO_get_bufsiz(f)




Return the total number of bytes in the buffer, this is neither the
number that can be read, nor the amount of memory allocated to the
buffer. Rather it is what the operating system and/or implementation
happened to read() (or whatever) last time IO was requested.














Previous: perlapio "Fast gets" Functions, Up: perlapio DESCRIPTION   [Contents]





2.3.3 Other Functions





PerlIO_apply_layers(f,mode,layers)




The new interface to the USE_PERLIO implementation. The layers ":crlf"
and ":raw" are only ones allowed for other implementations and those
are silently ignored. (As of perl5.8 ":raw" is deprecated.)  Use
PerlIO_binmode() below for the portable case.





PerlIO_binmode(f,ptype,imode,layers)




The hook used by perl’s binmode operator.
ptype is perl’s character for the kind of IO:





’<’ read






’>’ write






’+’ read/write









imode is O_BINARY or O_TEXT.



layers is a string of layers to apply, only ":crlf" makes sense in
the non USE_PERLIO case. (As of perl5.8 ":raw" is deprecated in favour
of passing NULL.)



Portable cases are:



    PerlIO_binmode(f,ptype,O_BINARY,NULL);
and
    PerlIO_binmode(f,ptype,O_TEXT,":crlf");



On Unix these calls probably have no effect whatsoever.  Elsewhere
they alter "\n" to CR,LF translation and possibly cause a special text
"end of file" indicator to be written or honoured on read. The effect
of making the call after doing any IO to the handle depends on the
implementation. (It may be ignored, affect any data which is already
buffered as well, or only apply to subsequent data.)





PerlIO_debug(fmt,...)




PerlIO_debug is a printf()-like function which can be used for
debugging.  No return value. Its main use is inside PerlIO where using
real printf, warn() etc. would recursively call PerlIO and be a
problem.



PerlIO_debug writes to the file named by $ENV{’PERLIO_DEBUG’} typical
use might be



  Bourne shells (sh, ksh, bash, zsh, ash, ...):
   PERLIO_DEBUG=/dev/tty ./perl somescript some args

  Csh/Tcsh:
   setenv PERLIO_DEBUG /dev/tty
   ./perl somescript some args

  If you have the "env" utility:
   env PERLIO_DEBUG=/dev/tty ./perl somescript some args

  Win32:
   set PERLIO_DEBUG=CON
   perl somescript some args



If $ENV{’PERLIO_DEBUG’} is not set PerlIO_debug() is a no-op.














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3 perlartistic





• perlartistic NAME:  



• perlartistic SYNOPSIS:  



• perlartistic DESCRIPTION:  



• perlartistic The "Artistic License":  












Next: , Up: perlartistic   [Contents]





3.1 NAME



perlartistic - the Perl Artistic License









Next: , Previous: perlartistic NAME, Up: perlartistic   [Contents]





3.2 SYNOPSIS



 You can refer to this document in Pod via "L<perlartistic>"
 Or you can see this document by entering "perldoc perlartistic"









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3.3 DESCRIPTION



Perl is free software; you can redistribute it and/or modify
it under the terms of either:



        a) the GNU General Public License as published by the Free
        Software Foundation; either version 1, or (at your option) any
        later version, or

        b) the "Artistic License" which comes with this Kit.



This is "The Artistic License".
It’s here so that modules, programs, etc., that want to declare
this as their distribution license can link to it.



For the GNU General Public License, see perlgpl NAME.









Previous: perlartistic DESCRIPTION, Up: perlartistic   [Contents]





3.4 The "Artistic License"





• perlartistic Preamble:  



• perlartistic Definitions:  



• perlartistic Conditions:  












Next: , Up: perlartistic The "Artistic License"   [Contents]





3.4.1 Preamble



The intent of this document is to state the conditions under which a
Package may be copied, such that the Copyright Holder maintains some
semblance of artistic control over the development of the package,
while giving the users of the package the right to use and distribute
the Package in a more-or-less customary fashion, plus the right to make
reasonable modifications.









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3.4.2 Definitions





"Package"




refers to the collection of files distributed by the
Copyright Holder, and derivatives of that collection of files created
through textual modification.





"Standard Version"




refers to such a Package if it has not been
modified, or has been modified in accordance with the wishes of the
Copyright Holder as specified below.





"Copyright Holder"




is whoever is named in the copyright or
copyrights for the package.





"You"




is you, if you’re thinking about copying or distributing this Package.





"Reasonable copying fee"




is whatever you can justify on the basis
of media cost, duplication charges, time of people involved, and so on.
(You will not be required to justify it to the Copyright Holder, but
only to the computing community at large as a market that must bear the
fee.)





"Freely Available"




means that no fee is charged for the item
itself, though there may be fees involved in handling the item. It also
means that recipients of the item may redistribute it under the same
conditions they received it.














Previous: perlartistic Definitions, Up: perlartistic The "Artistic License"   [Contents]





3.4.3 Conditions



    

  1.  You may make and give away verbatim copies of the source form of the
Standard Version of this Package without restriction, provided that you
duplicate all of the original copyright notices and associated disclaimers.


  2.  You may apply bug fixes, portability fixes and other modifications
derived from the Public Domain or from the Copyright Holder.  A Package
modified in such a way shall still be considered the Standard Version.


  3.  You may otherwise modify your copy of this Package in any way, provided
that you insert a prominent notice in each changed file stating how and
when you changed that file, and provided that you do at least ONE of the
following:


    

    a)
    

    

    place your modifications in the Public Domain or otherwise make them
Freely Available, such as by posting said modifications to Usenet or an
equivalent medium, or placing the modifications on a major archive site
such as uunet.uu.net, or by allowing the Copyright Holder to include
your modifications in the Standard Version of the Package.

    

    

    b)
    

    

    use the modified Package only within your corporation or organization.

    

    

    c)
    

    

    rename any non-standard executables so the names do not conflict with
standard executables, which must also be provided, and provide a
separate manual page for each non-standard executable that clearly
documents how it differs from the Standard Version.

    

    

    d)
    

    

    make other distribution arrangements with the Copyright Holder.

    

    

    


  4.  You may distribute the programs of this Package in object code or
executable form, provided that you do at least ONE of the following:


    

    a)
    

    

    distribute a Standard Version of the executables and library files,
together with instructions (in the manual page or equivalent) on where
to get the Standard Version.

    

    

    b)
    

    

    accompany the distribution with the machine-readable source of the
Package with your modifications.

    

    

    c)
    

    

    give non-standard executables non-standard names, and clearly
document the differences in manual pages (or equivalent), together with
instructions on where to get the Standard Version.

    

    

    d)
    

    

    make other distribution arrangements with the Copyright Holder.

    

    

    


  5.  You may charge a reasonable copying fee for any distribution of this
Package.  You may charge any fee you choose for support of this
Package.  You may not charge a fee for this Package itself.  However,
you may distribute this Package in aggregate with other (possibly
commercial) programs as part of a larger (possibly commercial) software
distribution provided that you do not advertise this Package as a
product of your own.  You may embed this Package’s interpreter within
an executable of yours (by linking); this shall be construed as a mere
form of aggregation, provided that the complete Standard Version of the
interpreter is so embedded.


  6.  The scripts and library files supplied as input to or produced as
output from the programs of this Package do not automatically fall
under the copyright of this Package, but belong to whoever generated
them, and may be sold commercially, and may be aggregated with this
Package.  If such scripts or library files are aggregated with this
Package via the so-called "undump" or "unexec" methods of producing a
binary executable image, then distribution of such an image shall
neither be construed as a distribution of this Package nor shall it
fall under the restrictions of Paragraphs 3 and 4, provided that you do
not represent such an executable image as a Standard Version of this
Package.


  7.  C subroutines (or comparably compiled subroutines in other
languages) supplied by you and linked into this Package in order to
emulate subroutines and variables of the language defined by this
Package shall not be considered part of this Package, but are the
equivalent of input as in Paragraph 6, provided these subroutines do
not change the language in any way that would cause it to fail the
regression tests for the language.


  8.  Aggregation of this Package with a commercial distribution is always
permitted provided that the use of this Package is embedded; that is,
when no overt attempt is made to make this Package’s interfaces visible
to the end user of the commercial distribution.  Such use shall not be
construed as a distribution of this Package.


  9.  The name of the Copyright Holder may not be used to endorse or promote
products derived from this software without specific prior written permission.


  10.  THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.





The End









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4 perlbook





• perlbook NAME:  



• perlbook DESCRIPTION:  












Next: , Up: perlbook   [Contents]





4.1 NAME



perlbook - Books about and related to Perl









Previous: perlbook NAME, Up: perlbook   [Contents]





4.2 DESCRIPTION



There are many books on Perl and Perl-related. A few of these are
good, some are OK, but many aren’t worth your money. There is a list
of these books, some with extensive reviews, at http://books.perl.org/
. We list some of the books here, and while listing a book implies our
endorsement, don’t think that not including a book means anything.



Most of these books are available online through Safari Books Online
( http://safaribooksonline.com/ ).





• perlbook The most popular books:  



• perlbook References:  



• perlbook Tutorials:  



• perlbook Task-Oriented:  



• perlbook Special Topics:  



• perlbook Free (as in beer) books:  



• perlbook Other interesting, non-Perl books:  



• perlbook A note on freshness:  



• perlbook Get your book listed:  












Next: , Up: perlbook DESCRIPTION   [Contents]





4.2.1 The most popular books



The major reference book on Perl, written by the creator of Perl, is
Programming Perl:





Programming Perl (the "Camel Book"):




        by Larry Wall, Tom Christiansen, and Jon Orwant
        ISBN 978-0-596-00027-1  [3rd edition July 2000]
        http://oreilly.com/catalog/9780596000271/








The Ram is a cookbook with hundreds of examples of using Perl to
accomplish specific tasks:





The Perl Cookbook (the "Ram Book"):




        by Tom Christiansen and Nathan Torkington,
            with Foreword by Larry Wall
        ISBN 978-0-596-00313-5 [2nd Edition August 2003]
        http://oreilly.com/catalog/9780596003135/








If you want to learn the basics of Perl, you might start with the
Llama book, which assumes that you already know a little about
programming:





Learning Perl  (the "Llama Book")




        by Randal L. Schwartz, Tom Phoenix, and brian d foy
        ISBN 978-0-596-52011-3 [5th edition June 2008]
        http://oreilly.com/catalog/9780596520113








The tutorial started in the Llama continues in the Alpaca, which
introduces the intermediate features of references, data structures,
object-oriented programming, and modules:





Intermediate Perl (the "Alpaca Book")




        by Randal L. Schwartz and brian d foy, with Tom Phoenix
                foreword by Damian Conway
        ISBN 978-0-596-00478-1 [1st edition March 2006]
        http://oreilly.com/catalog/9780596004781/














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4.2.2 References



You might want to keep these desktop references close by your keyboard:





Perl 5 Pocket Reference




        by Johan Vromans
        ISBN 978-0-596-00374-6 [4th edition July 2002]
        http://oreilly.com/catalog/9780596003746/





Perl Debugger Pocket Reference




        by Richard Foley
        ISBN 978-0-596-00503-0 [1st edition January 2004]
        http://oreilly.com/catalog/9780596005030/





Regular Expression Pocket Reference




        by Tony Stubblebine
        ISBN 978-0-596-51427-3 [July 2007]
        http://oreilly.com/catalog/9780596514273/














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4.2.3 Tutorials





Beginning Perl




        by James Lee
        ISBN 1-59059-391-X [3rd edition April 2010]
        http://www.apress.com/9781430227939





Learning Perl




        by Randal L. Schwartz, Tom Phoenix, and brian d foy
        ISBN 978-0-596-52010-6 [5th edition June 2008]
        http://oreilly.com/catalog/9780596520106





Intermediate Perl (the "Alpaca Book")




        by Randal L. Schwartz and brian d foy, with Tom Phoenix
                foreword by Damian Conway
        ISBN 0-596-10206-2 [1st edition March 2006]
        http://oreilly.com/catalog/9780596102067





Mastering Perl




        by brian d foy
        ISBN 978-0-596-10206-7 [1st edition July 2007]
        http://www.oreilly.com/catalog/9780596527242





Effective Perl Programming




        by Joseph N. Hall, Joshua A. McAdams, brian d foy
        ISBN 0-321-49694-9 [2nd edition 2010]
        http://www.effectiveperlprogramming.com/














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4.2.4 Task-Oriented





Writing Perl Modules for CPAN




        by Sam Tregar
        ISBN 1-59059-018-X [1st edition August 2002]
        http://www.apress.com/9781590590188





The Perl Cookbook




        by Tom Christiansen and Nathan Torkington
            with foreword by Larry Wall
        ISBN 1-56592-243-3 [2nd edition August 2003]
        http://oreilly.com/catalog/9780596003135





Automating System Administration with Perl




        by David N. Blank-Edelman
        ISBN 978-0-596-00639-6 [2nd edition May 2009]
        http://oreilly.com/catalog/9780596006396





Real World SQL Server Administration with Perl




        by Linchi Shea
        ISBN 1-59059-097-X [1st edition July 2003]
        http://www.apress.com/9781590590973














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4.2.5 Special Topics





Regular Expressions Cookbook




        by Jan Goyvaerts and Steven Levithan
        ISBN 978-0-596-52069-4 [May 2009]
        http://oreilly.com/catalog/9780596520694





Programming the Perl DBI




        by Tim Bunce and Alligator Descartes
        ISBN 978-1-56592-699-8 [February 2000]
        http://oreilly.com/catalog/9781565926998





Perl Best Practices




        by Damian Conway
        ISBN: 978-0-596-00173-5 [1st edition July 2005]
        http://oreilly.com/catalog/9780596001735





Higher-Order Perl




        by Mark-Jason Dominus
        ISBN: 1-55860-701-3 [1st edition March 2005]
        http://hop.perl.plover.com/





Mastering Regular Expressions




        by Jeffrey E. F. Friedl
        ISBN 978-0-596-52812-6 [3rd edition August 2006]
        http://oreilly.com/catalog/9780596528126





Network Programming with Perl




        by Lincoln Stein
        ISBN 0-201-61571-1 [1st edition 2001]
        http://www.pearsonhighered.com/educator/product/Network-Programming-with-Perl/9780201615715.page





Perl Template Toolkit




        by Darren Chamberlain, Dave Cross, and Andy Wardley
        ISBN 978-0-596-00476-7 [December 2003]
        http://oreilly.com/catalog/9780596004767





Object Oriented Perl




        by Damian Conway
            with foreword by Randal L. Schwartz
        ISBN 1-884777-79-1 [1st edition August 1999]
        http://www.manning.com/conway/





Data Munging with Perl




        by Dave Cross
        ISBN 1-930110-00-6 [1st edition 2001]
        http://www.manning.com/cross





Mastering Perl/Tk




        by Steve Lidie and Nancy Walsh
        ISBN 978-1-56592-716-2 [1st edition January 2002]
        http://oreilly.com/catalog/9781565927162





Extending and Embedding Perl




        by Tim Jenness and Simon Cozens
        ISBN 1-930110-82-0 [1st edition August 2002]
        http://www.manning.com/jenness





Pro Perl Debugging




        by Richard Foley with Andy Lester
        ISBN 1-59059-454-1 [1st edition July 2005]
        http://www.apress.com/9781590594544














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4.2.6 Free (as in beer) books



Some of these books are available as free downloads.



Higher-Order Perl: http://hop.perl.plover.com/









Next: , Previous: perlbook Free (as in beer) books, Up: perlbook DESCRIPTION   [Contents]





4.2.7 Other interesting, non-Perl books



You might notice several familiar Perl concepts in this collection of
ACM columns from Jon Bentley. The similarity to the title of the major
Perl book (which came later) is not completely accidental:





Programming Pearls




        by Jon Bentley
        ISBN 978-0-201-65788-3 [2 edition, October 1999]





More Programming Pearls




        by Jon Bentley
        ISBN 0-201-11889-0 [January 1988]














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4.2.8 A note on freshness



Each version of Perl comes with the documentation that was current at
the time of release. This poses a problem for content such as book
lists. There are probably very nice books published after this list
was included in your Perl release, and you can check the latest
released version at http://perldoc.perl.org/perlbook.html .



Some of the books we’ve listed appear almost ancient in internet
scale, but we’ve included those books because they still describe the
current way of doing things. Not everything in Perl changes every day.
Many of the beginner-level books, too, go over basic features and
techniques that are still valid today. In general though, we try to
limit this list to books published in the past five years.









Previous: perlbook A note on freshness, Up: perlbook DESCRIPTION   [Contents]





4.2.9 Get your book listed



If your Perl book isn’t listed and you think it should be, let us know.









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5 perlboot





• perlboot NAME:  



• perlboot DESCRIPTION:  












Next: , Up: perlboot   [Contents]





5.1 NAME



perlboot - This document has been deleted









Previous: perlboot NAME, Up: perlboot   [Contents]





5.2 DESCRIPTION



For information on OO programming with Perl, please see perlootut NAME
and (perlobj).









Next: , Previous: perlboot, Up: Top   [Contents]





6 perlbot





• perlbot NAME:  



• perlbot DESCRIPTION:  












Next: , Up: perlbot   [Contents]





6.1 NAME



perlbot - This document has been deleted









Previous: perlbot NAME, Up: perlbot   [Contents]





6.2 DESCRIPTION



For information on OO programming with Perl, please see perlootut NAME
and (perlobj).









Next: , Previous: perlbot, Up: Top   [Contents]





7 perlcall





• perlcall NAME:  



• perlcall DESCRIPTION:  



• perlcall THE CALL_ FUNCTIONS:  



• perlcall FLAG VALUES:  



• perlcall EXAMPLES:  



• perlcall LIGHTWEIGHT CALLBACKS:  



• perlcall SEE ALSO:  



• perlcall AUTHOR:  



• perlcall DATE:  












Next: , Up: perlcall   [Contents]





7.1 NAME



perlcall - Perl calling conventions from C









Next: , Previous: perlcall NAME, Up: perlcall   [Contents]





7.2 DESCRIPTION



The purpose of this document is to show you how to call Perl subroutines
directly from C, i.e., how to write callbacks.



Apart from discussing the C interface provided by Perl for writing
callbacks the document uses a series of examples to show how the
interface actually works in practice.  In addition some techniques for
coding callbacks are covered.



Examples where callbacks are necessary include





Although the techniques described here are applicable when embedding
Perl in a C program, this is not the primary goal of this document.
There are other details that must be considered and are specific to
embedding Perl. For details on embedding Perl in C refer to
perlembed NAME.



Before you launch yourself head first into the rest of this document,
it would be a good idea to have read the following two documents–(perlxs)
and perlguts NAME.









Next: , Previous: perlcall DESCRIPTION, Up: perlcall   [Contents]





7.3 THE CALL_ FUNCTIONS



Although this stuff is easier to explain using examples, you first need
be aware of a few important definitions.



Perl has a number of C functions that allow you to call Perl
subroutines.  They are



    I32 call_sv(SV* sv, I32 flags);
    I32 call_pv(char *subname, I32 flags);
    I32 call_method(char *methname, I32 flags);
    I32 call_argv(char *subname, I32 flags, register char **argv);



The key function is call_sv.  All the other functions are
fairly simple wrappers which make it easier to call Perl subroutines in
special cases. At the end of the day they will all call call_sv
to invoke the Perl subroutine.



All the call_* functions have a flags parameter which is
used to pass a bit mask of options to Perl.  This bit mask operates
identically for each of the functions.  The settings available in the
bit mask are discussed in FLAG VALUES.



Each of the functions will now be discussed in turn.





call_sv




call_sv takes two parameters. The first, sv, is an SV*.
This allows you to specify the Perl subroutine to be called either as a
C string (which has first been converted to an SV) or a reference to a
subroutine. The section, Using call_sv, shows how you can make
use of call_sv.





call_pv




The function, call_pv, is similar to call_sv except it
expects its first parameter to be a C char* which identifies the Perl
subroutine you want to call, e.g., call_pv("fred", 0).  If the
subroutine you want to call is in another package, just include the
package name in the string, e.g., "pkg::fred".





call_method




The function call_method is used to call a method from a Perl
class.  The parameter methname corresponds to the name of the method
to be called.  Note that the class that the method belongs to is passed
on the Perl stack rather than in the parameter list. This class can be
either the name of the class (for a static method) or a reference to an
object (for a virtual method).  See (perlobj) for more information on
static and virtual methods and Using call_method for an example
of using call_method.





call_argv




call_argv calls the Perl subroutine specified by the C string
stored in the subname parameter. It also takes the usual flags
parameter.  The final parameter, argv, consists of a NULL-terminated
list of C strings to be passed as parameters to the Perl subroutine.
See Using call_argv.








All the functions return an integer. This is a count of the number of
items returned by the Perl subroutine. The actual items returned by the
subroutine are stored on the Perl stack.



As a general rule you should always check the return value from
these functions.  Even if you are expecting only a particular number of
values to be returned from the Perl subroutine, there is nothing to
stop someone from doing something unexpected–don’t say you haven’t
been warned.









Next: , Previous: perlcall THE CALL_ FUNCTIONS, Up: perlcall   [Contents]





7.4 FLAG VALUES



The flags parameter in all the call_* functions is one of G_VOID,
G_SCALAR, or G_ARRAY, which indicate the call context, OR’ed together
with a bit mask of any combination of the other G_* symbols defined below.





• perlcall G_VOID:  



• perlcall G_SCALAR:  



• perlcall G_ARRAY:  



• perlcall G_DISCARD:  



• perlcall G_NOARGS:  



• perlcall G_EVAL:  



• perlcall G_KEEPERR:  



• perlcall Determining the Context:  












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7.4.1 G_VOID



Calls the Perl subroutine in a void context.



This flag has 2 effects:



    

  1.  It indicates to the subroutine being called that it is executing in
a void context (if it executes wantarray the result will be the
undefined value).


  2.  It ensures that nothing is actually returned from the subroutine.





The value returned by the call_* function indicates how many
items have been returned by the Perl subroutine–in this case it will
be 0.









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7.4.2 G_SCALAR



Calls the Perl subroutine in a scalar context.  This is the default
context flag setting for all the call_* functions.



This flag has 2 effects:



    

  1.  It indicates to the subroutine being called that it is executing in a
scalar context (if it executes wantarray the result will be false).


  2.  It ensures that only a scalar is actually returned from the subroutine.
The subroutine can, of course,  ignore the wantarray and return a
list anyway. If so, then only the last element of the list will be
returned.





The value returned by the call_* function indicates how many
items have been returned by the Perl subroutine - in this case it will
be either 0 or 1.



If 0, then you have specified the G_DISCARD flag.



If 1, then the item actually returned by the Perl subroutine will be
stored on the Perl stack - the section Returning a Scalar shows how
to access this value on the stack.  Remember that regardless of how
many items the Perl subroutine returns, only the last one will be
accessible from the stack - think of the case where only one value is
returned as being a list with only one element.  Any other items that
were returned will not exist by the time control returns from the
call_* function.  The section Returning a list in a scalar
context shows an example of this behavior.









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7.4.3 G_ARRAY



Calls the Perl subroutine in a list context.



As with G_SCALAR, this flag has 2 effects:



    

  1.  It indicates to the subroutine being called that it is executing in a
list context (if it executes wantarray the result will be true).


  2.  It ensures that all items returned from the subroutine will be
accessible when control returns from the call_* function.





The value returned by the call_* function indicates how many
items have been returned by the Perl subroutine.



If 0, then you have specified the G_DISCARD flag.



If not 0, then it will be a count of the number of items returned by
the subroutine. These items will be stored on the Perl stack.  The
section Returning a list of values gives an example of using the
G_ARRAY flag and the mechanics of accessing the returned items from the
Perl stack.









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7.4.4 G_DISCARD



By default, the call_* functions place the items returned from
by the Perl subroutine on the stack.  If you are not interested in
these items, then setting this flag will make Perl get rid of them
automatically for you.  Note that it is still possible to indicate a
context to the Perl subroutine by using either G_SCALAR or G_ARRAY.



If you do not set this flag then it is very important that you make
sure that any temporaries (i.e., parameters passed to the Perl
subroutine and values returned from the subroutine) are disposed of
yourself.  The section Returning a Scalar gives details of how to
dispose of these temporaries explicitly and the section Using Perl to
dispose of temporaries discusses the specific circumstances where you
can ignore the problem and let Perl deal with it for you.









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7.4.5 G_NOARGS



Whenever a Perl subroutine is called using one of the call_*
functions, it is assumed by default that parameters are to be passed to
the subroutine.  If you are not passing any parameters to the Perl
subroutine, you can save a bit of time by setting this flag.  It has
the effect of not creating the @_ array for the Perl subroutine.



Although the functionality provided by this flag may seem
straightforward, it should be used only if there is a good reason to do
so.  The reason for being cautious is that, even if you have specified
the G_NOARGS flag, it is still possible for the Perl subroutine that
has been called to think that you have passed it parameters.



In fact, what can happen is that the Perl subroutine you have called
can access the @_ array from a previous Perl subroutine.  This will
occur when the code that is executing the call_* function has
itself been called from another Perl subroutine. The code below
illustrates this



    sub fred
      { print "@_\n"  }

    sub joe
      { &fred }

    &joe(1,2,3);



This will print



    1 2 3



What has happened is that fred accesses the @_ array which
belongs to joe.









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7.4.6 G_EVAL



It is possible for the Perl subroutine you are calling to terminate
abnormally, e.g., by calling die explicitly or by not actually
existing.  By default, when either of these events occurs, the
process will terminate immediately.  If you want to trap this
type of event, specify the G_EVAL flag.  It will put an eval { }
around the subroutine call.



Whenever control returns from the call_* function you need to
check the $@ variable as you would in a normal Perl script.



The value returned from the call_* function is dependent on
what other flags have been specified and whether an error has
occurred.  Here are all the different cases that can occur:





See Using G_EVAL for details on using G_EVAL.









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7.4.7 G_KEEPERR



Using the G_EVAL flag described above will always set $@: clearing
it if there was no error, and setting it to describe the error if there
was an error in the called code.  This is what you want if your intention
is to handle possible errors, but sometimes you just want to trap errors
and stop them interfering with the rest of the program.



This scenario will mostly be applicable to code that is meant to be called
from within destructors, asynchronous callbacks, and signal handlers.
In such situations, where the code being called has little relation to the
surrounding dynamic context, the main program needs to be insulated from
errors in the called code, even if they can’t be handled intelligently.
It may also be useful to do this with code for __DIE__ or __WARN__
hooks, and tie functions.



The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
call_* functions that are used to implement such code, or with
eval_sv.  This flag has no effect on the call_* functions when
G_EVAL is not used.



When G_KEEPERR is used, any error in the called code will terminate the
call as usual, and the error will not propagate beyond the call (as usual
for G_EVAL), but it will not go into $@.  Instead the error will be
converted into a warning, prefixed with the string "\t(in cleanup)".
This can be disabled using no warnings 'misc'.  If there is no error,
$@ will not be cleared.



Note that the G_KEEPERR flag does not propagate into inner evals; these
may still set $@.



The G_KEEPERR flag was introduced in Perl version 5.002.



See Using G_KEEPERR for an example of a situation that warrants the
use of this flag.









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7.4.8 Determining the Context



As mentioned above, you can determine the context of the currently
executing subroutine in Perl with wantarray.  The equivalent test
can be made in C by using the GIMME_V macro, which returns
G_ARRAY if you have been called in a list context, G_SCALAR if
in a scalar context, or G_VOID if in a void context (i.e., the
return value will not be used).  An older version of this macro is
called GIMME; in a void context it returns G_SCALAR instead of
G_VOID.  An example of using the GIMME_V macro is shown in
section Using GIMME_V.









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7.5 EXAMPLES



Enough of the definition talk! Let’s have a few examples.



Perl provides many macros to assist in accessing the Perl stack.
Wherever possible, these macros should always be used when interfacing
to Perl internals.  We hope this should make the code less vulnerable
to any changes made to Perl in the future.



Another point worth noting is that in the first series of examples I
have made use of only the call_pv function.  This has been done
to keep the code simpler and ease you into the topic.  Wherever
possible, if the choice is between using call_pv and
call_sv, you should always try to use call_sv.  See
Using call_sv for details.





• perlcall No Parameters, Nothing Returned:  



• perlcall Passing Parameters:  



• perlcall Returning a Scalar:  



• perlcall Returning a List of Values:  



• perlcall Returning a List in a Scalar Context:  



• perlcall Returning Data from Perl via the Parameter List:  



• perlcall Using G_EVAL:  



• perlcall Using G_KEEPERR:  



• perlcall Using call_sv:  



• perlcall Using call_argv:  



• perlcall Using call_method:  



• perlcall Using GIMME_V:  



• perlcall Using Perl to Dispose of Temporaries:  



• perlcall Strategies for Storing Callback Context Information:  



• perlcall Alternate Stack Manipulation:  



• perlcall Creating and Calling an Anonymous Subroutine in C:  












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7.5.1 No Parameters, Nothing Returned



This first trivial example will call a Perl subroutine, PrintUID, to
print out the UID of the process.



    sub PrintUID
    {
        print "UID is $<\n";
    }



and here is a C function to call it



    static void
    call_PrintUID()
    {
        dSP;

        PUSHMARK(SP);
        call_pv("PrintUID", G_DISCARD|G_NOARGS);
    }



Simple, eh?



A few points to note about this example:



    

  1.  Ignore dSP and PUSHMARK(SP) for now. They will be discussed in
the next example.


  2.  We aren’t passing any parameters to PrintUID so G_NOARGS can be
specified.


  3.  We aren’t interested in anything returned from PrintUID, so
G_DISCARD is specified. Even if PrintUID was changed to
return some value(s), having specified G_DISCARD will mean that they
will be wiped by the time control returns from call_pv.


  4.  As call_pv is being used, the Perl subroutine is specified as a
C string. In this case the subroutine name has been ’hard-wired’ into the
code.


  5.  Because we specified G_DISCARD, it is not necessary to check the value
returned from call_pv. It will always be 0.











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7.5.2 Passing Parameters



Now let’s make a slightly more complex example. This time we want to
call a Perl subroutine, LeftString, which will take 2 parameters–a
string ($s) and an integer ($n).  The subroutine will simply
print the first $n characters of the string.



So the Perl subroutine would look like this:



    sub LeftString
    {
        my($s, $n) = @_;
        print substr($s, 0, $n), "\n";
    }



The C function required to call LeftString would look like this:



    static void
    call_LeftString(a, b)
    char * a;
    int b;
    {
        dSP;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSVpv(a, 0)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        call_pv("LeftString", G_DISCARD);

        FREETMPS;
        LEAVE;
    }



Here are a few notes on the C function call_LeftString.



    

  1.  Parameters are passed to the Perl subroutine using the Perl stack.
This is the purpose of the code beginning with the line dSP and
ending with the line PUTBACK.  The dSP declares a local copy
of the stack pointer.  This local copy should always be accessed
as SP.


  2.  If you are going to put something onto the Perl stack, you need to know
where to put it. This is the purpose of the macro dSP–it declares
and initializes a local copy of the Perl stack pointer.


    All the other macros which will be used in this example require you to
have used this macro.

    

    The exception to this rule is if you are calling a Perl subroutine
directly from an XSUB function. In this case it is not necessary to
use the dSP macro explicitly–it will be declared for you
automatically.

    

  3.  Any parameters to be pushed onto the stack should be bracketed by the
PUSHMARK and PUTBACK macros.  The purpose of these two macros, in
this context, is to count the number of parameters you are
pushing automatically.  Then whenever Perl is creating the @_ array for the
subroutine, it knows how big to make it.


    The PUSHMARK macro tells Perl to make a mental note of the current
stack pointer. Even if you aren’t passing any parameters (like the
example shown in the section No Parameters, Nothing Returned) you
must still call the PUSHMARK macro before you can call any of the
call_* functions–Perl still needs to know that there are no
parameters.

    

    The PUTBACK macro sets the global copy of the stack pointer to be
the same as our local copy. If we didn’t do this, call_pv
wouldn’t know where the two parameters we pushed were–remember that
up to now all the stack pointer manipulation we have done is with our
local copy, not the global copy.

    

  4.  Next, we come to XPUSHs. This is where the parameters actually get
pushed onto the stack. In this case we are pushing a string and an
integer.


    See perlguts XSUBs and the Argument Stack for details
on how the XPUSH macros work.

    

  5.  Because we created temporary values (by means of sv_2mortal() calls)
we will have to tidy up the Perl stack and dispose of mortal SVs.


    This is the purpose of

    

        ENTER;
    SAVETMPS;

    

    at the start of the function, and

    

        FREETMPS;
    LEAVE;

    

    at the end. The ENTER/SAVETMPS pair creates a boundary for any
temporaries we create.  This means that the temporaries we get rid of
will be limited to those which were created after these calls.

    

    The FREETMPS/LEAVE pair will get rid of any values returned by
the Perl subroutine (see next example), plus it will also dump the
mortal SVs we have created.  Having ENTER/SAVETMPS at the
beginning of the code makes sure that no other mortals are destroyed.

    

    Think of these macros as working a bit like { and } in Perl
to limit the scope of local variables.

    

    See the section Using Perl to Dispose of Temporaries for details of
an alternative to using these macros.

    

  6.  Finally, LeftString can now be called via the call_pv function.
The only flag specified this time is G_DISCARD. Because we are passing
2 parameters to the Perl subroutine this time, we have not specified
G_NOARGS.











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7.5.3 Returning a Scalar



Now for an example of dealing with the items returned from a Perl
subroutine.



Here is a Perl subroutine, Adder, that takes 2 integer parameters
and simply returns their sum.



    sub Adder
    {
        my($a, $b) = @_;
        $a + $b;
    }



Because we are now concerned with the return value from Adder, the C
function required to call it is now a bit more complex.



    static void
    call_Adder(a, b)
    int a;
    int b;
    {
        dSP;
        int count;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        count = call_pv("Adder", G_SCALAR);

        SPAGAIN;

        if (count != 1)
            croak("Big trouble\n");

        printf ("The sum of %d and %d is %d\n", a, b, POPi);

        PUTBACK;
        FREETMPS;
        LEAVE;
    }



Points to note this time are



    

  1.  The only flag specified this time was G_SCALAR. That means that the @_
array will be created and that the value returned by Adder will
still exist after the call to call_pv.


  2.  The purpose of the macro SPAGAIN is to refresh the local copy of the
stack pointer. This is necessary because it is possible that the memory
allocated to the Perl stack has been reallocated during the
call_pv call.


    If you are making use of the Perl stack pointer in your code you must
always refresh the local copy using SPAGAIN whenever you make use
of the call_* functions or any other Perl internal function.

    

  3.  Although only a single value was expected to be returned from Adder,
it is still good practice to check the return code from call_pv
anyway.


    Expecting a single value is not quite the same as knowing that there
will be one. If someone modified Adder to return a list and we
didn’t check for that possibility and take appropriate action the Perl
stack would end up in an inconsistent state. That is something you
really don’t want to happen ever.

    

  4.  The POPi macro is used here to pop the return value from the stack.
In this case we wanted an integer, so POPi was used.


    Here is the complete list of POP macros available, along with the types
they return.

    

        POPs        SV
    POPp        pointer
    POPn        double
    POPi        integer
    POPl        long

    

  5.  The final PUTBACK is used to leave the Perl stack in a consistent
state before exiting the function.  This is necessary because when we
popped the return value from the stack with POPi it updated only our
local copy of the stack pointer.  Remember, PUTBACK sets the global
stack pointer to be the same as our local copy.











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7.5.4 Returning a List of Values



Now, let’s extend the previous example to return both the sum of the
parameters and the difference.



Here is the Perl subroutine



    sub AddSubtract
    {
       my($a, $b) = @_;
       ($a+$b, $a-$b);
    }



and this is the C function



    static void
    call_AddSubtract(a, b)
    int a;
    int b;
    {
        dSP;
        int count;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        count = call_pv("AddSubtract", G_ARRAY);

        SPAGAIN;

        if (count != 2)
            croak("Big trouble\n");

        printf ("%d - %d = %d\n", a, b, POPi);
        printf ("%d + %d = %d\n", a, b, POPi);

        PUTBACK;
        FREETMPS;
        LEAVE;
    }



If call_AddSubtract is called like this



    call_AddSubtract(7, 4);



then here is the output



    7 - 4 = 3
    7 + 4 = 11



Notes



    

  1.  We wanted list context, so G_ARRAY was used.


  2.  Not surprisingly POPi is used twice this time because we were
retrieving 2 values from the stack. The important thing to note is that
when using the POP* macros they come off the stack in reverse
order.











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7.5.5 Returning a List in a Scalar Context



Say the Perl subroutine in the previous section was called in a scalar
context, like this



    static void
    call_AddSubScalar(a, b)
    int a;
    int b;
    {
        dSP;
        int count;
        int i;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        count = call_pv("AddSubtract", G_SCALAR);

        SPAGAIN;

        printf ("Items Returned = %d\n", count);

        for (i = 1; i <= count; ++i)
            printf ("Value %d = %d\n", i, POPi);

        PUTBACK;
        FREETMPS;
        LEAVE;
    }



The other modification made is that call_AddSubScalar will print the
number of items returned from the Perl subroutine and their value (for
simplicity it assumes that they are integer).  So if
call_AddSubScalar is called



    call_AddSubScalar(7, 4);



then the output will be



    Items Returned = 1
    Value 1 = 3



In this case the main point to note is that only the last item in the
list is returned from the subroutine. AddSubtract actually made it back to
call_AddSubScalar.









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7.5.6 Returning Data from Perl via the Parameter List



It is also possible to return values directly via the parameter
list–whether it is actually desirable to do it is another matter entirely.



The Perl subroutine, Inc, below takes 2 parameters and increments
each directly.



    sub Inc
    {
        ++ $_[0];
        ++ $_[1];
    }



and here is a C function to call it.



    static void
    call_Inc(a, b)
    int a;
    int b;
    {
        dSP;
        int count;
        SV * sva;
        SV * svb;

        ENTER;
        SAVETMPS;

        sva = sv_2mortal(newSViv(a));
        svb = sv_2mortal(newSViv(b));

        PUSHMARK(SP);
        XPUSHs(sva);
        XPUSHs(svb);
        PUTBACK;

        count = call_pv("Inc", G_DISCARD);

        if (count != 0)
            croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
                   count);

        printf ("%d + 1 = %d\n", a, SvIV(sva));
        printf ("%d + 1 = %d\n", b, SvIV(svb));

        FREETMPS;
        LEAVE;
    }



To be able to access the two parameters that were pushed onto the stack
after they return from call_pv it is necessary to make a note
of their addresses–thus the two variables sva and svb.



The reason this is necessary is that the area of the Perl stack which
held them will very likely have been overwritten by something else by
the time control returns from call_pv.









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7.5.7 Using G_EVAL



Now an example using G_EVAL. Below is a Perl subroutine which computes
the difference of its 2 parameters. If this would result in a negative
result, the subroutine calls die.



    sub Subtract
    {
        my ($a, $b) = @_;

        die "death can be fatal\n" if $a < $b;

        $a - $b;
    }



and some C to call it



    static void
    call_Subtract(a, b)
    int a;
    int b;
    {
        dSP;
        int count;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        count = call_pv("Subtract", G_EVAL|G_SCALAR);

        SPAGAIN;

        /* Check the eval first */
        if (SvTRUE(ERRSV))
        {
            printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
            POPs;
        }
        else
        {
            if (count != 1)
               croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
                        count);

            printf ("%d - %d = %d\n", a, b, POPi);
        }

        PUTBACK;
        FREETMPS;
        LEAVE;
    }



If call_Subtract is called thus



    call_Subtract(4, 5)



the following will be printed



    Uh oh - death can be fatal



Notes



    

  1.  We want to be able to catch the die so we have used the G_EVAL
flag.  Not specifying this flag would mean that the program would
terminate immediately at the die statement in the subroutine
Subtract.


  2.  The code


        if (SvTRUE(ERRSV))
    {
        printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
        POPs;
    }

    

    is the direct equivalent of this bit of Perl

    

        print "Uh oh - $@\n" if $@;

    

    PL_errgv is a perl global of type GV * that points to the
symbol table entry containing the error.  ERRSV therefore
refers to the C equivalent of $@.

    

  3.  Note that the stack is popped using POPs in the block where
SvTRUE(ERRSV) is true.  This is necessary because whenever a
call_* function invoked with G_EVAL|G_SCALAR returns an error,
the top of the stack holds the value undef. Because we want the
program to continue after detecting this error, it is essential that
the stack be tidied up by removing the undef.











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7.5.8 Using G_KEEPERR



Consider this rather facetious example, where we have used an XS
version of the call_Subtract example above inside a destructor:



    package Foo;
    sub new { bless {}, $_[0] }
    sub Subtract {
        my($a,$b) = @_;
        die "death can be fatal" if $a < $b;
        $a - $b;
    }
    sub DESTROY { call_Subtract(5, 4); }
    sub foo { die "foo dies"; }

    package main;
    {
        my $foo = Foo->new;
        eval { $foo->foo };
    }
    print "Saw: $@" if $@;             # should be, but isn't



This example will fail to recognize that an error occurred inside the
eval {}.  Here’s why: the call_Subtract code got executed while perl
was cleaning up temporaries when exiting the outer braced block, and because
call_Subtract is implemented with call_pv using the G_EVAL
flag, it promptly reset $@.  This results in the failure of the
outermost test for $@, and thereby the failure of the error trap.



Appending the G_KEEPERR flag, so that the call_pv call in
call_Subtract reads:



        count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);



will preserve the error and restore reliable error handling.









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7.5.9 Using call_sv



In all the previous examples I have ’hard-wired’ the name of the Perl
subroutine to be called from C.  Most of the time though, it is more
convenient to be able to specify the name of the Perl subroutine from
within the Perl script.



Consider the Perl code below



    sub fred
    {
        print "Hello there\n";
    }

    CallSubPV("fred");



Here is a snippet of XSUB which defines CallSubPV.



    void
    CallSubPV(name)
        char *  name
        CODE:
        PUSHMARK(SP);
        call_pv(name, G_DISCARD|G_NOARGS);



That is fine as far as it goes. The thing is, the Perl subroutine
can be specified as only a string.  For Perl 4 this was adequate,
but Perl 5 allows references to subroutines and anonymous subroutines.
This is where call_sv is useful.



The code below for CallSubSV is identical to CallSubPV except
that the name parameter is now defined as an SV* and we use
call_sv instead of call_pv.



    void
    CallSubSV(name)
        SV *    name
        CODE:
        PUSHMARK(SP);
        call_sv(name, G_DISCARD|G_NOARGS);



Because we are using an SV to call fred the following can all be used:



    CallSubSV("fred");
    CallSubSV(\&fred);
    $ref = \&fred;
    CallSubSV($ref);
    CallSubSV( sub { print "Hello there\n" } );



As you can see, call_sv gives you much greater flexibility in
how you can specify the Perl subroutine.



You should note that, if it is necessary to store the SV (name in the
example above) which corresponds to the Perl subroutine so that it can
be used later in the program, it not enough just to store a copy of the
pointer to the SV. Say the code above had been like this:



    static SV * rememberSub;

    void
    SaveSub1(name)
        SV *    name
        CODE:
        rememberSub = name;

    void
    CallSavedSub1()
        CODE:
        PUSHMARK(SP);
        call_sv(rememberSub, G_DISCARD|G_NOARGS);



The reason this is wrong is that, by the time you come to use the
pointer rememberSub in CallSavedSub1, it may or may not still refer
to the Perl subroutine that was recorded in SaveSub1.  This is
particularly true for these cases:



    SaveSub1(\&fred);
    CallSavedSub1();

    SaveSub1( sub { print "Hello there\n" } );
    CallSavedSub1();



By the time each of the SaveSub1 statements above has been executed,
the SV*s which corresponded to the parameters will no longer exist.
Expect an error message from Perl of the form



    Can't use an undefined value as a subroutine reference at ...



for each of the CallSavedSub1 lines.



Similarly, with this code



    $ref = \&fred;
    SaveSub1($ref);
    $ref = 47;
    CallSavedSub1();



you can expect one of these messages (which you actually get is dependent on
the version of Perl you are using)



    Not a CODE reference at ...
    Undefined subroutine &main::47 called ...



The variable $ref may have referred to the subroutine fred
whenever the call to SaveSub1 was made but by the time
CallSavedSub1 gets called it now holds the number 47. Because we
saved only a pointer to the original SV in SaveSub1, any changes to
$ref will be tracked by the pointer rememberSub. This means that
whenever CallSavedSub1 gets called, it will attempt to execute the
code which is referenced by the SV* rememberSub.  In this case
though, it now refers to the integer 47, so expect Perl to complain
loudly.



A similar but more subtle problem is illustrated with this code:



    $ref = \&fred;
    SaveSub1($ref);
    $ref = \&joe;
    CallSavedSub1();



This time whenever CallSavedSub1 gets called it will execute the Perl
subroutine joe (assuming it exists) rather than fred as was
originally requested in the call to SaveSub1.



To get around these problems it is necessary to take a full copy of the
SV.  The code below shows SaveSub2 modified to do that.



    static SV * keepSub = (SV*)NULL;

    void
    SaveSub2(name)
        SV *    name
        CODE:
        /* Take a copy of the callback */
        if (keepSub == (SV*)NULL)
            /* First time, so create a new SV */
            keepSub = newSVsv(name);
        else
            /* Been here before, so overwrite */
            SvSetSV(keepSub, name);

    void
    CallSavedSub2()
        CODE:
        PUSHMARK(SP);
        call_sv(keepSub, G_DISCARD|G_NOARGS);



To avoid creating a new SV every time SaveSub2 is called,
the function first checks to see if it has been called before.  If not,
then space for a new SV is allocated and the reference to the Perl
subroutine name is copied to the variable keepSub in one
operation using newSVsv.  Thereafter, whenever SaveSub2 is called,
the existing SV, keepSub, is overwritten with the new value using
SvSetSV.









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7.5.10 Using call_argv



Here is a Perl subroutine which prints whatever parameters are passed
to it.



    sub PrintList
    {
        my(@list) = @_;

        foreach (@list) { print "$_\n" }
    }



And here is an example of call_argv which will call
PrintList.



    static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};

    static void
    call_PrintList()
    {
        dSP;

        call_argv("PrintList", G_DISCARD, words);
    }



Note that it is not necessary to call PUSHMARK in this instance.
This is because call_argv will do it for you.









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7.5.11 Using call_method



Consider the following Perl code:



    {
        package Mine;

        sub new
        {
            my($type) = shift;
            bless [@_]
        }

        sub Display
        {
            my ($self, $index) = @_;
            print "$index: $$self[$index]\n";
        }

        sub PrintID
        {
            my($class) = @_;
            print "This is Class $class version 1.0\n";
        }
    }



It implements just a very simple class to manage an array.  Apart from
the constructor, new, it declares methods, one static and one
virtual. The static method, PrintID, prints out simply the class
name and a version number. The virtual method, Display, prints out a
single element of the array.  Here is an all-Perl example of using it.



    $a = Mine->new('red', 'green', 'blue');
    $a->Display(1);
    Mine->PrintID;



will print



    1: green
    This is Class Mine version 1.0



Calling a Perl method from C is fairly straightforward. The following
things are required:





Here is a simple XSUB which illustrates the mechanics of calling both
the PrintID and Display methods from C.



    void
    call_Method(ref, method, index)
        SV *    ref
        char *  method
        int             index
        CODE:
        PUSHMARK(SP);
        XPUSHs(ref);
        XPUSHs(sv_2mortal(newSViv(index)));
        PUTBACK;

        call_method(method, G_DISCARD);

    void
    call_PrintID(class, method)
        char *  class
        char *  method
        CODE:
        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSVpv(class, 0)));
        PUTBACK;

        call_method(method, G_DISCARD);



So the methods PrintID and Display can be invoked like this:



    $a = Mine->new('red', 'green', 'blue');
    call_Method($a, 'Display', 1);
    call_PrintID('Mine', 'PrintID');



The only thing to note is that, in both the static and virtual methods,
the method name is not passed via the stack–it is used as the first
parameter to call_method.









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7.5.12 Using GIMME_V



Here is a trivial XSUB which prints the context in which it is
currently executing.



    void
    PrintContext()
        CODE:
        I32 gimme = GIMME_V;
        if (gimme == G_VOID)
            printf ("Context is Void\n");
        else if (gimme == G_SCALAR)
            printf ("Context is Scalar\n");
        else
            printf ("Context is Array\n");



And here is some Perl to test it.



    PrintContext;
    $a = PrintContext;
    @a = PrintContext;



The output from that will be



    Context is Void
    Context is Scalar
    Context is Array









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7.5.13 Using Perl to Dispose of Temporaries



In the examples given to date, any temporaries created in the callback
(i.e., parameters passed on the stack to the call_* function or
values returned via the stack) have been freed by one of these methods:





There is another method which can be used, namely letting Perl do it
for you automatically whenever it regains control after the callback
has terminated.  This is done by simply not using the



    ENTER;
    SAVETMPS;
    ...
    FREETMPS;
    LEAVE;



sequence in the callback (and not, of course, specifying the G_DISCARD
flag).



If you are going to use this method you have to be aware of a possible
memory leak which can arise under very specific circumstances.  To
explain these circumstances you need to know a bit about the flow of
control between Perl and the callback routine.



The examples given at the start of the document (an error handler and
an event driven program) are typical of the two main sorts of flow
control that you are likely to encounter with callbacks.  There is a
very important distinction between them, so pay attention.



In the first example, an error handler, the flow of control could be as
follows.  You have created an interface to an external library.
Control can reach the external library like this



    perl --> XSUB --> external library



Whilst control is in the library, an error condition occurs. You have
previously set up a Perl callback to handle this situation, so it will
get executed. Once the callback has finished, control will drop back to
Perl again.  Here is what the flow of control will be like in that
situation



    perl --> XSUB --> external library
                      ...
                      error occurs
                      ...
                      external library --> call_* --> perl
                                                          |
    perl <-- XSUB <-- external library <-- call_* <----+



After processing of the error using call_* is completed,
control reverts back to Perl more or less immediately.



In the diagram, the further right you go the more deeply nested the
scope is.  It is only when control is back with perl on the extreme
left of the diagram that you will have dropped back to the enclosing
scope and any temporaries you have left hanging around will be freed.



In the second example, an event driven program, the flow of control
will be more like this



    perl --> XSUB --> event handler
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+



In this case the flow of control can consist of only the repeated
sequence



    event handler --> call_* --> perl



for practically the complete duration of the program.  This means that
control may never drop back to the surrounding scope in Perl at the
extreme left.



So what is the big problem? Well, if you are expecting Perl to tidy up
those temporaries for you, you might be in for a long wait.  For Perl
to dispose of your temporaries, control must drop back to the
enclosing scope at some stage.  In the event driven scenario that may
never happen.  This means that, as time goes on, your program will
create more and more temporaries, none of which will ever be freed. As
each of these temporaries consumes some memory your program will
eventually consume all the available memory in your system–kapow!



So here is the bottom line–if you are sure that control will revert
back to the enclosing Perl scope fairly quickly after the end of your
callback, then it isn’t absolutely necessary to dispose explicitly of
any temporaries you may have created. Mind you, if you are at all
uncertain about what to do, it doesn’t do any harm to tidy up anyway.









Next: , Previous: perlcall Using Perl to Dispose of Temporaries, Up: perlcall EXAMPLES   [Contents]





7.5.14 Strategies for Storing Callback Context Information



Potentially one of the trickiest problems to overcome when designing a
callback interface can be figuring out how to store the mapping between
the C callback function and the Perl equivalent.



To help understand why this can be a real problem first consider how a
callback is set up in an all C environment.  Typically a C API will
provide a function to register a callback.  This will expect a pointer
to a function as one of its parameters.  Below is a call to a
hypothetical function register_fatal which registers the C function
to get called when a fatal error occurs.



    register_fatal(cb1);



The single parameter cb1 is a pointer to a function, so you must
have defined cb1 in your code, say something like this



    static void
    cb1()
    {
        printf ("Fatal Error\n");
        exit(1);
    }



Now change that to call a Perl subroutine instead



    static SV * callback = (SV*)NULL;

    static void
    cb1()
    {
        dSP;

        PUSHMARK(SP);

        /* Call the Perl sub to process the callback */
        call_sv(callback, G_DISCARD);
    }


    void
    register_fatal(fn)
        SV *    fn
        CODE:
        /* Remember the Perl sub */
        if (callback == (SV*)NULL)
            callback = newSVsv(fn);
        else
            SvSetSV(callback, fn);

        /* register the callback with the external library */
        register_fatal(cb1);



where the Perl equivalent of register_fatal and the callback it
registers, pcb1, might look like this



    # Register the sub pcb1
    register_fatal(\&pcb1);

    sub pcb1
    {
        die "I'm dying...\n";
    }



The mapping between the C callback and the Perl equivalent is stored in
the global variable callback.



This will be adequate if you ever need to have only one callback
registered at any time. An example could be an error handler like the
code sketched out above. Remember though, repeated calls to
register_fatal will replace the previously registered callback
function with the new one.



Say for example you want to interface to a library which allows asynchronous
file i/o.  In this case you may be able to register a callback whenever
a read operation has completed. To be of any use we want to be able to
call separate Perl subroutines for each file that is opened.  As it
stands, the error handler example above would not be adequate as it
allows only a single callback to be defined at any time. What we
require is a means of storing the mapping between the opened file and
the Perl subroutine we want to be called for that file.



Say the i/o library has a function asynch_read which associates a C
function ProcessRead with a file handle fh–this assumes that it
has also provided some routine to open the file and so obtain the file
handle.



    asynch_read(fh, ProcessRead)



This may expect the C ProcessRead function of this form



    void
    ProcessRead(fh, buffer)
    int fh;
    char *      buffer;
    {
         ...
    }



To provide a Perl interface to this library we need to be able to map
between the fh parameter and the Perl subroutine we want called.  A
hash is a convenient mechanism for storing this mapping.  The code
below shows a possible implementation



    static HV * Mapping = (HV*)NULL;

    void
    asynch_read(fh, callback)
        int     fh
        SV *    callback
        CODE:
        /* If the hash doesn't already exist, create it */
        if (Mapping == (HV*)NULL)
            Mapping = newHV();

        /* Save the fh -> callback mapping */
        hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0);

        /* Register with the C Library */
        asynch_read(fh, asynch_read_if);



and asynch_read_if could look like this



    static void
    asynch_read_if(fh, buffer)
    int fh;
    char *      buffer;
    {
        dSP;
        SV ** sv;

        /* Get the callback associated with fh */
        sv =  hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE);
        if (sv == (SV**)NULL)
            croak("Internal error...\n");

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(fh)));
        XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
        PUTBACK;

        /* Call the Perl sub */
        call_sv(*sv, G_DISCARD);
    }



For completeness, here is asynch_close.  This shows how to remove
the entry from the hash Mapping.



    void
    asynch_close(fh)
        int     fh
        CODE:
        /* Remove the entry from the hash */
        (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD);

        /* Now call the real asynch_close */
        asynch_close(fh);



So the Perl interface would look like this



    sub callback1
    {
        my($handle, $buffer) = @_;
    }

    # Register the Perl callback
    asynch_read($fh, \&callback1);

    asynch_close($fh);



The mapping between the C callback and Perl is stored in the global
hash Mapping this time. Using a hash has the distinct advantage that
it allows an unlimited number of callbacks to be registered.



What if the interface provided by the C callback doesn’t contain a
parameter which allows the file handle to Perl subroutine mapping?  Say
in the asynchronous i/o package, the callback function gets passed only
the buffer parameter like this



    void
    ProcessRead(buffer)
    char *      buffer;
    {
        ...
    }



Without the file handle there is no straightforward way to map from the
C callback to the Perl subroutine.



In this case a possible way around this problem is to predefine a
series of C functions to act as the interface to Perl, thus



    #define MAX_CB              3
    #define NULL_HANDLE -1
    typedef void (*FnMap)();

    struct MapStruct {
        FnMap    Function;
        SV *     PerlSub;
        int      Handle;
      };

    static void  fn1();
    static void  fn2();
    static void  fn3();

    static struct MapStruct Map [MAX_CB] =
        {
            { fn1, NULL, NULL_HANDLE },
            { fn2, NULL, NULL_HANDLE },
            { fn3, NULL, NULL_HANDLE }
        };

    static void
    Pcb(index, buffer)
    int index;
    char * buffer;
    {
        dSP;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
        PUTBACK;

        /* Call the Perl sub */
        call_sv(Map[index].PerlSub, G_DISCARD);
    }

    static void
    fn1(buffer)
    char * buffer;
    {
        Pcb(0, buffer);
    }

    static void
    fn2(buffer)
    char * buffer;
    {
        Pcb(1, buffer);
    }

    static void
    fn3(buffer)
    char * buffer;
    {
        Pcb(2, buffer);
    }

    void
    array_asynch_read(fh, callback)
        int             fh
        SV *    callback
        CODE:
        int index;
        int null_index = MAX_CB;

        /* Find the same handle or an empty entry */
        for (index = 0; index < MAX_CB; ++index)
        {
            if (Map[index].Handle == fh)
                break;

            if (Map[index].Handle == NULL_HANDLE)
                null_index = index;
        }

        if (index == MAX_CB && null_index == MAX_CB)
            croak ("Too many callback functions registered\n");

        if (index == MAX_CB)
            index = null_index;

        /* Save the file handle */
        Map[index].Handle = fh;

        /* Remember the Perl sub */
        if (Map[index].PerlSub == (SV*)NULL)
            Map[index].PerlSub = newSVsv(callback);
        else
            SvSetSV(Map[index].PerlSub, callback);

        asynch_read(fh, Map[index].Function);

    void
    array_asynch_close(fh)
        int     fh
        CODE:
        int index;

        /* Find the file handle */
        for (index = 0; index < MAX_CB; ++ index)
            if (Map[index].Handle == fh)
                break;

        if (index == MAX_CB)
            croak ("could not close fh %d\n", fh);

        Map[index].Handle = NULL_HANDLE;
        SvREFCNT_dec(Map[index].PerlSub);
        Map[index].PerlSub = (SV*)NULL;

        asynch_close(fh);



In this case the functions fn1, fn2, and fn3 are used to
remember the Perl subroutine to be called. Each of the functions holds
a separate hard-wired index which is used in the function Pcb to
access the Map array and actually call the Perl subroutine.



There are some obvious disadvantages with this technique.



Firstly, the code is considerably more complex than with the previous
example.



Secondly, there is a hard-wired limit (in this case 3) to the number of
callbacks that can exist simultaneously. The only way to increase the
limit is by modifying the code to add more functions and then
recompiling.  None the less, as long as the number of functions is
chosen with some care, it is still a workable solution and in some
cases is the only one available.



To summarize, here are a number of possible methods for you to consider
for storing the mapping between C and the Perl callback





1. Ignore the problem - Allow only 1 callback




For a lot of situations, like interfacing to an error handler, this may
be a perfectly adequate solution.





2. Create a sequence of callbacks - hard wired limit




If it is impossible to tell from the parameters passed back from the C
callback what the context is, then you may need to create a sequence of C
callback interface functions, and store pointers to each in an array.





3. Use a parameter to map to the Perl callback




A hash is an ideal mechanism to store the mapping between C and Perl.














Next: , Previous: perlcall Strategies for Storing Callback Context Information, Up: perlcall EXAMPLES   [Contents]





7.5.15 Alternate Stack Manipulation



Although I have made use of only the POP* macros to access values
returned from Perl subroutines, it is also possible to bypass these
macros and read the stack using the ST macro (See (perlxs) for a
full description of the ST macro).



Most of the time the POP* macros should be adequate; the main
problem with them is that they force you to process the returned values
in sequence. This may not be the most suitable way to process the
values in some cases. What we want is to be able to access the stack in
a random order. The ST macro as used when coding an XSUB is ideal
for this purpose.



The code below is the example given in the section Returning a List
of Values recoded to use ST instead of POP*.



    static void
    call_AddSubtract2(a, b)
    int a;
    int b;
    {
        dSP;
        I32 ax;
        int count;

        ENTER;
        SAVETMPS;

        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK;

        count = call_pv("AddSubtract", G_ARRAY);

        SPAGAIN;
        SP -= count;
        ax = (SP - PL_stack_base) + 1;

        if (count != 2)
            croak("Big trouble\n");

        printf ("%d + %d = %d\n", a, b, SvIV(ST(0)));
        printf ("%d - %d = %d\n", a, b, SvIV(ST(1)));

        PUTBACK;
        FREETMPS;
        LEAVE;
    }



Notes



    

  1.  Notice that it was necessary to define the variable ax.  This is
because the ST macro expects it to exist.  If we were in an XSUB it
would not be necessary to define ax as it is already defined for
us.


  2.  The code


            SPAGAIN;
        SP -= count;
        ax = (SP - PL_stack_base) + 1;

    

    sets the stack up so that we can use the ST macro.

    

  3.  Unlike the original coding of this example, the returned
values are not accessed in reverse order.  So ST(0) refers to the
first value returned by the Perl subroutine and ST(count-1)
refers to the last.











Previous: perlcall Alternate Stack Manipulation, Up: perlcall EXAMPLES   [Contents]





7.5.16 Creating and Calling an Anonymous Subroutine in C



As we’ve already shown, call_sv can be used to invoke an
anonymous subroutine.  However, our example showed a Perl script
invoking an XSUB to perform this operation.  Let’s see how it can be
done inside our C code:



 ...

 SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);

 ...

 call_sv(cvrv, G_VOID|G_NOARGS);



eval_pv is used to compile the anonymous subroutine, which
will be the return value as well (read more about eval_pv in
(perlapi)eval_pv).  Once this code reference is in hand, it
can be mixed in with all the previous examples we’ve shown.









Next: , Previous: perlcall EXAMPLES, Up: perlcall   [Contents]





7.6 LIGHTWEIGHT CALLBACKS



Sometimes you need to invoke the same subroutine repeatedly.
This usually happens with a function that acts on a list of
values, such as Perl’s built-in sort(). You can pass a
comparison function to sort(), which will then be invoked
for every pair of values that needs to be compared. The first()
and reduce() functions from (List-Util) follow a similar
pattern.



In this case it is possible to speed up the routine (often
quite substantially) by using the lightweight callback API.
The idea is that the calling context only needs to be
created and destroyed once, and the sub can be called
arbitrarily many times in between.



It is usual to pass parameters using global variables (typically
$_ for one parameter, or $a and $b for two parameters) rather
than via @_. (It is possible to use the @_ mechanism if you know
what you’re doing, though there is as yet no supported API for
it. It’s also inherently slower.)



The pattern of macro calls is like this:



    dMULTICALL;                 /* Declare local variables */
    I32 gimme = G_SCALAR;       /* context of the call: G_SCALAR,
                                 * G_ARRAY, or G_VOID */

    PUSH_MULTICALL(cv);         /* Set up the context for calling cv,
                                   and set local vars appropriately */

    /* loop */ {
        /* set the value(s) af your parameter variables */
        MULTICALL;              /* Make the actual call */
    } /* end of loop */

    POP_MULTICALL;              /* Tear down the calling context */



For some concrete examples, see the implementation of the
first() and reduce() functions of List::Util 1.18. There you
will also find a header file that emulates the multicall API
on older versions of perl.









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7.7 SEE ALSO



(perlxs), perlguts NAME, perlembed NAME









Next: , Previous: perlcall SEE ALSO, Up: perlcall   [Contents]





7.8 AUTHOR



Paul Marquess 



Special thanks to the following people who assisted in the creation of
the document.



Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
and Larry Wall.









Previous: perlcall AUTHOR, Up: perlcall   [Contents]





7.9 DATE



Version 1.3, 14th Apr 1997









Next: , Previous: perlcall, Up: Top   [Contents]





8 perlcheat





• perlcheat NAME:  



• perlcheat DESCRIPTION:  



• perlcheat ACKNOWLEDGEMENTS:  



• perlcheat AUTHOR:  



• perlcheat SEE ALSO:  












Next: , Up: perlcheat   [Contents]





8.1 NAME



perlcheat - Perl 5 Cheat Sheet









Next: , Previous: perlcheat NAME, Up: perlcheat   [Contents]





8.2 DESCRIPTION



This ’cheat sheet’ is a handy reference, meant for beginning Perl
programmers. Not everything is mentioned, but 195 features may
already be overwhelming.





• perlcheat The sheet:  












Up: perlcheat DESCRIPTION   [Contents]





8.2.1 The sheet



  CONTEXTS  SIGILS  ref        ARRAYS        HASHES
  void      $scalar SCALAR     @array        %hash
  scalar    @array  ARRAY      @array[0, 2]  @hash{'a', 'b'}
  list      %hash   HASH       $array[0]     $hash{'a'}
            &sub    CODE
            *glob   GLOB       SCALAR VALUES
                    FORMAT     number, string, ref, glob, undef
  REFERENCES
  \      reference       $$foo[1]       aka $foo->[1]
  $@%&*  dereference     $$foo{bar}     aka $foo->{bar}
  []     anon. arrayref  ${$$foo[1]}[2] aka $foo->[1]->[2]
  {}     anon. hashref   ${$$foo[1]}[2] aka $foo->[1][2]
  \()    list of refs
                         NUMBERS vs STRINGS    LINKS
  OPERATOR PRECEDENCE    =          =          perldoc.perl.org
  ->                     +          .           search.cpan.org
  ++ --                  == !=      eq ne              cpan.org
  **                     < > <= >=  lt gt le ge          pm.org
  ! ~ \ u+ u-            <=>        cmp                p3rl.org
  =~ !~                                           perlmonks.org
  * / % x                SYNTAX
  + - .                  foreach (LIST) { }     for (a;b;c) { }
  << >>                  while   (e) { }        until (e)   { }
  named uops             if      (e) { } elsif (e) { } else { }
  < > <= >= lt gt le ge  unless  (e) { } elsif (e) { } else { }
  == != <=> eq ne cmp ~~ given   (e) { when (e) {} default {} }
  &
  | ^             REGEX METACHARS          REGEX MODIFIERS
  &&              ^      string begin      /i case insensitive
  || //           $      str end (bfr \n)  /m line based ^$
  .. ...          +      one or more       /s . includes \n
  ?:              *      zero or more      /x ignore wh.space
  = += -= *= etc  ?      zero or one       /p preserve
  , =>            {3,7}  repeat in range   /a ASCII    /aa safe
  list ops        |      alternation       /l locale   /d  dual
  not             []     character class   /u Unicode
  and             \b     word boundary     /e evaluate /ee rpts
  or xor          \z     string end        /g global
                  ()     capture           /o compile pat once
  DEBUG           (?:p)  no capture
   -MO=Deparse    (?#t)  comment           REGEX CHARCLASSES
   -MO=Terse      (?=p)  ZW pos ahead      .   [^\n]
   -D##           (?!p)  ZW neg ahead      \s  whitespace
   -d:Trace       (?<=p) ZW pos behind \K  \w  word chars
                  (?<!p) ZW neg behind     \d  digits
  CONFIGURATION   (?>p)  no backtrack      \pP named property
  perl -V:ivsize  (?|p|p)branch reset      \h  horiz.wh.space
                  (?&NM) cap to name       \R  linebreak
                                           \S \W \D \H negate
  FUNCTION RETURN LISTS
  stat      localtime    caller         SPECIAL VARIABLES
   0 dev    0 second      0 package     $_    default variable
   1 ino    1 minute      1 filename    $0    program name
   2 mode   2 hour        2 line        $/    input separator
   3 nlink  3 day         3 subroutine  $\    output separator
   4 uid    4 month-1     4 hasargs     $|    autoflush
   5 gid    5 year-1900   5 wantarray   $!    sys/libcall error
   6 rdev   6 weekday     6 evaltext    $@    eval error
   7 size   7 yearday     7 is_require  $$    process ID
   8 atime  8 is_dst      8 hints       $.    line number
   9 mtime                9 bitmask     @ARGV command line args
  10 ctime               10 hinthash    @INC  include paths
  11 blksz               3..10 only     @_    subroutine args
  12 blcks               with EXPR      %ENV  environment









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8.3 ACKNOWLEDGEMENTS



The first version of this document appeared on Perl Monks, where several
people had useful suggestions. Thank you, Perl Monks.



A special thanks to Damian Conway, who didn’t only suggest important changes,
but also took the time to count the number of listed features and make a
Perl 6 version to show that Perl will stay Perl.









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8.4 AUTHOR



Juerd Waalboer <#####@juerd.nl>, with the help of many Perl Monks.









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8.5 SEE ALSO











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9 perlclib





• perlclib NAME:  



• perlclib DESCRIPTION:  



• perlclib SEE ALSO:  












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9.1 NAME



perlclib - Internal replacements for standard C library functions









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9.2 DESCRIPTION



One thing Perl porters should note is that perl doesn’t tend to use that
much of the C standard library internally; you’ll see very little use of, 
for example, the ctype.h functions in there. This is because Perl
tends to reimplement or abstract standard library functions, so that we
know exactly how they’re going to operate.



This is a reference card for people who are familiar with the C library
and who want to do things the Perl way; to tell them which functions
they ought to use instead of the more normal C functions. 





• perlclib Conventions:  



• perlclib File Operations:  



• perlclib File Input and Output:  



• perlclib File Positioning:  



• perlclib Memory Management and String Handling:  



• perlclib Character Class Tests:  



• perlclib stdlib.h functions:  



• perlclib Miscellaneous functions:  












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9.2.1 Conventions



In the following tables:





t




is a type.





p




is a pointer.





n




is a number.





s




is a string.








sv, av, hv, etc. represent variables of their respective types.









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9.2.2 File Operations



Instead of the stdio.h functions, you should use the Perl abstraction
layer. Instead of FILE* types, you need to be handling PerlIO*
types.  Don’t forget that with the new PerlIO layered I/O abstraction 
FILE* types may not even be available. See also the perlapio
documentation for more information about the following functions:



    Instead Of:                 Use:

    stdin                       PerlIO_stdin()
    stdout                      PerlIO_stdout()
    stderr                      PerlIO_stderr()

    fopen(fn, mode)             PerlIO_open(fn, mode)
    freopen(fn, mode, stream)   PerlIO_reopen(fn, mode, perlio) (Deprecated)
    fflush(stream)              PerlIO_flush(perlio)
    fclose(stream)              PerlIO_close(perlio)









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9.2.3 File Input and Output



    Instead Of:                 Use:

    fprintf(stream, fmt, ...)   PerlIO_printf(perlio, fmt, ...)

    [f]getc(stream)             PerlIO_getc(perlio)
    [f]putc(stream, n)          PerlIO_putc(perlio, n)
    ungetc(n, stream)           PerlIO_ungetc(perlio, n)



Note that the PerlIO equivalents of fread and fwrite are slightly
different from their C library counterparts:



    fread(p, size, n, stream)   PerlIO_read(perlio, buf, numbytes)
    fwrite(p, size, n, stream)  PerlIO_write(perlio, buf, numbytes)

    fputs(s, stream)            PerlIO_puts(perlio, s)



There is no equivalent to fgets; one should use sv_gets instead:



    fgets(s, n, stream)         sv_gets(sv, perlio, append)









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9.2.4 File Positioning



    Instead Of:                 Use:

    feof(stream)                PerlIO_eof(perlio)
    fseek(stream, n, whence)    PerlIO_seek(perlio, n, whence)
    rewind(stream)              PerlIO_rewind(perlio)

    fgetpos(stream, p)          PerlIO_getpos(perlio, sv)
    fsetpos(stream, p)          PerlIO_setpos(perlio, sv)

    ferror(stream)              PerlIO_error(perlio)
    clearerr(stream)            PerlIO_clearerr(perlio)









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9.2.5 Memory Management and String Handling



    Instead Of:                         Use:

    t* p = malloc(n)                    Newx(id, p, n, t)
    t* p = calloc(n, s)                 Newxz(id, p, n, t)
    p = realloc(p, n)                   Renew(p, n, t)
    memcpy(dst, src, n)                 Copy(src, dst, n, t)
    memmove(dst, src, n)                Move(src, dst, n, t)
    memcpy(dst, src, sizeof(t))         StructCopy(src, dst, t)
    memset(dst, 0, n * sizeof(t))       Zero(dst, n, t)
    memzero(dst, 0)                     Zero(dst, n, char)
    free(p)                             Safefree(p)

    strdup(p)                   savepv(p)
    strndup(p, n)               savepvn(p, n) (Hey, strndup doesn't exist!)

    strstr(big, little)         instr(big, little)
    strcmp(s1, s2)              strLE(s1, s2) / strEQ(s1, s2) / strGT(s1,s2)
    strncmp(s1, s2, n)          strnNE(s1, s2, n) / strnEQ(s1, s2, n)



Notice the different order of arguments to Copy and Move than used
in memcpy and memmove.



Most of the time, though, you’ll want to be dealing with SVs internally
instead of raw char * strings:



    strlen(s)                   sv_len(sv)
    strcpy(dt, src)             sv_setpv(sv, s)
    strncpy(dt, src, n)         sv_setpvn(sv, s, n)
    strcat(dt, src)             sv_catpv(sv, s)
    strncat(dt, src)            sv_catpvn(sv, s)
    sprintf(s, fmt, ...)        sv_setpvf(sv, fmt, ...)



Note also the existence of sv_catpvf and sv_vcatpvfn, combining
concatenation with formatting.



Sometimes instead of zeroing the allocated heap by using Newxz() you
should consider "poisoning" the data.  This means writing a bit
pattern into it that should be illegal as pointers (and floating point
numbers), and also hopefully surprising enough as integers, so that
any code attempting to use the data without forethought will break
sooner rather than later.  Poisoning can be done using the Poison()
macros, which have similar arguments to Zero():



    PoisonWith(dst, n, t, b)    scribble memory with byte b
    PoisonNew(dst, n, t)        equal to PoisonWith(dst, n, t, 0xAB)
    PoisonFree(dst, n, t)       equal to PoisonWith(dst, n, t, 0xEF)
    Poison(dst, n, t)           equal to PoisonFree(dst, n, t)









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9.2.6 Character Class Tests



There are two types of character class tests that Perl implements: one
type deals in chars and are thus not Unicode aware (and hence
deprecated unless you know you should use them) and the other type
deal in UVs and know about Unicode properties. In the following
table, c is a char, and u is a Unicode codepoint.



    Instead Of:                 Use:            But better use:

    isalnum(c)                  isALNUM(c)      isALNUM_uni(u)
    isalpha(c)                  isALPHA(c)      isALPHA_uni(u)
    iscntrl(c)                  isCNTRL(c)      isCNTRL_uni(u)
    isdigit(c)                  isDIGIT(c)      isDIGIT_uni(u)
    isgraph(c)                  isGRAPH(c)      isGRAPH_uni(u)
    islower(c)                  isLOWER(c)      isLOWER_uni(u)
    isprint(c)                  isPRINT(c)      isPRINT_uni(u)
    ispunct(c)                  isPUNCT(c)      isPUNCT_uni(u)
    isspace(c)                  isSPACE(c)      isSPACE_uni(u)
    isupper(c)                  isUPPER(c)      isUPPER_uni(u)
    isxdigit(c)                 isXDIGIT(c)     isXDIGIT_uni(u)

    tolower(c)                  toLOWER(c)      toLOWER_uni(u)
    toupper(c)                  toUPPER(c)      toUPPER_uni(u)









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9.2.7 stdlib.h functions



    Instead Of:                 Use: 

    atof(s)                     Atof(s)
    atol(s)                     Atol(s)
    strtod(s, &p)               Nothing.  Just don't use it.
    strtol(s, &p, n)            Strtol(s, &p, n)
    strtoul(s, &p, n)           Strtoul(s, &p, n)



Notice also the grok_bin, grok_hex, and grok_oct functions in
numeric.c for converting strings representing numbers in the respective
bases into NVs.



In theory Strtol and Strtoul may not be defined if the machine perl is
built on doesn’t actually have strtol and strtoul. But as those 2
functions are part of the 1989 ANSI C spec we suspect you’ll find them
everywhere by now.



    int rand()                  double Drand01()
    srand(n)                    { seedDrand01((Rand_seed_t)n); 
                                  PL_srand_called = TRUE; }

    exit(n)                     my_exit(n)
    system(s)                   Don't. Look at pp_system or use my_popen

    getenv(s)                   PerlEnv_getenv(s)
    setenv(s, val)              my_putenv(s, val)









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9.2.8 Miscellaneous functions



You should not even want to use setjmp.h functions, but if you
think you do, use the JMPENV stack in scope.h instead.



For signal/sigaction, use rsignal(signo, handler).









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9.3 SEE ALSO



(perlapi), perlapio NAME, perlguts NAME









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10 perlcommunity





• perlcommunity NAME:  



• perlcommunity DESCRIPTION:  



• perlcommunity AUTHOR:  












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10.1 NAME



perlcommunity - a brief overview of the Perl community









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10.2 DESCRIPTION



This document aims to provide an overview of the vast perl community, which is
far too large and diverse to provide a detailed listing. If any specific niche
has been forgotten, it is not meant as an insult but an omission for the sake
of brevity.



The Perl community is as diverse as Perl, and there is a large amount of
evidence that the Perl users apply TMTOWTDI to all endeavors, not just
programming. From websites, to IRC, to mailing lists, there is more than one
way to get involved in the community.





• perlcommunity Where to Find the Community:  



• perlcommunity Mailing Lists and Newsgroups:  



• perlcommunity IRC:  



• perlcommunity Websites:  



• perlcommunity User Groups:  



• perlcommunity Workshops:  



• perlcommunity Hackathons:  



• perlcommunity Conventions:  



• perlcommunity Calendar of Perl Events:  












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10.2.1 Where to Find the Community



There is a central directory for the Perl community: http://perl.org
maintained by the Perl Foundation (http://www.perlfoundation.org/),
which tracks and provides services for a variety of other community sites.









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10.2.2 Mailing Lists and Newsgroups



Perl runs on e-mail; there is no doubt about it. The Camel book was originally
written mostly over e-mail and today Perl’s development is co-ordinated through
mailing lists. The largest repository of Perl mailing lists is located at
http://lists.perl.org.



Most Perl-related projects set up mailing lists for both users and
contributors. If you don’t see a certain project listed at
http://lists.perl.org, check the particular website for that project.
Most mailing lists are archived at http://nntp.perl.org/.



There are also plenty of Perl related newsgroups located under
comp.lang.perl.*.









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10.2.3 IRC



The Perl community has a rather large IRC presence. For starters, it has its
own IRC network, irc://irc.perl.org. General (not help-oriented) chat can be
found at irc://irc.perl.org/#perl. Many other more specific chats are also
hosted on the network. Information about irc.perl.org is located on the
network’s website: http://www.irc.perl.org. For a more help-oriented #perl,
check out irc://irc.freenode.net/#perl. Perl 6 development also has a
presence in irc://irc.freenode.net/#perl6. Most Perl-related channels will
be kind enough to point you in the right direction if you ask nicely.



Any large IRC network (Dalnet, EFnet) is also likely to have a #perl channel,
with varying activity levels.









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10.2.4 Websites



Perl websites come in a variety of forms, but they fit into two large
categories: forums and news websites. There are many Perl-related
websites, so only a few of the community’s largest are mentioned here.





• perlcommunity News sites:  



• perlcommunity Forums:  












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10.2.4.1 News sites





http://perl.com/




Run by O’Reilly Media (the publisher of the Camel Book, among other
Perl-related literature), perl.com provides current Perl news, articles, and
resources for Perl developers as well as a directory of other useful websites.





http://use.perl.org/




use Perl; provides a slashdot-style Perl news website covering all things Perl,
from minutes of the meetings of the Perl 6 Design team to conference
announcements with (ir)relevant discussion.














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10.2.4.2 Forums





http://www.perlmonks.org/




PerlMonks is one of the largest Perl forums, and describes itself as "A place
for individuals to polish, improve, and showcase their Perl skills." and "A
community which allows everyone to grow and learn from each other."














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10.2.5 User Groups



Many cities around the world have local Perl Mongers chapters. A Perl Mongers
chapter is a local user group which typically holds regular in-person meetings,
both social and technical; helps organize local conferences, workshops, and
hackathons; and provides a mailing list or other continual contact method for
its members to keep in touch.



To find your local Perl Mongers (or PM as they’re commonly abbreviated) group
check the international Perl Mongers directory at http://www.pm.org/.









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10.2.6 Workshops



Perl workshops are, as the name might suggest, workshops where Perl is taught
in a variety of ways. At the workshops, subjects range from a beginner’s
introduction (such as the Pittsburgh Perl Workshop’s "Zero To Perl") to much
more advanced subjects.



There are several great resources for locating workshops: the
websites mentioned above, the
calendar mentioned below, and the YAPC Europe
website, http://www.yapceurope.org/, which is probably the best resource for
European Perl events.









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10.2.7 Hackathons



Hackathons are a very different kind of gathering where Perl hackers gather to
do just that, hack nonstop for an extended (several day) period on a specific
project or projects. Information about hackathons can be located in the same
place as information about workshops as well as in
irc://irc.perl.org/#perl.



If you have never been to a hackathon, here are a few basic things you need to
know before attending: have a working laptop and know how to use it; check out
the involved projects beforehand; have the necessary version control client;
and bring backup equipment (an extra LAN cable, additional power strips, etc.)
because someone will forget.









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10.2.8 Conventions



Perl has two major annual conventions: The Perl Conference (now part of OSCON),
put on by O’Reilly, and Yet Another Perl Conference or YAPC (pronounced
yap-see), which is localized into several regional YAPCs (North America,
Europe, Asia) in a stunning grassroots display by the Perl community. For more
information about either conference, check out their respective web pages:
OSCON http://conferences.oreillynet.com/; YAPC http://www.yapc.org.



A relatively new conference franchise with a large Perl portion is the
Open Source Developers Conference or OSDC. First held in Australia it has
recently also spread to Israel and France. More information can be found at:
http://www.osdc.com.au/ for Australia, http://www.osdc.org.il
for Israel, and http://www.osdc.fr/ for France.









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10.2.9 Calendar of Perl Events



The Perl Review, http://www.theperlreview.com maintains a website
and Google calendar
(http://www.theperlreview.com/community_calendar) for tracking
workshops, hackathons, Perl Mongers meetings, and other events. Views
of this calendar are at http://www.perl.org/events.html and
http://www.yapc.org.



Not every event or Perl Mongers group is on that calendar, so don’t lose
heart if you don’t see yours posted. To have your event or group listed,
contact brian d foy (brian@theperlreview.com).









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10.3 AUTHOR



Edgar "Trizor" Bering <trizor@gmail.com>









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11 perldata





• perldata NAME:  



• perldata DESCRIPTION:  



• perldata SEE ALSO:  












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11.1 NAME



perldata - Perl data types









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11.2 DESCRIPTION





• perldata Variable names:  



• perldata Context:  



• perldata Scalar values:  



• perldata Scalar value constructors:  



• perldata List value constructors:  



• perldata Subscripts:  



• perldata Multi-dimensional array emulation:  



• perldata Slices:  



• perldata Typeglobs and Filehandles:  












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11.2.1 Variable names







Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes".  A scalar is a 
single string (of any size, limited only by the available memory),
number, or a reference to something (which will be discussed
in (perlref)).  Normal arrays are ordered lists of scalars indexed
by number, starting with 0.  Hashes are unordered collections of scalar 
values indexed by their associated string key.




Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data
structure it refers.  The rest of the name tells you the particular
value to which it refers.  Usually this name is a single identifier,
that is, a string beginning with a letter or underscore, and
containing letters, underscores, and digits.  In some cases, it may
be a chain of identifiers, separated by :: (or by the slightly
archaic '); all but the last are interpreted as names of packages,
to locate the namespace in which to look up the final identifier
(see perlmod Packages for details).  It’s possible to substitute
for a simple identifier, an expression that produces a reference
to the value at runtime.   This is described in more detail below
and in (perlref).





Perl also has its own built-in variables whose names don’t follow
these rules.  They have strange names so they don’t accidentally
collide with one of your normal variables.  Strings that match
parenthesized parts of a regular expression are saved under names
containing only digits after the $ (see (perlop) and (perlre)).
In addition, several special variables that provide windows into
the inner working of Perl have names containing punctuation characters
and control characters.  These are documented in perlvar NAME.





Scalar values are always named with ’$’, even when referring to a
scalar that is part of an array or a hash.  The ’$’ symbol works
semantically like the English word "the" in that it indicates a
single value is expected.




    $days               # the simple scalar value "days"
    $days[28]           # the 29th element of array @days
    $days{'Feb'}        # the 'Feb' value from hash %days
    $#days              # the last index of array @days




Entire arrays (and slices of arrays and hashes) are denoted by ’@’,
which works much as the word "these" or "those" does in English,
in that it indicates multiple values are expected.




    @days               # ($days[0], $days[1],... $days[n])
    @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
    @days{'a','c'}      # same as ($days{'a'},$days{'c'})




Entire hashes are denoted by ’%’:




    %days               # (key1, val1, key2, val2 ...)



In addition, subroutines are named with an initial ’&’, though this
is optional when unambiguous, just as the word "do" is often redundant
in English.  Symbol table entries can be named with an initial ’*’,
but you don’t really care about that yet (if ever :-).




Every variable type has its own namespace, as do several
non-variable identifiers.  This means that you can, without fear
of conflict, use the same name for a scalar variable, an array, or
a hash–or, for that matter, for a filehandle, a directory handle, a
subroutine name, a format name, or a label.  This means that $foo
and @foo are two different variables.  It also means that $foo[1]
is a part of @foo, not a part of $foo.  This may seem a bit weird,
but that’s okay, because it is weird.






Because variable references always start with ’$’, ’@’, or ’%’, the
"reserved" words aren’t in fact reserved with respect to variable
names.  They are reserved with respect to labels and filehandles,
however, which don’t have an initial special character.  You can’t
have a filehandle named "log", for instance.  Hint: you could say
open(LOG,'logfile') rather than open(log,'logfile').  Using
uppercase filehandles also improves readability and protects you
from conflict with future reserved words.  Case is significant–"FOO",
"Foo", and "foo" are all different names.  Names that start with a
letter or underscore may also contain digits and underscores.




It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type.  For a description
of this, see (perlref).



Names that start with a digit may contain only more digits.  Names
that do not start with a letter, underscore, digit or a caret (i.e.
a control character) are limited to one character, e.g.,  $% or
$$.  (Most of these one character names have a predefined
significance to Perl.  For instance, $$ is the current process
id.)









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11.2.2 Context






The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
There are two major contexts: list and scalar.  Certain operations
return list values in contexts wanting a list, and scalar values
otherwise.  If this is true of an operation it will be mentioned in
the documentation for that operation.  In other words, Perl overloads
certain operations based on whether the expected return value is
singular or plural.  Some words in English work this way, like "fish"
and "sheep".



In a reciprocal fashion, an operation provides either a scalar or a
list context to each of its arguments.  For example, if you say



    int( <STDIN> )



the integer operation provides scalar context for the <>
operator, which responds by reading one line from STDIN and passing it
back to the integer operation, which will then find the integer value
of that line and return that.  If, on the other hand, you say



    sort( <STDIN> )



then the sort operation provides list context for <>, which
will proceed to read every line available up to the end of file, and
pass that list of lines back to the sort routine, which will then
sort those lines and return them as a list to whatever the context
of the sort was.



Assignment is a little bit special in that it uses its left argument
to determine the context for the right argument.  Assignment to a
scalar evaluates the right-hand side in scalar context, while
assignment to an array or hash evaluates the righthand side in list
context.  Assignment to a list (or slice, which is just a list
anyway) also evaluates the right-hand side in list context.



When you use the use warnings pragma or Perl’s -w command-line 
option, you may see warnings
about useless uses of constants or functions in "void context".
Void context just means the value has been discarded, such as a
statement containing only "fred"; or getpwuid(0);.  It still
counts as scalar context for functions that care whether or not
they’re being called in list context.



User-defined subroutines may choose to care whether they are being
called in a void, scalar, or list context.  Most subroutines do not
need to bother, though.  That’s because both scalars and lists are
automatically interpolated into lists.  See (perlfunc)wantarray
for how you would dynamically discern your function’s calling
context.









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11.2.3 Scalar values







All data in Perl is a scalar, an array of scalars, or a hash of
scalars.  A scalar may contain one single value in any of three
different flavors: a number, a string, or a reference.  In general,
conversion from one form to another is transparent.  Although a
scalar may not directly hold multiple values, it may contain a
reference to an array or hash which in turn contains multiple values.



Scalars aren’t necessarily one thing or another.  There’s no place
to declare a scalar variable to be of type "string", type "number",
type "reference", or anything else.  Because of the automatic
conversion of scalars, operations that return scalars don’t need
to care (and in fact, cannot care) whether their caller is looking
for a string, a number, or a reference.  Perl is a contextually
polymorphic language whose scalars can be strings, numbers, or
references (which includes objects).  Although strings and numbers
are considered pretty much the same thing for nearly all purposes,
references are strongly-typed, uncastable pointers with builtin
reference-counting and destructor invocation.








A scalar value is interpreted as FALSE in the Boolean sense
if it is undefined, the null string or the number 0 (or its
string equivalent, "0"), and TRUE if it is anything else.  The
Boolean context is just a special kind of scalar context where no 
conversion to a string or a number is ever performed.









There are actually two varieties of null strings (sometimes referred
to as "empty" strings), a defined one and an undefined one.  The
defined version is just a string of length zero, such as "".
The undefined version is the value that indicates that there is
no real value for something, such as when there was an error, or
at end of file, or when you refer to an uninitialized variable or
element of an array or hash.  Although in early versions of Perl,
an undefined scalar could become defined when first used in a
place expecting a defined value, this no longer happens except for
rare cases of autovivification as explained in (perlref).  You can
use the defined() operator to determine whether a scalar value is
defined (this has no meaning on arrays or hashes), and the undef()
operator to produce an undefined value.




To find out whether a given string is a valid non-zero number, it’s
sometimes enough to test it against both numeric 0 and also lexical
"0" (although this will cause noises if warnings are on).  That’s 
because strings that aren’t numbers count as 0, just as they do in awk:



    if ($str == 0 && $str ne "0")  {
        warn "That doesn't look like a number";
    }



That method may be best because otherwise you won’t treat IEEE
notations like NaN or Infinity properly.  At other times, you
might prefer to determine whether string data can be used numerically
by calling the POSIX::strtod() function or by inspecting your string
with a regular expression (as documented in (perlre)).



    warn "has nondigits"        if     /\D/;
    warn "not a natural number" unless /^\d+$/;             # rejects -3
    warn "not an integer"       unless /^-?\d+$/;           # rejects +3
    warn "not an integer"       unless /^[+-]?\d+$/;
    warn "not a decimal number" unless /^-?\d+\.?\d*$/;     # rejects .2
    warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
    warn "not a C float"
        unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;





The length of an array is a scalar value.  You may find the length
of array @days by evaluating $#days, as in csh.  However, this
isn’t the length of the array; it’s the subscript of the last element,
which is a different value since there is ordinarily a 0th element.
Assigning to $#days actually changes the length of the array.
Shortening an array this way destroys intervening values.  Lengthening
an array that was previously shortened does not recover values
that were in those elements.  (It used to do so in Perl 4, but we
had to break this to make sure destructors were called when expected.)




You can also gain some minuscule measure of efficiency by pre-extending
an array that is going to get big.  You can also extend an array
by assigning to an element that is off the end of the array.  You
can truncate an array down to nothing by assigning the null list
() to it.  The following are equivalent:



    @whatever = ();
    $#whatever = -1;




If you evaluate an array in scalar context, it returns the length
of the array.  (Note that this is not true of lists, which return
the last value, like the C comma operator, nor of built-in functions,
which return whatever they feel like returning.)  The following is
always true:




    scalar(@whatever) == $#whatever + 1;



Some programmers choose to use an explicit conversion so as to 
leave nothing to doubt:



    $element_count = scalar(@whatever);






If you evaluate a hash in scalar context, it returns false if the
hash is empty.  If there are any key/value pairs, it returns true;
more precisely, the value returned is a string consisting of the
number of used buckets and the number of allocated buckets, separated
by a slash.  This is pretty much useful only to find out whether
Perl’s internal hashing algorithm is performing poorly on your data
set.  For example, you stick 10,000 things in a hash, but evaluating
%HASH in scalar context reveals "1/16", which means only one out
of sixteen buckets has been touched, and presumably contains all
10,000 of your items.  This isn’t supposed to happen.  If a tied hash
is evaluated in scalar context, the SCALAR method is called (with a
fallback to FIRSTKEY).




You can preallocate space for a hash by assigning to the keys() function.
This rounds up the allocated buckets to the next power of two:



    keys(%users) = 1000;                # allocate 1024 buckets









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11.2.4 Scalar value constructors





Numeric literals are specified in any of the following floating point or
integer formats:



    12345
    12345.67
    .23E-10             # a very small number
    3.14_15_92          # a very important number
    4_294_967_296       # underscore for legibility
    0xff                # hex
    0xdead_beef         # more hex   
    0377                # octal (only numbers, begins with 0)
    0b011011            # binary




You are allowed to use underscores (underbars) in numeric literals
between digits for legibility (but not multiple underscores in a row:
23__500 is not legal; 23_500 is).
You could, for example, group binary
digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.





String literals are usually delimited by either single or double
quotes.  They work much like quotes in the standard Unix shells:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for \' and
\\).  The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
forms.  See (perlop)Quote and Quote-like Operators for a list.




Hexadecimal, octal, or binary, representations in string literals
(e.g. ’0xff’) are not automatically converted to their integer
representation.  The hex() and oct() functions make these conversions
for you.  See (perlfunc)hex and (perlfunc)oct for more details.




You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin.  This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
another line containing the quote character, which may be much further
on in the script.  Variable substitution inside strings is limited to
scalar variables, arrays, and array or hash slices.  (In other words,
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.)  The following code segment prints out "The
price is $100."




    $Price = '$100';    # not interpolated
    print "The price is $Price.\n";     # interpolated



There is no double interpolation in Perl, so the $100 is left as is.



By default floating point numbers substituted inside strings use the
dot (".")  as the decimal separator.  If use locale is in effect,
and POSIX::setlocale() has been called, the character used for the
decimal separator is affected by the LC_NUMERIC locale.
See perllocale NAME and (POSIX).




As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics (and underscores).
You must also do
this when interpolating a variable into a string to separate the
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:




    $who = "Larry";
    print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
    print "We use ${who}speak when ${who}'s here.\n";



Without the braces, Perl would have looked for a $whospeak, a
$who::0, and a $who's variable.  The last two would be the
$0 and the $s variables in the (presumably) non-existent package
who.



In fact, an identifier within such curlies is forced to be a string,
as is any simple identifier within a hash subscript.  Neither need
quoting.  Our earlier example, $days{'Feb'} can be written as
$days{Feb} and the quotes will be assumed automatically.  But
anything more complicated in the subscript will be interpreted as an
expression.  This means for example that $version{2.0}++ is
equivalent to $version{2}++, not to $version{'2.0'}++.





• perldata Version Strings:  



• perldata Special Literals:  



• perldata Barewords:  



• perldata Array Interpolation:  












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11.2.4.1 Version Strings






A literal of the form v1.20.300.4000 is parsed as a string composed
of characters with the specified ordinals.  This form, known as
v-strings, provides an alternative, more readable way to construct
strings, rather than use the somewhat less readable interpolation form
"\x{1}\x{14}\x{12c}\x{fa0}".  This is useful for representing
Unicode strings, and for comparing version "numbers" using the string
comparison operators, cmp, gt, lt etc.  If there are two or
more dots in the literal, the leading v may be omitted.



    print v9786;              # prints SMILEY, "\x{263a}"
    print v102.111.111;       # prints "foo"
    print 102.111.111;        # same



Such literals are accepted by both require and use for
doing a version check.  Note that using the v-strings for IPv4
addresses is not portable unless you also use the
inet_aton()/inet_ntoa() routines of the Socket package.



Note that since Perl 5.8.1 the single-number v-strings (like v65)
are not v-strings before the => operator (which is usually used
to separate a hash key from a hash value); instead they are interpreted
as literal strings (’v65’).  They were v-strings from Perl 5.6.0 to
Perl 5.8.0, but that caused more confusion and breakage than good.
Multi-number v-strings like v65.66 and 65.66.67 continue to
be v-strings always.









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11.2.4.2 Special Literals



















The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
point in your program.  __SUB__ gives a reference to the current
subroutine.  They may be used only as separate tokens; they
will not be interpolated into strings.  If there is no current package
(due to an empty package; directive), __PACKAGE__ is the undefined
value. (But the empty package; is no longer supported, as of version
5.10.)  Outside of a subroutine, __SUB__ is the undefined value.  __SUB__
is only available in 5.16 or higher, and only with a use v5.16 or
use feature "current_sub" declaration.




The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
may be used to indicate the logical end of the script before the actual
end of file.  Any following text is ignored.



Text after __DATA__ may be read via the filehandle PACKNAME::DATA,
where PACKNAME is the package that was current when the __DATA__
token was encountered.  The filehandle is left open pointing to the
line after __DATA__.  The program should close DATA when it is done
reading from it.  (Leaving it open leaks filehandles if the module is
reloaded for any reason, so it’s a safer practice to close it.)  For
compatibility with older scripts written before __DATA__ was
introduced, __END__ behaves like __DATA__ in the top level script (but
not in files loaded with require or do) and leaves the remaining
contents of the file accessible via main::DATA.



See (SelfLoader) for more description of __DATA__, and
an example of its use.  Note that you cannot read from the DATA
filehandle in a BEGIN block: the BEGIN block is executed as soon
as it is seen (during compilation), at which point the corresponding
__DATA__ (or __END__) token has not yet been seen.









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11.2.4.3 Barewords




A word that has no other interpretation in the grammar will
be treated as if it were a quoted string.  These are known as
"barewords".  As with filehandles and labels, a bareword that consists
entirely of lowercase letters risks conflict with future reserved
words, and if you use the use warnings pragma or the -w switch, 
Perl will warn you about any such words.  Perl limits barewords (like
identifiers) to about 250 characters.  Future versions of Perl are likely
to eliminate these arbitrary limitations.



Some people may wish to outlaw barewords entirely.  If you
say



    use strict 'subs';



then any bareword that would NOT be interpreted as a subroutine call
produces a compile-time error instead.  The restriction lasts to the
end of the enclosing block.  An inner block may countermand this
by saying no strict 'subs'.









Previous: perldata Barewords, Up: perldata Scalar value constructors   [Contents]





11.2.4.4 Array Interpolation






Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the $"
variable ($LIST_SEPARATOR if "use English;" is specified), 
space by default.  The following are equivalent:



    $temp = join($", @ARGV);
    system "echo $temp";

    system "echo @ARGV";



Within search patterns (which also undergo double-quotish substitution)
there is an unfortunate ambiguity:  Is /$foo[bar]/ to be interpreted as
/${foo}[bar]/ (where [bar] is a character class for the regular
expression) or as /${foo[bar]}/ (where [bar] is the subscript to array
@foo)?  If @foo doesn’t otherwise exist, then it’s obviously a
character class.  If @foo exists, Perl takes a good guess about [bar],
and is almost always right.  If it does guess wrong, or if you’re just
plain paranoid, you can force the correct interpretation with curly
braces as above.



If you’re looking for the information on how to use here-documents,
which used to be here, that’s been moved to
(perlop)Quote and Quote-like Operators.









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11.2.5 List value constructors




List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):



    (LIST)



In a context not requiring a list value, the value of what appears
to be a list literal is simply the value of the final element, as
with the C comma operator.  For example,



    @foo = ('cc', '-E', $bar);



assigns the entire list value to array @foo, but



    $foo = ('cc', '-E', $bar);



assigns the value of variable $bar to the scalar variable $foo.
Note that the value of an actual array in scalar context is the
length of the array; the following assigns the value 3 to $foo:



    @foo = ('cc', '-E', $bar);
    $foo = @foo;                # $foo gets 3



You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:



    @foo = (
        1,
        2,
        3,
    );



To use a here-document to assign an array, one line per element,
you might use an approach like this:



    @sauces = <<End_Lines =~ m/(\S.*\S)/g;
        normal tomato
        spicy tomato
        green chile
        pesto
        white wine
    End_Lines



LISTs do automatic interpolation of sublists.  That is, when a LIST is
evaluated, each element of the list is evaluated in list context, and
the resulting list value is interpolated into LIST just as if each
individual element were a member of LIST.  Thus arrays and hashes lose their
identity in a LIST–the list



    (@foo,@bar,&SomeSub,%glarch)



contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub 
called in list context, followed by the key/value pairs of %glarch.
To make a list reference that does NOT interpolate, see (perlref).



The null list is represented by ().  Interpolating it in a list
has no effect.  Thus ((),(),()) is equivalent to ().  Similarly,
interpolating an array with no elements is the same as if no
array had been interpolated at that point.



This interpolation combines with the facts that the opening
and closing parentheses are optional (except when necessary for
precedence) and lists may end with an optional comma to mean that
multiple commas within lists are legal syntax. The list 1,,3 is a
concatenation of two lists, 1, and 3, the first of which ends
with that optional comma.  1,,3 is (1,),(3) is 1,3 (And
similarly for 1,,,3 is (1,),(,),3 is 1,3 and so on.)  Not that
we’d advise you to use this obfuscation.



A list value may also be subscripted like a normal array.  You must
put the list in parentheses to avoid ambiguity.  For example:



    # Stat returns list value.
    $time = (stat($file))[8];

    # SYNTAX ERROR HERE.
    $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES

    # Find a hex digit.
    $hexdigit = ('a','b','c','d','e','f')[$digit-10];

    # A "reverse comma operator".
    return (pop(@foo),pop(@foo))[0];



Lists may be assigned to only when each element of the list
is itself legal to assign to:



    ($a, $b, $c) = (1, 2, 3);

    ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);



An exception to this is that you may assign to undef in a list.
This is useful for throwing away some of the return values of a
function:



    ($dev, $ino, undef, undef, $uid, $gid) = stat($file);



List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:



    $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
    $x = (($foo,$bar) = f());           # set $x to f()'s return count



This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.



It’s also the source of a useful idiom for executing a function or
performing an operation in list context and then counting the number of
return values, by assigning to an empty list and then using that
assignment in scalar context. For example, this code:



    $count = () = $string =~ /\d+/g;



will place into $count the number of digit groups found in $string.
This happens because the pattern match is in list context (since it
is being assigned to the empty list), and will therefore return a list
of all matching parts of the string. The list assignment in scalar
context will translate that into the number of elements (here, the
number of times the pattern matched) and assign that to $count. Note
that simply using



    $count = $string =~ /\d+/g;



would not have worked, since a pattern match in scalar context will
only return true or false, rather than a count of matches.



The final element of a list assignment may be an array or a hash:



    ($a, $b, @rest) = split;
    my($a, $b, %rest) = @_;



You can actually put an array or hash anywhere in the list, but the first one
in the list will soak up all the values, and anything after it will become
undefined.  This may be useful in a my() or local().



A hash can be initialized using a literal list holding pairs of
items to be interpreted as a key and a value:



    # same as map assignment above
    %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);



While literal lists and named arrays are often interchangeable, that’s
not the case for hashes.  Just because you can subscript a list value like
a normal array does not mean that you can subscript a list value as a
hash.  Likewise, hashes included as parts of other lists (including
parameters lists and return lists from functions) always flatten out into
key/value pairs.  That’s why it’s good to use references sometimes.



It is often more readable to use the => operator between key/value
pairs.  The => operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to be
interpreted as a string if it’s a bareword that would be a legal simple
identifier. => doesn’t quote compound identifiers, that contain
double colons. This makes it nice for initializing hashes:



    %map = (
                 red   => 0x00f,
                 blue  => 0x0f0,
                 green => 0xf00,
   );



or for initializing hash references to be used as records:



    $rec = {
                witch => 'Mable the Merciless',
                cat   => 'Fluffy the Ferocious',
                date  => '10/31/1776',
    };



or for using call-by-named-parameter to complicated functions:



   $field = $query->radio_group(
               name      => 'group_name',
               values    => ['eenie','meenie','minie'],
               default   => 'meenie',
               linebreak => 'true',
               labels    => \%labels
   );



Note that just because a hash is initialized in that order doesn’t
mean that it comes out in that order.  See (perlfunc)sort for examples
of how to arrange for an output ordering.









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11.2.6 Subscripts



An array can be accessed one scalar at a
time by specifying a dollar sign ($), then the
name of the array (without the leading @), then the subscript inside
square brackets.  For example:



    @myarray = (5, 50, 500, 5000);
    print "The Third Element is", $myarray[2], "\n";



The array indices start with 0. A negative subscript retrieves its 
value from the end.  In our example, $myarray[-1] would have been 
5000, and $myarray[-2] would have been 500.



Hash subscripts are similar, only instead of square brackets curly brackets
are used. For example:



    %scientists = 
    (
        "Newton" => "Isaac",
        "Einstein" => "Albert",
        "Darwin" => "Charles",
        "Feynman" => "Richard",
    );

    print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";



You can also subscript a list to get a single element from it:



    $dir = (getpwnam("daemon"))[7];









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11.2.7 Multi-dimensional array emulation



Multidimensional arrays may be emulated by subscripting a hash with a
list. The elements of the list are joined with the subscript separator
(see perlvar $;).



    $foo{$a,$b,$c}



is equivalent to



    $foo{join($;, $a, $b, $c)}



The default subscript separator is "\034", the same as SUBSEP in awk.









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11.2.8 Slices






A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts.  It’s more convenient
than writing out the individual elements as a list of separate
scalar values.



    ($him, $her)   = @folks[0,-1];              # array slice
    @them          = @folks[0 .. 3];            # array slice
    ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
    ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice



Since you can assign to a list of variables, you can also assign to
an array or hash slice.



    @days[3..5]    = qw/Wed Thu Fri/;
    @colors{'red','blue','green'} 
                   = (0xff0000, 0x0000ff, 0x00ff00);
    @folks[0, -1]  = @folks[-1, 0];



The previous assignments are exactly equivalent to



    ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
    ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                   = (0xff0000, 0x0000ff, 0x00ff00);
    ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);



Since changing a slice changes the original array or hash that it’s
slicing, a foreach construct will alter some–or even all–of the
values of the array or hash.



    foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } 

    foreach (@hash{qw[key1 key2]}) {
        s/^\s+//;           # trim leading whitespace
        s/\s+$//;           # trim trailing whitespace
        s/(\w+)/\u\L$1/g;   # "titlecase" words
    }



A slice of an empty list is still an empty list.  Thus:



    @a = ()[1,0];           # @a has no elements
    @b = (@a)[0,1];         # @b has no elements
    @c = (0,1)[2,3];        # @c has no elements



But:



    @a = (1)[1,0];          # @a has two elements
    @b = (1,undef)[1,0,2];  # @b has three elements



This makes it easy to write loops that terminate when a null list
is returned:



    while ( ($home, $user) = (getpwent)[7,0]) {
        printf "%-8s %s\n", $user, $home;
    }



As noted earlier in this document, the scalar sense of list assignment
is the number of elements on the right-hand side of the assignment.
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.



Slices in scalar context return the last item of the slice.



    @a = qw/first second third/;
    %h = (first => 'A', second => 'B');
    $t = @a[0, 1];                  # $t is now 'second'
    $u = @h{'first', 'second'};     # $u is now 'B'



If you’re confused about why you use an ’@’ there on a hash slice
instead of a ’%’, think of it like this.  The type of bracket (square
or curly) governs whether it’s an array or a hash being looked at.
On the other hand, the leading symbol (’$’ or ’@’) on the array or
hash indicates whether you are getting back a singular value (a
scalar) or a plural one (a list).









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11.2.9 Typeglobs and Filehandles






Perl uses an internal type called a typeglob to hold an entire
symbol table entry.  The type prefix of a typeglob is a *, because
it represents all types.  This used to be the preferred way to
pass arrays and hashes by reference into a function, but now that
we have real references, this is seldom needed.  



The main use of typeglobs in modern Perl is create symbol table aliases.
This assignment:



    *this = *that;



makes $this an alias for $that, @this an alias for @that, %this an alias
for %that, &this an alias for &that, etc.  Much safer is to use a reference.
This:



    local *Here::blue = \$There::green;



temporarily makes $Here::blue an alias for $There::green, but doesn’t
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc.  See perlmod Symbol Tables for more examples
of this.  Strange though this may seem, this is the basis for the whole
module import/export system.



Another use for typeglobs is to pass filehandles into a function or
to create new filehandles.  If you need to use a typeglob to save away
a filehandle, do it this way:



    $fh = *STDOUT;



or perhaps as a real reference, like this:



    $fh = \*STDOUT;



See (perlsub) for examples of using these as indirect filehandles
in functions.



Typeglobs are also a way to create a local filehandle using the local()
operator.  These last until their block is exited, but may be passed back.
For example:



    sub newopen {
        my $path = shift;
        local  *FH;  # not my!
        open   (FH, $path)          or  return undef;
        return *FH;
    }
    $fh = newopen('/etc/passwd');



Now that we have the *foo{THING} notation, typeglobs aren’t used as much
for filehandle manipulations, although they’re still needed to pass brand
new file and directory handles into or out of functions. That’s because
*HANDLE{IO} only works if HANDLE has already been used as a handle.
In other words, *FH must be used to create new symbol table entries;
*foo{THING} cannot.  When in doubt, use *FH.



All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
automatically create an anonymous filehandle if the handle passed to
them is an uninitialized scalar variable. This allows the constructs
such as open(my $fh, ...) and open(local $fh,...) to be used to
create filehandles that will conveniently be closed automatically when
the scope ends, provided there are no other references to them. This
largely eliminates the need for typeglobs when opening filehandles
that must be passed around, as in the following example:



    sub myopen {
        open my $fh, "@_"
             or die "Can't open '@_': $!";
        return $fh;
    }

    {
        my $f = myopen("</etc/motd");
        print <$f>;
        # $f implicitly closed here
    }



Note that if an initialized scalar variable is used instead the
result is different: my $fh='zzz'; open($fh, ...) is equivalent
to open( *{'zzz'}, ...).
use strict 'refs' forbids such practice.



Another way to create anonymous filehandles is with the Symbol
module or with the IO::Handle module and its ilk.  These modules
have the advantage of not hiding different types of the same name
during the local().  See the bottom of (perlfunc)open for an
example.









Previous: perldata DESCRIPTION, Up: perldata   [Contents]





11.3 SEE ALSO



See perlvar NAME for a description of Perl’s built-in variables and
a discussion of legal variable names.  See (perlref), (perlsub),
and perlmod Symbol Tables for more discussion on typeglobs and
the *foo{THING} syntax.









Next: , Previous: perldata, Up: Top   [Contents]





12 perldbmfilter





• perldbmfilter NAME:  



• perldbmfilter SYNOPSIS:  



• perldbmfilter DESCRIPTION:  



• perldbmfilter SEE ALSO:  



• perldbmfilter AUTHOR:  












Next: , Up: perldbmfilter   [Contents]





12.1 NAME



perldbmfilter - Perl DBM Filters









Next: , Previous: perldbmfilter NAME, Up: perldbmfilter   [Contents]





12.2 SYNOPSIS



    $db = tie %hash, 'DBM', ...

    $old_filter = $db->filter_store_key  ( sub { ... } );
    $old_filter = $db->filter_store_value( sub { ... } );
    $old_filter = $db->filter_fetch_key  ( sub { ... } );
    $old_filter = $db->filter_fetch_value( sub { ... } );









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12.3 DESCRIPTION



The four filter_* methods shown above are available in all the DBM
modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
ODBM_File and SDBM_File.



Each of the methods works identically, and is used to install (or
uninstall) a single DBM Filter. The only difference between them is the
place that the filter is installed.



To summarise:





filter_store_key




If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.





filter_store_value




If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.





filter_fetch_key




If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.





filter_fetch_value




If a filter has been installed with this method, it will be invoked
every time you read a value from a DBM database.








You can use any combination of the methods from none to all four.



All filter methods return the existing filter, if present, or undef
if not.



To delete a filter pass undef to it.





• perldbmfilter The Filter:  



• perldbmfilter An Example: the NULL termination problem.:  



• perldbmfilter Another Example: Key is a C int.:  












Next: , Up: perldbmfilter DESCRIPTION   [Contents]





12.3.1 The Filter



When each filter is called by Perl, a local copy of $_ will contain
the key or value to be filtered. Filtering is achieved by modifying
the contents of $_. The return code from the filter is ignored.









Next: , Previous: perldbmfilter The Filter, Up: perldbmfilter DESCRIPTION   [Contents]





12.3.2 An Example: the NULL termination problem.



DBM Filters are useful for a class of problems where you always
want to make the same transformation to all keys, all values or both.



For example, consider the following scenario. You have a DBM database
that you need to share with a third-party C application. The C application
assumes that all keys and values are NULL terminated. Unfortunately
when Perl writes to DBM databases it doesn’t use NULL termination, so
your Perl application will have to manage NULL termination itself. When
you write to the database you will have to use something like this:



    $hash{"$key\0"} = "$value\0";



Similarly the NULL needs to be taken into account when you are considering
the length of existing keys/values.



It would be much better if you could ignore the NULL terminations issue
in the main application code and have a mechanism that automatically
added the terminating NULL to all keys and values whenever you write to
the database and have them removed when you read from the database. As I’m
sure you have already guessed, this is a problem that DBM Filters can
fix very easily.



    use strict;
    use warnings;
    use SDBM_File;
    use Fcntl;

    my %hash;
    my $filename = "filt";
    unlink $filename;

    my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
      or die "Cannot open $filename: $!\n";

    # Install DBM Filters
    $db->filter_fetch_key  ( sub { s/\0$//    } );
    $db->filter_store_key  ( sub { $_ .= "\0" } );
    $db->filter_fetch_value( 
        sub { no warnings 'uninitialized'; s/\0$// } );
    $db->filter_store_value( sub { $_ .= "\0" } );

    $hash{"abc"} = "def";
    my $a = $hash{"ABC"};
    # ...
    undef $db;
    untie %hash;



The code above uses SDBM_File, but it will work with any of the DBM
modules.



Hopefully the contents of each of the filters should be
self-explanatory. Both "fetch" filters remove the terminating NULL,
and both "store" filters add a terminating NULL.









Previous: perldbmfilter An Example: the NULL termination problem., Up: perldbmfilter DESCRIPTION   [Contents]





12.3.3 Another Example: Key is a C int.



Here is another real-life example. By default, whenever Perl writes to
a DBM database it always writes the key and value as strings. So when
you use this:



    $hash{12345} = "something";



the key 12345 will get stored in the DBM database as the 5 byte string
"12345". If you actually want the key to be stored in the DBM database
as a C int, you will have to use pack when writing, and unpack
when reading.



Here is a DBM Filter that does it:



    use strict;
    use warnings;
    use DB_File;
    my %hash;
    my $filename = "filt";
    unlink $filename;


    my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH 
      or die "Cannot open $filename: $!\n";

    $db->filter_fetch_key  ( sub { $_ = unpack("i", $_) } );
    $db->filter_store_key  ( sub { $_ = pack ("i", $_) } );
    $hash{123} = "def";
    # ...
    undef $db;
    untie %hash;



The code above uses DB_File, but again it will work with any of the
DBM modules.



This time only two filters have been used; we only need to manipulate
the contents of the key, so it wasn’t necessary to install any value
filters.









Next: , Previous: perldbmfilter DESCRIPTION, Up: perldbmfilter   [Contents]





12.4 SEE ALSO



(DB_File), (GDBM_File), (NDBM_File), (ODBM_File) and (SDBM_File).









Previous: perldbmfilter SEE ALSO, Up: perldbmfilter   [Contents]





12.5 AUTHOR



Paul Marquess









Next: , Previous: perldbmfilter, Up: Top   [Contents]





13 perldebguts





• perldebguts NAME:  



• perldebguts DESCRIPTION:  



• perldebguts Debugger Internals:  



• perldebguts Frame Listing Output Examples:  



• perldebguts Debugging Regular Expressions:  



• perldebguts Debugging Perl Memory Usage:  



• perldebguts SEE ALSO:  












Next: , Up: perldebguts   [Contents]





13.1 NAME



perldebguts - Guts of Perl debugging 









Next: , Previous: perldebguts NAME, Up: perldebguts   [Contents]





13.2 DESCRIPTION



This is not (perldebug), which tells you how to use
the debugger.  This manpage describes low-level details concerning
the debugger’s internals, which range from difficult to impossible
to understand for anyone who isn’t incredibly intimate with Perl’s guts.
Caveat lector.









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13.3 Debugger Internals



Perl has special debugging hooks at compile-time and run-time used
to create debugging environments.  These hooks are not to be confused
with the perl -Dxxx command described in perlrun NAME, which is
usable only if a special Perl is built per the instructions in the
INSTALL podpage in the Perl source tree.



For example, whenever you call Perl’s built-in caller function
from the package DB, the arguments that the corresponding stack
frame was called with are copied to the @DB::args array.  These
mechanisms are enabled by calling Perl with the -d switch.
Specifically, the following additional features are enabled
(cf. perlvar $^P):





Note that if &DB::sub needs external data for it to work, no
subroutine call is possible without it. As an example, the standard
debugger’s &DB::sub depends on the $DB::deep variable
(it defines how many levels of recursion deep into the debugger you can go
before a mandatory break).  If $DB::deep is not defined, subroutine
calls are not possible, even though &DB::sub exists.





• perldebguts Writing Your Own Debugger:  












Up: perldebguts Debugger Internals   [Contents]





13.3.1 Writing Your Own Debugger





• perldebguts Environment Variables:  



• perldebguts Debugger Internal Variables:  



• perldebguts Debugger Customization Functions:  












Next: , Up: perldebguts Writing Your Own Debugger   [Contents]





13.3.1.1 Environment Variables



The PERL5DB environment variable can be used to define a debugger.
For example, the minimal "working" debugger (it actually doesn’t do anything)
consists of one line:



  sub DB::DB {}



It can easily be defined like this:



  $ PERL5DB="sub DB::DB {}" perl -d your-script



Another brief debugger, slightly more useful, can be created
with only the line:



  sub DB::DB {print ++$i; scalar <STDIN>}



This debugger prints a number which increments for each statement
encountered and waits for you to hit a newline before continuing
to the next statement.



The following debugger is actually useful:



  {
    package DB;
    sub DB  {}
    sub sub {print ++$i, " $sub\n"; &$sub}
  }



It prints the sequence number of each subroutine call and the name of the
called subroutine.  Note that &DB::sub is being compiled into the
package DB through the use of the package directive.



When it starts, the debugger reads your rc file (./.perldb or
~/.perldb under Unix), which can set important options.
(A subroutine (&afterinit) can be defined here as well; it is executed
after the debugger completes its own initialization.)



After the rc file is read, the debugger reads the PERLDB_OPTS
environment variable and uses it to set debugger options. The
contents of this variable are treated as if they were the argument
of an o ... debugger command (q.v. in (perldebug)Configurable Options).









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13.3.1.2 Debugger Internal Variables



In addition to the file and subroutine-related variables mentioned above,
the debugger also maintains various magical internal variables.











Previous: perldebguts Debugger Internal Variables, Up: perldebguts Writing Your Own Debugger   [Contents]





13.3.1.3 Debugger Customization Functions



Some functions are provided to simplify customization.





Note that any variables and functions that are not documented in
this manpages (or in (perldebug)) are considered for internal   
use only, and as such are subject to change without notice.









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13.4 Frame Listing Output Examples



The frame option can be used to control the output of frame 
information.  For example, contrast this expression trace:



 $ perl -de 42
 Stack dump during die enabled outside of evals.

 Loading DB routines from perl5db.pl patch level 0.94
 Emacs support available.

 Enter h or 'h h' for help.

 main::(-e:1):   0
   DB<1> sub foo { 14 }

   DB<2> sub bar { 3 }

   DB<3> t print foo() * bar()
 main::((eval 172):3):   print foo() + bar();
 main::foo((eval 168):2):
 main::bar((eval 170):2):
 42



with this one, once the option frame=2 has been set:



   DB<4> o f=2
                frame = '2'
   DB<5> t print foo() * bar()
 3:      foo() * bar()
 entering main::foo
  2:     sub foo { 14 };
 exited main::foo
 entering main::bar
  2:     sub bar { 3 };
 exited main::bar
 42



By way of demonstration, we present below a laborious listing
resulting from setting your PERLDB_OPTS environment variable to
the value f=n N, and running perl -d -V from the command line.
Examples using various values of n are shown to give you a feel
for the difference between settings.  Long though it may be, this
is not a complete listing, but only excerpts.



    

  1.  

      entering main::BEGIN
   entering Config::BEGIN
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   Package lib/Config.pm.
   entering Config::TIEHASH
   entering Exporter::import
    entering Exporter::export
  entering Config::myconfig
   entering Config::FETCH
   entering Config::FETCH
   entering Config::FETCH
   entering Config::FETCH

    

  2.  

      entering main::BEGIN
   entering Config::BEGIN
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   exited Config::BEGIN
   Package lib/Config.pm.
   entering Config::TIEHASH
   exited Config::TIEHASH
   entering Exporter::import
    entering Exporter::export
    exited Exporter::export
   exited Exporter::import
  exited main::BEGIN
  entering Config::myconfig
   entering Config::FETCH
   exited Config::FETCH
   entering Config::FETCH
   exited Config::FETCH
   entering Config::FETCH

    

  3.  

      in  $=main::BEGIN() from /dev/null:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
  in  @=Config::myconfig() from /dev/null:0
   in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574

    

  4.  

      in  $=main::BEGIN() from /dev/null:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   out $=Config::BEGIN() from lib/Config.pm:0
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   out $=Config::TIEHASH('Config') from lib/Config.pm:644
   in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
    out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
   out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
  out $=main::BEGIN() from /dev/null:0
  in  @=Config::myconfig() from /dev/null:0
   in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574

    

  5.  

      in  $=main::BEGIN() from /dev/null:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   out $=Config::BEGIN() from lib/Config.pm:0
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   out $=Config::TIEHASH('Config') from lib/Config.pm:644
   in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
    out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
   out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
  out $=main::BEGIN() from /dev/null:0
  in  @=Config::myconfig() from /dev/null:0
   in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
   out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
   in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
   out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

    

  6.  

      in  $=CODE(0x15eca4)() from /dev/null:0
   in  $=CODE(0x182528)() from lib/Config.pm:2
    Package lib/Exporter.pm.
   out $=CODE(0x182528)() from lib/Config.pm:0
   scalar context return from CODE(0x182528): undef
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:628
   out $=Config::TIEHASH('Config') from lib/Config.pm:628
   scalar context return from Config::TIEHASH:   empty hash
   in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
    out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
    scalar context return from Exporter::export: ''
   out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
   scalar context return from Exporter::import: ''

    




In all cases shown above, the line indentation shows the call tree.
If bit 2 of frame is set, a line is printed on exit from a
subroutine as well.  If bit 4 is set, the arguments are printed
along with the caller info.  If bit 8 is set, the arguments are
printed even if they are tied or references.  If bit 16 is set, the
return value is printed, too.



When a package is compiled, a line like this



    Package lib/Carp.pm.



is printed with proper indentation.









Next: , Previous: perldebguts Frame Listing Output Examples, Up: perldebguts   [Contents]





13.5 Debugging Regular Expressions



There are two ways to enable debugging output for regular expressions.



If your perl is compiled with -DDEBUGGING, you may use the
-Dr flag on the command line.



Otherwise, one can use re 'debug', which has effects at
compile time and run time.  Since Perl 5.9.5, this pragma is lexically
scoped.





• perldebguts Compile-time Output:  



• perldebguts Types of Nodes:  



• perldebguts Run-time Output:  












Next: , Up: perldebguts Debugging Regular Expressions   [Contents]





13.5.1 Compile-time Output



The debugging output at compile time looks like this:



  Compiling REx '[bc]d(ef*g)+h[ij]k$'
  size 45 Got 364 bytes for offset annotations.
  first at 1
  rarest char g at 0
  rarest char d at 0
     1: ANYOF[bc](12)
    12: EXACT <d>(14)
    14: CURLYX[0] {1,32767}(28)
    16:   OPEN1(18)
    18:     EXACT <e>(20)
    20:     STAR(23)
    21:       EXACT <f>(0)
    23:     EXACT <g>(25)
    25:   CLOSE1(27)
    27:   WHILEM[1/1](0)
    28: NOTHING(29)
    29: EXACT <h>(31)
    31: ANYOF[ij](42)
    42: EXACT <k>(44)
    44: EOL(45)
    45: END(0)
  anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating) 
        stclass 'ANYOF[bc]' minlen 7 
  Offsets: [45]
        1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
        0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
        11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
        0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]  
  Omitting $` $& $' support.



The first line shows the pre-compiled form of the regex.  The second
shows the size of the compiled form (in arbitrary units, usually
4-byte words) and the total number of bytes allocated for the
offset/length table, usually 4+size*8.  The next line shows the
label id of the first node that does a match.



The 



  anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating) 
        stclass 'ANYOF[bc]' minlen 7 



line (split into two lines above) contains optimizer
information.  In the example shown, the optimizer found that the match 
should contain a substring de at offset 1, plus substring gh
at some offset between 3 and infinity.  Moreover, when checking for
these substrings (to abandon impossible matches quickly), Perl will check
for the substring gh before checking for the substring de.  The
optimizer may also use the knowledge that the match starts (at the
first id) with a character class, and no string 
shorter than 7 characters can possibly match.



The fields of interest which may appear in this line are





anchored STRING at POS






floating STRING at POS1..POS2




See above.





matching floating/anchored




Which substring to check first.





minlen




The minimal length of the match.





stclass TYPE




Type of first matching node.





noscan




Don’t scan for the found substrings.





isall




Means that the optimizer information is all that the regular
expression contains, and thus one does not need to enter the regex engine at
all.





GPOS




Set if the pattern contains \G.





plus




Set if the pattern starts with a repeated char (as in x+y).





implicit




Set if the pattern starts with .*.





with eval




Set if the pattern contain eval-groups, such as (?{ code }) and
(??{ code }).





anchored(TYPE)




If the pattern may match only at a handful of places, with TYPE
being BOL, MBOL, or GPOS.  See the table below.








If a substring is known to match at end-of-line only, it may be
followed by $, as in floating 'k'$.



The optimizer-specific information is used to avoid entering (a slow) regex
engine on strings that will not definitely match.  If the isall flag
is set, a call to the regex engine may be avoided even when the optimizer
found an appropriate place for the match.



Above the optimizer section is the list of nodes of the compiled
form of the regex.  Each line has format 



   id: TYPE OPTIONAL-INFO (next-id)









Next: , Previous: perldebguts Compile-time Output, Up: perldebguts Debugging Regular Expressions   [Contents]





13.5.2 Types of Nodes



Here are the possible types, with short descriptions:



 # TYPE arg-description [num-args] [longjump-len] DESCRIPTION

 # Exit points
 END   no        End of program.
 SUCCEED   no        Return from a subroutine, basically.

 # Anchors:

 BOL        no      Match "" at beginning of line.
 MBOL       no      Same, assuming multiline.
 SBOL       no      Same, assuming singleline.
 EOS        no      Match "" at end of string.
 EOL        no      Match "" at end of line.
 MEOL       no      Same, assuming multiline.
 SEOL       no      Same, assuming singleline.
 BOUND      no      Match "" at any word boundary using native charset
                    semantics for non-utf8
 BOUNDL     no      Match "" at any locale word boundary
 BOUNDU     no      Match "" at any word boundary using Unicode semantics
 BOUNDA     no      Match "" at any word boundary using ASCII semantics
 NBOUND     no      Match "" at any word non-boundary using native charset
                    semantics for non-utf8
 NBOUNDL    no      Match "" at any locale word non-boundary
 NBOUNDU    no      Match "" at any word non-boundary using Unicode semantics
 NBOUNDA    no      Match "" at any word non-boundary using ASCII semantics
 GPOS       no      Matches where last m//g left off.

 # [Special] alternatives:

 REG_ANY    no      Match any one character (except newline).
 SANY       no      Match any one character.
 CANY       no      Match any one byte.
 ANYOF      sv      Match character in (or not in) this class, single char
                    match only
 ANYOFV     sv      Match character in (or not in) this class, can
                    match-multiple chars
 ALNUM      no      Match any alphanumeric character using native charset
                    semantics for non-utf8
 ALNUML     no      Match any alphanumeric char in locale
 ALNUMU     no      Match any alphanumeric char using Unicode semantics
 ALNUMA     no      Match [A-Za-z_0-9]
 NALNUM     no      Match any non-alphanumeric character using native charset
                    semantics for non-utf8
 NALNUML    no      Match any non-alphanumeric char in locale
 NALNUMU    no      Match any non-alphanumeric char using Unicode semantics
 NALNUMA    no      Match [^A-Za-z_0-9]
 SPACE      no      Match any whitespace character using native charset
                    semantics for non-utf8
 SPACEL     no      Match any whitespace char in locale
 SPACEU     no      Match any whitespace char using Unicode semantics
 SPACEA     no      Match [ \t\n\f\r]
 NSPACE     no      Match any non-whitespace character using native charset
                    semantics for non-utf8
 NSPACEL    no      Match any non-whitespace char in locale
 NSPACEU    no      Match any non-whitespace char using Unicode semantics
 NSPACEA    no      Match [^ \t\n\f\r]
 DIGIT      no      Match any numeric character using native charset semantics
                    for non-utf8
 DIGITL     no      Match any numeric character in locale
 DIGITA     no      Match [0-9]
 NDIGIT     no      Match any non-numeric character using native charset
 i                  semantics for non-utf8
 NDIGITL    no      Match any non-numeric character in locale
 NDIGITA    no      Match [^0-9]
 CLUMP      no      Match any extended grapheme cluster sequence

 # Alternation

 # BRANCH        The set of branches constituting a single choice are hooked
 #               together with their "next" pointers, since precedence prevents
 #               anything being concatenated to any individual branch.  The
 #               "next" pointer of the last BRANCH in a choice points to the
 #               thing following the whole choice.  This is also where the
 #               final "next" pointer of each individual branch points; each
 #               branch starts with the operand node of a BRANCH node.
 #
 BRANCH node        Match this alternative, or the next...

 # Back pointer

 # BACK          Normal "next" pointers all implicitly point forward; BACK
 #               exists to make loop structures possible.
 # not used
 BACK       no      Match "", "next" ptr points backward.

 # Literals

 EXACT      str     Match this string (preceded by length).
 EXACTF     str     Match this string, folded, native charset semantics for
                    non-utf8 (prec. by length).
 EXACTFL    str     Match this string, folded in locale (w/len).
 EXACTFU    str     Match this string, folded, Unicode semantics for non-utf8
                    (prec. by length).
 EXACTFA    str     Match this string, folded, Unicode semantics for non-utf8,
                    but no ASCII-range character matches outside ASCII (prec.
                    by length),.

 # Do nothing types

 NOTHING    no        Match empty string.
 # A variant of above which delimits a group, thus stops optimizations
 TAIL       no        Match empty string. Can jump here from outside.

 # Loops

 # STAR,PLUS    '?', and complex '*' and '+', are implemented as circular
 #               BRANCH structures using BACK.  Simple cases (one character
 #               per match) are implemented with STAR and PLUS for speed
 #               and to minimize recursive plunges.
 #
 STAR       node    Match this (simple) thing 0 or more times.
 PLUS       node    Match this (simple) thing 1 or more times.

 CURLY      sv 2    Match this simple thing {n,m} times.
 CURLYN     no 2    Capture next-after-this simple thing
 CURLYM     no 2    Capture this medium-complex thing {n,m} times.
 CURLYX     sv 2    Match this complex thing {n,m} times.

 # This terminator creates a loop structure for CURLYX
 WHILEM     no      Do curly processing and see if rest matches.

 # Buffer related

 # OPEN,CLOSE,GROUPP     ...are numbered at compile time.
 OPEN       num 1   Mark this point in input as start of #n.
 CLOSE      num 1   Analogous to OPEN.

 REF        num 1   Match some already matched string
 REFF       num 1   Match already matched string, folded using native charset
                    semantics for non-utf8
 REFFL      num 1   Match already matched string, folded in loc.
 REFFU      num 1   Match already matched string, folded using unicode
                    semantics for non-utf8
 REFFA      num 1   Match already matched string, folded using unicode
                    semantics for non-utf8, no mixing ASCII, non-ASCII

 # Named references.  Code in regcomp.c assumes that these all are after the
 # numbered references
 NREF       no-sv 1 Match some already matched string
 NREFF      no-sv 1 Match already matched string, folded using native charset
                    semantics for non-utf8
 NREFFL     no-sv 1 Match already matched string, folded in loc.
 NREFFU     num   1 Match already matched string, folded using unicode
                    semantics for non-utf8
 NREFFA     num   1 Match already matched string, folded using unicode
                    semantics for non-utf8, no mixing ASCII, non-ASCII

 IFMATCH    off 1 2 Succeeds if the following matches.
 UNLESSM    off 1 2 Fails if the following matches.
 SUSPEND    off 1 1 "Independent" sub-RE.
 IFTHEN     off 1 1 Switch, should be preceded by switcher.
 GROUPP     num 1   Whether the group matched.

 # Support for long RE

 LONGJMP    off 1 1 Jump far away.
 BRANCHJ    off 1 1 BRANCH with long offset.

 # The heavy worker

 EVAL       evl 1   Execute some Perl code.

 # Modifiers

 MINMOD     no      Next operator is not greedy.
 LOGICAL    no      Next opcode should set the flag only.

 # This is not used yet
 RENUM      off 1 1 Group with independently numbered parens.

 # Trie Related

 # Behave the same as A|LIST|OF|WORDS would. The '..C' variants have
 # inline charclass data (ascii only), the 'C' store it in the structure.
 # NOTE: the relative order of the TRIE-like regops  is significant

 TRIE       trie 1    Match many EXACT(F[ALU]?)? at once. flags==type
 TRIEC      charclass Same as TRIE, but with embedded charclass data

 # For start classes, contains an added fail table.
 AHOCORASICK trie 1   Aho Corasick stclass. flags==type
 AHOCORASICKC charclass Same as AHOCORASICK, but with embedded charclass data

 # Regex Subroutines
 GOSUB      num/ofs 2L recurse to paren arg1 at (signed) ofs arg2
 GOSTART    no         recurse to start of pattern

 # Special conditionals
 NGROUPP    no-sv 1   Whether the group matched.
 INSUBP     num 1     Whether we are in a specific recurse.
 DEFINEP    none 1    Never execute directly.

 # Backtracking Verbs
 ENDLIKE    none      Used only for the type field of verbs
 OPFAIL     none      Same as (?!)
 ACCEPT     parno 1   Accepts the current matched string.


 # Verbs With Arguments
 VERB       no-sv 1   Used only for the type field of verbs
 PRUNE      no-sv 1   Pattern fails at this startpoint if no-backtracking through this
 MARKPOINT  no-sv 1   Push the current location for rollback by cut.
 SKIP       no-sv 1   On failure skip forward (to the mark) before retrying
 COMMIT     no-sv 1   Pattern fails outright if backtracking through this
 CUTGROUP   no-sv 1   On failure go to the next alternation in the group

 # Control what to keep in $&.
 KEEPS      no        $& begins here.

 # New charclass like patterns
 LNBREAK    none      generic newline pattern
 VERTWS     none      vertical whitespace         (Perl 6)
 NVERTWS    none      not vertical whitespace     (Perl 6)
 HORIZWS    none      horizontal whitespace       (Perl 6)
 NHORIZWS   none      not horizontal whitespace   (Perl 6)

 FOLDCHAR   codepoint 1 codepoint with tricky case folding properties.

 # SPECIAL  REGOPS

 # This is not really a node, but an optimized away piece of a "long" node.
 # To simplify debugging output, we mark it as if it were a node
 OPTIMIZED  off       Placeholder for dump.

 # Special opcode with the property that no opcode in a compiled program
 # will ever be of this type. Thus it can be used as a flag value that
 # no other opcode has been seen. END is used similarly, in that an END
 # node cant be optimized. So END implies "unoptimizable" and PSEUDO mean
 # "not seen anything to optimize yet".
 PSEUDO     off       Pseudo opcode for internal use.



Following the optimizer information is a dump of the offset/length
table, here split across several lines:



  Offsets: [45]
        1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
        0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
        11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
        0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]  



The first line here indicates that the offset/length table contains 45
entries.  Each entry is a pair of integers, denoted by offset[length].
Entries are numbered starting with 1, so entry #1 here is 1[4] and
entry #12 is 5[1].  1[4] indicates that the node labeled 1:
(the 1: ANYOF[bc]) begins at character position 1 in the
pre-compiled form of the regex, and has a length of 4 characters.
5[1] in position 12 
indicates that the node labeled 12:
(the 12: EXACT <d>) begins at character position 5 in the
pre-compiled form of the regex, and has a length of 1 character.
12[1] in position 14 
indicates that the node labeled 14:
(the 14: CURLYX[0] {1,32767}) begins at character position 12 in the
pre-compiled form of the regex, and has a length of 1 character—that
is, it corresponds to the + symbol in the precompiled regex.



0[0] items indicate that there is no corresponding node.









Previous: perldebguts Types of Nodes, Up: perldebguts Debugging Regular Expressions   [Contents]





13.5.3 Run-time Output



First of all, when doing a match, one may get no run-time output even
if debugging is enabled.  This means that the regex engine was never
entered and that all of the job was therefore done by the optimizer.



If the regex engine was entered, the output may look like this:



  Matching '[bc]d(ef*g)+h[ij]k$' against 'abcdefg__gh__'
    Setting an EVAL scope, savestack=3
     2 <ab> <cdefg__gh_>    |  1: ANYOF
     3 <abc> <defg__gh_>    | 11: EXACT <d>
     4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
     4 <abcd> <efg__gh_>    | 26:   WHILEM
                                0 out of 1..32767  cc=effff31c
     4 <abcd> <efg__gh_>    | 15:     OPEN1
     4 <abcd> <efg__gh_>    | 17:     EXACT <e>
     5 <abcde> <fg__gh_>    | 19:     STAR
                             EXACT <f> can match 1 times out of 32767...
    Setting an EVAL scope, savestack=3
     6 <bcdef> <g__gh__>    | 22:       EXACT <g>
     7 <bcdefg> <__gh__>    | 24:       CLOSE1
     7 <bcdefg> <__gh__>    | 26:       WHILEM
                                    1 out of 1..32767  cc=effff31c
    Setting an EVAL scope, savestack=12
     7 <bcdefg> <__gh__>    | 15:         OPEN1
     7 <bcdefg> <__gh__>    | 17:         EXACT <e>
       restoring \1 to 4(4)..7
                                    failed, try continuation...
     7 <bcdefg> <__gh__>    | 27:         NOTHING
     7 <bcdefg> <__gh__>    | 28:         EXACT <h>
                                    failed...
                                failed...



The most significant information in the output is about the particular node
of the compiled regex that is currently being tested against the target string.
The format of these lines is



    STRING-OFFSET <PRE-STRING> <POST-STRING>   |ID:  TYPE



The TYPE info is indented with respect to the backtracking level.
Other incidental information appears interspersed within.









Next: , Previous: perldebguts Debugging Regular Expressions, Up: perldebguts   [Contents]





13.6 Debugging Perl Memory Usage



Perl is a profligate wastrel when it comes to memory use.  There
is a saying that to estimate memory usage of Perl, assume a reasonable
algorithm for memory allocation, multiply that estimate by 10, and
while you still may miss the mark, at least you won’t be quite so
astonished.  This is not absolutely true, but may provide a good
grasp of what happens.



Assume that an integer cannot take less than 20 bytes of memory, a
float cannot take less than 24 bytes, a string cannot take less
than 32 bytes (all these examples assume 32-bit architectures, the
result are quite a bit worse on 64-bit architectures).  If a variable
is accessed in two of three different ways (which require an integer,
a float, or a string), the memory footprint may increase yet another
20 bytes.  A sloppy malloc(3) implementation can inflate these
numbers dramatically.



On the opposite end of the scale, a declaration like



  sub foo;



may take up to 500 bytes of memory, depending on which release of Perl
you’re running.



Anecdotal estimates of source-to-compiled code bloat suggest an
eightfold increase.  This means that the compiled form of reasonable
(normally commented, properly indented etc.) code will take
about eight times more space in memory than the code took
on disk.



The -DL command-line switch is obsolete since circa Perl 5.6.0
(it was available only if Perl was built with -DDEBUGGING).
The switch was used to track Perl’s memory allocations and possible
memory leaks.  These days the use of malloc debugging tools like
Purify or valgrind is suggested instead.  See also
perlhacktips PERL_MEM_LOG.



One way to find out how much memory is being used by Perl data
structures is to install the Devel::Size module from CPAN: it gives
you the minimum number of bytes required to store a particular data
structure.  Please be mindful of the difference between the size()
and total_size().



If Perl has been compiled using Perl’s malloc you can analyze Perl
memory usage by setting $ENV{PERL_DEBUG_MSTATS}.





• perldebguts Using $ENV{PERL_DEBUG_MSTATS}:  












Up: perldebguts Debugging Perl Memory Usage   [Contents]





13.6.1 Using $ENV{PERL_DEBUG_MSTATS}



If your perl is using Perl’s malloc() and was compiled with the
necessary switches (this is the default), then it will print memory
usage statistics after compiling your code when $ENV{PERL_DEBUG_MSTATS}
> 1, and before termination of the program when $ENV{PERL_DEBUG_MSTATS} >= 1.  The report format is similar to
the following example:



  $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
  Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
     14216 free:   130   117    28     7     9   0   2     2   1 0 0
                437    61    36     0     5
     60924 used:   125   137   161    55     7   8   6    16   2 0 1
                 74   109   304    84    20
  Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
  Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
     30888 free:   245    78    85    13     6   2   1     3   2 0 1
                315   162    39    42    11
    175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                196   178  1066   798    39
  Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.



It is possible to ask for such a statistic at arbitrary points in
your execution using the mstat() function out of the standard
Devel::Peek module.



Here is some explanation of that format:





buckets SMALLEST(APPROX)..GREATEST(APPROX)




Perl’s malloc() uses bucketed allocations.  Every request is rounded
up to the closest bucket size available, and a bucket is taken from
the pool of buckets of that size.



The line above describes the limits of buckets currently in use.
Each bucket has two sizes: memory footprint and the maximal size
of user data that can fit into this bucket.  Suppose in the above
example that the smallest bucket were size 4.  The biggest bucket
would have usable size 8188, and the memory footprint would be 8192.



In a Perl built for debugging, some buckets may have negative usable
size.  This means that these buckets cannot (and will not) be used.
For larger buckets, the memory footprint may be one page greater
than a power of 2.  If so, the corresponding power of two is
printed in the APPROX field above.





Free/Used




The 1 or 2 rows of numbers following that correspond to the number
of buckets of each size between SMALLEST and GREATEST.  In
the first row, the sizes (memory footprints) of buckets are powers
of two–or possibly one page greater.  In the second row, if present,
the memory footprints of the buckets are between the memory footprints
of two buckets "above".



For example, suppose under the previous example, the memory footprints
were



     free:    8     16    32    64    128  256 512 1024 2048 4096 8192
           4     12    24    48    80



With a non-DEBUGGING perl, the buckets starting from 128 have
a 4-byte overhead, and thus an 8192-long bucket may take up to
8188-byte allocations.





Total sbrk(): SBRKed/SBRKs:CONTINUOUS




The first two fields give the total amount of memory perl sbrk(2)ed
(ess-broken? :-) and number of sbrk(2)s used.  The third number is
what perl thinks about continuity of returned chunks.  So long as
this number is positive, malloc() will assume that it is probable
that sbrk(2) will provide continuous memory.



Memory allocated by external libraries is not counted.





pad: 0




The amount of sbrk(2)ed memory needed to keep buckets aligned.





heads: 2192




Although memory overhead of bigger buckets is kept inside the bucket, for
smaller buckets, it is kept in separate areas.  This field gives the
total size of these areas.





chain: 0




malloc() may want to subdivide a bigger bucket into smaller buckets.
If only a part of the deceased bucket is left unsubdivided, the rest
is kept as an element of a linked list.  This field gives the total
size of these chunks.





tail: 6144




To minimize the number of sbrk(2)s, malloc() asks for more memory.  This
field gives the size of the yet unused part, which is sbrk(2)ed, but
never touched.














Previous: perldebguts Debugging Perl Memory Usage, Up: perldebguts   [Contents]





13.7 SEE ALSO



(perldebug),
perlguts NAME,
perlrun NAME
(re),
and
(Devel-DProf).









Next: , Previous: perldebguts, Up: Top   [Contents]





14 perldebtut





• perldebtut NAME:  



• perldebtut DESCRIPTION:  



• perldebtut use strict:  



• perldebtut Looking at data and -w and v:  



• perldebtut help:  



• perldebtut Stepping through code:  



• perldebtut Placeholder for a, w, t, T:  



• perldebtut REGULAR EXPRESSIONS:  



• perldebtut OUTPUT TIPS:  



• perldebtut CGI:  



• perldebtut GUIs:  



• perldebtut SUMMARY:  



• perldebtut SEE ALSO:  



• perldebtut AUTHOR:  



• perldebtut CONTRIBUTORS:  












Next: , Up: perldebtut   [Contents]





14.1 NAME



perldebtut - Perl debugging tutorial









Next: , Previous: perldebtut NAME, Up: perldebtut   [Contents]





14.2 DESCRIPTION



A (very) lightweight introduction in the use of the perl debugger, and a
pointer to existing, deeper sources of information on the subject of debugging
perl programs.  



There’s an extraordinary number of people out there who don’t appear to know
anything about using the perl debugger, though they use the language every
day.  
This is for them.  









Next: , Previous: perldebtut DESCRIPTION, Up: perldebtut   [Contents]





14.3 use strict



First of all, there’s a few things you can do to make your life a lot more
straightforward when it comes to debugging perl programs, without using the
debugger at all.  To demonstrate, here’s a simple script, named "hello", with
a problem:



        #!/usr/bin/perl

        $var1 = 'Hello World'; # always wanted to do that :-)
        $var2 = "$varl\n";

        print $var2; 
        exit;



While this compiles and runs happily, it probably won’t do what’s expected,
namely it doesn’t print "Hello World\n" at all;  It will on the other hand do
exactly what it was told to do, computers being a bit that way inclined.  That
is, it will print out a newline character, and you’ll get what looks like a
blank line.  It looks like there’s 2 variables when (because of the typo)
there’s really 3:



        $var1 = 'Hello World';
        $varl = undef;
        $var2 = "\n";



To catch this kind of problem, we can force each variable to be declared
before use by pulling in the strict module, by putting ’use strict;’ after the
first line of the script.



Now when you run it, perl complains about the 3 undeclared variables and we
get four error messages because one variable is referenced twice:



 Global symbol "$var1" requires explicit package name at ./t1 line 4.
 Global symbol "$var2" requires explicit package name at ./t1 line 5.
 Global symbol "$varl" requires explicit package name at ./t1 line 5.
 Global symbol "$var2" requires explicit package name at ./t1 line 7.
 Execution of ./hello aborted due to compilation errors.     



Luvverly! and to fix this we declare all variables explicitly and now our
script looks like this:	



        #!/usr/bin/perl
        use strict;

        my $var1 = 'Hello World';
        my $varl = undef;
        my $var2 = "$varl\n";

        print $var2; 
        exit;



We then do (always a good idea) a syntax check before we try to run it again:



        > perl -c hello
        hello syntax OK 



And now when we run it, we get "\n" still, but at least we know why.  Just
getting this script to compile has exposed the ’$varl’ (with the letter ’l’)
variable, and simply changing $varl to $var1 solves the problem.









Next: , Previous: perldebtut use strict, Up: perldebtut   [Contents]





14.4 Looking at data and -w and v



Ok, but how about when you want to really see your data, what’s in that
dynamic variable, just before using it?



        #!/usr/bin/perl 
        use strict;

        my $key = 'welcome';
        my %data = (
                'this' => qw(that), 
                'tom' => qw(and jerry),
                'welcome' => q(Hello World),
                'zip' => q(welcome),
        );
        my @data = keys %data;

        print "$data{$key}\n";
        exit;                               



Looks OK, after it’s been through the syntax check (perl -c scriptname), we
run it and all we get is a blank line again!  Hmmmm.



One common debugging approach here, would be to liberally sprinkle a few print
statements, to add a check just before we print out our data, and another just
after:



        print "All OK\n" if grep($key, keys %data);
        print "$data{$key}\n";
        print "done: '$data{$key}'\n";



And try again:



        > perl data
        All OK     

        done: ''



After much staring at the same piece of code and not seeing the wood for the
trees for some time, we get a cup of coffee and try another approach.  That
is, we bring in the cavalry by giving perl the ’-d’ switch on the command
line:



        > perl -d data 
        Default die handler restored.

        Loading DB routines from perl5db.pl version 1.07
        Editor support available.

        Enter h or `h h' for help, or `man perldebug' for more help.

        main::(./data:4):     my $key = 'welcome';   



Now, what we’ve done here is to launch the built-in perl debugger on our
script.  It’s stopped at the first line of executable code and is waiting for
input.



Before we go any further, you’ll want to know how to quit the debugger: use
just the letter ’q’, not the words ’quit’ or ’exit’:



        DB<1> q
        >



That’s it, you’re back on home turf again.









Next: , Previous: perldebtut Looking at data and -w and v, Up: perldebtut   [Contents]





14.5 help



Fire the debugger up again on your script and we’ll look at the help menu. 
There’s a couple of ways of calling help: a simple ’h’ will get the summary 
help list, ’|h’ (pipe-h) will pipe the help through your pager (which is 
(probably ’more’ or ’less’), and finally, ’h h’ (h-space-h) will give you 
the entire help screen.  Here is the summary page:



D1h



 List/search source lines:               Control script execution:
  l [ln|sub]  List source code            T           Stack trace
  - or .      List previous/current line  s [expr]    Single step [in expr]
  v [line]    View around line            n [expr]    Next, steps over subs
  f filename  View source in file         <CR/Enter>  Repeat last n or s
  /pattern/ ?patt?   Search forw/backw    r           Return from subroutine
  M           Show module versions        c [ln|sub]  Continue until position
 Debugger controls:                       L           List break/watch/actions
  o [...]     Set debugger options        t [expr]    Toggle trace [trace expr]
  <[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint
  ! [N|pat]   Redo a previous command     B ln|*      Delete a/all breakpoints
  H [-num]    Display last num commands   a [ln] cmd  Do cmd before line
  = [a val]   Define/list an alias        A ln|*      Delete a/all actions
  h [db_cmd]  Get help on command         w expr      Add a watch expression
  h h         Complete help page          W expr|*    Delete a/all watch exprs
  |[|]db_cmd  Send output to pager        ![!] syscmd Run cmd in a subprocess
  q or ^D     Quit                        R           Attempt a restart
 Data Examination:     expr     Execute perl code, also see: s,n,t expr
  x|m expr       Evals expr in list context, dumps the result or lists methods.
  p expr         Print expression (uses script's current package).
  S [[!]pat]     List subroutine names [not] matching pattern
  V [Pk [Vars]]  List Variables in Package.  Vars can be ~pattern or !pattern.
  X [Vars]       Same as "V current_package [Vars]".
  y [n [Vars]]   List lexicals in higher scope <n>.  Vars same as V.
 For more help, type h cmd_letter, or run man perldebug for all docs. 



More confusing options than you can shake a big stick at!  It’s not as bad as
it looks and it’s very useful to know more about all of it, and fun too!



There’s a couple of useful ones to know about straight away.  You wouldn’t
think we’re using any libraries at all at the moment, but ’M’ will show
which modules are currently loaded, and their version number, while ’m’ 
will show the methods, and ’S’ shows all subroutines (by pattern) as 
shown below.  ’V’ and ’X’ show variables in the program by package 
scope and can be constrained by pattern. 



        DB<2>S str 
        dumpvar::stringify
        strict::bits
        strict::import
        strict::unimport  



Using ’X’ and cousins requires you not to use the type identifiers ($@%), just
the ’name’:



        DM<3>X ~err
        FileHandle(stderr) => fileno(2)    



Remember we’re in our tiny program with a problem, we should have a look at
where we are, and what our data looks like. First of all let’s view some code 
at our present position (the first line of code in this case), via ’v’:



        DB<4> v
        1       #!/usr/bin/perl
        2:      use strict;
        3
        4==>    my $key = 'welcome';
        5:      my %data = (
        6               'this' => qw(that),
        7               'tom' => qw(and jerry),
        8               'welcome' => q(Hello World),
        9               'zip' => q(welcome),
        10      );                                 



At line number 4 is a helpful pointer, that tells you where you are now.  To
see more code, type ’v’ again:



        DB<4> v
        8               'welcome' => q(Hello World),
        9               'zip' => q(welcome),
        10      );
        11:     my @data = keys %data;
        12:     print "All OK\n" if grep($key, keys %data);
        13:     print "$data{$key}\n";
        14:     print "done: '$data{$key}'\n";
        15:     exit;      



And if you wanted to list line 5 again, type ’l 5’, (note the space):



        DB<4> l 5
        5:      my %data = (



In this case, there’s not much to see, but of course normally there’s pages of
stuff to wade through, and ’l’ can be very useful.  To reset your view to the
line we’re about to execute, type a lone period ’.’:



        DB<5> .
        main::(./data_a:4):     my $key = 'welcome';  



The line shown is the one that is about to be executed next, it hasn’t
happened yet.  So while we can print a variable with the letter ’p’, at
this point all we’d get is an empty (undefined) value back.  What we need to
do is to step through the next executable statement with an ’s’:



        DB<6> s
        main::(./data_a:5):     my %data = (
        main::(./data_a:6):             'this' => qw(that),
        main::(./data_a:7):             'tom' => qw(and jerry),
        main::(./data_a:8):             'welcome' => q(Hello World),
        main::(./data_a:9):             'zip' => q(welcome),
        main::(./data_a:10):    );   



Now we can have a look at that first ($key) variable:



        DB<7> p $key 
        welcome 



line 13 is where the action is, so let’s continue down to there via the letter
’c’, which by the way, inserts a ’one-time-only’ breakpoint at the given
line or sub routine:



        DB<8> c 13
        All OK
        main::(./data_a:13):    print "$data{$key}\n";



We’ve gone past our check (where ’All OK’ was printed) and have stopped just
before the meat of our task.  We could try to print out a couple of variables
to see what is happening:



        DB<9> p $data{$key}



Not much in there, lets have a look at our hash:



        DB<10> p %data
        Hello Worldziptomandwelcomejerrywelcomethisthat 

        DB<11> p keys %data
        Hello Worldtomwelcomejerrythis  



Well, this isn’t very easy to read, and using the helpful manual (h h), the
’x’ command looks promising:



        DB<12> x %data
        0  'Hello World'
        1  'zip'
        2  'tom'
        3  'and'
        4  'welcome'
        5  undef
        6  'jerry'
        7  'welcome'
        8  'this'
        9  'that'     



That’s not much help, a couple of welcomes in there, but no indication of
which are keys, and which are values, it’s just a listed array dump and, in
this case, not particularly helpful.  The trick here, is to use a reference
to the data structure:



        DB<13> x \%data
        0  HASH(0x8194bc4)
           'Hello World' => 'zip'
           'jerry' => 'welcome'
           'this' => 'that'
           'tom' => 'and'
           'welcome' => undef  



The reference is truly dumped and we can finally see what we’re dealing with. 
Our quoting was perfectly valid but wrong for our purposes, with ’and jerry’
being treated as 2 separate words rather than a phrase, thus throwing the
evenly paired hash structure out of alignment.



The ’-w’ switch would have told us about this, had we used it at the start,
and saved us a lot of trouble: 



        > perl -w data
        Odd number of elements in hash assignment at ./data line 5.    



We fix our quoting: ’tom’ => q(and jerry), and run it again, this time we get
our expected output:



        > perl -w data
        Hello World



While we’re here, take a closer look at the ’x’ command, it’s really useful
and will merrily dump out nested references, complete objects, partial objects
- just about whatever you throw at it:



Let’s make a quick object and x-plode it, first we’ll start the debugger:
it wants some form of input from STDIN, so we give it something non-committal,
a zero:



        > perl -de 0
        Default die handler restored.

        Loading DB routines from perl5db.pl version 1.07
        Editor support available.

        Enter h or `h h' for help, or `man perldebug' for more help.

        main::(-e:1):   0                       



Now build an on-the-fly object over a couple of lines (note the backslash):



        DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \
        cont:   {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')



And let’s have a look at it:



        DB<2> x $obj
        0  MY_class=HASH(0x828ad98)
                'attr' => HASH(0x828ad68)
        'col' => 'black'
        'things' => ARRAY(0x828abb8)
                0  'this'
                1  'that'
                2  'etc'
                'unique_id' => 123       
        DB<3>



Useful, huh?  You can eval nearly anything in there, and experiment with bits
of code or regexes until the cows come home:



        DB<3> @data = qw(this that the other atheism leather theory scythe)

        DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
        atheism
        leather
        other
        scythe
        the
        theory  
        saw -> 6



If you want to see the command History, type an ’H’:



        DB<5> H
        4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
        3: @data = qw(this that the other atheism leather theory scythe)
        2: x $obj
        1: $obj = bless({'unique_id'=>'123', 'attr'=>
        {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
        DB<5>



And if you want to repeat any previous command, use the exclamation: ’!’:



        DB<5> !4
        p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
        atheism
        leather
        other
        scythe
        the
        theory  
        saw -> 12



For more on references see (perlref) and perlreftut NAME









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14.6 Stepping through code



Here’s a simple program which converts between Celsius and Fahrenheit, it too
has a problem:



        #!/usr/bin/perl -w
        use strict;

        my $arg = $ARGV[0] || '-c20';

        if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) {
                my ($deg, $num) = ($1, $2);
                my ($in, $out) = ($num, $num);
                if ($deg eq 'c') {
                        $deg = 'f';
                        $out = &c2f($num);
                } else {
                        $deg = 'c';
                        $out = &f2c($num);
                }
                $out = sprintf('%0.2f', $out);
                $out =~ s/^((\-|\+)*\d+)\.0+$/$1/;
                print "$out $deg\n";
        } else {
                print "Usage: $0 -[c|f] num\n";
        }
        exit;

        sub f2c {
                my $f = shift;
                my $c = 5 * $f - 32 / 9;
                return $c;
        }

        sub c2f {
                my $c = shift;
                my $f = 9 * $c / 5 + 32;
                return $f;
        }



For some reason, the Fahrenheit to Celsius conversion fails to return the
expected output.  This is what it does:



        > temp -c0.72
        33.30 f

        > temp -f33.3
        162.94 c



Not very consistent!  We’ll set a breakpoint in the code manually and run it
under the debugger to see what’s going on.  A breakpoint is a flag, to which
the debugger will run without interruption, when it reaches the breakpoint, it
will stop execution and offer a prompt for further interaction.  In normal
use, these debugger commands are completely ignored, and they are safe - if a
little messy, to leave in production code.



        my ($in, $out) = ($num, $num);
        $DB::single=2; # insert at line 9!
        if ($deg eq 'c') 
                ...

        > perl -d temp -f33.3
        Default die handler restored.

        Loading DB routines from perl5db.pl version 1.07
        Editor support available.

        Enter h or `h h' for help, or `man perldebug' for more help.

        main::(temp:4): my $arg = $ARGV[0] || '-c100';     



We’ll simply continue down to our pre-set breakpoint with a ’c’:



        DB<1> c
        main::(temp:10):                if ($deg eq 'c') {   



Followed by a view command to see where we are:



        DB<1> v
        7:              my ($deg, $num) = ($1, $2);
        8:              my ($in, $out) = ($num, $num);
        9:              $DB::single=2;
        10==>           if ($deg eq 'c') {
        11:                     $deg = 'f';
        12:                     $out = &c2f($num);
        13              } else {
        14:                     $deg = 'c';
        15:                     $out = &f2c($num);
        16              }                             



And a print to show what values we’re currently using:



        DB<1> p $deg, $num
        f33.3



We can put another break point on any line beginning with a colon, we’ll use
line 17 as that’s just as we come out of the subroutine, and we’d like to
pause there later on:



        DB<2> b 17



There’s no feedback from this, but you can see what breakpoints are set by
using the list ’L’ command:



        DB<3> L
        temp:
                17:            print "$out $deg\n";
                break if (1)     



Note that to delete a breakpoint you use ’B’.



Now we’ll continue down into our subroutine, this time rather than by line
number, we’ll use the subroutine name, followed by the now familiar ’v’:



        DB<3> c f2c
        main::f2c(temp:30):             my $f = shift;  

        DB<4> v
        24:     exit;
        25
        26      sub f2c {
        27==>           my $f = shift;
        28:             my $c = 5 * $f - 32 / 9; 
        29:             return $c;
        30      }
        31
        32      sub c2f {
        33:             my $c = shift;   



Note that if there was a subroutine call between us and line 29, and we wanted
to single-step through it, we could use the ’s’ command, and to step
over it we would use ’n’ which would execute the sub, but not descend into
it for inspection.  In this case though, we simply continue down to line 29:



        DB<4> c 29  
        main::f2c(temp:29):             return $c;



And have a look at the return value:



        DB<5> p $c
        162.944444444444



This is not the right answer at all, but the sum looks correct.  I wonder if
it’s anything to do with operator precedence?  We’ll try a couple of other
possibilities with our sum:



        DB<6> p (5 * $f - 32 / 9)
        162.944444444444

        DB<7> p 5 * $f - (32 / 9) 
        162.944444444444

        DB<8> p (5 * $f) - 32 / 9
        162.944444444444

        DB<9> p 5 * ($f - 32) / 9
        0.722222222222221



:-) that’s more like it!  Ok, now we can set our return variable and we’ll
return out of the sub with an ’r’:



        DB<10> $c = 5 * ($f - 32) / 9

        DB<11> r
        scalar context return from main::f2c: 0.722222222222221



Looks good, let’s just continue off the end of the script:



        DB<12> c
        0.72 c 
        Debugged program terminated.  Use q to quit or R to restart,
        use O inhibit_exit to avoid stopping after program termination,
        h q, h R or h O to get additional info.   



A quick fix to the offending line (insert the missing parentheses) in the
actual program and we’re finished.









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14.7 Placeholder for a, w, t, T



Actions, watch variables, stack traces etc.: on the TODO list.



        a 

        w 

        t 

        T









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14.8 REGULAR EXPRESSIONS



Ever wanted to know what a regex looked like?  You’ll need perl compiled with
the DEBUGGING flag for this one:



        > perl -Dr -e '/^pe(a)*rl$/i'
        Compiling REx `^pe(a)*rl$'
        size 17 first at 2
        rarest char
         at 0
           1: BOL(2)
           2: EXACTF <pe>(4)
           4: CURLYN[1] {0,32767}(14)
           6:   NOTHING(8)
           8:   EXACTF <a>(0)
          12:   WHILEM(0)
          13: NOTHING(14)
          14: EXACTF <rl>(16)
          16: EOL(17)
          17: END(0)
        floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'
anchored(BOL) minlen 4
        Omitting $` $& $' support.

        EXECUTING...

        Freeing REx: `^pe(a)*rl$'  



Did you really want to know? :-)
For more gory details on getting regular expressions to work, have a look at
(perlre), perlretut NAME, and to decode the mysterious labels (BOL and CURLYN,
etc. above), see perldebguts NAME.









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14.9 OUTPUT TIPS



To get all the output from your error log, and not miss any messages via
helpful operating system buffering, insert a line like this, at the start of
your script:



        $|=1;   



To watch the tail of a dynamically growing logfile, (from the command line):



        tail -f $error_log



Wrapping all die calls in a handler routine can be useful to see how, and from
where, they’re being called, perlvar NAME has more information:



        BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }



Various useful techniques for the redirection of STDOUT and STDERR filehandles
are explained in perlopentut NAME and (perlfaq8).









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14.10 CGI



Just a quick hint here for all those CGI programmers who can’t figure out how
on earth to get past that ’waiting for input’ prompt, when running their CGI
script from the command-line, try something like this:



        > perl -d my_cgi.pl -nodebug 



Of course (CGI) and (perlfaq9) will tell you more.









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14.11 GUIs



The command line interface is tightly integrated with an emacs extension
and there’s a vi interface too.  



You don’t have to do this all on the command line, though, there are a few GUI
options out there.  The nice thing about these is you can wave a mouse over a
variable and a dump of its data will appear in an appropriate window, or in a
popup balloon, no more tiresome typing of ’x $varname’ :-)



In particular have a hunt around for the following:



ptkdb perlTK based wrapper for the built-in debugger



ddd data display debugger



PerlDevKit and PerlBuilder are NT specific



NB. (more info on these and others would be appreciated).









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14.12 SUMMARY



We’ve seen how to encourage good coding practices with use strict and
-w.  We can run the perl debugger perl -d scriptname to inspect your
data from within the perl debugger with the p and x commands.  You can
walk through your code, set breakpoints with b and step through that code
with s or n, continue with c and return from a sub with r.  Fairly
intuitive stuff when you get down to it.  



There is of course lots more to find out about, this has just scratched the
surface.  The best way to learn more is to use perldoc to find out more about
the language, to read the on-line help ((perldebug) is probably the next
place to go), and of course, experiment.  









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14.13 SEE ALSO



(perldebug), 
perldebguts NAME, 
perldiag NAME,
perlrun NAME









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14.14 AUTHOR



Richard Foley <richard.foley@rfi.net> Copyright (c) 2000









Previous: perldebtut AUTHOR, Up: perldebtut   [Contents]





14.15 CONTRIBUTORS



Various people have made helpful suggestions and contributions, in particular:



Ronald J Kimball <rjk@linguist.dartmouth.edu>



Hugo van der Sanden <hv@crypt0.demon.co.uk>



Peter Scott <Peter@PSDT.com>









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15 NAME debug debugger





• NAME debug debugger NAME:  



• NAME debug debugger DESCRIPTION:  



• NAME debug debugger The Perl Debugger:  



• NAME debug debugger Debugging Regular Expressions:  



• NAME debug debugger Debugging Memory Usage:  



• NAME debug debugger SEE ALSO:  



• NAME debug debugger BUGS:  












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15.1 NAME





perldebug - Perl debugging









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15.2 DESCRIPTION



First of all, have you tried using the -w switch?



If you’re new to the Perl debugger, you may prefer to read
perldebtut NAME, which is a tutorial introduction to the debugger.









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15.3 The Perl Debugger




If you invoke Perl with the -d switch, your script runs under the
Perl source debugger.  This works like an interactive Perl
environment, prompting for debugger commands that let you examine
source code, set breakpoints, get stack backtraces, change the values of
variables, etc.  This is so convenient that you often fire up
the debugger all by itself just to test out Perl constructs
interactively to see what they do.  For example:




    $ perl -d -e 42



In Perl, the debugger is not a separate program the way it usually is in the
typical compiled environment.  Instead, the -d flag tells the compiler
to insert source information into the parse trees it’s about to hand off
to the interpreter.  That means your code must first compile correctly
for the debugger to work on it.  Then when the interpreter starts up, it
preloads a special Perl library file containing the debugger.



The program will halt right before the first run-time executable
statement (but see below regarding compile-time statements) and ask you
to enter a debugger command.  Contrary to popular expectations, whenever
the debugger halts and shows you a line of code, it always displays the
line it’s about to execute, rather than the one it has just executed.



Any command not recognized by the debugger is directly executed
(eval’d) as Perl code in the current package.  (The debugger
uses the DB package for keeping its own state information.)



Note that the said eval is bound by an implicit scope. As a
result any newly introduced lexical variable or any modified
capture buffer content is lost after the eval. The debugger is a
nice environment to learn Perl, but if you interactively experiment using
material which should be in the same scope, stuff it in one line.



For any text entered at the debugger prompt, leading and trailing whitespace
is first stripped before further processing.  If a debugger command
coincides with some function in your own program, merely precede the
function with something that doesn’t look like a debugger command, such
as a leading ; or perhaps a +, or by wrapping it with parentheses
or braces.





• NAME debug debugger Calling the Debugger:  



• NAME debug debugger Debugger Commands:  



• NAME debug debugger Configurable Options:  



• NAME debug debugger Debugger Input/Output:  



• NAME debug debugger Debugging Compile-Time Statements:  



• NAME debug debugger Debugger Customization:  



• NAME debug debugger Readline Support / History in the Debugger:  



• NAME debug debugger Editor Support for Debugging:  



• NAME debug debugger The Perl Profiler:  












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15.3.1 Calling the Debugger



There are several ways to call the debugger:





perl -d program_name




On the given program identified by program_name.





perl -d -e 0




Interactively supply an arbitrary expression using -e.





perl -d:Ptkdb program_name




Debug a given program via the Devel::Ptkdb GUI.





perl -dt threaded_program_name




Debug a given program using threads (experimental).














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15.3.2 Debugger Commands



The interactive debugger understands the following commands:





h





Prints out a summary help message





h [command]




Prints out a help message for the given debugger command.





h h




The special argument of h h produces the entire help page, which is quite long.



If the output of the h h command (or any command, for that matter) scrolls
past your screen, precede the command with a leading pipe symbol so
that it’s run through your pager, as in



    DB> |h h



You may change the pager which is used via o pager=... command.





p expr





Same as print {$DB::OUT} expr in the current package.  In particular,
because this is just Perl’s own print function, this means that nested
data structures and objects are not dumped, unlike with the x command.



The DB::OUT filehandle is opened to /dev/tty, regardless of
where STDOUT may be redirected to.





x [maxdepth] expr





Evaluates its expression in list context and dumps out the result in a
pretty-printed fashion.  Nested data structures are printed out
recursively, unlike the real print function in Perl.  When dumping
hashes, you’ll probably prefer ’x \%h’ rather than ’x %h’.
See (Dumpvalue) if you’d like to do this yourself.



The output format is governed by multiple options described under
Configurable Options.



If the maxdepth is included, it must be a numeral N; the value is
dumped only N levels deep, as if the dumpDepth option had been
temporarily set to N.





V [pkg [vars]]





Display all (or some) variables in package (defaulting to main)
using a data pretty-printer (hashes show their keys and values so
you see what’s what, control characters are made printable, etc.).
Make sure you don’t put the type specifier (like $) there, just
the symbol names, like this:



    V DB filename line



Use ~pattern and !pattern for positive and negative regexes.



This is similar to calling the x command on each applicable var.





X [vars]





Same as V currentpackage [vars].





y [level [vars]]





Display all (or some) lexical variables (mnemonic: mY variables)
in the current scope or level scopes higher.  You can limit the
variables that you see with vars which works exactly as it does
for the V and X commands.  Requires the PadWalker module
version 0.08 or higher; will warn if this isn’t installed.  Output
is pretty-printed in the same style as for V and the format is
controlled by the same options.





T







Produce a stack backtrace.  See below for details on its output.





s [expr]






Single step.  Executes until the beginning of another
statement, descending into subroutine calls.  If an expression is
supplied that includes function calls, it too will be single-stepped.





n [expr]





Next.  Executes over subroutine calls, until the beginning
of the next statement.  If an expression is supplied that includes
function calls, those functions will be executed with stops before
each statement.





r





Continue until the return from the current subroutine.
Dump the return value if the PrintRet option is set (default).





<CR>




Repeat last n or s command.





c [line|sub]





Continue, optionally inserting a one-time-only breakpoint
at the specified line or subroutine.





l





List next window of lines.





l min+incr




List incr+1 lines starting at min.





l min-max




List lines min through max.  l - is synonymous to -.





l line




List a single line.





l subname




List first window of lines from subroutine.  subname may
be a variable that contains a code reference.





-





List previous window of lines.





v [line]





View a few lines of code around the current line.





.





Return the internal debugger pointer to the line last
executed, and print out that line.





f filename





Switch to viewing a different file or eval statement.  If filename
is not a full pathname found in the values of %INC, it is considered
a regex.



evaled strings (when accessible) are considered to be filenames:
f (eval 7) and f eval 7\b access the body of the 7th evaled string
(in the order of execution).  The bodies of the currently executed eval
and of evaled strings that define subroutines are saved and thus
accessible.





/pattern/




Search forwards for pattern (a Perl regex); final / is optional.
The search is case-insensitive by default.





?pattern?




Search backwards for pattern; final ? is optional.
The search is case-insensitive by default.





L [abw]





List (default all) actions, breakpoints and watch expressions





S [[!]regex]





List subroutine names [not] matching the regex.





t [n]





Toggle trace mode (see also the AutoTrace option).
Optional argument is the maximum number of levels to trace below
the current one; anything deeper than that will be silent.





t [n] expr





Trace through execution of expr.
Optional first argument is the maximum number of levels to trace below
the current one; anything deeper than that will be silent.
See perldebguts Frame Listing Output Examples for examples.





b






Sets breakpoint on current line





b [line] [condition]






Set a breakpoint before the given line.  If a condition
is specified, it’s evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true.  Breakpoints may
only be set on lines that begin an executable statement.  Conditions
don’t use if:



    b 237 $x > 30
    b 237 ++$count237 < 11
    b 33 /pattern/i



If the line number is ., sets a breakpoint on the current line:



    b . $n > 100





b [file]:[line] [condition]






Set a breakpoint before the given line in a (possibly different) file.  If a
condition is specified, it’s evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true.  Breakpoints may only be set
on lines that begin an executable statement.  Conditions don’t use if:



    b lib/MyModule.pm:237 $x > 30
    b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11





b subname [condition]






Set a breakpoint before the first line of the named subroutine.  subname may
be a variable containing a code reference (in this case condition
is not supported).





b postpone subname [condition]






Set a breakpoint at first line of subroutine after it is compiled.





b load filename






Set a breakpoint before the first executed line of the filename,
which should be a full pathname found amongst the %INC values.





b compile subname






Sets a breakpoint before the first statement executed after the specified
subroutine is compiled.





B line






Delete a breakpoint from the specified line.





B *






Delete all installed breakpoints.





disable [file]:[line]







Disable the breakpoint so it won’t stop the execution of the program. 
Breakpoints are enabled by default and can be re-enabled using the enable
command.





disable [line]







Disable the breakpoint so it won’t stop the execution of the program. 
Breakpoints are enabled by default and can be re-enabled using the enable
command.



This is done for a breakpoint in the current file.





enable [file]:[line]







Enable the breakpoint so it will stop the execution of the program. 





enable [line]







Enable the breakpoint so it will stop the execution of the program. 



This is done for a breakpoint in the current file.





a [line] command





Set an action to be done before the line is executed.  If line is
omitted, set an action on the line about to be executed.
The sequence of steps taken by the debugger is



  1. check for a breakpoint at this line
  2. print the line if necessary (tracing)
  3. do any actions associated with that line
  4. prompt user if at a breakpoint or in single-step
  5. evaluate line



For example, this will print out $foo every time line
53 is passed:



    a 53 print "DB FOUND $foo\n"





A line





Delete an action from the specified line.





A *





Delete all installed actions.





w expr





Add a global watch-expression. Whenever a watched global changes the
debugger will stop and display the old and new values.





W expr





Delete watch-expression





W *





Delete all watch-expressions.





o





Display all options.





o booloption ...





Set each listed Boolean option to the value 1.





o anyoption? ...





Print out the value of one or more options.





o option=value ...





Set the value of one or more options.  If the value has internal
whitespace, it should be quoted.  For example, you could set o
pager="less -MQeicsNfr" to call less with those specific options.
You may use either single or double quotes, but if you do, you must
escape any embedded instances of same sort of quote you began with,
as well as any escaping any escapes that immediately precede that
quote but which are not meant to escape the quote itself.  In other
words, you follow single-quoting rules irrespective of the quote;
eg: o option='this isn\'t bad' or o option="She said, \"Isn't
it?\"".



For historical reasons, the =value is optional, but defaults to
1 only where it is safe to do so–that is, mostly for Boolean
options.  It is always better to assign a specific value using =.
The option can be abbreviated, but for clarity probably should
not be.  Several options can be set together.  See Configurable Options
for a list of these.





< ?





List out all pre-prompt Perl command actions.





< [ command ]





Set an action (Perl command) to happen before every debugger prompt.
A multi-line command may be entered by backslashing the newlines.





< *





Delete all pre-prompt Perl command actions.





<< command





Add an action (Perl command) to happen before every debugger prompt.
A multi-line command may be entered by backwhacking the newlines.





> ?





List out post-prompt Perl command actions.





> command





Set an action (Perl command) to happen after the prompt when you’ve
just given a command to return to executing the script.  A multi-line
command may be entered by backslashing the newlines (we bet you
couldn’t have guessed this by now).





> *





Delete all post-prompt Perl command actions.





>> command





Adds an action (Perl command) to happen after the prompt when you’ve
just given a command to return to executing the script.  A multi-line
command may be entered by backslashing the newlines.





{ ?





List out pre-prompt debugger commands.





{ [ command ]




Set an action (debugger command) to happen before every debugger prompt.
A multi-line command may be entered in the customary fashion.



Because this command is in some senses new, a warning is issued if
you appear to have accidentally entered a block instead.  If that’s
what you mean to do, write it as with ;{ ... } or even
do { ... }.





{ *





Delete all pre-prompt debugger commands.





{{ command





Add an action (debugger command) to happen before every debugger prompt.
A multi-line command may be entered, if you can guess how: see above.





! number





Redo a previous command (defaults to the previous command).





! -number





Redo number’th previous command.





! pattern





Redo last command that started with pattern.
See o recallCommand, too.





!! cmd





Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
o shellBang, also.  Note that the user’s current shell (well,
their $ENV{SHELL} variable) will be used, which can interfere
with proper interpretation of exit status or signal and coredump
information.





source file





Read and execute debugger commands from file.
file may itself contain source commands.





H -number





Display last n commands.  Only commands longer than one character are
listed.  If number is omitted, list them all.





q or ^D






Quit.  ("quit" doesn’t work for this, unless you’ve made an alias)
This is the only supported way to exit the debugger, though typing
exit twice might work.



Set the inhibit_exit option to 0 if you want to be able to step
off the end the script.  You may also need to set $finished to 0
if you want to step through global destruction.





R





Restart the debugger by exec()ing a new session.  We try to maintain
your history across this, but internal settings and command-line options
may be lost.



The following setting are currently preserved: history, breakpoints,
actions, debugger options, and the Perl command-line
options -w, -I, and -e.





|dbcmd





Run the debugger command, piping DB::OUT into your current pager.





||dbcmd





Same as |dbcmd but DB::OUT is temporarily selected as well.





= [alias value]





Define a command alias, like



    = quit q



or list current aliases.





command




Execute command as a Perl statement.  A trailing semicolon will be
supplied.  If the Perl statement would otherwise be confused for a
Perl debugger, use a leading semicolon, too.





m expr





List which methods may be called on the result of the evaluated
expression.  The expression may evaluated to a reference to a
blessed object, or to a package name.





M





Display all loaded modules and their versions.





man [manpage]





Despite its name, this calls your system’s default documentation
viewer on the given page, or on the viewer itself if manpage is
omitted.  If that viewer is man, the current Config information
is used to invoke man using the proper MANPATH or -M manpath option.  Failed lookups of the form XXX that match
known manpages of the form perlXXX will be retried.  This lets
you type man debug or man op from the debugger.



On systems traditionally bereft of a usable man command, the
debugger invokes perldoc.  Occasionally this determination is
incorrect due to recalcitrant vendors or rather more felicitously,
to enterprising users.  If you fall into either category, just
manually set the $DB::doccmd variable to whatever viewer to view
the Perl documentation on your system.  This may be set in an rc
file, or through direct assignment.  We’re still waiting for a
working example of something along the lines of:



    $DB::doccmd = 'netscape -remote http://something.here/';














Next: , Previous: NAME debug debugger Debugger Commands, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.3 Configurable Options



The debugger has numerous options settable using the o command,
either interactively or from the environment or an rc file.
(./.perldb or ~/.perldb under Unix.)





recallCommand, ShellBang






The characters used to recall command or spawn shell.  By
default, both are set to !, which is unfortunate.





pager





Program to use for output of pager-piped commands (those beginning
with a | character.)  By default, $ENV{PAGER} will be used.
Because the debugger uses your current terminal characteristics
for bold and underlining, if the chosen pager does not pass escape
sequences through unchanged, the output of some debugger commands
will not be readable when sent through the pager.





tkRunning





Run Tk while prompting (with ReadLine).





signalLevel, warnLevel, dieLevel







Level of verbosity.  By default, the debugger leaves your exceptions
and warnings alone, because altering them can break correctly running
programs.  It will attempt to print a message when uncaught INT, BUS, or
SEGV signals arrive.  (But see the mention of signals in BUGS below.)



To disable this default safe mode, set these values to something higher
than 0.  At a level of 1, you get backtraces upon receiving any kind
of warning (this is often annoying) or exception (this is
often valuable).  Unfortunately, the debugger cannot discern fatal
exceptions from non-fatal ones.  If dieLevel is even 1, then your
non-fatal exceptions are also traced and unceremoniously altered if they
came from eval'ed strings or from any kind of eval within modules
you’re attempting to load.  If dieLevel is 2, the debugger doesn’t
care where they came from:  It usurps your exception handler and prints
out a trace, then modifies all exceptions with its own embellishments.
This may perhaps be useful for some tracing purposes, but tends to hopelessly
destroy any program that takes its exception handling seriously.





AutoTrace





Trace mode (similar to t command, but can be put into
PERLDB_OPTS).





LineInfo





File or pipe to print line number info to.  If it is a pipe (say,
|visual_perl_db), then a short message is used.  This is the
mechanism used to interact with a slave editor or visual debugger,
such as the special vi or emacs hooks, or the ddd graphical
debugger.





inhibit_exit





If 0, allows stepping off the end of the script.





PrintRet





Print return value after r command if set (default).





ornaments





Affects screen appearance of the command line (see (Term-ReadLine)).
There is currently no way to disable these, which can render
some output illegible on some displays, or with some pagers.
This is considered a bug.





frame





Affects the printing of messages upon entry and exit from subroutines.  If
frame & 2 is false, messages are printed on entry only. (Printing
on exit might be useful if interspersed with other messages.)



If frame & 4, arguments to functions are printed, plus context
and caller info.  If frame & 8, overloaded stringify and
tied FETCH is enabled on the printed arguments.  If frame
& 16, the return value from the subroutine is printed.



The length at which the argument list is truncated is governed by the
next option:





maxTraceLen





Length to truncate the argument list when the frame option’s
bit 4 is set.





windowSize





Change the size of code list window (default is 10 lines).








The following options affect what happens with V, X, and x
commands:





arrayDepth, hashDepth






Print only first N elements (” for all).





dumpDepth





Limit recursion depth to N levels when dumping structures.
Negative values are interpreted as infinity.  Default: infinity.





compactDump, veryCompact






Change the style of array and hash output.  If compactDump, short array
may be printed on one line.





globPrint





Whether to print contents of globs.





DumpDBFiles





Dump arrays holding debugged files.





DumpPackages





Dump symbol tables of packages.





DumpReused





Dump contents of "reused" addresses.





quote, HighBit, undefPrint







Change the style of string dump.  The default value for quote
is auto; one can enable double-quotish or single-quotish format
by setting it to " or ', respectively.  By default, characters
with their high bit set are printed verbatim.





UsageOnly





Rudimentary per-package memory usage dump.  Calculates total
size of strings found in variables in the package.  This does not
include lexicals in a module’s file scope, or lost in closures.








After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
environment variable and parses this as the remainder of a "O ..."
line as one might enter at the debugger prompt.  You may place the
initialization options TTY, noTTY, ReadLine, and NonStop
there.



If your rc file contains:



  parse_options("NonStop=1 LineInfo=db.out AutoTrace");



then your script will run without human intervention, putting trace
information into the file db.out.  (If you interrupt it, you’d
better reset LineInfo to /dev/tty if you expect to see anything.)





TTY





The TTY to use for debugging I/O.





noTTY





If set, the debugger goes into NonStop mode and will not connect to a TTY.  If
interrupted (or if control goes to the debugger via explicit setting of
$DB::signal or $DB::single from the Perl script), it connects to a TTY
specified in the TTY option at startup, or to a tty found at
runtime using the Term::Rendezvous module of your choice.



This module should implement a method named new that returns an object
with two methods: IN and OUT.  These should return filehandles to use
for debugging input and output correspondingly.  The new method should
inspect an argument containing the value of $ENV{PERLDB_NOTTY} at
startup, or "$ENV{HOME}/.perldbtty$$" otherwise.  This file is not
inspected for proper ownership, so security hazards are theoretically
possible.





ReadLine





If false, readline support in the debugger is disabled in order
to debug applications that themselves use ReadLine.





NonStop





If set, the debugger goes into non-interactive mode until interrupted, or
programmatically by setting $DB::signal or $DB::single.








Here’s an example of using the $ENV{PERLDB_OPTS} variable:



    $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram



That will run the script myprogram without human intervention,
printing out the call tree with entry and exit points.  Note that
NonStop=1 frame=2 is equivalent to N f=2, and that originally,
options could be uniquely abbreviated by the first letter (modulo
the Dump* options).  It is nevertheless recommended that you
always spell them out in full for legibility and future compatibility.



Other examples include



    $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram



which runs script non-interactively, printing info on each entry
into a subroutine and each executed line into the file named listing.
(If you interrupt it, you would better reset LineInfo to something
"interactive"!)



Other examples include (using standard shell syntax to show environment
variable settings):



  $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
      perl -d myprogram )



which may be useful for debugging a program that uses Term::ReadLine
itself.  Do not forget to detach your shell from the TTY in the window that
corresponds to /dev/ttyXX, say, by issuing a command like



  $ sleep 1000000



See perldebguts Debugger Internals for details.









Next: , Previous: NAME debug debugger Configurable Options, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.4 Debugger Input/Output





Prompt




The debugger prompt is something like



    DB<8>



or even



    DB<<17>>



where that number is the command number, and which you’d use to
access with the built-in csh-like history mechanism.  For example,
!17 would repeat command number 17.  The depth of the angle
brackets indicates the nesting depth of the debugger.  You could
get more than one set of brackets, for example, if you’d already
at a breakpoint and then printed the result of a function call that
itself has a breakpoint, or you step into an expression via s/n/t
expression command.





Multiline commands




If you want to enter a multi-line command, such as a subroutine
definition with several statements or a format, escape the newline
that would normally end the debugger command with a backslash.
Here’s an example:



      DB<1> for (1..4) {         \
      cont:     print "ok\n";   \
      cont: }
      ok
      ok
      ok
      ok



Note that this business of escaping a newline is specific to interactive
commands typed into the debugger.





Stack backtrace






Here’s an example of what a stack backtrace via T command might
look like:



    $ = main::infested called from file 'Ambulation.pm' line 10
    @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea' line 7
    $ = main::pests('bactrian', 4) called from file 'camel_flea' line 4



The left-hand character up there indicates the context in which the
function was called, with $ and @ meaning scalar or list
contexts respectively, and . meaning void context (which is
actually a sort of scalar context).  The display above says
that you were in the function main::infested when you ran the
stack dump, and that it was called in scalar context from line
10 of the file Ambulation.pm, but without any arguments at all,
meaning it was called as &infested.  The next stack frame shows
that the function Ambulation::legs was called in list context
from the camel_flea file with four arguments.  The last stack
frame shows that main::pests was called in scalar context,
also from camel_flea, but from line 4.



If you execute the T command from inside an active use
statement, the backtrace will contain both a require frame and
an eval frame.





Line Listing Format




This shows the sorts of output the l command can produce:



    DB<<13>> l
  101:                @i{@i} = ();
  102:b               @isa{@i,$pack} = ()
  103                     if(exists $i{$prevpack} || exists $isa{$pack});
  104             }
  105
  106             next
  107==>              if(exists $isa{$pack});
  108
  109:a           if ($extra-- > 0) {
  110:                %isa = ($pack,1);



Breakable lines are marked with :.  Lines with breakpoints are
marked by b and those with actions by a.  The line that’s
about to be executed is marked by ==>.



Please be aware that code in debugger listings may not look the same
as your original source code.  Line directives and external source
filters can alter the code before Perl sees it, causing code to move
from its original positions or take on entirely different forms.





Frame listing




When the frame option is set, the debugger would print entered (and
optionally exited) subroutines in different styles.  See perldebguts NAME
for incredibly long examples of these.














Next: , Previous: NAME debug debugger Debugger Input/Output, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.5 Debugging Compile-Time Statements



If you have compile-time executable statements (such as code within
BEGIN, UNITCHECK and CHECK blocks or use statements), these will
not be stopped by debugger, although requires and INIT blocks
will, and compile-time statements can be traced with the AutoTrace
option set in PERLDB_OPTS).  From your own Perl code, however, you
can transfer control back to the debugger using the following
statement, which is harmless if the debugger is not running:



    $DB::single = 1;



If you set $DB::single to 2, it’s equivalent to having
just typed the n command, whereas a value of 1 means the s
command.  The $DB::trace  variable should be set to 1 to simulate
having typed the t command.



Another way to debug compile-time code is to start the debugger, set a
breakpoint on the load of some module:



    DB<7> b load f:/perllib/lib/Carp.pm
  Will stop on load of 'f:/perllib/lib/Carp.pm'.



and then restart the debugger using the R command (if possible).  One can use b
compile subname for the same purpose.









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15.3.6 Debugger Customization



The debugger probably contains enough configuration hooks that you
won’t ever have to modify it yourself.  You may change the behaviour
of the debugger from within the debugger using its o command, from
the command line via the PERLDB_OPTS environment variable, and
from customization files.



You can do some customization by setting up a .perldb file, which
contains initialization code.  For instance, you could make aliases
like these (the last one is one people expect to be there):



    $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
    $DB::alias{'stop'} = 's/^stop (at|in)/b/';
    $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
    $DB::alias{'quit'} = 's/^quit(\s*)/exit/';



You can change options from .perldb by using calls like this one;



    parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");



The code is executed in the package DB.  Note that .perldb is
processed before processing PERLDB_OPTS.  If .perldb defines the
subroutine afterinit, that function is called after debugger
initialization ends.  .perldb may be contained in the current
directory, or in the home directory.  Because this file is sourced
in by Perl and may contain arbitrary commands, for security reasons,
it must be owned by the superuser or the current user, and writable
by no one but its owner.



You can mock TTY input to debugger by adding arbitrary commands to
@DB::typeahead. For example, your .perldb file might contain:



    sub afterinit { push @DB::typeahead, "b 4", "b 6"; }



Which would attempt to set breakpoints on lines 4 and 6 immediately
after debugger initialization. Note that @DB::typeahead is not a supported
interface and is subject to change in future releases.



If you want to modify the debugger, copy perl5db.pl from the
Perl library to another name and hack it to your heart’s content.
You’ll then want to set your PERL5DB environment variable to say
something like this:



    BEGIN { require "myperl5db.pl" }



As a last resort, you could also use PERL5DB to customize the debugger
by directly setting internal variables or calling debugger functions.



Note that any variables and functions that are not documented in
this document (or in perldebguts NAME) are considered for internal
use only, and as such are subject to change without notice.









Next: , Previous: NAME debug debugger Debugger Customization, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.7 Readline Support / History in the Debugger



As shipped, the only command-line history supplied is a simplistic one
that checks for leading exclamation points.  However, if you install
the Term::ReadKey and Term::ReadLine modules from CPAN (such as
Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
have full editing capabilities much like those GNU readline(3) provides.
Look for these in the modules/by-module/Term directory on CPAN.
These do not support normal vi command-line editing, however.



A rudimentary command-line completion is also available, including
lexical variables in the current scope if the PadWalker module
is installed.



Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
"^[[D"", "^H", ... when using the arrow keys and/or the backspace key.









Next: , Previous: NAME debug debugger Readline Support / History in the Debugger, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.8 Editor Support for Debugging



If you have the FSF’s version of emacs installed on your system,
it can interact with the Perl debugger to provide an integrated
software development environment reminiscent of its interactions
with C debuggers.



Recent versions of Emacs come with a
start file for making emacs act like a
syntax-directed editor that understands (some of) Perl’s syntax.
See (perlfaq3).



A similar setup by Tom Christiansen for interacting with any
vendor-shipped vi and the X11 window system is also available.
This works similarly to the integrated multiwindow support that
emacs provides, where the debugger drives the editor.  At the
time of this writing, however, that tool’s eventual location in the
Perl distribution was uncertain.



Users of vi should also look into vim and gvim, the mousey
and windy version, for coloring of Perl keywords.



Note that only perl can truly parse Perl, so all such CASE tools
fall somewhat short of the mark, especially if you don’t program
your Perl as a C programmer might.









Previous: NAME debug debugger Editor Support for Debugging, Up: NAME debug debugger The Perl Debugger   [Contents]





15.3.9 The Perl Profiler






If you wish to supply an alternative debugger for Perl to run,
invoke your script with a colon and a package argument given to the
-d flag.  Perl’s alternative debuggers include a Perl profiler,
(Devel-NYTProf), which is available separately as a CPAN
distribution.  To profile your Perl program in the file mycode.pl,
just type:



    $ perl -d:NYTProf mycode.pl



When the script terminates the profiler will create a database of the
profile information that you can turn into reports using the profiler’s
tools. See <perlperf> for details.









Next: , Previous: NAME debug debugger The Perl Debugger, Up: NAME debug debugger   [Contents]





15.4 Debugging Regular Expressions






use re 'debug' enables you to see the gory details of how the Perl
regular expression engine works. In order to understand this typically
voluminous output, one must not only have some idea about how regular
expression matching works in general, but also know how Perl’s regular
expressions are internally compiled into an automaton. These matters
are explored in some detail in
perldebguts Debugging Regular Expressions.









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15.5 Debugging Memory Usage




Perl contains internal support for reporting its own memory usage,
but this is a fairly advanced concept that requires some understanding
of how memory allocation works.
See perldebguts Debugging Perl Memory Usage for the details.









Next: , Previous: NAME debug debugger Debugging Memory Usage, Up: NAME debug debugger   [Contents]





15.6 SEE ALSO



You did try the -w switch, didn’t you?



perldebtut NAME,
perldebguts NAME,
(re),
(DB),
(Devel-NYTProf),
(Dumpvalue),
and
perlrun NAME.



When debugging a script that uses #! and is thus normally found in
$PATH, the -S option causes perl to search $PATH for it, so you don’t
have to type the path or which $scriptname.



  $ perl -Sd foo.pl









Previous: NAME debug debugger SEE ALSO, Up: NAME debug debugger   [Contents]





15.7 BUGS



You cannot get stack frame information or in any fashion debug functions
that were not compiled by Perl, such as those from C or C++ extensions.



If you alter your @_ arguments in a subroutine (such as with shift
or pop), the stack backtrace will not show the original values.



The debugger does not currently work in conjunction with the -W
command-line switch, because it itself is not free of warnings.



If you’re in a slow syscall (like waiting, accepting, or reading
from your keyboard or a socket) and haven’t set up your own $SIG{INT}
handler, then you won’t be able to CTRL-C your way back to the debugger,
because the debugger’s own $SIG{INT} handler doesn’t understand that
it needs to raise an exception to longjmp(3) out of slow syscalls.









Next: , Previous: NAME debug debugger, Up: Top   [Contents]





16 perldiag





• perldiag NAME:  



• perldiag DESCRIPTION:  



• perldiag SEE ALSO:  












Next: , Up: perldiag   [Contents]





16.1 NAME



perldiag - various Perl diagnostics









Next: , Previous: perldiag NAME, Up: perldiag   [Contents]





16.2 DESCRIPTION



These messages are classified as follows (listed in increasing order of
desperation):



    (W) A warning (optional).
    (D) A deprecation (enabled by default).
    (S) A severe warning (enabled by default).
    (F) A fatal error (trappable).
    (P) An internal error you should never see (trappable).
    (X) A very fatal error (nontrappable).
    (A) An alien error message (not generated by Perl).



The majority of messages from the first three classifications above
(W, D & S) can be controlled using the warnings pragma.



If a message can be controlled by the warnings pragma, its warning
category is included with the classification letter in the description
below.



Optional warnings are enabled by using the warnings pragma or the -w
and -W switches.  Warnings may be captured by setting $SIG{__WARN__}
to a reference to a routine that will be called on each warning instead
of printing it.  See perlvar NAME.



Severe warnings are always enabled, unless they are explicitly disabled
with the warnings pragma or the -X switch.



Trappable errors may be trapped using the eval operator.  See
(perlfunc)eval.  In almost all cases, warnings may be selectively
disabled or promoted to fatal errors using the warnings pragma.
See (warnings).



The messages are in alphabetical order, without regard to upper or
lower-case.  Some of these messages are generic.  Spots that vary are
denoted with a %s or other printf-style escape.  These escapes are
ignored by the alphabetical order, as are all characters other than
letters.  To look up your message, just ignore anything that is not a
letter.





accept() on closed socket %s




(W closed) You tried to do an accept on a closed socket.  Did you forget
to check the return value of your socket() call?  See
(perlfunc)accept.





Allocation too large: %x




(X) You can’t allocate more than 64K on an MS-DOS machine.





’%c’ allowed only after types %s




(F) The modifiers ’!’, ’<’ and ’>’ are allowed in pack() or unpack() only
after certain types.  See (perlfunc)pack.





Ambiguous call resolved as CORE::%s(), qualify as such or use &




(W ambiguous) A subroutine you have declared has the same name as a Perl
keyword, and you have used the name without qualification for calling
one or the other.  Perl decided to call the builtin because the
subroutine is not imported.



To force interpretation as a subroutine call, either put an ampersand
before the subroutine name, or qualify the name with its package.
Alternatively, you can import the subroutine (or pretend that it’s
imported with the use subs pragma).



To silently interpret it as the Perl operator, use the CORE:: prefix
on the operator (e.g. CORE::log($x)) or declare the subroutine
to be an object method (see (perlsub)Subroutine Attributes or
(attributes)).





Ambiguous range in transliteration operator




(F) You wrote something like tr/a-z-0// which doesn’t mean anything at
all.  To include a - character in a transliteration, put it either
first or last.  (In the past, tr/a-z-0// was synonymous with
tr/a-y//, which was probably not what you would have expected.)





Ambiguous use of %s resolved as %s




(W ambiguous)(S) You said something that may not be interpreted the way
you thought.  Normally it’s pretty easy to disambiguate it by supplying
a missing quote, operator, parenthesis pair or declaration.





Ambiguous use of %c resolved as operator %c




(W ambiguous) %, &, and * are both infix operators (modulus,
bitwise and, and multiplication) and initial special characters
(denoting hashes, subroutines and typeglobs), and you said something
like *foo * foo that might be interpreted as either of them.  We
assumed you meant the infix operator, but please try to make it more
clear – in the example given, you might write *foo * foo() if you
really meant to multiply a glob by the result of calling a function.





Ambiguous use of %c{%s} resolved to %c%s




(W ambiguous) You wrote something like @{foo}, which might be
asking for the variable @foo, or it might be calling a function
named foo, and dereferencing it as an array reference.  If you wanted
the variable, you can just write @foo.  If you wanted to call the
function, write @{foo()} ... or you could just not have a variable
and a function with the same name, and save yourself a lot of trouble.





Ambiguous use of %c{%s[...]} resolved to %c%s[...]






Ambiguous use of %c{%s{...}} resolved to %c%s{...}




(W ambiguous) You wrote something like ${foo[2]} (where foo represents
the name of a Perl keyword), which might be looking for element number
2 of the array named @foo, in which case please write $foo[2], or you
might have meant to pass an anonymous arrayref to the function named
foo, and then do a scalar deref on the value it returns.  If you meant
that, write ${foo([2])}.



In regular expressions, the ${foo[2]} syntax is sometimes necessary
to disambiguate between array subscripts and character classes.
/$length[2345]/, for instance, will be interpreted as $length followed
by the character class [2345].  If an array subscript is what you
want, you can avoid the warning by changing /${length[2345]}/ to the
unsightly /${\$length[2345]}/, by renaming your array to something
that does not coincide with a built-in keyword, or by simply turning
off warnings with no warnings 'ambiguous';.





Ambiguous use of -%s resolved as -&%s()




(W ambiguous) You wrote something like -foo, which might be the
string "-foo", or a call to the function foo, negated.  If you meant
the string, just write "-foo".  If you meant the function call,
write -foo().





Ambiguous use of ’s//le...’ resolved as ’s// le...’; Rewrite as ’s//el’ if you meant ’use locale rules and evaluate rhs as an expression’.  In Perl 5.18, it will be resolved the other way




(W deprecated, ambiguous) You wrote a pattern match with substitution
immediately followed by "le".  In Perl 5.16 and earlier, this is
resolved as meaning to take the result of the substitution, and see if
it is stringwise less-than-or-equal-to what follows in the expression.
Having the "le" immediately following a pattern is deprecated behavior,
so in Perl 5.18, this expression will be resolved as meaning to do the
pattern match using the rules of the current locale, and evaluate the
rhs as an expression when doing the substitution.  In 5.14, and 5.16 if
you want the latter interpretation, you can simply write "el" instead.
But note that the /l modifier should not be used explicitly anyway;
you should use use locale instead.  See perllocale NAME.





’|’ and ’<’ may not both be specified on command line




(F) An error peculiar to VMS.  Perl does its own command line
redirection, and found that STDIN was a pipe, and that you also tried to
redirect STDIN using ’<’.  Only one STDIN stream to a customer, please.





’|’ and ’>’ may not both be specified on command line




(F) An error peculiar to VMS.  Perl does its own command line
redirection, and thinks you tried to redirect stdout both to a file and
into a pipe to another command.  You need to choose one or the other,
though nothing’s stopping you from piping into a program or Perl script
which ’splits’ output into two streams, such as



    open(OUT,">$ARGV[0]") or die "Can't write to $ARGV[0]: $!";
    while (<STDIN>) {
        print;
        print OUT;
    }
    close OUT;





Applying %s to %s will act on scalar(%s)




(W misc) The pattern match (//), substitution (s///), and
transliteration (tr///) operators work on scalar values.  If you apply
one of them to an array or a hash, it will convert the array or hash to
a scalar value (the length of an array, or the population info of a
hash) and then work on that scalar value.  This is probably not what
you meant to do.  See (perlfunc)grep and (perlfunc)map for
alternatives.





Arg too short for msgsnd




(F) msgsnd() requires a string at least as long as sizeof(long).





%s argument is not a HASH or ARRAY element or a subroutine




(F) The argument to exists() must be a hash or array element or a
subroutine with an ampersand, such as:



    $foo{$bar}
    $ref->{"susie"}[12]
    &do_something





%s argument is not a HASH or ARRAY element or slice




(F) The argument to delete() must be either a hash or array element,
such as:



    $foo{$bar}
    $ref->{"susie"}[12]



or a hash or array slice, such as:



    @foo[$bar, $baz, $xyzzy]
    @{$ref->[12]}{"susie", "queue"}





%s argument is not a subroutine name




(F) The argument to exists() for exists &sub must be a subroutine
name, and not a subroutine call.  exists &sub() will generate this
error.





Argument "%s" isn’t numeric%s




(W numeric) The indicated string was fed as an argument to an operator
that expected a numeric value instead.  If you’re fortunate the message
will identify which operator was so unfortunate.





Argument list not closed for PerlIO layer "%s"




(W layer) When pushing a layer with arguments onto the Perl I/O
system you forgot the ) that closes the argument list.  (Layers
take care of transforming data between external and internal
representations.)  Perl stopped parsing the layer list at this
point and did not attempt to push this layer.  If your program
didn’t explicitly request the failing operation, it may be the
result of the value of the environment variable PERLIO.





Array @%s missing the @ in argument %d of %s()




(D deprecated) Really old Perl let you omit the @ on array names in some
spots.  This is now heavily deprecated.





assertion botched: %s




(X) The malloc package that comes with Perl had an internal failure.





Assertion failed: file "%s"




(X) A general assertion failed.  The file in question must be examined.





Assigning non-zero to $[ is no longer possible




(F) When the "array_base" feature is disabled (e.g., under use v5.16;)
the special variable $[, which is deprecated, is now a fixed zero value.





Assignment to both a list and a scalar




(F) If you assign to a conditional operator, the 2nd and 3rd arguments
must either both be scalars or both be lists.  Otherwise Perl won’t
know which context to supply to the right side.





A thread exited while %d threads were running




(W threads)(S) When using threaded Perl, a thread (not necessarily
the main thread) exited while there were still other threads running.
Usually it’s a good idea first to collect the return values of the
created threads by joining them, and only then to exit from the main
thread.  See (threads).





Attempt to access disallowed key ’%s’ in a restricted hash




(F) The failing code has attempted to get or set a key which is not in
the current set of allowed keys of a restricted hash.





Attempt to bless into a reference




(F) The CLASSNAME argument to the bless() operator is expected to be
the name of the package to bless the resulting object into.  You’ve
supplied instead a reference to something: perhaps you wrote



    bless $self, $proto;



when you intended



    bless $self, ref($proto) || $proto;



If you actually want to bless into the stringified version
of the reference supplied, you need to stringify it yourself, for
example by:



    bless $self, "$proto";





Attempt to clear deleted array




(S debugging) An array was assigned to when it was being freed.
Freed values are not supposed to be visible to Perl code.  This
can also happen if XS code calls av_clear from a custom magic
callback on the array.





Attempt to delete disallowed key ’%s’ from a restricted hash




(F) The failing code attempted to delete from a restricted hash a key
which is not in its key set.





Attempt to delete readonly key ’%s’ from a restricted hash




(F) The failing code attempted to delete a key whose value has been
declared readonly from a restricted hash.





Attempt to free non-arena SV: 0x%x




(S internal) All SV objects are supposed to be allocated from arenas
that will be garbage collected on exit.  An SV was discovered to be
outside any of those arenas.





Attempt to free nonexistent shared string ’%s’%s




(S internal) Perl maintains a reference-counted internal table of
strings to optimize the storage and access of hash keys and other
strings.  This indicates someone tried to decrement the reference count
of a string that can no longer be found in the table.





Attempt to free temp prematurely: SV 0x%x




(S debugging) Mortalized values are supposed to be freed by the
free_tmps() routine.  This indicates that something else is freeing the
SV before the free_tmps() routine gets a chance, which means that the
free_tmps() routine will be freeing an unreferenced scalar when it does
try to free it.





Attempt to free unreferenced glob pointers




(S internal) The reference counts got screwed up on symbol aliases.





Attempt to free unreferenced scalar: SV 0x%x




(W internal) Perl went to decrement the reference count of a scalar to
see if it would go to 0, and discovered that it had already gone to 0
earlier, and should have been freed, and in fact, probably was freed.
This could indicate that SvREFCNT_dec() was called too many times, or
that SvREFCNT_inc() was called too few times, or that the SV was
mortalized when it shouldn’t have been, or that memory has been
corrupted.





Attempt to join self




(F) You tried to join a thread from within itself, which is an
impossible task.  You may be joining the wrong thread, or you may need
to move the join() to some other thread.





Attempt to pack pointer to temporary value




(W pack) You tried to pass a temporary value (like the result of a
function, or a computed expression) to the "p" pack() template.  This
means the result contains a pointer to a location that could become
invalid anytime, even before the end of the current statement.  Use
literals or global values as arguments to the "p" pack() template to
avoid this warning.





Attempt to reload %s aborted.




(F) You tried to load a file with use or require that failed to
compile once already.  Perl will not try to compile this file again
unless you delete its entry from %INC.  See (perlfunc)require and
perlvar %INC.





Attempt to set length of freed array




(W) You tried to set the length of an array which has been freed.  You
can do this by storing a reference to the scalar representing the last index
of an array and later assigning through that reference.  For example



    $r = do {my @a; \$#a};
    $$r = 503





Attempt to use reference as lvalue in substr




(W substr) You supplied a reference as the first argument to substr()
used as an lvalue, which is pretty strange.  Perhaps you forgot to
dereference it first.  See (perlfunc)substr.





Attribute "locked" is deprecated




(D deprecated) You have used the attributes pragma to modify the
"locked" attribute on a code reference.  The :locked attribute is
obsolete, has had no effect since 5005 threads were removed, and
will be removed in a future release of Perl 5.





Attribute "unique" is deprecated




(D deprecated) You have used the attributes pragma to modify
the "unique" attribute on an array, hash or scalar reference.
The :unique attribute has had no effect since Perl 5.8.8, and
will be removed in a future release of Perl 5.





av_reify called on tied array




(S debugging) This indicates that something went wrong and Perl got very
confused about @_ or @DB::args being tied.





Bad arg length for %s, is %u, should be %d




(F) You passed a buffer of the wrong size to one of msgctl(), semctl()
or shmctl().  In C parlance, the correct sizes are, respectively,
sizeof(struct msqid_ds *), sizeof(struct semid_ds *), and
sizeof(struct shmid_ds *).





Bad evalled substitution pattern




(F) You’ve used the /e switch to evaluate the replacement for a
substitution, but perl found a syntax error in the code to evaluate,
most likely an unexpected right brace ’}’.





Bad filehandle: %s




(F) A symbol was passed to something wanting a filehandle, but the
symbol has no filehandle associated with it.  Perhaps you didn’t do an
open(), or did it in another package.





Bad free() ignored




(S malloc) An internal routine called free() on something that had never
been malloc()ed in the first place.  Mandatory, but can be disabled by
setting environment variable PERL_BADFREE to 0.



This message can be seen quite often with DB_File on systems with "hard"
dynamic linking, like AIX and OS/2.  It is a bug of Berkeley DB
which is left unnoticed if DB uses forgiving system malloc().





Bad hash




(P) One of the internal hash routines was passed a null HV pointer.





Badly placed ()’s




(A) You’ve accidentally run your script through csh instead
of Perl.  Check the #! line, or manually feed your script into
Perl yourself.





Bad name after %s




(F) You started to name a symbol by using a package prefix, and then
didn’t finish the symbol.  In particular, you can’t interpolate outside
of quotes, so



    $var = 'myvar';
    $sym = mypack::$var;



is not the same as



    $var = 'myvar';
    $sym = "mypack::$var";





Bad plugin affecting keyword ’%s’




(F) An extension using the keyword plugin mechanism violated the
plugin API.





Bad realloc() ignored




(S malloc) An internal routine called realloc() on something that
had never been malloc()ed in the first place.  Mandatory, but can
be disabled by setting the environment variable PERL_BADFREE to 1.





Bad symbol for array




(P) An internal request asked to add an array entry to something that
wasn’t a symbol table entry.





Bad symbol for dirhandle




(P) An internal request asked to add a dirhandle entry to something
that wasn’t a symbol table entry.





Bad symbol for filehandle




(P) An internal request asked to add a filehandle entry to something
that wasn’t a symbol table entry.





Bad symbol for hash




(P) An internal request asked to add a hash entry to something that
wasn’t a symbol table entry.





Bareword found in conditional




(W bareword) The compiler found a bareword where it expected a
conditional, which often indicates that an || or && was parsed as part
of the last argument of the previous construct, for example:



    open FOO || die;



It may also indicate a misspelled constant that has been interpreted as
a bareword:



    use constant TYPO => 1;
    if (TYOP) { print "foo" }



The strict pragma is useful in avoiding such errors.





Bareword "%s" not allowed while "strict subs" in use




(F) With "strict subs" in use, a bareword is only allowed as a
subroutine identifier, in curly brackets or to the left of the "=>"
symbol.  Perhaps you need to predeclare a subroutine?





Bareword "%s" refers to nonexistent package




(W bareword) You used a qualified bareword of the form Foo::, but the
compiler saw no other uses of that namespace before that point.  Perhaps
you need to predeclare a package?





BEGIN failed–compilation aborted




(F) An untrapped exception was raised while executing a BEGIN
subroutine.  Compilation stops immediately and the interpreter is
exited.





BEGIN not safe after errors–compilation aborted




(F) Perl found a BEGIN {} subroutine (or a use directive, which
implies a BEGIN {}) after one or more compilation errors had already
occurred.  Since the intended environment for the BEGIN {} could not
be guaranteed (due to the errors), and since subsequent code likely
depends on its correct operation, Perl just gave up.





\1 better written as $1




(W syntax) Outside of patterns, backreferences live on as variables.
The use of backslashes is grandfathered on the right-hand side of a
substitution, but stylistically it’s better to use the variable form
because other Perl programmers will expect it, and it works better if
there are more than 9 backreferences.





Binary number > 0b11111111111111111111111111111111 non-portable




(W portable) The binary number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
perlport NAME for more on portability concerns.





bind() on closed socket %s




(W closed) You tried to do a bind on a closed socket.  Did you forget to
check the return value of your socket() call?  See (perlfunc)bind.





binmode() on closed filehandle %s




(W unopened) You tried binmode() on a filehandle that was never opened.
Check your control flow and number of arguments.





"\b{" is deprecated; use "\b\{" instead






"\B{" is deprecated; use "\B\{" instead




(W deprecated, regexp) Use of an unescaped "{" immediately following a
\b or \B is now deprecated so as to reserve its use for Perl
itself in a future release.





Bit vector size > 32 non-portable




(W portable) Using bit vector sizes larger than 32 is non-portable.





Bizarre copy of %s




(P) Perl detected an attempt to copy an internal value that is not
copiable.





Buffer overflow in prime_env_iter: %s




(W internal) A warning peculiar to VMS.  While Perl was preparing to
iterate over %ENV, it encountered a logical name or symbol definition
which was too long, so it was truncated to the string shown.





Bizarre SvTYPE [%d]




(P) When starting a new thread or return values from a thread, Perl
encountered an invalid data type.





Callback called exit




(F) A subroutine invoked from an external package via call_sv()
exited by calling exit.





%s() called too early to check prototype




(W prototype) You’ve called a function that has a prototype before the
parser saw a definition or declaration for it, and Perl could not check
that the call conforms to the prototype.  You need to either add an
early prototype declaration for the subroutine in question, or move the
subroutine definition ahead of the call to get proper prototype
checking.  Alternatively, if you are certain that you’re calling the
function correctly, you may put an ampersand before the name to avoid
the warning.  See (perlsub).





Cannot compress integer in pack




(F) An argument to pack("w",...) was too large to compress.  The BER
compressed integer format can only be used with positive integers, and you
attempted to compress Infinity or a very large number (> 1e308).
See (perlfunc)pack.





Cannot compress negative numbers in pack




(F) An argument to pack("w",...) was negative.  The BER compressed integer
format can only be used with positive integers.  See (perlfunc)pack.





Cannot convert a reference to %s to typeglob




(F) You manipulated Perl’s symbol table directly, stored a reference
in it, then tried to access that symbol via conventional Perl syntax.
The access triggers Perl to autovivify that typeglob, but it there is
no legal conversion from that type of reference to a typeglob.





Cannot copy to %s




(P) Perl detected an attempt to copy a value to an internal type that cannot
be directly assigned to.





Cannot find encoding "%s"




(S io) You tried to apply an encoding that did not exist to a filehandle,
either with open() or binmode().





Cannot set tied @DB::args




(F) caller tried to set @DB::args, but found it tied.  Tying @DB::args
is not supported.  (Before this error was added, it used to crash.)





Cannot tie unreifiable array




(P) You somehow managed to call tie on an array that does not
keep a reference count on its arguments and cannot be made to
do so.  Such arrays are not even supposed to be accessible to
Perl code, but are only used internally.





Can only compress unsigned integers in pack




(F) An argument to pack("w",...) was not an integer.  The BER compressed
integer format can only be used with positive integers, and you attempted
to compress something else.  See (perlfunc)pack.





Can’t bless non-reference value




(F) Only hard references may be blessed.  This is how Perl "enforces"
encapsulation of objects.  See (perlobj).





Can’t "break" in a loop topicalizer




(F) You called break, but you’re in a foreach block rather than
a given block.  You probably meant to use next or last.





Can’t "break" outside a given block




(F) You called break, but you’re not inside a given block.





Can’t call method "%s" on an undefined value




(F) You used the syntax of a method call, but the slot filled by the
object reference or package name contains an undefined value.  Something
like this will reproduce the error:



    $BADREF = undef;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);





Can’t call method "%s" on unblessed reference




(F) A method call must know in what package it’s supposed to run.  It
ordinarily finds this out from the object reference you supply, but you
didn’t supply an object reference in this case.  A reference isn’t an
object reference until it has been blessed.  See (perlobj).





Can’t call method "%s" without a package or object reference




(F) You used the syntax of a method call, but the slot filled by the
object reference or package name contains an expression that returns a
defined value which is neither an object reference nor a package name.
Something like this will reproduce the error:



    $BADREF = 42;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);





Can’t chdir to %s




(F) You called perl -x/foo/bar, but /foo/bar is not a directory
that you can chdir to, possibly because it doesn’t exist.





Can’t check filesystem of script "%s" for nosuid




(P) For some reason you can’t check the filesystem of the script for
nosuid.





Can’t coerce %s to %s in %s




(F) Certain types of SVs, in particular real symbol table entries
(typeglobs), can’t be forced to stop being what they are.  So you can’t
say things like:



    *foo += 1;



You CAN say



    $foo = *foo;
    $foo += 1;



but then $foo no longer contains a glob.





Can’t "continue" outside a when block




(F) You called continue, but you’re not inside a when
or default block.





Can’t create pipe mailbox




(P) An error peculiar to VMS.  The process is suffering from exhausted
quotas or other plumbing problems.





Can’t declare %s in "%s"




(F) Only scalar, array, and hash variables may be declared as "my", "our" or
"state" variables.  They must have ordinary identifiers as names.





Can’t "default" outside a topicalizer




(F) You have used a default block that is neither inside a
foreach loop nor a given block.  (Note that this error is
issued on exit from the default block, so you won’t get the
error if you use an explicit continue.)





Can’t do inplace edit: %s is not a regular file




(S inplace) You tried to use the -i switch on a special file, such as
a file in /dev, or a FIFO.  The file was ignored.





Can’t do inplace edit on %s: %s




(S inplace) The creation of the new file failed for the indicated
reason.





Can’t do inplace edit without backup




(F) You’re on a system such as MS-DOS that gets confused if you try
reading from a deleted (but still opened) file.  You have to say
-i.bak, or some such.





Can’t do inplace edit: %s would not be unique




(S inplace) Your filesystem does not support filenames longer than 14
characters and Perl was unable to create a unique filename during
inplace editing with the -i switch.  The file was ignored.





Can’t do {n,m} with n > m in regex; marked by <– HERE in m/%s/




(F) Minima must be less than or equal to maxima.  If you really
want your regexp to match something 0 times, just put {0}.  The
<– HERE shows in the regular expression about where the problem
was discovered.  See (perlre).





Can’t do waitpid with flags




(F) This machine doesn’t have either waitpid() or wait4(), so only
waitpid() without flags is emulated.





Can’t emulate -%s on #! line




(F) The #! line specifies a switch that doesn’t make sense at this
point.  For example, it’d be kind of silly to put a -x on the #!
line.





Can’t %s %s-endian %ss on this platform




(F) Your platform’s byte-order is neither big-endian nor little-endian,
or it has a very strange pointer size.  Packing and unpacking big- or
little-endian floating point values and pointers may not be possible.
See (perlfunc)pack.





Can’t exec "%s": %s




(W exec) A system(), exec(), or piped open call could not execute the
named program for the indicated reason.  Typical reasons include: the
permissions were wrong on the file, the file wasn’t found in
$ENV{PATH}, the executable in question was compiled for another
architecture, or the #! line in a script points to an interpreter that
can’t be run for similar reasons.  (Or maybe your system doesn’t support
#! at all.)





Can’t exec %s




(F) Perl was trying to execute the indicated program for you because
that’s what the #! line said.  If that’s not what you wanted, you may
need to mention "perl" on the #! line somewhere.





Can’t execute %s




(F) You used the -S switch, but the copies of the script to execute
found in the PATH did not have correct permissions.





Can’t find an opnumber for "%s"




(F) A string of a form CORE::word was given to prototype(), but there
is no builtin with the name word.





Can’t find %s character property "%s"




(F) You used \p{} or \P{} but the character property by that name
could not be found.  Maybe you misspelled the name of the property?
See (perluniprops)Properties accessible through \p{} and \P{}
for a complete list of available properties.





Can’t find label %s




(F) You said to goto a label that isn’t mentioned anywhere that it’s
possible for us to go to.  See (perlfunc)goto.





Can’t find %s on PATH




(F) You used the -S switch, but the script to execute could not be
found in the PATH.





Can’t find %s on PATH, ’.’ not in PATH




(F) You used the -S switch, but the script to execute could not be
found in the PATH, or at least not with the correct permissions.  The
script exists in the current directory, but PATH prohibits running it.





Can’t find string terminator %s anywhere before EOF




(F) Perl strings can stretch over multiple lines.  This message means
that the closing delimiter was omitted.  Because bracketed quotes count
nesting levels, the following is missing its final parenthesis:



    print q(The character '(' starts a side comment.);



If you’re getting this error from a here-document, you may have
included unseen whitespace before or after your closing tag or there
may not be a linebreak after it.  A good programmer’s editor will have
a way to help you find these characters (or lack of characters).  See
(perlop) for the full details on here-documents.





Can’t find Unicode property definition "%s"




(F) You may have tried to use \p which means a Unicode
property (for example \p{Lu} matches all uppercase
letters).  If you did mean to use a Unicode property, see
(perluniprops)Properties accessible through \p{} and \P{}
for a complete list of available properties.  If you didn’t
mean to use a Unicode property, escape the \p, either by
\\p (just the \p) or by \Q\p (the rest of the string, or
until \E).





Can’t fork: %s




(F) A fatal error occurred while trying to fork while opening a
pipeline.





Can’t fork, trying again in 5 seconds




(W pipe) A fork in a piped open failed with EAGAIN and will be retried
after five seconds.





Can’t get filespec - stale stat buffer?




(S) A warning peculiar to VMS.  This arises because of the difference
between access checks under VMS and under the Unix model Perl assumes.
Under VMS, access checks are done by filename, rather than by bits in
the stat buffer, so that ACLs and other protections can be taken into
account.  Unfortunately, Perl assumes that the stat buffer contains all
the necessary information, and passes it, instead of the filespec, to
the access-checking routine.  It will try to retrieve the filespec using
the device name and FID present in the stat buffer, but this works only
if you haven’t made a subsequent call to the CRTL stat() routine,
because the device name is overwritten with each call.  If this warning
appears, the name lookup failed, and the access-checking routine gave up
and returned FALSE, just to be conservative.  (Note: The access-checking
routine knows about the Perl stat operator and file tests, so you
shouldn’t ever see this warning in response to a Perl command; it arises
only if some internal code takes stat buffers lightly.)





Can’t get pipe mailbox device name




(P) An error peculiar to VMS.  After creating a mailbox to act as a
pipe, Perl can’t retrieve its name for later use.





Can’t get SYSGEN parameter value for MAXBUF




(P) An error peculiar to VMS.  Perl asked $GETSYI how big you want your
mailbox buffers to be, and didn’t get an answer.





Can’t "goto" into the middle of a foreach loop




(F) A "goto" statement was executed to jump into the middle of a foreach
loop.  You can’t get there from here.  See (perlfunc)goto.





Can’t "goto" out of a pseudo block




(F) A "goto" statement was executed to jump out of what might look like
a block, except that it isn’t a proper block.  This usually occurs if
you tried to jump out of a sort() block or subroutine, which is a no-no.
See (perlfunc)goto.





Can’t goto subroutine from a sort sub (or similar callback)




(F) The "goto subroutine" call can’t be used to jump out of the
comparison sub for a sort(), or from a similar callback (such
as the reduce() function in List::Util).





Can’t goto subroutine from an eval-%s




(F) The "goto subroutine" call can’t be used to jump out of an eval
"string" or block.





Can’t goto subroutine outside a subroutine




(F) The deeply magical "goto subroutine" call can only replace one
subroutine call for another.  It can’t manufacture one out of whole
cloth.  In general you should be calling it out of only an AUTOLOAD
routine anyway.  See (perlfunc)goto.





Can’t ignore signal CHLD, forcing to default




(W signal) Perl has detected that it is being run with the SIGCHLD
signal (sometimes known as SIGCLD) disabled.  Since disabling this
signal will interfere with proper determination of exit status of child
processes, Perl has reset the signal to its default value.  This
situation typically indicates that the parent program under which Perl
may be running (e.g. cron) is being very careless.





Can’t kill a non-numeric process ID




(F) Process identifiers must be (signed) integers.  It is a fatal error to
attempt to kill() an undefined, empty-string or otherwise non-numeric
process identifier.





Can’t "last" outside a loop block




(F) A "last" statement was executed to break out of the current block,
except that there’s this itty bitty problem called there isn’t a current
block.  Note that an "if" or "else" block doesn’t count as a "loopish"
block, as doesn’t a block given to sort(), map() or grep().  You can
usually double the curlies to get the same effect though, because the
inner curlies will be considered a block that loops once.  See
(perlfunc)last.





Can’t linearize anonymous symbol table




(F) Perl tried to calculate the method resolution order (MRO) of a
package, but failed because the package stash has no name.





Can’t load ’%s’ for module %s




(F) The module you tried to load failed to load a dynamic extension.
This may either mean that you upgraded your version of perl to one
that is incompatible with your old dynamic extensions (which is known
to happen between major versions of perl), or (more likely) that your
dynamic extension was built against an older version of the library
that is installed on your system.  You may need to rebuild your old
dynamic extensions.





Can’t localize lexical variable %s




(F) You used local on a variable name that was previously declared as a
lexical variable using "my" or "state".  This is not allowed.  If you
want to localize a package variable of the same name, qualify it with
the package name.





Can’t localize through a reference




(F) You said something like local $$ref, which Perl can’t currently
handle, because when it goes to restore the old value of whatever $ref
pointed to after the scope of the local() is finished, it can’t be sure
that $ref will still be a reference.





Can’t locate %s




(F) You said to do (or require, or use) a file that couldn’t be found.
Perl looks for the file in all the locations mentioned in @INC, unless
the file name included the full path to the file.  Perhaps you need
to set the PERL5LIB or PERL5OPT environment variable to say where the
extra library is, or maybe the script needs to add the library name
to @INC.  Or maybe you just misspelled the name of the file.  See
(perlfunc)require and (lib).





Can’t locate auto/%s.al in @INC




(F) A function (or method) was called in a package which allows
autoload, but there is no function to autoload.  Most probable causes
are a misprint in a function/method name or a failure to AutoSplit
the file, say, by doing make install.





Can’t locate loadable object for module %s in @INC




(F) The module you loaded is trying to load an external library, like
for example, foo.so or bar.dll, but the (DynaLoader) module was
unable to locate this library.  See (DynaLoader).





Can’t locate object method "%s" via package "%s"




(F) You called a method correctly, and it correctly indicated a package
functioning as a class, but that package doesn’t define that particular
method, nor does any of its base classes.  See (perlobj).





Can’t locate package %s for @%s::ISA




(W syntax) The @ISA array contained the name of another package that
doesn’t seem to exist.





Can’t locate PerlIO%s




(F) You tried to use in open() a PerlIO layer that does not exist,
e.g. open(FH, ">:nosuchlayer", "somefile").





Can’t make list assignment to %ENV on this system




(F) List assignment to %ENV is not supported on some systems, notably
VMS.





Can’t modify %s in %s




(F) You aren’t allowed to assign to the item indicated, or otherwise try
to change it, such as with an auto-increment.





Can’t modify nonexistent substring




(P) The internal routine that does assignment to a substr() was handed
a NULL.





Can’t modify non-lvalue subroutine call




(F) Subroutines meant to be used in lvalue context should be declared as
such.  See (perlsub)Lvalue subroutines.





Can’t msgrcv to read-only var




(F) The target of a msgrcv must be modifiable to be used as a receive
buffer.





Can’t "next" outside a loop block




(F) A "next" statement was executed to reiterate the current block, but
there isn’t a current block.  Note that an "if" or "else" block doesn’t
count as a "loopish" block, as doesn’t a block given to sort(), map() or
grep().  You can usually double the curlies to get the same effect
though, because the inner curlies will be considered a block that loops
once.  See (perlfunc)next.





Can’t open %s




(F) You tried to run a perl built with MAD support with
the PERL_XMLDUMP environment variable set, but the file
named by that variable could not be opened.





Can’t open %s: %s




(S inplace) The implicit opening of a file through use of the <>
filehandle, either implicitly under the -n or -p command-line
switches, or explicitly, failed for the indicated reason.  Usually
this is because you don’t have read permission for a file which
you named on the command line.



(F) You tried to call perl with the -e switch, but /dev/null (or
your operating system’s equivalent) could not be opened.





Can’t open a reference




(W io) You tried to open a scalar reference for reading or writing,
using the 3-arg open() syntax:



    open FH, '>', $ref;



but your version of perl is compiled without perlio, and this form of
open is not supported.





Can’t open bidirectional pipe




(W pipe) You tried to say open(CMD, "|cmd|"), which is not supported.
You can try any of several modules in the Perl library to do this, such
as IPC::Open2.  Alternately, direct the pipe’s output to a file using
">", and then read it in under a different file handle.





Can’t open error file %s as stderr




(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn’t open the file specified after ’2>’ or ’2>>’ on
the command line for writing.





Can’t open input file %s as stdin




(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn’t open the file specified after ’<’ on the
command line for reading.





Can’t open output file %s as stdout




(F) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn’t open the file specified after ’>’ or ’>>’ on
the command line for writing.





Can’t open output pipe (name: %s)




(P) An error peculiar to VMS.  Perl does its own command line
redirection, and couldn’t open the pipe into which to send data destined
for stdout.





Can’t open perl script "%s": %s




(F) The script you specified can’t be opened for the indicated reason.



If you’re debugging a script that uses #!, and normally relies on the
shell’s $PATH search, the -S option causes perl to do that search, so
you don’t have to type the path or `which $scriptname`.





Can’t read CRTL environ




(S) A warning peculiar to VMS.  Perl tried to read an element of %ENV
from the CRTL’s internal environment array and discovered the array was
missing.  You need to figure out where your CRTL misplaced its environ
or define PERL_ENV_TABLES (see perlvms NAME) so that environ is not
searched.





Can’t "redo" outside a loop block




(F) A "redo" statement was executed to restart the current block, but
there isn’t a current block.  Note that an "if" or "else" block doesn’t
count as a "loopish" block, as doesn’t a block given to sort(), map()
or grep().  You can usually double the curlies to get the same effect
though, because the inner curlies will be considered a block that
loops once.  See (perlfunc)redo.





Can’t remove %s: %s, skipping file




(S inplace) You requested an inplace edit without creating a backup
file.  Perl was unable to remove the original file to replace it with
the modified file.  The file was left unmodified.





Can’t rename %s to %s: %s, skipping file




(S inplace) The rename done by the -i switch failed for some reason,
probably because you don’t have write permission to the directory.





Can’t reopen input pipe (name: %s) in binary mode




(P) An error peculiar to VMS.  Perl thought stdin was a pipe, and tried
to reopen it to accept binary data.  Alas, it failed.





Can’t reset %ENV on this system




(F) You called reset('E') or similar, which tried to reset
all variables in the current package beginning with "E".  In
the main package, that includes %ENV.  Resetting %ENV is not
supported on some systems, notably VMS.





Can’t resolve method "%s" overloading "%s" in package "%s"




(F)(P) Error resolving overloading specified by a method name (as
opposed to a subroutine reference): no such method callable via the
package.  If the method name is ???, this is an internal error.





Can’t return %s from lvalue subroutine




(F) Perl detected an attempt to return illegal lvalues (such as
temporary or readonly values) from a subroutine used as an lvalue.  This
is not allowed.





Can’t return outside a subroutine




(F) The return statement was executed in mainline code, that is, where
there was no subroutine call to return out of.  See (perlsub).





Can’t return %s to lvalue scalar context




(F) You tried to return a complete array or hash from an lvalue
subroutine, but you called the subroutine in a way that made Perl
think you meant to return only one value.  You probably meant to
write parentheses around the call to the subroutine, which tell
Perl that the call should be in list context.





Can’t stat script "%s"




(P) For some reason you can’t fstat() the script even though you have it
open already.  Bizarre.





Can’t take log of %g




(F) For ordinary real numbers, you can’t take the logarithm of a
negative number or zero.  There’s a Math::Complex package that comes
standard with Perl, though, if you really want to do that for the
negative numbers.





Can’t take sqrt of %g




(F) For ordinary real numbers, you can’t take the square root of a
negative number.  There’s a Math::Complex package that comes standard
with Perl, though, if you really want to do that.





Can’t undef active subroutine




(F) You can’t undefine a routine that’s currently running.  You can,
however, redefine it while it’s running, and you can even undef the
redefined subroutine while the old routine is running.  Go figure.





Can’t upgrade %s (%d) to %d




(P) The internal sv_upgrade routine adds "members" to an SV, making it
into a more specialized kind of SV.  The top several SV types are so
specialized, however, that they cannot be interconverted.  This message
indicates that such a conversion was attempted.





Can’t use ’%c’ after -mname




(F) You tried to call perl with the -m switch, but you put something
other than "=" after the module name.





Can’t use anonymous symbol table for method lookup




(F) The internal routine that does method lookup was handed a symbol
table that doesn’t have a name.  Symbol tables can become anonymous
for example by undefining stashes: undef %Some::Package::.





Can’t use an undefined value as %s reference




(F) A value used as either a hard reference or a symbolic reference must
be a defined value.  This helps to delurk some insidious errors.





Can’t use bareword ("%s") as %s ref while "strict refs" in use




(F) Only hard references are allowed by "strict refs".  Symbolic
references are disallowed.  See (perlref).





Can’t use %! because Errno.pm is not available




(F) The first time the %! hash is used, perl automatically loads the
Errno.pm module.  The Errno module is expected to tie the %! hash to
provide symbolic names for $! errno values.





Can’t use both ’<’ and ’>’ after type ’%c’ in %s




(F) A type cannot be forced to have both big-endian and little-endian
byte-order at the same time, so this combination of modifiers is not
allowed.  See (perlfunc)pack.





Can’t use %s for loop variable




(F) Only a simple scalar variable may be used as a loop variable on a
foreach.





Can’t use global %s in "%s"




(F) You tried to declare a magical variable as a lexical variable.  This
is not allowed, because the magic can be tied to only one location
(namely the global variable) and it would be incredibly confusing to
have variables in your program that looked like magical variables but
weren’t.





Can’t use ’%c’ in a group with different byte-order in %s




(F) You attempted to force a different byte-order on a type
that is already inside a group with a byte-order modifier.
For example you cannot force little-endianness on a type that
is inside a big-endian group.





Can’t use "my %s" in sort comparison




(F) The global variables $a and $b are reserved for sort comparisons.
You mentioned $a or $b in the same line as the <=> or cmp operator,
and the variable had earlier been declared as a lexical variable.
Either qualify the sort variable with the package name, or rename the
lexical variable.





Can’t use %s ref as %s ref




(F) You’ve mixed up your reference types.  You have to dereference a
reference of the type needed.  You can use the ref() function to
test the type of the reference, if need be.





Can’t use string ("%s") as %s ref while "strict refs" in use




(F) Only hard references are allowed by "strict refs".  Symbolic
references are disallowed.  See (perlref).





Can’t use subscript on %s




(F) The compiler tried to interpret a bracketed expression as a
subscript.  But to the left of the brackets was an expression that
didn’t look like a hash or array reference, or anything else subscriptable.





Can’t use \%c to mean $%c in expression




(W syntax) In an ordinary expression, backslash is a unary operator that
creates a reference to its argument.  The use of backslash to indicate a
backreference to a matched substring is valid only as part of a regular
expression pattern.  Trying to do this in ordinary Perl code produces a
value that prints out looking like SCALAR(0xdecaf).  Use the $1 form
instead.





Can’t weaken a nonreference




(F) You attempted to weaken something that was not a reference.  Only
references can be weakened.





Can’t "when" outside a topicalizer




(F) You have used a when() block that is neither inside a foreach
loop nor a given block.  (Note that this error is issued on exit
from the when block, so you won’t get the error if the match fails,
or if you use an explicit continue.)





Can’t x= to read-only value




(F) You tried to repeat a constant value (often the undefined value)
with an assignment operator, which implies modifying the value itself.
Perhaps you need to copy the value to a temporary, and repeat that.





Character following "\c" must be ASCII




(F)(W deprecated, syntax) In \cX, X must be an ASCII character.
It is planned to make this fatal in all instances in Perl 5.18.  In the
cases where it isn’t fatal, the character this evaluates to is
derived by exclusive or’ing the code point of this character with 0x40.



Note that non-alphabetic ASCII characters are discouraged here as well.





Character in ’C’ format wrapped in pack




(W pack) You said



    pack("C", $x)



where $x is either less than 0 or more than 255; the "C" format is
only for encoding native operating system characters (ASCII, EBCDIC,
and so on) and not for Unicode characters, so Perl behaved as if you meant



    pack("C", $x & 255)



If you actually want to pack Unicode codepoints, use the "U" format
instead.





Character in ’W’ format wrapped in pack




(W pack) You said



    pack("U0W", $x)



where $x is either less than 0 or more than 255.  However, U0-mode
expects all values to fall in the interval [0, 255], so Perl behaved
as if you meant:



    pack("U0W", $x & 255)





Character in ’c’ format wrapped in pack




(W pack) You said



    pack("c", $x)



where $x is either less than -128 or more than 127; the "c" format
is only for encoding native operating system characters (ASCII, EBCDIC,
and so on) and not for Unicode characters, so Perl behaved as if you meant



    pack("c", $x & 255);



If you actually want to pack Unicode codepoints, use the "U" format
instead.





Character in ’%c’ format wrapped in unpack




(W unpack) You tried something like



   unpack("H", "\x{2a1}")



where the format expects to process a byte (a character with a value
below 256), but a higher value was provided instead.  Perl uses the
value modulus 256 instead, as if you had provided:



   unpack("H", "\x{a1}")





Character(s) in ’%c’ format wrapped in pack




(W pack) You tried something like



   pack("u", "\x{1f3}b")



where the format expects to process a sequence of bytes (character with a
value below 256), but some of the characters had a higher value.  Perl
uses the character values modulus 256 instead, as if you had provided:



   pack("u", "\x{f3}b")





Character(s) in ’%c’ format wrapped in unpack




(W unpack) You tried something like



   unpack("s", "\x{1f3}b")



where the format expects to process a sequence of bytes (character with a
value below 256), but some of the characters had a higher value.  Perl
uses the character values modulus 256 instead, as if you had provided:



   unpack("s", "\x{f3}b")





"\c{" is deprecated and is more clearly written as ";"




(D deprecated, syntax) The \cX construct is intended to be a way
to specify non-printable characters.  You used it with a "{" which
evaluates to ";", which is printable.  It is planned to remove the
ability to specify a semi-colon this way in Perl 5.18.  Just use a
semi-colon or a backslash-semi-colon without the "\c".





"\c%c" is more clearly written simply as "%s"




(W syntax) The \cX construct is intended to be a way to specify
non-printable characters.  You used it for a printable one, which is better
written as simply itself, perhaps preceded by a backslash for non-word
characters.





Cloning substitution context is unimplemented




(F) Creating a new thread inside the s/// operator is not supported.





close() on unopened filehandle %s




(W unopened) You tried to close a filehandle that was never opened.





closedir() attempted on invalid dirhandle %s




(W io) The dirhandle you tried to close is either closed or not really
a dirhandle.  Check your control flow.





Closure prototype called




(F) If a closure has attributes, the subroutine passed to an attribute
handler is the prototype that is cloned when a new closure is created.
This subroutine cannot be called.





Code missing after ’/’




(F) You had a (sub-)template that ends with a ’/’.  There must be
another template code following the slash.  See (perlfunc)pack.





Code point 0x%X is not Unicode, may not be portable






Code point 0x%X is not Unicode, all \p{} matches fail; all \P{} matches succeed




(W utf8, non_unicode) You had a code point above the Unicode maximum
of U+10FFFF.



Perl allows strings to contain a superset of Unicode code points, up
to the limit of what is storable in an unsigned integer on your system,
but these may not be accepted by other languages/systems.  At one time,
it was legal in some standards to have code points up to 0x7FFF_FFFF,
but not higher.  Code points above 0xFFFF_FFFF require larger than a
32 bit word.



None of the Unicode or Perl-defined properties will match a non-Unicode
code point.  For example,



    chr(0x7FF_FFFF) =~ /\p{Any}/



will not match, because the code point is not in Unicode.  But



    chr(0x7FF_FFFF) =~ /\P{Any}/



will match.



This may be counterintuitive at times, as both these fail:



 chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails.
 chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Also fails!



and both these succeed:



 chr(0x110000) =~ \P{ASCII_Hex_Digit=True}      # Succeeds.
 chr(0x110000) =~ \P{ASCII_Hex_Digit=False}     # Also succeeds!





%s: Command not found




(A) You’ve accidentally run your script through csh or another shell
shell instead of Perl.  Check the #! line, or manually feed your script
into Perl yourself.  The #! line at the top of your file could look like



  #!/usr/bin/perl -w





Compilation failed in require




(F) Perl could not compile a file specified in a require statement.
Perl uses this generic message when none of the errors that it
encountered were severe enough to halt compilation immediately.





Complex regular subexpression recursion limit (%d) exceeded




(W regexp) The regular expression engine uses recursion in complex
situations where back-tracking is required.  Recursion depth is limited
to 32766, or perhaps less in architectures where the stack cannot grow
arbitrarily.  ("Simple" and "medium" situations are handled without
recursion and are not subject to a limit.)  Try shortening the string
under examination; looping in Perl code (e.g. with while) rather than
in the regular expression engine; or rewriting the regular expression so
that it is simpler or backtracks less.  (See (perlfaq2) for information
on Mastering Regular Expressions.)





cond_broadcast() called on unlocked variable




(W threads) Within a thread-enabled program, you tried to
call cond_broadcast() on a variable which wasn’t locked.
The cond_broadcast() function is used to wake up another thread
that is waiting in a cond_wait().  To ensure that the signal isn’t
sent before the other thread has a chance to enter the wait, it
is usual for the signaling thread first to wait for a lock on
variable.  This lock attempt will only succeed after the other
thread has entered cond_wait() and thus relinquished the lock.





cond_signal() called on unlocked variable




(W threads) Within a thread-enabled program, you tried to
call cond_signal() on a variable which wasn’t locked.  The
cond_signal() function is used to wake up another thread that
is waiting in a cond_wait().  To ensure that the signal isn’t
sent before the other thread has a chance to enter the wait, it
is usual for the signaling thread first to wait for a lock on
variable.  This lock attempt will only succeed after the other
thread has entered cond_wait() and thus relinquished the lock.





connect() on closed socket %s




(W closed) You tried to do a connect on a closed socket.  Did you forget
to check the return value of your socket() call?  See
(perlfunc)connect.





Constant(%s)%s: %s




(F) The parser found inconsistencies either while attempting to define
an overloaded constant, or when trying to find the character name
specified in the \N{...} escape.  Perhaps you forgot to load the
corresponding (overload) pragma?.





Constant(%s)%s: %s in regex; marked by <– HERE in m/%s/




(F) The parser found inconsistencies while attempting to find
the character name specified in the \N{...} escape.





Constant is not %s reference




(F) A constant value (perhaps declared using the use constant pragma)
is being dereferenced, but it amounts to the wrong type of reference.
The message indicates the type of reference that was expected.  This
usually indicates a syntax error in dereferencing the constant value.
See (perlsub)Constant Functions and (constant).





Constant subroutine %s redefined




(W redefine)(S) You redefined a subroutine which had previously
been eligible for inlining.  See (perlsub)Constant Functions
for commentary and workarounds.





Constant subroutine %s undefined




(W misc) You undefined a subroutine which had previously been eligible
for inlining.  See (perlsub)Constant Functions for commentary and
workarounds.





Copy method did not return a reference




(F) The method which overloads "=" is buggy.  See
(overload)Copy Constructor.





&CORE::%s cannot be called directly




(F) You tried to call a subroutine in the CORE:: namespace
with &foo syntax or through a reference.  Some subroutines
in this package cannot yet be called that way, but must be
called as barewords.  Something like this will work:



    BEGIN { *shove = \&CORE::push; }
    shove @array, 1,2,3; # pushes on to @array





CORE::%s is not a keyword




(F) The CORE:: namespace is reserved for Perl keywords.





corrupted regexp pointers




(P) The regular expression engine got confused by what the regular
expression compiler gave it.





corrupted regexp program




(P) The regular expression engine got passed a regexp program without a
valid magic number.





Corrupt malloc ptr 0x%x at 0x%x




(P) The malloc package that comes with Perl had an internal failure.





Count after length/code in unpack




(F) You had an unpack template indicating a counted-length string, but
you have also specified an explicit size for the string.  See
(perlfunc)pack.





Deep recursion on anonymous subroutine






Deep recursion on subroutine "%s"




(W recursion) This subroutine has called itself (directly or indirectly)
100 times more than it has returned.  This probably indicates an
infinite recursion, unless you’re writing strange benchmark programs, in
which case it indicates something else.



This threshold can be changed from 100, by recompiling the perl binary,
setting the C pre-processor macro PERL_SUB_DEPTH_WARN to the desired value.





defined(@array) is deprecated




(D deprecated) defined() is not usually useful on arrays because it
checks for an undefined scalar value.  If you want to see if the
array is empty, just use if (@array) { # not empty } for example.





defined(%hash) is deprecated




(D deprecated) defined() is not usually right on hashes and has been
discouraged since 5.004.



Although defined %hash is false on a plain not-yet-used hash, it
becomes true in several non-obvious circumstances, including iterators,
weak references, stash names, even remaining true after undef %hash.
These things make defined %hash fairly useless in practice.



If a check for non-empty is what you wanted then just put it in boolean
context (see perldata Scalar values):



    if (%hash) {
       # not empty
    }



If you had defined %Foo::Bar::QUUX to check whether such a package
variable exists then that’s never really been reliable, and isn’t
a good way to enquire about the features of a package, or whether
it’s loaded, etc.





(?(DEFINE)....) does not allow branches in regex; marked by <– HERE in m/%s/




(F) You used something like (?(DEFINE)...|..) which is illegal.  The
most likely cause of this error is that you left out a parenthesis inside
of the .... part.



The <– HERE shows in the regular expression about where the problem was
discovered.





%s defines neither package nor VERSION–version check failed




(F) You said something like "use Module 42" but in the Module file
there are neither package declarations nor a $VERSION.





Delimiter for here document is too long




(F) In a here document construct like <<FOO, the label FOO is too
long for Perl to handle.  You have to be seriously twisted to write code
that triggers this error.





Deprecated character in \N{...}; marked by <– HERE  in \N{%s<– HERE %s




(D deprecated) Just about anything is legal for the ... in \N{...}.
But starting in 5.12, non-reasonable ones that don’t look like names
are deprecated.  A reasonable name begins with an alphabetic character
and continues with any combination of alphanumerics, dashes, spaces,
parentheses or colons.





Deprecated use of my() in false conditional




(D deprecated) You used a declaration similar to my $x if 0.  There
has been a long-standing bug in Perl that causes a lexical variable
not to be cleared at scope exit when its declaration includes a false
conditional.  Some people have exploited this bug to achieve a kind of
static variable.  Since we intend to fix this bug, we don’t want people
relying on this behavior.  You can achieve a similar static effect by
declaring the variable in a separate block outside the function, eg



    sub f { my $x if 0; return $x++ }



becomes



    { my $x; sub f { return $x++ } }



Beginning with perl 5.9.4, you can also use state variables to have
lexicals that are initialized only once (see (feature)):



    sub f { state $x; return $x++ }





DESTROY created new reference to dead object ’%s’




(F) A DESTROY() method created a new reference to the object which is
just being DESTROYed.  Perl is confused, and prefers to abort rather
than to create a dangling reference.





Did not produce a valid header




See Server error.





%s did not return a true value




(F) A required (or used) file must return a true value to indicate that
it compiled correctly and ran its initialization code correctly.  It’s
traditional to end such a file with a "1;", though any true value would
do.  See (perlfunc)require.





(Did you mean &%s instead?)




(W misc) You probably referred to an imported subroutine &FOO as $FOO or
some such.





(Did you mean "local" instead of "our"?)




(W misc) Remember that "our" does not localize the declared global
variable.  You have declared it again in the same lexical scope, which
seems superfluous.





(Did you mean $ or @ instead of %?)




(W) You probably said %hash{$key} when you meant $hash{$key} or
@hash{@keys}.  On the other hand, maybe you just meant %hash and got
carried away.





Died




(F) You passed die() an empty string (the equivalent of die "") or
you called it with no args and $@ was empty.





Document contains no data




See Server error.





%s does not define %s::VERSION–version check failed




(F) You said something like "use Module 42" but the Module did not
define a $VERSION.





’/’ does not take a repeat count




(F) You cannot put a repeat count of any kind right after the ’/’ code.
See (perlfunc)pack.





Don’t know how to handle magic of type ’%s’




(P) The internal handling of magical variables has been cursed.





do_study: out of memory




(P) This should have been caught by safemalloc() instead.





(Do you need to predeclare %s?)




(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  It often means a subroutine or module
name is being referenced that hasn’t been declared yet.  This may be
because of ordering problems in your file, or because of a missing
"sub", "package", "require", or "use" statement.  If you’re referencing
something that isn’t defined yet, you don’t actually have to define the
subroutine or package before the current location.  You can use an empty
"sub foo;" or "package FOO;" to enter a "forward" declaration.





dump() better written as CORE::dump()




(W misc) You used the obsolescent dump() built-in function, without fully
qualifying it as CORE::dump().  Maybe it’s a typo.  See (perlfunc)dump.





dump is not supported




(F) Your machine doesn’t support dump/undump.





Duplicate free() ignored




(S malloc) An internal routine called free() on something that had
already been freed.





Duplicate modifier ’%c’ after ’%c’ in %s




(W) You have applied the same modifier more than once after a type
in a pack template.  See (perlfunc)pack.





elseif should be elsif




(S syntax) There is no keyword "elseif" in Perl because Larry thinks
it’s ugly.  Your code will be interpreted as an attempt to call a method
named "elseif" for the class returned by the following block.  This is
unlikely to be what you want.





Empty %s




(F) \p and \P are used to introduce a named Unicode property, as
described in perlunicode NAME and (perlre).  You used \p or \P in
a regular expression without specifying the property name.





entering effective %s failed




(F) While under the use filetest pragma, switching the real and
effective uids or gids failed.





%ENV is aliased to %s




(F) You’re running under taint mode, and the %ENV variable has been
aliased to another hash, so it doesn’t reflect anymore the state of the
program’s environment.  This is potentially insecure.





Error converting file specification %s




(F) An error peculiar to VMS.  Because Perl may have to deal with file
specifications in either VMS or Unix syntax, it converts them to a
single form when it must operate on them directly.  Either you’ve passed
an invalid file specification to Perl, or you’ve found a case the
conversion routines don’t handle.  Drat.





%s: Eval-group in insecure regular expression




(F) Perl detected tainted data when trying to compile a regular
expression that contains the (?{ ... }) zero-width assertion, which
is unsafe.  See (perlre)(?{ code }), and perlsec NAME.





%s: Eval-group not allowed at runtime, use re ’eval’




(F) Perl tried to compile a regular expression containing the
(?{ ... }) zero-width assertion at run time, as it would when the
pattern contains interpolated values.  Since that is a security risk,
it is not allowed.  If you insist, you may still do this by using the
re 'eval' pragma or by explicitly building the pattern from an
interpolated string at run time and using that in an eval().  See
(perlre)(?{ code }).





%s: Eval-group not allowed, use re ’eval’




(F) A regular expression contained the (?{ ... }) zero-width
assertion, but that construct is only allowed when the use re 'eval'
pragma is in effect.  See (perlre)(?{ code }).





EVAL without pos change exceeded limit in regex; marked by <– HERE in m/%s/




(F) You used a pattern that nested too many EVAL calls without consuming
any text.  Restructure the pattern so that text is consumed.



The <– HERE shows in the regular expression about where the problem was
discovered.





Excessively long <> operator




(F) The contents of a <> operator may not exceed the maximum size of a
Perl identifier.  If you’re just trying to glob a long list of
filenames, try using the glob() operator, or put the filenames into a
variable and glob that.





exec? I’m not *that* kind of operating system




(F) The exec function is not implemented on some systems, e.g., Symbian
OS.  See perlport NAME.





Execution of %s aborted due to compilation errors.




(F) The final summary message when a Perl compilation fails.





Exiting eval via %s




(W exiting) You are exiting an eval by unconventional means, such as a
goto, or a loop control statement.





Exiting format via %s




(W exiting) You are exiting a format by unconventional means, such as a
goto, or a loop control statement.





Exiting pseudo-block via %s




(W exiting) You are exiting a rather special block construct (like a
sort block or subroutine) by unconventional means, such as a goto, or a
loop control statement.  See (perlfunc)sort.





Exiting subroutine via %s




(W exiting) You are exiting a subroutine by unconventional means, such
as a goto, or a loop control statement.





Exiting substitution via %s




(W exiting) You are exiting a substitution by unconventional means, such
as a return, a goto, or a loop control statement.





Explicit blessing to ” (assuming package main)




(W misc) You are blessing a reference to a zero length string.  This has
the effect of blessing the reference into the package main.  This is
usually not what you want.  Consider providing a default target package,
e.g. bless($ref, $p || ’MyPackage’);





%s: Expression syntax




(A) You’ve accidentally run your script through csh instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.





%s failed–call queue aborted




(F) An untrapped exception was raised while executing a UNITCHECK,
CHECK, INIT, or END subroutine.  Processing of the remainder of the
queue of such routines has been prematurely ended.





False [] range "%s" in regex; marked by <– HERE in m/%s/




(W regexp) A character class range must start and end at a literal
character, not another character class like \d or [:alpha:].  The "-"
in your false range is interpreted as a literal "-".  Consider quoting the
"-", "\-".  The <– HERE shows in the regular expression about where the
problem was discovered.  See (perlre).





Fatal VMS error (status=%d) at %s, line %d




(P) An error peculiar to VMS.  Something untoward happened in a VMS
system service or RTL routine; Perl’s exit status should provide more
details.  The filename in "at %s" and the line number in "line %d" tell
you which section of the Perl source code is distressed.





fcntl is not implemented




(F) Your machine apparently doesn’t implement fcntl().  What is this, a
PDP-11 or something?





FETCHSIZE returned a negative value




(F) A tied array claimed to have a negative number of elements, which
is not possible.





Field too wide in ’u’ format in pack




(W pack) Each line in an uuencoded string start with a length indicator
which can’t encode values above 63.  So there is no point in asking for
a line length bigger than that.  Perl will behave as if you specified
u63 as the format.





Filehandle %s opened only for input




(W io) You tried to write on a read-only filehandle.  If you intended
it to be a read-write filehandle, you needed to open it with "+<" or
"+>" or "+>>" instead of with "<" or nothing.  If you intended only to
write the file, use ">" or ">>".  See (perlfunc)open.





Filehandle %s opened only for output




(W io) You tried to read from a filehandle opened only for writing, If
you intended it to be a read/write filehandle, you needed to open it
with "+<" or "+>" or "+>>" instead of with ">".  If you intended only to
read from the file, use "<".  See (perlfunc)open.  Another possibility
is that you attempted to open filedescriptor 0 (also known as STDIN) for
output (maybe you closed STDIN earlier?).





Filehandle %s reopened as %s only for input




(W io) You opened for reading a filehandle that got the same filehandle id
as STDOUT or STDERR.  This occurred because you closed STDOUT or STDERR
previously.





Filehandle STDIN reopened as %s only for output




(W io) You opened for writing a filehandle that got the same filehandle id
as STDIN.  This occurred because you closed STDIN previously.





Final $ should be \$ or $name




(F) You must now decide whether the final $ in a string was meant to be
a literal dollar sign, or was meant to introduce a variable name that
happens to be missing.  So you have to put either the backslash or the
name.





flock() on closed filehandle %s




(W closed) The filehandle you’re attempting to flock() got itself closed
some time before now.  Check your control flow.  flock() operates on
filehandles.  Are you attempting to call flock() on a dirhandle by the
same name?





Format not terminated




(F) A format must be terminated by a line with a solitary dot.  Perl got
to the end of your file without finding such a line.





Format %s redefined




(W redefine) You redefined a format.  To suppress this warning, say



    {
        no warnings 'redefine';
        eval "format NAME =...";
    }





Found = in conditional, should be ==




(W syntax) You said



    if ($foo = 123)



when you meant



    if ($foo == 123)



(or something like that).





%s found where operator expected




(S syntax) The Perl lexer knows whether to expect a term or an operator.
If it sees what it knows to be a term when it was expecting to see an
operator, it gives you this warning.  Usually it indicates that an
operator or delimiter was omitted, such as a semicolon.





gdbm store returned %d, errno %d, key "%s"




(S) A warning from the GDBM_File extension that a store failed.





gethostent not implemented




(F) Your C library apparently doesn’t implement gethostent(), probably
because if it did, it’d feel morally obligated to return every hostname
on the Internet.





get%sname() on closed socket %s




(W closed) You tried to get a socket or peer socket name on a closed
socket.  Did you forget to check the return value of your socket() call?





getpwnam returned invalid UIC %#o for user "%s"




(S) A warning peculiar to VMS.  The call to sys$getuai underlying the
getpwnam operator returned an invalid UIC.





getsockopt() on closed socket %s




(W closed) You tried to get a socket option on a closed socket.  Did you
forget to check the return value of your socket() call?  See
(perlfunc)getsockopt.





Global symbol "%s" requires explicit package name




(F) You’ve said "use strict" or "use strict vars", which indicates 
that all variables must either be lexically scoped (using "my" or "state"), 
declared beforehand using "our", or explicitly qualified to say 
which package the global variable is in (using "::").





glob failed (%s)




(S glob) Something went wrong with the external program(s) used
for glob and <*.c>.  Usually, this means that you supplied a glob
pattern that caused the external program to fail and exit with a
nonzero status.  If the message indicates that the abnormal exit
resulted in a coredump, this may also mean that your csh (C shell)
is broken.  If so, you should change all of the csh-related variables
in config.sh:  If you have tcsh, make the variables refer to it as
if it were csh (e.g. full_csh='/usr/bin/tcsh'); otherwise, make them
all empty (except that d_csh should be 'undef') so that Perl will
think csh is missing.  In either case, after editing config.sh, run
./Configure -S and rebuild Perl.





Glob not terminated




(F) The lexer saw a left angle bracket in a place where it was expecting
a term, so it’s looking for the corresponding right angle bracket, and
not finding it.  Chances are you left some needed parentheses out
earlier in the line, and you really meant a "less than".





gmtime(%f) too large




(W overflow) You called gmtime with a number that was larger than
it can reliably handle and gmtime probably returned the wrong
date.  This warning is also triggered with NaN (the special
not-a-number value).





gmtime(%f) too small




(W overflow) You called gmtime with a number that was smaller than
it can reliably handle and gmtime probably returned the wrong date.





Got an error from DosAllocMem




(P) An error peculiar to OS/2.  Most probably you’re using an obsolete
version of Perl, and this should not happen anyway.





goto must have label




(F) Unlike with "next" or "last", you’re not allowed to goto an
unspecified destination.  See (perlfunc)goto.





Goto undefined subroutine%s




(F) You tried to call a subroutine with goto &sub syntax, but
the indicated subroutine hasn’t been defined, or if it was, it
has since been undefined.





()-group starts with a count




(F) A ()-group started with a count.  A count is supposed to follow
something: a template character or a ()-group.  See (perlfunc)pack.





%s had compilation errors.




(F) The final summary message when a perl -c fails.





Had to create %s unexpectedly




(S internal) A routine asked for a symbol from a symbol table that ought
to have existed already, but for some reason it didn’t, and had to be
created on an emergency basis to prevent a core dump.





Hash %%s missing the % in argument %d of %s()




(D deprecated) Really old Perl let you omit the % on hash names in some
spots.  This is now heavily deprecated.





%s has too many errors




(F) The parser has given up trying to parse the program after 10 errors.
Further error messages would likely be uninformative.





Having no space between pattern and following word is deprecated




(D syntax)



You had a word that isn’t a regex modifier immediately following
a pattern without an intervening space.  If you are trying to use
the /le flags on a substitution, use /el instead.  Otherwise, add
white space between the pattern and following word to eliminate
the warning.  As an example of the latter, the two constructs:



 $a =~ m/$foo/sand $bar
 $a =~ m/$foo/s and $bar



both currently mean the same thing, but it is planned to disallow
the first form in Perl 5.18.  And,



 $a =~ m/$foo/and $bar



will be disallowed too.





Hexadecimal number > 0xffffffff non-portable




(W portable) The hexadecimal number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
perlport NAME for more on portability concerns.





Identifier too long




(F) Perl limits identifiers (names for variables, functions, etc.) to
about 250 characters for simple names, and somewhat more for compound
names (like $A::B).  You’ve exceeded Perl’s limits.  Future versions
of Perl are likely to eliminate these arbitrary limitations.





Ignoring zero length \N{} in character class




(W) Named Unicode character escapes (\N{...}) may return a zero-length
sequence.  When such an escape is used in a character class its
behaviour is not well defined.  Check that the correct escape has
been used, and the correct charname handler is in scope.





Illegal binary digit %s




(F) You used a digit other than 0 or 1 in a binary number.





Illegal binary digit %s ignored




(W digit) You may have tried to use a digit other than 0 or 1 in a
binary number.  Interpretation of the binary number stopped before the
offending digit.





Illegal character after ’_’ in prototype for %s : %s




(W illegalproto) An illegal character was found in a prototype declaration.
Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +.





Illegal character \%o (carriage return)




(F) Perl normally treats carriage returns in the program text as it
would any other whitespace, which means you should never see this error
when Perl was built using standard options.  For some reason, your
version of Perl appears to have been built without this support.  Talk
to your Perl administrator.





Illegal character in prototype for %s : %s




(W illegalproto) An illegal character was found in a prototype declaration.
Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +.





Illegal declaration of anonymous subroutine




(F) When using the sub keyword to construct an anonymous subroutine,
you must always specify a block of code.  See (perlsub).





Illegal declaration of subroutine %s




(F) A subroutine was not declared correctly.  See (perlsub).





Illegal division by zero




(F) You tried to divide a number by 0.  Either something was wrong in
your logic, or you need to put a conditional in to guard against
meaningless input.





Illegal hexadecimal digit %s ignored




(W digit) You may have tried to use a character other than 0 - 9 or
A - F, a - f in a hexadecimal number.  Interpretation of the hexadecimal
number stopped before the illegal character.





Illegal modulus zero




(F) You tried to divide a number by 0 to get the remainder.  Most
numbers don’t take to this kindly.





Illegal number of bits in vec




(F) The number of bits in vec() (the third argument) must be a power of
two from 1 to 32 (or 64, if your platform supports that).





Illegal octal digit %s




(F) You used an 8 or 9 in an octal number.





Illegal octal digit %s ignored




(W digit) You may have tried to use an 8 or 9 in an octal number.
Interpretation of the octal number stopped before the 8 or 9.





Illegal switch in PERL5OPT: -%c




(X) The PERL5OPT environment variable may only be used to set the
following switches: -[CDIMUdmtw].





Ill-formed CRTL environ value "%s"




(W internal) A warning peculiar to VMS.  Perl tried to read the CRTL’s
internal environ array, and encountered an element without the =
delimiter used to separate keys from values.  The element is ignored.





Ill-formed message in prime_env_iter: |%s|




(W internal) A warning peculiar to VMS.  Perl tried to read a logical
name or CLI symbol definition when preparing to iterate over %ENV, and
didn’t see the expected delimiter between key and value, so the line was
ignored.





(in cleanup) %s




(W misc) This prefix usually indicates that a DESTROY() method raised
the indicated exception.  Since destructors are usually called by the
system at arbitrary points during execution, and often a vast number of
times, the warning is issued only once for any number of failures that
would otherwise result in the same message being repeated.



Failure of user callbacks dispatched using the G_KEEPERR flag could
also result in this warning.  See perlcall G_KEEPERR.





Inconsistent hierarchy during C3 merge of class ’%s’: merging failed on parent ’%s’




(F) The method resolution order (MRO) of the given class is not
C3-consistent, and you have enabled the C3 MRO for this class.  See the C3
documentation in (mro) for more information.





In EBCDIC the v-string components cannot exceed 2147483647




(F) An error peculiar to EBCDIC.  Internally, v-strings are stored as
Unicode code points, and encoded in EBCDIC as UTF-EBCDIC.  The UTF-EBCDIC
encoding is limited to code points no larger than 2147483647 (0x7FFFFFFF).





Infinite recursion in regex; marked by <– HERE in m/%s/




(F) You used a pattern that references itself without consuming any input
text.  You should check the pattern to ensure that recursive patterns
either consume text or fail.



The <– HERE shows in the regular expression about where the problem was
discovered.





Initialization of state variables in list context currently forbidden




(F) Currently the implementation of "state" only permits the
initialization of scalar variables in scalar context.  Re-write
state ($a) = 42 as state $a = 42 to change from list to scalar
context.  Constructions such as state (@a) = foo() will be
supported in a future perl release.





Insecure dependency in %s




(F) You tried to do something that the tainting mechanism didn’t like.
The tainting mechanism is turned on when you’re running setuid or
setgid, or when you specify -T to turn it on explicitly.  The
tainting mechanism labels all data that’s derived directly or indirectly
from the user, who is considered to be unworthy of your trust.  If any
such data is used in a "dangerous" operation, you get this error.  See
perlsec NAME for more information.





Insecure directory in %s




(F) You can’t use system(), exec(), or a piped open in a setuid or
setgid script if $ENV{PATH} contains a directory that is writable by
the world.  Also, the PATH must not contain any relative directory.
See perlsec NAME.





Insecure $ENV{%s} while running %s




(F) You can’t use system(), exec(), or a piped open in a setuid or
setgid script if any of $ENV{PATH}, $ENV{IFS}, $ENV{CDPATH},
$ENV{ENV}, $ENV{BASH_ENV} or $ENV{TERM} are derived from data
supplied (or potentially supplied) by the user.  The script must set
the path to a known value, using trustworthy data.  See perlsec NAME.





Insecure user-defined property %s




(F) Perl detected tainted data when trying to compile a regular
expression that contains a call to a user-defined character property
function, i.e. \p{IsFoo} or \p{InFoo}.
See perlunicode User-Defined Character Properties and perlsec NAME.





Integer overflow in format string for %s




(F) The indexes and widths specified in the format string of printf()
or sprintf() are too large.  The numbers must not overflow the size of
integers for your architecture.





Integer overflow in %s number




(W overflow) The hexadecimal, octal or binary number you have specified
either as a literal or as an argument to hex() or oct() is too big for
your architecture, and has been converted to a floating point number.
On a 32-bit architecture the largest hexadecimal, octal or binary number
representable without overflow is 0xFFFFFFFF, 037777777777, or
0b11111111111111111111111111111111 respectively.  Note that Perl
transparently promotes all numbers to a floating point representation
internally–subject to loss of precision errors in subsequent
operations.





Integer overflow in version




(F) Some portion of a version initialization is too large for the
size of integers for your architecture.  This is not a warning
because there is no rational reason for a version to try and use a
element larger than typically 2**32.  This is usually caused by
trying to use some odd mathematical operation as a version, like
100/9.





Internal disaster in regex; marked by <– HERE in m/%s/




(P) Something went badly wrong in the regular expression parser.
The <– HERE shows in the regular expression about where the problem was
discovered.





Internal inconsistency in tracking vforks




(S) A warning peculiar to VMS.  Perl keeps track of the number of times
you’ve called fork and exec, to determine whether the current call
to exec should affect the current script or a subprocess (see
perlvms exec LIST).  Somehow, this count has become scrambled, so
Perl is making a guess and treating this exec as a request to
terminate the Perl script and execute the specified command.





Internal urp in regex; marked by <– HERE in m/%s/




(P) Something went badly awry in the regular expression parser.  The
<– HERE shows in the regular expression about where the problem was
discovered.





%s (...) interpreted as function




(W syntax) You’ve run afoul of the rule that says that any list operator
followed by parentheses turns into a function, with all the list
operators arguments found inside the parentheses.  See
(perlop)Terms and List Operators (Leftward).





Invalid %s attribute: %s




(F) The indicated attribute for a subroutine or variable was not recognized
by Perl or by a user-supplied handler.  See (attributes).





Invalid %s attributes: %s




(F) The indicated attributes for a subroutine or variable were not
recognized by Perl or by a user-supplied handler.  See (attributes).





Invalid conversion in %s: "%s"




(W printf) Perl does not understand the given format conversion.  See
(perlfunc)sprintf.





Invalid escape in the specified encoding in regex; marked by <– HERE in m/%s/




(W regexp) The numeric escape (for example \xHH) of value < 256
didn’t correspond to a single character through the conversion
from the encoding specified by the encoding pragma.
The escape was replaced with REPLACEMENT CHARACTER (U+FFFD) instead.
The <– HERE shows in the regular expression about where the
escape was discovered.





Invalid hexadecimal number in \N{U+...}




(F) The character constant represented by ... is not a valid hexadecimal
number.  Either it is empty, or you tried to use a character other than
0 - 9 or A - F, a - f in a hexadecimal number.





Invalid module name %s with -%c option: contains single ’:’




(F) The module argument to perl’s -m and -M command-line options
cannot contain single colons in the module name, but only in the
arguments after "=".  In other words, -MFoo::Bar=:baz is ok, but
-MFoo:Bar=baz is not.





Invalid mro name: ’%s’




(F) You tried to mro::set_mro("classname", "foo") or use mro 'foo',
where foo is not a valid method resolution order (MRO).  Currently,
the only valid ones supported are dfs and c3, unless you have loaded
a module that is a MRO plugin.  See (mro) and perlmroapi NAME.





invalid option -D%c, use -D” to see choices




(F) Perl was called with invalid debugger flags.  Call perl with
the -D option with no flags to see the list of acceptable values.
See also perlrun -Dletters.





Invalid [] range "%s" in regex; marked by <– HERE in m/%s/




(F) The range specified in a character class had a minimum character
greater than the maximum character.  One possibility is that you forgot the
{} from your ending \x{} - \x without the curly braces can go only
up to ff.  The <– HERE shows in the regular expression about where the
problem was discovered.  See (perlre).





Invalid range "%s" in transliteration operator




(F) The range specified in the tr/// or y/// operator had a minimum
character greater than the maximum character.  See (perlop).





Invalid separator character %s in attribute list




(F) Something other than a colon or whitespace was seen between the
elements of an attribute list.  If the previous attribute had a
parenthesised parameter list, perhaps that list was terminated too soon.
See (attributes).





Invalid separator character %s in PerlIO layer specification %s




(W layer) When pushing layers onto the Perl I/O system, something other
than a colon or whitespace was seen between the elements of a layer list.
If the previous attribute had a parenthesised parameter list, perhaps that
list was terminated too soon.





Invalid strict version format (%s)




(F) A version number did not meet the "strict" criteria for versions.
A "strict" version number is a positive decimal number (integer or
decimal-fraction) without exponentiation or else a dotted-decimal
v-string with a leading ’v’ character and at least three components.
The parenthesized text indicates which criteria were not met.
See the (version) module for more details on allowed version formats.





Invalid type ’%s’ in %s




(F) The given character is not a valid pack or unpack type.
See (perlfunc)pack.



(W) The given character is not a valid pack or unpack type but used to be
silently ignored.





Invalid version format (%s)




(F) A version number did not meet the "lax" criteria for versions.
A "lax" version number is a positive decimal number (integer or
decimal-fraction) without exponentiation or else a dotted-decimal
v-string.  If the v-string has fewer than three components, it
must have a leading ’v’ character.  Otherwise, the leading ’v’ is
optional.  Both decimal and dotted-decimal versions may have a
trailing "alpha" component separated by an underscore character
after a fractional or dotted-decimal component.  The parenthesized
text indicates which criteria were not met.  See the (version) module
for more details on allowed version formats.





Invalid version object




(F) The internal structure of the version object was invalid.
Perhaps the internals were modified directly in some way or
an arbitrary reference was blessed into the "version" class.





ioctl is not implemented




(F) Your machine apparently doesn’t implement ioctl(), which is pretty
strange for a machine that supports C.





ioctl() on unopened %s




(W unopened) You tried ioctl() on a filehandle that was never opened.
Check your control flow and number of arguments.





IO layers (like ’%s’) unavailable




(F) Your Perl has not been configured to have PerlIO, and therefore
you cannot use IO layers.  To have PerlIO, Perl must be configured
with ’useperlio’.





IO::Socket::atmark not implemented on this architecture




(F) Your machine doesn’t implement the sockatmark() functionality,
neither as a system call nor an ioctl call (SIOCATMARK).





$* is no longer supported




(D deprecated, syntax) The special variable $*, deprecated in older
perls, has been removed as of 5.9.0 and is no longer supported.  In
previous versions of perl the use of $* enabled or disabled multi-line
matching within a string.



Instead of using $* you should use the /m (and maybe /s) regexp
modifiers.  You can enable /m for a lexical scope (even a whole file)
with use re '/m'.  (In older versions: when $* was set to a true value
then all regular expressions behaved as if they were written using /m.)





$# is no longer supported




(D deprecated, syntax) The special variable $#, deprecated in older
perls, has been removed as of 5.9.3 and is no longer supported.  You
should use the printf/sprintf functions instead.





’%s’ is not a code reference




(W overload) The second (fourth, sixth, ...) argument of
overload::constant needs to be a code reference.  Either
an anonymous subroutine, or a reference to a subroutine.





’%s’ is not an overloadable type




(W overload) You tried to overload a constant type the overload package is
unaware of.





junk on end of regexp




(P) The regular expression parser is confused.





Label not found for "last %s"




(F) You named a loop to break out of, but you’re not currently in a loop
of that name, not even if you count where you were called from.  See
(perlfunc)last.





Label not found for "next %s"




(F) You named a loop to continue, but you’re not currently in a loop of
that name, not even if you count where you were called from.  See
(perlfunc)last.





Label not found for "redo %s"




(F) You named a loop to restart, but you’re not currently in a loop of
that name, not even if you count where you were called from.  See
(perlfunc)last.





leaving effective %s failed




(F) While under the use filetest pragma, switching the real and
effective uids or gids failed.





length/code after end of string in unpack




(F) While unpacking, the string buffer was already used up when an unpack
length/code combination tried to obtain more data.  This results in
an undefined value for the length.  See (perlfunc)pack.





length() used on %s




(W syntax) You used length() on either an array or a hash when you
probably wanted a count of the items.



Array size can be obtained by doing:



    scalar(@array);



The number of items in a hash can be obtained by doing:



    scalar(keys %hash);





Lexing code attempted to stuff non-Latin-1 character into Latin-1 input




(F) An extension is attempting to insert text into the current parse
(using (perlapi)lex_stuff_pvn or similar), but tried to insert a character that
couldn’t be part of the current input.  This is an inherent pitfall
of the stuffing mechanism, and one of the reasons to avoid it.  Where
it is necessary to stuff, stuffing only plain ASCII is recommended.





Lexing code internal error (%s)




(F) Lexing code supplied by an extension violated the lexer’s API in a
detectable way.





listen() on closed socket %s




(W closed) You tried to do a listen on a closed socket.  Did you forget
to check the return value of your socket() call?  See
(perlfunc)listen.





List form of piped open not implemented




(F) On some platforms, notably Windows, the three-or-more-arguments
form of open does not support pipes, such as open($pipe, '|-', @args).
Use the two-argument open($pipe, '|prog arg1 arg2...') form instead.





localtime(%f) too large




(W overflow) You called localtime with a number that was larger
than it can reliably handle and localtime probably returned the
wrong date.  This warning is also triggered with NaN (the special
not-a-number value).





localtime(%f) too small




(W overflow) You called localtime with a number that was smaller
than it can reliably handle and localtime probably returned the
wrong date.





Lookbehind longer than %d not implemented in regex m/%s/




(F) There is currently a limit on the length of string which lookbehind can
handle.  This restriction may be eased in a future release. 





Lost precision when %s %f by 1




(W) The value you attempted to increment or decrement by one is too large
for the underlying floating point representation to store accurately,
hence the target of ++ or -- is unchanged.  Perl issues this warning
because it has already switched from integers to floating point when values
are too large for integers, and now even floating point is insufficient.
You may wish to switch to using (Math-BigInt) explicitly.





lstat() on filehandle%s




(W io) You tried to do an lstat on a filehandle.  What did you mean
by that?  lstat() makes sense only on filenames.  (Perl did a fstat()
instead on the filehandle.)





lvalue attribute %s already-defined subroutine




(W misc) Although (attributes)attributes.pm allows this, turning the lvalue
attribute on or off on a Perl subroutine that is already defined
does not always work properly.  It may or may not do what you
want, depending on what code is inside the subroutine, with exact
details subject to change between Perl versions.  Only do this
if you really know what you are doing.





lvalue attribute ignored after the subroutine has been defined




(W misc) Using the :lvalue declarative syntax to make a Perl
subroutine an lvalue subroutine after it has been defined is
not permitted.  To make the subroutine an lvalue subroutine,
add the lvalue attribute to the definition, or put the sub
foo :lvalue; declaration before the definition.



See also (attributes)attributes.pm.





Malformed integer in [] in pack




(F) Between the brackets enclosing a numeric repeat count only digits
are permitted.  See (perlfunc)pack.





Malformed integer in [] in unpack




(F) Between the brackets enclosing a numeric repeat count only digits
are permitted.  See (perlfunc)pack.





Malformed PERLLIB_PREFIX




(F) An error peculiar to OS/2.  PERLLIB_PREFIX should be of the form



    prefix1;prefix2



or
    prefix1 prefix2



with nonempty prefix1 and prefix2.  If prefix1 is indeed a prefix of
a builtin library search path, prefix2 is substituted.  The error may
appear if components are not found, or are too long.  See
"PERLLIB_PREFIX" in (perlos2).





Malformed prototype for %s: %s




(F) You tried to use a function with a malformed prototype.  The
syntax of function prototypes is given a brief compile-time check for
obvious errors like invalid characters.  A more rigorous check is run
when the function is called.





Malformed UTF-8 character (%s)




(S utf8)(F) Perl detected a string that didn’t comply with UTF-8
encoding rules, even though it had the UTF8 flag on.



One possible cause is that you set the UTF8 flag yourself for data that
you thought to be in UTF-8 but it wasn’t (it was for example legacy
8-bit data).  To guard against this, you can use Encode::decode_utf8.



If you use the :encoding(UTF-8) PerlIO layer for input, invalid byte
sequences are handled gracefully, but if you use :utf8, the flag is
set without validating the data, possibly resulting in this error
message.



See also (Encode)Handling Malformed Data.





Malformed UTF-8 returned by \N




(F) The charnames handler returned malformed UTF-8.





Malformed UTF-8 string in ’%c’ format in unpack




(F) You tried to unpack something that didn’t comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.





Malformed UTF-8 string in pack




(F) You tried to pack something that didn’t comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.





Malformed UTF-8 string in unpack




(F) You tried to unpack something that didn’t comply with UTF-8 encoding
rules and perl was unable to guess how to make more progress.





Malformed UTF-16 surrogate




(F) Perl thought it was reading UTF-16 encoded character data but while
doing it Perl met a malformed Unicode surrogate.





%s matches null string many times in regex; marked by <– HERE in m/%s/




(W regexp) The pattern you’ve specified would be an infinite loop if the
regular expression engine didn’t specifically check for that.  The <– HERE
shows in the regular expression about where the problem was discovered.
See (perlre).





Maximal count of pending signals (%u) exceeded




(F) Perl aborted due to too high a number of signals pending.  This
usually indicates that your operating system tried to deliver signals
too fast (with a very high priority), starving the perl process from
resources it would need to reach a point where it can process signals
safely.  (See perlipc Deferred Signals (Safe Signals).)





"%s" may clash with future reserved word




(W) This warning may be due to running a perl5 script through a perl4
interpreter, especially if the word that is being warned about is
"use" or "my".





’%’ may not be used in pack




(F) You can’t pack a string by supplying a checksum, because the
checksumming process loses information, and you can’t go the other way.
See (perlfunc)unpack.





Method for operation %s not found in package %s during blessing




(F) An attempt was made to specify an entry in an overloading table that
doesn’t resolve to a valid subroutine.  See (overload).





Method %s not permitted




See Server error.





Might be a runaway multi-line %s string starting on line %d




(S) An advisory indicating that the previous error may have been caused
by a missing delimiter on a string or pattern, because it eventually
ended earlier on the current line.





Misplaced _ in number




(W syntax) An underscore (underbar) in a numeric constant did not
separate two digits.





Missing argument in %s




(W uninitialized) A printf-type format required more arguments than were
supplied.





Missing argument to -%c




(F) The argument to the indicated command line switch must follow
immediately after the switch, without intervening spaces.





Missing braces on \N{}




(F) Wrong syntax of character name literal \N{charname} within
double-quotish context.  This can also happen when there is a space
(or comment) between the \N and the { in a regex with the /x modifier.
This modifier does not change the requirement that the brace immediately
follow the \N.





Missing braces on \o{}




(F) A \o must be followed immediately by a { in double-quotish context.





Missing comma after first argument to %s function




(F) While certain functions allow you to specify a filehandle or an
"indirect object" before the argument list, this ain’t one of them.





Missing command in piped open




(W pipe) You used the open(FH, "| command") or
open(FH, "command |") construction, but the command was missing or
blank.





Missing control char name in \c




(F) A double-quoted string ended with "\c", without the required control
character name.





Missing name in "my sub"




(F) The reserved syntax for lexically scoped subroutines requires that
they have a name with which they can be found.





Missing $ on loop variable




(F) Apparently you’ve been programming in csh too much.  Variables
are always mentioned with the $ in Perl, unlike in the shells, where it
can vary from one line to the next.





(Missing operator before %s?)




(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  Often the missing operator is a comma.





Missing right brace on %s




(F) Missing right brace in \x{...}, \p{...}, \P{...}, or \N{...}.





Missing right brace on \N{} or unescaped left brace after \N




(F) \N has two meanings.



The traditional one has it followed by a name enclosed in braces,
meaning the character (or sequence of characters) given by that
name.  Thus \N{ASTERISK} is another way of writing *, valid in both
double-quoted strings and regular expression patterns.  In patterns,
it doesn’t have the meaning an unescaped * does.



Starting in Perl 5.12.0, \N also can have an additional meaning (only)
in patterns, namely to match a non-newline character.  (This is short
for [^\n], and like . but is not affected by the /s regex modifier.)



This can lead to some ambiguities.  When \N is not followed immediately
by a left brace, Perl assumes the [^\n] meaning.  Also, if the braces
form a valid quantifier such as \N{3} or \N{5,}, Perl assumes that this
means to match the given quantity of non-newlines (in these examples,
3; and 5 or more, respectively).  In all other case, where there is a
\N{ and a matching }, Perl assumes that a character name is desired.



However, if there is no matching }, Perl doesn’t know if it was
mistakenly omitted, or if [^\n]{ was desired, and raises this error.
If you meant the former, add the right brace; if you meant the latter,
escape the brace with a backslash, like so: \N\{





Missing right curly or square bracket




(F) The lexer counted more opening curly or square brackets than closing
ones.  As a general rule, you’ll find it’s missing near the place you
were last editing.





(Missing semicolon on previous line?)




(S syntax) This is an educated guess made in conjunction with the message
"%s found where operator expected".  Don’t automatically put a semicolon on
the previous line just because you saw this message.





Modification of a read-only value attempted




(F) You tried, directly or indirectly, to change the value of a
constant.  You didn’t, of course, try "2 = 1", because the compiler
catches that.  But an easy way to do the same thing is:



    sub mod { $_[0] = 1 }
    mod(2);



Another way is to assign to a substr() that’s off the end of the string.



Yet another way is to assign to a foreach loop VAR when VAR
is aliased to a constant in the look LIST:



    $x = 1;
    foreach my $n ($x, 2) {
        $n *= 2; # modifies the $x, but fails on attempt to
    }            # modify the 2





Modification of non-creatable array value attempted, %s




(F) You tried to make an array value spring into existence, and the
subscript was probably negative, even counting from end of the array
backwards.





Modification of non-creatable hash value attempted, %s




(P) You tried to make a hash value spring into existence, and it
couldn’t be created for some peculiar reason.





Module name must be constant




(F) Only a bare module name is allowed as the first argument to a "use".





Module name required with -%c option




(F) The -M or -m options say that Perl should load some module, but
you omitted the name of the module.  Consult perlrun NAME for full details
about -M and -m.





More than one argument to ’%s’ open




(F) The open function has been asked to open multiple files.  This
can happen if you are trying to open a pipe to a command that takes a
list of arguments, but have forgotten to specify a piped open mode.
See (perlfunc)open for details.





msg%s not implemented




(F) You don’t have System V message IPC on your system.





Multidimensional syntax %s not supported




(W syntax) Multidimensional arrays aren’t written like $foo[1,2,3].
They’re written like $foo[1][2][3], as in C.





’/’ must follow a numeric type in unpack




(F) You had an unpack template that contained a ’/’, but this did not
follow some unpack specification producing a numeric value.
See (perlfunc)pack.





"my sub" not yet implemented




(F) Lexically scoped subroutines are not yet implemented.  Don’t try
that yet.





"my" variable %s can’t be in a package




(F) Lexically scoped variables aren’t in a package, so it doesn’t make
sense to try to declare one with a package qualifier on the front.  Use
local() if you want to localize a package variable.





Name "%s::%s" used only once: possible typo




(W once) Typographical errors often show up as unique variable names.
If you had a good reason for having a unique name, then just mention it
again somehow to suppress the message.  The our declaration is
provided for this purpose.



NOTE: This warning detects symbols that have been used only once so $c, @c,
%c, *c, &c, sub c{}, c(), and c (the filehandle or format) are considered
the same; if a program uses $c only once but also uses any of the others it
will not trigger this warning.





\N in a character class must be a named character: \N{...}




(F) The new (5.12) meaning of \N as [^\n] is not valid in a bracketed
character class, for the same reason that . in a character class loses
its specialness: it matches almost everything, which is probably not
what you want.





\N{NAME} must be resolved by the lexer




(F) When compiling a regex pattern, an unresolved named character or
sequence was encountered.  This can happen in any of several ways that
bypass the lexer, such as using single-quotish context, or an extra
backslash in double-quotish:



    $re = '\N{SPACE}';  # Wrong!
    $re = "\\N{SPACE}"; # Wrong!
    /$re/;



Instead, use double-quotes with a single backslash:



    $re = "\N{SPACE}";  # ok
    /$re/;



The lexer can be bypassed as well by creating the pattern from smaller
components:



    $re = '\N';
    /${re}{SPACE}/;     # Wrong!



It’s not a good idea to split a construct in the middle like this, and it
doesn’t work here.  Instead use the solution above.



Finally, the message also can happen under the /x regex modifier when the
\N is separated by spaces from the {, in which case, remove the spaces.



    /\N {SPACE}/x;      # Wrong!
    /\N{SPACE}/x;       # ok





Negative ’/’ count in unpack




(F) The length count obtained from a length/code unpack operation was
negative.  See (perlfunc)pack.





Negative length




(F) You tried to do a read/write/send/recv operation with a buffer
length that is less than 0.  This is difficult to imagine.





Negative offset to vec in lvalue context




(F) When vec is called in an lvalue context, the second argument must be
greater than or equal to zero.





Nested quantifiers in regex; marked by <– HERE in m/%s/




(F) You can’t quantify a quantifier without intervening parentheses.
So things like ** or +* or ?* are illegal.  The <– HERE shows in the
regular expression about where the problem was discovered.



Note that the minimal matching quantifiers, *?, +?, and
?? appear to be nested quantifiers, but aren’t.  See (perlre).





%s never introduced




(S internal) The symbol in question was declared but somehow went out of
scope before it could possibly have been used.





next::method/next::can/maybe::next::method cannot find enclosing method




(F) next::method needs to be called within the context of a
real method in a real package, and it could not find such a context.
See (mro).





No %s allowed while running setuid




(F) Certain operations are deemed to be too insecure for a setuid or
setgid script to even be allowed to attempt.  Generally speaking there
will be another way to do what you want that is, if not secure, at least
securable.  See perlsec NAME.





No code specified for -%c




(F) Perl’s -e and -E command-line options require an argument.  If
you want to run an empty program, pass the empty string as a separate
argument or run a program consisting of a single 0 or 1:



    perl -e ""
    perl -e0
    perl -e1





No comma allowed after %s




(F) A list operator that has a filehandle or "indirect object" is
not allowed to have a comma between that and the following arguments.
Otherwise it’d be just another one of the arguments.



One possible cause for this is that you expected to have imported
a constant to your name space with use or import while no such
importing took place, it may for example be that your operating
system does not support that particular constant.  Hopefully you did
use an explicit import list for the constants you expect to see;
please see (perlfunc)use and (perlfunc)import.  While an
explicit import list would probably have caught this error earlier
it naturally does not remedy the fact that your operating system
still does not support that constant.  Maybe you have a typo in
the constants of the symbol import list of use or import or in the
constant name at the line where this error was triggered?





No command into which to pipe on command line




(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a ’|’ at the end of the command line, so it
doesn’t know where you want to pipe the output from this command.





No DB::DB routine defined




(F) The currently executing code was compiled with the -d switch, but
for some reason the current debugger (e.g. perl5db.pl or a Devel::
module) didn’t define a routine to be called at the beginning of each
statement.





No dbm on this machine




(P) This is counted as an internal error, because every machine should
supply dbm nowadays, because Perl comes with SDBM.  See (SDBM_File).





No DB::sub routine defined




(F) The currently executing code was compiled with the -d switch, but
for some reason the current debugger (e.g. perl5db.pl or a Devel::
module) didn’t define a DB::sub routine to be called at the beginning
of each ordinary subroutine call.





No directory specified for -I




(F) The -I command-line switch requires a directory name as part of the
same argument.  Use -Ilib, for instance.  -I lib won’t work.





No error file after 2> or 2>> on command line




(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a ’2>’ or a ’2>>’ on the command line, but can’t
find the name of the file to which to write data destined for stderr.





No group ending character ’%c’ found in template




(F) A pack or unpack template has an opening ’(’ or ’[’ without its
matching counterpart.  See (perlfunc)pack.





No input file after < on command line




(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a ’<’ on the command line, but can’t find the
name of the file from which to read data for stdin.





No next::method ’%s’ found for %s




(F) next::method found no further instances of this method name
in the remaining packages of the MRO of this class.  If you don’t want
it throwing an exception, use maybe::next::method
or next::can.  See (mro).





"no" not allowed in expression




(F) The "no" keyword is recognized and executed at compile time, and
returns no useful value.  See perlmod NAME.





No output file after > on command line




(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a lone ’>’ at the end of the command line, so it
doesn’t know where you wanted to redirect stdout.





No output file after > or >> on command line




(F) An error peculiar to VMS.  Perl handles its own command line
redirection, and found a ’>’ or a ’>>’ on the command line, but can’t
find the name of the file to which to write data destined for stdout.





No package name allowed for variable %s in "our"




(F) Fully qualified variable names are not allowed in "our"
declarations, because that doesn’t make much sense under existing
semantics.  Such syntax is reserved for future extensions.





No Perl script found in input




(F) You called perl -x, but no line was found in the file beginning
with #! and containing the word "perl".





No setregid available




(F) Configure didn’t find anything resembling the setregid() call for
your system.





No setreuid available




(F) Configure didn’t find anything resembling the setreuid() call for
your system.





No such class field "%s" in variable %s of type %s




(F) You tried to access a key from a hash through the indicated typed
variable but that key is not allowed by the package of the same type.
The indicated package has restricted the set of allowed keys using the
(fields) pragma.





No such class %s




(F) You provided a class qualifier in a "my", "our" or "state"
declaration, but this class doesn’t exist at this point in your program.





No such hook: %s




(F) You specified a signal hook that was not recognized by Perl.
Currently, Perl accepts __DIE__ and __WARN__ as valid signal hooks.





No such pipe open




(P) An error peculiar to VMS.  The internal routine my_pclose() tried to
close a pipe which hadn’t been opened.  This should have been caught
earlier as an attempt to close an unopened filehandle.





No such signal: SIG%s




(W signal) You specified a signal name as a subscript to %SIG that was
not recognized.  Say kill -l in your shell to see the valid signal
names on your system.





Not a CODE reference




(F) Perl was trying to evaluate a reference to a code value (that is, a
subroutine), but found a reference to something else instead.  You can
use the ref() function to find out what kind of ref it really was.  See
also (perlref).





Not a format reference




(F) I’m not sure how you managed to generate a reference to an anonymous
format, but this indicates you did, and that it didn’t exist.





Not a GLOB reference




(F) Perl was trying to evaluate a reference to a "typeglob" (that is, a
symbol table entry that looks like *foo), but found a reference to
something else instead.  You can use the ref() function to find out what
kind of ref it really was.  See (perlref).





Not a HASH reference




(F) Perl was trying to evaluate a reference to a hash value, but found a
reference to something else instead.  You can use the ref() function to
find out what kind of ref it really was.  See (perlref).





Not an ARRAY reference




(F) Perl was trying to evaluate a reference to an array value, but found
a reference to something else instead.  You can use the ref() function
to find out what kind of ref it really was.  See (perlref).





Not an unblessed ARRAY reference




(F) You passed a reference to a blessed array to push, shift or
another array function.  These only accept unblessed array references
or arrays beginning explicitly with @.





Not a SCALAR reference




(F) Perl was trying to evaluate a reference to a scalar value, but found
a reference to something else instead.  You can use the ref() function
to find out what kind of ref it really was.  See (perlref).





Not a subroutine reference




(F) Perl was trying to evaluate a reference to a code value (that is, a
subroutine), but found a reference to something else instead.  You can
use the ref() function to find out what kind of ref it really was.  See
also (perlref).





Not a subroutine reference in overload table




(F) An attempt was made to specify an entry in an overloading table that
doesn’t somehow point to a valid subroutine.  See (overload).





Not enough arguments for %s




(F) The function requires more arguments than you specified.





Not enough format arguments




(W syntax) A format specified more picture fields than the next line
supplied.  See (perlform).





%s: not found




(A) You’ve accidentally run your script through the Bourne shell instead
of Perl.  Check the #! line, or manually feed your script into Perl
yourself.





no UTC offset information; assuming local time is UTC




(S) A warning peculiar to VMS.  Perl was unable to find the local
timezone offset, so it’s assuming that local system time is equivalent
to UTC.  If it’s not, define the logical name
SYS$TIMEZONE_DIFFERENTIAL to translate to the number of seconds which
need to be added to UTC to get local time.





Non-octal character ’%c’.  Resolved as "%s"




(W digit) In parsing an octal numeric constant, a character was
unexpectedly encountered that isn’t octal.  The resulting value
is as indicated.





Non-string passed as bitmask




(W misc) A number has been passed as a bitmask argument to select().
Use the vec() function to construct the file descriptor bitmasks for
select.  See (perlfunc)select.





Null filename used




(F) You can’t require the null filename, especially because on many
machines that means the current directory!  See (perlfunc)require.





NULL OP IN RUN




(S debugging) Some internal routine called run() with a null opcode
pointer.





Null picture in formline




(F) The first argument to formline must be a valid format picture
specification.  It was found to be empty, which probably means you
supplied it an uninitialized value.  See (perlform).





Null realloc




(P) An attempt was made to realloc NULL.





NULL regexp argument




(P) The internal pattern matching routines blew it big time.





NULL regexp parameter




(P) The internal pattern matching routines are out of their gourd.





Number too long




(F) Perl limits the representation of decimal numbers in programs to
about 250 characters.  You’ve exceeded that length.  Future
versions of Perl are likely to eliminate this arbitrary limitation.  In
the meantime, try using scientific notation (e.g. "1e6" instead of
"1_000_000").





Number with no digits




(F) Perl was looking for a number but found nothing that looked like
a number.  This happens, for example with \o{}, with no number between
the braces.





Octal number > 037777777777 non-portable




(W portable) The octal number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
perlport NAME for more on portability concerns.





Odd number of arguments for overload::constant




(W overload) The call to overload::constant contained an odd number of
arguments.  The arguments should come in pairs.





Odd number of elements in anonymous hash




(W misc) You specified an odd number of elements to initialize a hash,
which is odd, because hashes come in key/value pairs.





Odd number of elements in hash assignment




(W misc) You specified an odd number of elements to initialize a hash,
which is odd, because hashes come in key/value pairs.





Offset outside string




(F)(W layer) You tried to do a read/write/send/recv/seek operation
with an offset pointing outside the buffer.  This is difficult to
imagine.  The sole exceptions to this are that zero padding will
take place when going past the end of the string when either
sysread()ing a file, or when seeking past the end of a scalar opened
for I/O (in anticipation of future reads and to imitate the behaviour
with real files).





%s() on unopened %s




(W unopened) An I/O operation was attempted on a filehandle that was
never initialized.  You need to do an open(), a sysopen(), or a socket()
call, or call a constructor from the FileHandle package.





-%s on unopened filehandle %s




(W unopened) You tried to invoke a file test operator on a filehandle
that isn’t open.  Check your control flow.  See also (perlfunc)-X.





oops: oopsAV




(S internal) An internal warning that the grammar is screwed up.





oops: oopsHV




(S internal) An internal warning that the grammar is screwed up.





Opening dirhandle %s also as a file




(W io, deprecated) You used open() to associate a filehandle to
a symbol (glob or scalar) that already holds a dirhandle.
Although legal, this idiom might render your code confusing
and is deprecated.





Opening filehandle %s also as a directory




(W io, deprecated) You used opendir() to associate a dirhandle to
a symbol (glob or scalar) that already holds a filehandle.
Although legal, this idiom might render your code confusing
and is deprecated.





Operation "%s": no method found, %s




(F) An attempt was made to perform an overloaded operation for which no
handler was defined.  While some handlers can be autogenerated in terms
of other handlers, there is no default handler for any operation, unless
the fallback overloading key is specified to be true.  See (overload).





Operation "%s" returns its argument for non-Unicode code point 0x%X




(W utf8, non_unicode) You performed an operation requiring Unicode
semantics on a code point that is not in Unicode, so what it should do
is not defined.  Perl has chosen to have it do nothing, and warn you.



If the operation shown is "ToFold", it means that case-insensitive
matching in a regular expression was done on the code point.



If you know what you are doing you can turn off this warning by
no warnings 'non_unicode';.





Operation "%s" returns its argument for UTF-16 surrogate U+%X




(W utf8, surrogate) You performed an operation requiring Unicode
semantics on a Unicode surrogate.  Unicode frowns upon the use of
surrogates for anything but storing strings in UTF-16, but semantics
are (reluctantly) defined for the surrogates, and they are to do
nothing for this operation.  Because the use of surrogates can be
dangerous, Perl warns.



If the operation shown is "ToFold", it means that case-insensitive
matching in a regular expression was done on the code point.



If you know what you are doing you can turn off this warning by
no warnings 'surrogate';.





Operator or semicolon missing before %s




(S ambiguous) You used a variable or subroutine call where the parser
was expecting an operator.  The parser has assumed you really meant to
use an operator, but this is highly likely to be incorrect.  For
example, if you say "*foo *foo" it will be interpreted as if you said
"*foo * ’foo’".





"our" variable %s redeclared




(W misc) You seem to have already declared the same global once before
in the current lexical scope.





Out of memory!




(X) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request.  Perl has
no option but to exit immediately.



At least in Unix you may be able to get past this by increasing your
process datasize limits: in csh/tcsh use limit and
limit datasize n (where n is the number of kilobytes) to check
the current limits and change them, and in ksh/bash/zsh use ulimit -a
and ulimit -d n, respectively.





Out of memory during %s extend




(X) An attempt was made to extend an array, a list, or a string beyond
the largest possible memory allocation.





Out of memory during "large" request for %s




(F) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request.  However,
the request was judged large enough (compile-time default is 64K), so a
possibility to shut down by trapping this error is granted.





Out of memory during request for %s




(X)(F) The malloc() function returned 0, indicating there was
insufficient remaining memory (or virtual memory) to satisfy the
request.



The request was judged to be small, so the possibility to trap it
depends on the way perl was compiled.  By default it is not trappable.
However, if compiled for this, Perl may use the contents of $^M as an
emergency pool after die()ing with this message.  In this case the error
is trappable once, and the error message will include the line and file
where the failed request happened.





Out of memory during ridiculously large request




(F) You can’t allocate more than 2^31+"small amount" bytes.  This error
is most likely to be caused by a typo in the Perl program. e.g.,
$arr[time] instead of $arr[$time].





Out of memory for yacc stack




(F) The yacc parser wanted to grow its stack so it could continue
parsing, but realloc() wouldn’t give it more memory, virtual or
otherwise.





’.’ outside of string in pack




(F) The argument to a ’.’ in your template tried to move the working
position to before the start of the packed string being built.





’@’ outside of string in unpack




(F) You had a template that specified an absolute position outside
the string being unpacked.  See (perlfunc)pack.





’@’ outside of string with malformed UTF-8 in unpack




(F) You had a template that specified an absolute position outside
the string being unpacked.  The string being unpacked was also invalid
UTF-8.  See (perlfunc)pack.





overload arg ’%s’ is invalid




(W overload) The (overload) pragma was passed an argument it did not
recognize.  Did you mistype an operator?





Overloaded dereference did not return a reference




(F) An object with an overloaded dereference operator was dereferenced,
but the overloaded operation did not return a reference.  See
(overload).





Overloaded qr did not return a REGEXP




(F) An object with a qr overload was used as part of a match, but the
overloaded operation didn’t return a compiled regexp.  See (overload).





%s package attribute may clash with future reserved word: %s




(W reserved) A lowercase attribute name was used that had a
package-specific handler.  That name might have a meaning to Perl itself
some day, even though it doesn’t yet.  Perhaps you should use a
mixed-case attribute name, instead.  See (attributes).





pack/unpack repeat count overflow




(F) You can’t specify a repeat count so large that it overflows your
signed integers.  See (perlfunc)pack.





page overflow




(W io) A single call to write() produced more lines than can fit on a
page.  See (perlform).





panic: %s




(P) An internal error.





panic: attempt to call %s in %s




(P) One of the file test operators entered a code branch that calls
an ACL related-function, but that function is not available on this
platform.  Earlier checks mean that it should not be possible to
enter this branch on this platform.





panic: ck_grep, type=%u




(P) Failed an internal consistency check trying to compile a grep.





panic: ck_split, type=%u




(P) Failed an internal consistency check trying to compile a split.





panic: corrupt saved stack index %ld




(P) The savestack was requested to restore more localized values than
there are in the savestack.





panic: del_backref




(P) Failed an internal consistency check while trying to reset a weak
reference.





panic: die %s




(P) We popped the context stack to an eval context, and then discovered
it wasn’t an eval context.





panic: do_subst




(P) The internal pp_subst() routine was called with invalid operational
data.





panic: do_trans_%s




(P) The internal do_trans routines were called with invalid operational
data.





panic: fold_constants JMPENV_PUSH returned %d




(P) While attempting folding constants an exception other than an eval
failure was caught.





panic: frexp




(P) The library function frexp() failed, making printf("%f") impossible.





panic: goto, type=%u, ix=%ld




(P) We popped the context stack to a context with the specified label,
and then discovered it wasn’t a context we know how to do a goto in.





panic: gp_free failed to free glob pointer




(P) The internal routine used to clear a typeglob’s entries tried
repeatedly, but each time something re-created entries in the glob.
Most likely the glob contains an object with a reference back to
the glob and a destructor that adds a new object to the glob.





panic: INTERPCASEMOD, %s




(P) The lexer got into a bad state at a case modifier.





panic: INTERPCONCAT, %s




(P) The lexer got into a bad state parsing a string with brackets.





panic: kid popen errno read




(F) forked child returned an incomprehensible message about its errno.





panic: last, type=%u




(P) We popped the context stack to a block context, and then discovered
it wasn’t a block context.





panic: leave_scope clearsv




(P) A writable lexical variable became read-only somehow within the
scope.





panic: leave_scope inconsistency %u




(P) The savestack probably got out of sync.  At least, there was an
invalid enum on the top of it.





panic: magic_killbackrefs




(P) Failed an internal consistency check while trying to reset all weak
references to an object.





panic: malloc, %s




(P) Something requested a negative number of bytes of malloc.





panic: memory wrap




(P) Something tried to allocate more memory than possible.





panic: pad_alloc, %p!=%p




(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.





panic: pad_free curpad, %p!=%p




(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.





panic: pad_free po




(P) An invalid scratch pad offset was detected internally.





panic: pad_reset curpad, %p!=%p




(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.





panic: pad_sv po




(P) An invalid scratch pad offset was detected internally.





panic: pad_swipe curpad, %p!=%p




(P) The compiler got confused about which scratch pad it was allocating
and freeing temporaries and lexicals from.





panic: pad_swipe po




(P) An invalid scratch pad offset was detected internally.





panic: pp_iter, type=%u




(P) The foreach iterator got called in a non-loop context frame.





panic: pp_match%s




(P) The internal pp_match() routine was called with invalid operational
data.





panic: pp_split, pm=%p, s=%p




(P) Something terrible went wrong in setting up for the split.





panic: realloc, %s




(P) Something requested a negative number of bytes of realloc.





panic: reference miscount on nsv in sv_replace() (%d != 1)




(P) The internal sv_replace() function was handed a new SV with a
reference count other than 1.





panic: restartop in %s




(P) Some internal routine requested a goto (or something like it), and
didn’t supply the destination.





panic: return, type=%u




(P) We popped the context stack to a subroutine or eval context, and
then discovered it wasn’t a subroutine or eval context.





panic: scan_num, %s




(P) scan_num() got called on something that wasn’t a number.





panic: sv_chop %s




(P) The sv_chop() routine was passed a position that is not within the
scalar’s string buffer.





panic: sv_insert, midend=%p, bigend=%p




(P) The sv_insert() routine was told to remove more string than there
was string.





panic: strxfrm() gets absurd - a => %u, ab => %u




(P) The interpreter’s sanity check of the C function strxfrm() failed.
In your current locale the returned transformation of the string "ab" is
shorter than that of the string "a", which makes no sense.





panic: top_env




(P) The compiler attempted to do a goto, or something weird like that.





panic: unimplemented op %s (#%d) called




(P) The compiler is screwed up and attempted to use an op that isn’t
permitted at run time.





panic: utf16_to_utf8: odd bytelen




(P) Something tried to call utf16_to_utf8 with an odd (as opposed
to even) byte length.





panic: utf16_to_utf8_reversed: odd bytelen




(P) Something tried to call utf16_to_utf8_reversed with an odd (as opposed
to even) byte length.





panic: yylex, %s




(P) The lexer got into a bad state while processing a case modifier.





Parsing code internal error (%s)




(F) Parsing code supplied by an extension violated the parser’s API in
a detectable way.





Pattern subroutine nesting without pos change exceeded limit in regex; marked by <– HERE in m/%s/




(F) You used a pattern that uses too many nested subpattern calls without
consuming any text.  Restructure the pattern so text is consumed before
the nesting limit is exceeded.



The <– HERE shows in the regular expression about where the problem was
discovered.





Parentheses missing around "%s" list




(W parenthesis) You said something like



    my $foo, $bar = @_;



when you meant



    my ($foo, $bar) = @_;



Remember that "my", "our", "local" and "state" bind tighter than comma.





-p destination: %s




(F) An error occurred during the implicit output invoked by the -p
command-line switch.  (This output goes to STDOUT unless you’ve
redirected it with select().)





(perhaps you forgot to load "%s"?)




(F) This is an educated guess made in conjunction with the message
"Can’t locate object method \"%s\" via package \"%s\"".  It often means
that a method requires a package that has not been loaded.





Perl folding rules are not up-to-date for 0x%x; please use the perlbug utility to report




(W regex, deprecated) You used a regular expression with
case-insensitive matching, and there is a bug in Perl in which the
built-in regular expression folding rules are not accurate.  This may
lead to incorrect results.  Please report this as a bug using the
"perlbug" utility.  (This message is marked deprecated, so that it by
default will be turned-on.)





Perl_my_%s() not available




(F) Your platform has very uncommon byte-order and integer size,
so it was not possible to set up some or all fixed-width byte-order
conversion functions.  This is only a problem when you’re using the
’<’ or ’>’ modifiers in (un)pack templates.  See (perlfunc)pack.





Perl %s required (did you mean %s?)–this is only %s, stopped




(F) The code you are trying to run has asked for a newer version of
Perl than you are running.  Perhaps use 5.10 was written instead
of use 5.010 or use v5.10.  Without the leading v, the number is
interpreted as a decimal, with every three digits after the
decimal point representing a part of the version number.  So 5.10
is equivalent to v5.100.





Perl %s required–this is only version %s, stopped




(F) The module in question uses features of a version of Perl more
recent than the currently running version.  How long has it been since
you upgraded, anyway?  See (perlfunc)require.





PERL_SH_DIR too long




(F) An error peculiar to OS/2.  PERL_SH_DIR is the directory to find the
sh-shell in.  See "PERL_SH_DIR" in (perlos2).





PERL_SIGNALS illegal: "%s"




See perlrun PERL_SIGNALS for legal values.





Perls since %s too modern–this is %s, stopped




(F) The code you are trying to run claims it will not run
on the version of Perl you are using because it is too new.
Maybe the code needs to be updated, or maybe it is simply
wrong and the version check should just be removed.





perl: warning: Setting locale failed.




(S) The whole warning message will look something like:



        perl: warning: Setting locale failed.
        perl: warning: Please check that your locale settings:
                LC_ALL = "En_US",
                LANG = (unset)
            are supported and installed on your system.
        perl: warning: Falling back to the standard locale ("C").



Exactly what were the failed locale settings varies.  In the above the
settings were that the LC_ALL was "En_US" and the LANG had no value.
This error means that Perl detected that you and/or your operating
system supplier and/or system administrator have set up the so-called
locale system but Perl could not use those settings.  This was not
dead serious, fortunately: there is a "default locale" called "C" that
Perl can and will use, and the script will be run.  Before you really
fix the problem, however, you will get the same error message each
time you run Perl.  How to really fix the problem can be found in
perllocale NAME section LOCALE PROBLEMS.





pid %x not a child




(W exec) A warning peculiar to VMS.  Waitpid() was asked to wait for a
process which isn’t a subprocess of the current process.  While this is
fine from VMS’ perspective, it’s probably not what you intended.





’P’ must have an explicit size in unpack




(F) The unpack format P must have an explicit size, not "*".





POSIX class [:%s:] unknown in regex; marked by <– HERE in m/%s/




(F) The class in the character class [: :] syntax is unknown.  The <– HERE
shows in the regular expression about where the problem was discovered.
Note that the POSIX character classes do not have the is prefix
the corresponding C interfaces have: in other words, it’s [[:print:]],
not isprint.  See (perlre).





POSIX getpgrp can’t take an argument




(F) Your system has POSIX getpgrp(), which takes no argument, unlike
the BSD version, which takes a pid.





POSIX syntax [%s] belongs inside character classes in regex; marked by <– HERE in m/%s/




(W regexp) The character class constructs [: :], [= =], and [. .]  go
inside character classes, the [] are part of the construct, for example:
/[012[:alpha:]345]/.  Note that [= =] and [. .] are not currently
implemented; they are simply placeholders for future extensions and will
cause fatal errors.  The <– HERE shows in the regular expression about
where the problem was discovered.  See (perlre).





POSIX syntax [. .] is reserved for future extensions in regex; marked by <– HERE in m/%s/




(F regexp) Within regular expression character classes ([]) the syntax
beginning with "[." and ending with ".]" is reserved for future extensions.
If you need to represent those character sequences inside a regular
expression character class, just quote the square brackets with the
backslash: "\[." and ".\]".  The <– HERE shows in the regular expression
about where the problem was discovered.  See (perlre).





POSIX syntax [= =] is reserved for future extensions in regex; marked by <– HERE in m/%s/




(F) Within regular expression character classes ([]) the syntax beginning
with "[=" and ending with "=]" is reserved for future extensions.  If you
need to represent those character sequences inside a regular expression
character class, just quote the square brackets with the backslash: "\[="
and "=\]".  The <– HERE shows in the regular expression about where the
problem was discovered.  See (perlre).





Possible attempt to put comments in qw() list




(W qw) qw() lists contain items separated by whitespace; as with literal
strings, comment characters are not ignored, but are instead treated as
literal data.  (You may have used different delimiters than the
parentheses shown here; braces are also frequently used.)



You probably wrote something like this:



    @list = qw(
        a # a comment
        b # another comment
    );



when you should have written this:



    @list = qw(
        a
        b
    );



If you really want comments, build your list the
old-fashioned way, with quotes and commas:



    @list = (
        'a',    # a comment
        'b',    # another comment
    );





Possible attempt to separate words with commas




(W qw) qw() lists contain items separated by whitespace; therefore
commas aren’t needed to separate the items.  (You may have used
different delimiters than the parentheses shown here; braces are also
frequently used.)



You probably wrote something like this:



    qw! a, b, c !;



which puts literal commas into some of the list items.  Write it without
commas if you don’t want them to appear in your data:



    qw! a b c !;





Possible memory corruption: %s overflowed 3rd argument




(F) An ioctl() or fcntl() returned more than Perl was bargaining for.
Perl guesses a reasonable buffer size, but puts a sentinel byte at the
end of the buffer just in case.  This sentinel byte got clobbered, and
Perl assumes that memory is now corrupted.  See (perlfunc)ioctl.





Possible precedence problem on bitwise %c operator




(W precedence) Your program uses a bitwise logical operator in conjunction
with a numeric comparison operator, like this :



    if ($x & $y == 0) { ... }



This expression is actually equivalent to $x & ($y == 0), due to the
higher precedence of ==.  This is probably not what you want.  (If you
really meant to write this, disable the warning, or, better, put the
parentheses explicitly and write $x & ($y == 0)).





Possible unintended interpolation of $\ in regex




(W ambiguous) You said something like m/$\/ in a regex.
The regex m/foo$\s+bar/m translates to: match the word ’foo’, the output
record separator (see perlvar $\) and the letter ’s’ (one time or more)
followed by the word ’bar’.



If this is what you intended then you can silence the warning by using 
m/${\}/ (for example: m/foo${\}s+bar/).



If instead you intended to match the word ’foo’ at the end of the line
followed by whitespace and the word ’bar’ on the next line then you can use
m/$(?)\/ (for example: m/foo$(?)\s+bar/).





Possible unintended interpolation of %s in string




(W ambiguous) You said something like ’@foo’ in a double-quoted string
but there was no array @foo in scope at the time.  If you wanted a
literal @foo, then write it as \@foo; otherwise find out what happened
to the array you apparently lost track of.





Precedence problem: open %s should be open(%s)




(S precedence) The old irregular construct



    open FOO || die;



is now misinterpreted as



    open(FOO || die);



because of the strict regularization of Perl 5’s grammar into unary and
list operators.  (The old open was a little of both.)  You must put
parentheses around the filehandle, or use the new "or" operator instead
of "||".





Premature end of script headers




See Server error.





printf() on closed filehandle %s




(W closed) The filehandle you’re writing to got itself closed sometime
before now.  Check your control flow.





print() on closed filehandle %s




(W closed) The filehandle you’re printing on got itself closed sometime
before now.  Check your control flow.





Process terminated by SIG%s




(W) This is a standard message issued by OS/2 applications, while *nix
applications die in silence.  It is considered a feature of the OS/2
port.  One can easily disable this by appropriate sighandlers, see
perlipc Signals.  See also "Process terminated by SIGTERM/SIGINT"
in (perlos2).





Prototype after ’%c’ for %s : %s




(W illegalproto) A character follows % or @ in a prototype.  This is
useless, since % and @ gobble the rest of the subroutine arguments.





Prototype mismatch: %s vs %s




(S prototype) The subroutine being declared or defined had previously been
declared or defined with a different function prototype.





Prototype not terminated




(F) You’ve omitted the closing parenthesis in a function prototype
definition.





\p{} uses Unicode rules, not locale rules




(W) You compiled a regular expression that contained a Unicode property
match (\p or \P), but the regular expression is also being told to
use the run-time locale, not Unicode.  Instead, use a POSIX character
class, which should know about the locale’s rules.
(See (perlrecharclass)POSIX Character Classes.)



Even if the run-time locale is ISO 8859-1 (Latin1), which is a subset of
Unicode, some properties will give results that are not valid for that
subset.



Here are a couple of examples to help you see what’s going on.  If the
locale is ISO 8859-7, the character at code point 0xD7 is the "GREEK
CAPITAL LETTER CHI".  But in Unicode that code point means the
"MULTIPLICATION SIGN" instead, and \p always uses the Unicode
meaning.  That means that \p{Alpha} won’t match, but [[:alpha:]]
should.  Only in the Latin1 locale are all the characters in the same
positions as they are in Unicode.  But, even here, some properties give
incorrect results.  An example is \p{Changes_When_Uppercased} which
is true for "LATIN SMALL LETTER Y WITH DIAERESIS", but since the upper
case of that character is not in Latin1, in that locale it doesn’t
change when upper cased.





Quantifier follows nothing in regex; marked by <– HERE in m/%s/




(F) You started a regular expression with a quantifier.  Backslash it if
you meant it literally.  The <– HERE shows in the regular expression
about where the problem was discovered.  See (perlre).





Quantifier in {,} bigger than %d in regex; marked by <– HERE in m/%s/




(F) There is currently a limit to the size of the min and max values of
the {min,max} construct.  The <– HERE shows in the regular expression
about where the problem was discovered.  See (perlre).





Quantifier unexpected on zero-length expression; marked by <– HERE in m/%s/




(W regexp) You applied a regular expression quantifier in a place where
it makes no sense, such as on a zero-width assertion.  Try putting the
quantifier inside the assertion instead.  For example, the way to match
"abc" provided that it is followed by three repetitions of "xyz" is
/abc(?=(?:xyz){3})/, not /abc(?=xyz){3}/.



The <– HERE shows in the regular expression about where the problem was
discovered.





Range iterator outside integer range




(F) One (or both) of the numeric arguments to the range operator ".."
are outside the range which can be represented by integers internally.
One possible workaround is to force Perl to use magical string increment
by prepending "0" to your numbers.





readdir() attempted on invalid dirhandle %s




(W io) The dirhandle you’re reading from is either closed or not really
a dirhandle.  Check your control flow.





readline() on closed filehandle %s




(W closed) The filehandle you’re reading from got itself closed sometime
before now.  Check your control flow.





read() on closed filehandle %s




(W closed) You tried to read from a closed filehandle.





read() on unopened filehandle %s




(W unopened) You tried to read from a filehandle that was never opened.





Reallocation too large: %x




(F) You can’t allocate more than 64K on an MS-DOS machine.





realloc() of freed memory ignored




(S malloc) An internal routine called realloc() on something that had
already been freed.





Recompile perl with -DDEBUGGING to use -D switch




(F debugging) You can’t use the -D option unless the code to produce
the desired output is compiled into Perl, which entails some overhead,
which is why it’s currently left out of your copy.





Recursive call to Perl_load_module in PerlIO_find_layer




(P) It is currently not permitted to load modules when creating
a filehandle inside an %INC hook.  This can happen with open my
$fh, '<', \$scalar, which implicitly loads PerlIO::scalar.  Try
loading PerlIO::scalar explicitly first.





Recursive inheritance detected in package ’%s’




(F) While calculating the method resolution order (MRO) of a package, Perl
believes it found an infinite loop in the @ISA hierarchy.  This is a
crude check that bails out after 100 levels of @ISA depth.





refcnt_dec: fd %d%s






refcnt: fd %d%s






refcnt_inc: fd %d%s




(P) Perl’s I/O implementation failed an internal consistency check.  If
you see this message, something is very wrong.





Reference found where even-sized list expected




(W misc) You gave a single reference where Perl was expecting a list
with an even number of elements (for assignment to a hash).  This
usually means that you used the anon hash constructor when you meant
to use parens.  In any case, a hash requires key/value pairs.



    %hash = { one => 1, two => 2, };    # WRONG
    %hash = [ qw/ an anon array / ];    # WRONG
    %hash = ( one => 1, two => 2, );    # right
    %hash = qw( one 1 two 2 );                  # also fine





Reference is already weak




(W misc) You have attempted to weaken a reference that is already weak.
Doing so has no effect.





Reference to invalid group 0




(F) You used \g0 or similar in a regular expression.  You may refer
to capturing parentheses only with strictly positive integers
(normal backreferences) or with strictly negative integers (relative
backreferences).  Using 0 does not make sense.





Reference to nonexistent group in regex; marked by <– HERE in m/%s/




(F) You used something like \7 in your regular expression, but there are
not at least seven sets of capturing parentheses in the expression.  If
you wanted to have the character with ordinal 7 inserted into the regular
expression, prepend zeroes to make it three digits long: \007



The <– HERE shows in the regular expression about where the problem was
discovered.





Reference to nonexistent named group in regex; marked by <– HERE in m/%s/




(F) You used something like \k'NAME' or \k<NAME> in your regular
expression, but there is no corresponding named capturing parentheses
such as (?'NAME'...) or (?<NAME>...).  Check if the name has been
spelled correctly both in the backreference and the declaration.



The <– HERE shows in the regular expression about where the problem was
discovered.





Reference to nonexistent or unclosed group in regex; marked by <– HERE in m/%s/




(F) You used something like \g{-7} in your regular expression, but there
are not at least seven sets of closed capturing parentheses in the
expression before where the \g{-7} was located.



The <– HERE shows in the regular expression about where the problem was
discovered.





regexp memory corruption




(P) The regular expression engine got confused by what the regular
expression compiler gave it.





Regexp modifier "/%c" may appear a maximum of twice






Regexp modifier "/%c" may not appear twice




(F syntax, regexp) The regular expression pattern had too many occurrences
of the specified modifier.  Remove the extraneous ones.





Regexp modifier "%c" may not appear after the "-"




(F regexp) Turning off the given modifier has the side effect of turning
on another one.  Perl currently doesn’t allow this.  Reword the regular
expression to use the modifier you want to turn on (and place it before
the minus), instead of the one you want to turn off.





Regexp modifiers "/%c" and "/%c" are mutually exclusive




(F syntax, regexp) The regular expression pattern had more than one of these
mutually exclusive modifiers.  Retain only the modifier that is
supposed to be there.





Regexp out of space




(P) A "can’t happen" error, because safemalloc() should have caught it
earlier.





Repeated format line will never terminate (~~ and @# incompatible)




(F) Your format contains the ~~ repeat-until-blank sequence and a
numeric field that will never go blank so that the repetition never
terminates.  You might use ^# instead.  See (perlform).





Replacement list is longer than search list




(W misc) You have used a replacement list that is longer than the
search list.  So the additional elements in the replacement list
are meaningless.





Reversed %s= operator




(W syntax) You wrote your assignment operator backwards.  The = must
always come last, to avoid ambiguity with subsequent unary operators.





rewinddir() attempted on invalid dirhandle %s




(W io) The dirhandle you tried to do a rewinddir() on is either closed or not
really a dirhandle.  Check your control flow.





Scalars leaked: %d




(P) Something went wrong in Perl’s internal bookkeeping of scalars:
not all scalar variables were deallocated by the time Perl exited.
What this usually indicates is a memory leak, which is of course bad,
especially if the Perl program is intended to be long-running.





Scalar value @%s[%s] better written as $%s[%s]




(W syntax) You’ve used an array slice (indicated by @) to select a
single element of an array.  Generally it’s better to ask for a scalar
value (indicated by $).  The difference is that $foo[&bar] always
behaves like a scalar, both when assigning to it and when evaluating its
argument, while @foo[&bar] behaves like a list when you assign to it,
and provides a list context to its subscript, which can do weird things
if you’re expecting only one subscript.



On the other hand, if you were actually hoping to treat the array
element as a list, you need to look into how references work, because
Perl will not magically convert between scalars and lists for you.  See
(perlref).





Scalar value @%s{%s} better written as $%s{%s}




(W syntax) You’ve used a hash slice (indicated by @) to select a single
element of a hash.  Generally it’s better to ask for a scalar value
(indicated by $).  The difference is that $foo{&bar} always behaves
like a scalar, both when assigning to it and when evaluating its
argument, while @foo{&bar} behaves like a list when you assign to it,
and provides a list context to its subscript, which can do weird things
if you’re expecting only one subscript.



On the other hand, if you were actually hoping to treat the hash element
as a list, you need to look into how references work, because Perl will
not magically convert between scalars and lists for you.  See
(perlref).





Search pattern not terminated




(F) The lexer couldn’t find the final delimiter of a // or m{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading $ from a variable $m may cause this error.



Note that since Perl 5.9.0 a // can also be the defined-or
construct, not just the empty search pattern.  Therefore code written
in Perl 5.9.0 or later that uses the // as the defined-or can be
misparsed by pre-5.9.0 Perls as a non-terminated search pattern.





Search pattern not terminated or ternary operator parsed as search pattern




(F) The lexer couldn’t find the final delimiter of a ?PATTERN?
construct.



The question mark is also used as part of the ternary operator (as in
foo ? 0 : 1) leading to some ambiguous constructions being wrongly
parsed.  One way to disambiguate the parsing is to put parentheses around
the conditional expression, i.e. (foo) ? 0 : 1.





seekdir() attempted on invalid dirhandle %s




(W io) The dirhandle you are doing a seekdir() on is either closed or not
really a dirhandle.  Check your control flow.





%sseek() on unopened filehandle




(W unopened) You tried to use the seek() or sysseek() function on a
filehandle that was either never opened or has since been closed.





select not implemented




(F) This machine doesn’t implement the select() system call.





Self-ties of arrays and hashes are not supported




(F) Self-ties are of arrays and hashes are not supported in
the current implementation.





Semicolon seems to be missing




(W semicolon) A nearby syntax error was probably caused by a missing
semicolon, or possibly some other missing operator, such as a comma.





semi-panic: attempt to dup freed string




(S internal) The internal newSVsv() routine was called to duplicate a
scalar that had previously been marked as free.





sem%s not implemented




(F) You don’t have System V semaphore IPC on your system.





send() on closed socket %s




(W closed) The socket you’re sending to got itself closed sometime
before now.  Check your control flow.





Sequence (? incomplete in regex; marked by <– HERE in m/%s/




(F) A regular expression ended with an incomplete extension (?.  The
<– HERE shows in the regular expression about where the problem was
discovered.  See (perlre).





Sequence (?%s...) not implemented in regex; marked by <– HERE in m/%s/




(F) A proposed regular expression extension has the character reserved
but has not yet been written.  The <– HERE shows in the regular
expression about where the problem was discovered.  See (perlre).





Sequence (?%s...) not recognized in regex; marked by <– HERE in m/%s/




(F) You used a regular expression extension that doesn’t make sense.  The
<– HERE shows in the regular expression about where the problem was
discovered.  This happens when using the (?^...) construct to tell
Perl to use the default regular expression modifiers, and you
redundantly specify a default modifier.  For other
causes, see (perlre).





Sequence \%s... not terminated in regex; marked by <– HERE in m/%s/




(F) The regular expression expects a mandatory argument following the escape
sequence and this has been omitted or incorrectly written.





Sequence (?#... not terminated in regex; marked by <– HERE in m/%s/




(F) A regular expression comment must be terminated by a closing
parenthesis.  Embedded parentheses aren’t allowed.  The <– HERE shows in
the regular expression about where the problem was discovered.  See
(perlre).





Sequence (?{...}) not terminated or not {}-balanced in regex; marked by <– HERE in m/%s/




(F) If the contents of a (?{...}) clause contain braces, they
must balance for Perl to detect the end of the clause properly.
The <– HERE shows in the regular expression about where the
problem was discovered.  See (perlre).





500 Server error




See Server error.





Server error




(A) This is the error message generally seen in a browser window
when trying to run a CGI program (including SSI) over the web.  The
actual error text varies widely from server to server.  The most
frequently-seen variants are "500 Server error", "Method (something)
not permitted", "Document contains no data", "Premature end of script
headers", and "Did not produce a valid header".



This is a CGI error, not a Perl error.



You need to make sure your script is executable, is accessible by
the user CGI is running the script under (which is probably not the
user account you tested it under), does not rely on any environment
variables (like PATH) from the user it isn’t running under, and isn’t
in a location where the CGI server can’t find it, basically, more or
less.  Please see the following for more information:



        http://www.perl.org/CGI_MetaFAQ.html
        http://www.htmlhelp.org/faq/cgifaq.html
        http://www.w3.org/Security/Faq/



You should also look at (perlfaq9).





setegid() not implemented




(F) You tried to assign to $), and your operating system doesn’t
support the setegid() system call (or equivalent), or at least Configure
didn’t think so.





seteuid() not implemented




(F) You tried to assign to $>, and your operating system doesn’t
support the seteuid() system call (or equivalent), or at least Configure
didn’t think so.





setpgrp can’t take arguments




(F) Your system has the setpgrp() from BSD 4.2, which takes no
arguments, unlike POSIX setpgid(), which takes a process ID and process
group ID.





setrgid() not implemented




(F) You tried to assign to $(, and your operating system doesn’t
support the setrgid() system call (or equivalent), or at least Configure
didn’t think so.





setruid() not implemented




(F) You tried to assign to $<, and your operating system doesn’t
support the setruid() system call (or equivalent), or at least Configure
didn’t think so.





setsockopt() on closed socket %s




(W closed) You tried to set a socket option on a closed socket.  Did you
forget to check the return value of your socket() call?  See
(perlfunc)setsockopt.





shm%s not implemented




(F) You don’t have System V shared memory IPC on your system.





!=~ should be !~




(W syntax) The non-matching operator is !~, not !=~.  !=~ will be
interpreted as the != (numeric not equal) and ~ (1’s complement)
operators: probably not what you intended.





<> should be quotes




(F) You wrote require <file> when you should have written
require 'file'.





/%s/ should probably be written as "%s"




(W syntax) You have used a pattern where Perl expected to find a string,
as in the first argument to join.  Perl will treat the true or false
result of matching the pattern against $_ as the string, which is
probably not what you had in mind.





shutdown() on closed socket %s




(W closed) You tried to do a shutdown on a closed socket.  Seems a bit
superfluous.





SIG%s handler "%s" not defined




(W signal) The signal handler named in %SIG doesn’t, in fact, exist.
Perhaps you put it into the wrong package?





Smart matching a non-overloaded object breaks encapsulation




(F) You should not use the ~~ operator on an object that does not
overload it: Perl refuses to use the object’s underlying structure for
the smart match.





sort is now a reserved word




(F) An ancient error message that almost nobody ever runs into anymore.
But before sort was a keyword, people sometimes used it as a filehandle.





Sort subroutine didn’t return single value




(F) A sort comparison subroutine may not return a list value with more
or less than one element.  See (perlfunc)sort.





Source filters apply only to byte streams




(F) You tried to activate a source filter (usually by loading a
source filter module) within a string passed to eval.  This is
not permitted under the unicode_eval feature.  Consider using
evalbytes instead.  See (feature).





splice() offset past end of array




(W misc) You attempted to specify an offset that was past the end of
the array passed to splice().  Splicing will instead commence at the
end of the array, rather than past it.  If this isn’t what you want,
try explicitly pre-extending the array by assigning $#array = $offset.
See (perlfunc)splice.





Split loop




(P) The split was looping infinitely.  (Obviously, a split shouldn’t
iterate more times than there are characters of input, which is what
happened.)  See (perlfunc)split.





Statement unlikely to be reached




(W exec) You did an exec() with some statement after it other than a
die().  This is almost always an error, because exec() never returns
unless there was a failure.  You probably wanted to use system()
instead, which does return.  To suppress this warning, put the exec() in
a block by itself.





"state" variable %s can’t be in a package




(F) Lexically scoped variables aren’t in a package, so it doesn’t make
sense to try to declare one with a package qualifier on the front.  Use
local() if you want to localize a package variable.





stat() on unopened filehandle %s




(W unopened) You tried to use the stat() function on a filehandle that
was either never opened or has since been closed.





Stub found while resolving method "%s" overloading "%s" in package "%s"




(P) Overloading resolution over @ISA tree may be broken by importation
stubs.  Stubs should never be implicitly created, but explicit calls to
can may break this.





Subroutine %s redefined




(W redefine) You redefined a subroutine.  To suppress this warning, say



    {
        no warnings 'redefine';
        eval "sub name { ... }";
    }





Substitution loop




(P) The substitution was looping infinitely.  (Obviously, a substitution
shouldn’t iterate more times than there are characters of input, which
is what happened.)  See the discussion of substitution in
(perlop)Regexp Quote-Like Operators.





Substitution pattern not terminated




(F) The lexer couldn’t find the interior delimiter of an s/// or s{}{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading $ from variable $s may cause this error.





Substitution replacement not terminated




(F) The lexer couldn’t find the final delimiter of an s/// or s{}{}
construct.  Remember that bracketing delimiters count nesting level.
Missing the leading $ from variable $s may cause this error.





substr outside of string




(W substr)(F) You tried to reference a substr() that pointed outside of
a string.  That is, the absolute value of the offset was larger than the
length of the string.  See (perlfunc)substr.  This warning is fatal if
substr is used in an lvalue context (as the left hand side of an
assignment or as a subroutine argument for example).





sv_upgrade from type %d down to type %d




(P) Perl tried to force the upgrade of an SV to a type which was actually
inferior to its current type.





Switch (?(condition)... contains too many branches in regex; marked by <– HERE in m/%s/




(F) A (?(condition)if-clause|else-clause) construct can have at most
two branches (the if-clause and the else-clause).  If you want one or
both to contain alternation, such as using this|that|other, enclose
it in clustering parentheses:



    (?(condition)(?:this|that|other)|else-clause)



The <– HERE shows in the regular expression about where the problem
was discovered.  See (perlre).





Switch condition not recognized in regex; marked by <– HERE in m/%s/




(F) If the argument to the (?(...)if-clause|else-clause) construct is
a number, it can be only a number.  The <– HERE shows in the regular
expression about where the problem was discovered.  See (perlre).





switching effective %s is not implemented




(F) While under the use filetest pragma, we cannot switch the real
and effective uids or gids.





%s syntax OK




(F) The final summary message when a perl -c succeeds.





syntax error




(F) Probably means you had a syntax error.  Common reasons include:



    A keyword is misspelled.
    A semicolon is missing.
    A comma is missing.
    An opening or closing parenthesis is missing.
    An opening or closing brace is missing.
    A closing quote is missing.



Often there will be another error message associated with the syntax
error giving more information.  (Sometimes it helps to turn on -w.)
The error message itself often tells you where it was in the line when
it decided to give up.  Sometimes the actual error is several tokens
before this, because Perl is good at understanding random input.
Occasionally the line number may be misleading, and once in a blue moon
the only way to figure out what’s triggering the error is to call
perl -c repeatedly, chopping away half the program each time to see
if the error went away.  Sort of the cybernetic version of 20 questions.





syntax error at line %d: ’%s’ unexpected




(A) You’ve accidentally run your script through the Bourne shell instead
of Perl.  Check the #! line, or manually feed your script into Perl
yourself.





syntax error in file %s at line %d, next 2 tokens "%s"




(F) This error is likely to occur if you run a perl5 script through
a perl4 interpreter, especially if the next 2 tokens are "use strict"
or "my $var" or "our $var".





sysread() on closed filehandle %s




(W closed) You tried to read from a closed filehandle.





sysread() on unopened filehandle %s




(W unopened) You tried to read from a filehandle that was never opened.





System V %s is not implemented on this machine




(F) You tried to do something with a function beginning with "sem",
"shm", or "msg" but that System V IPC is not implemented in your
machine.  In some machines the functionality can exist but be
unconfigured.  Consult your system support.





syswrite() on closed filehandle %s




(W closed) The filehandle you’re writing to got itself closed sometime
before now.  Check your control flow.





-T and -B not implemented on filehandles




(F) Perl can’t peek at the stdio buffer of filehandles when it doesn’t
know about your kind of stdio.  You’ll have to use a filename instead.





Target of goto is too deeply nested




(F) You tried to use goto to reach a label that was too deeply nested
for Perl to reach.  Perl is doing you a favor by refusing.





telldir() attempted on invalid dirhandle %s




(W io) The dirhandle you tried to telldir() is either closed or not really
a dirhandle.  Check your control flow.





tell() on unopened filehandle




(W unopened) You tried to use the tell() function on a filehandle that
was either never opened or has since been closed.





That use of $[ is unsupported




(F) Assignment to $[ is now strictly circumscribed, and interpreted
as a compiler directive.  You may say only one of



    $[ = 0;
    $[ = 1;
    ...
    local $[ = 0;
    local $[ = 1;
    ...



This is to prevent the problem of one module changing the array base out
from under another module inadvertently.  See perlvar $[ and (arybase).





The crypt() function is unimplemented due to excessive paranoia




(F) Configure couldn’t find the crypt() function on your machine,
probably because your vendor didn’t supply it, probably because they
think the U.S. Government thinks it’s a secret, or at least that they
will continue to pretend that it is.  And if you quote me on that, I
will deny it.





The %s function is unimplemented




(F) The function indicated isn’t implemented on this architecture, according
to the probings of Configure.





The stat preceding %s wasn’t an lstat




(F) It makes no sense to test the current stat buffer for symbolic
linkhood if the last stat that wrote to the stat buffer already went
past the symlink to get to the real file.  Use an actual filename
instead.





The ’unique’ attribute may only be applied to ’our’ variables




(F) This attribute was never supported on my or sub declarations.





This Perl can’t reset CRTL environ elements (%s)






This Perl can’t set CRTL environ elements (%s=%s)




(W internal) Warnings peculiar to VMS.  You tried to change or delete an
element of the CRTL’s internal environ array, but your copy of Perl
wasn’t built with a CRTL that contained the setenv() function.  You’ll
need to rebuild Perl with a CRTL that does, or redefine
PERL_ENV_TABLES (see perlvms NAME) so that the environ array isn’t the
target of the change to
%ENV which produced the warning.





thread failed to start: %s




(W threads)(S) The entry point function of threads->create() failed for some reason.





times not implemented




(F) Your version of the C library apparently doesn’t do times().  I
suspect you’re not running on Unix.





"-T" is on the #! line, it must also be used on the command line




(X) The #! line (or local equivalent) in a Perl script contains
the -T option (or the -t option), but Perl was not invoked with
-T in its command line.  This is an error because, by the time
Perl discovers a -T in a script, it’s too late to properly taint
everything from the environment.  So Perl gives up.



If the Perl script is being executed as a command using the #!
mechanism (or its local equivalent), this error can usually be
fixed by editing the #! line so that the -%c option is a part of
Perl’s first argument: e.g. change perl -n -%c to perl -%c -n.



If the Perl script is being executed as perl scriptname, then the
-%c option must appear on the command line: perl -%c scriptname.





To%s: illegal mapping ’%s’




(F) You tried to define a customized To-mapping for lc(), lcfirst,
uc(), or ucfirst() (or their string-inlined versions), but you
specified an illegal mapping.
See perlunicode User-Defined Character Properties.





Too deeply nested ()-groups




(F) Your template contains ()-groups with a ridiculously deep nesting level.





Too few args to syscall




(F) There has to be at least one argument to syscall() to specify the
system call to call, silly dilly.





Too late for "-%s" option




(X) The #! line (or local equivalent) in a Perl script contains the
-M, -m or -C option.



In the case of -M and -m, this is an error because those options
are not intended for use inside scripts.  Use the use pragma instead.



The -C option only works if it is specified on the command line as
well (with the same sequence of letters or numbers following).  Either
specify this option on the command line, or, if your system supports
it, make your script executable and run it directly instead of passing
it to perl.





Too late to run %s block




(W void) A CHECK or INIT block is being defined during run time proper,
when the opportunity to run them has already passed.  Perhaps you are
loading a file with require or do when you should be using use
instead.  Or perhaps you should put the require or do inside a
BEGIN block.





Too many args to syscall




(F) Perl supports a maximum of only 14 args to syscall().





Too many arguments for %s




(F) The function requires fewer arguments than you specified.





Too many )’s




(A) You’ve accidentally run your script through csh instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.





Too many (’s




(A) You’ve accidentally run your script through csh instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.





Trailing \ in regex m/%s/




(F) The regular expression ends with an unbackslashed backslash.
Backslash it.   See (perlre).





Transliteration pattern not terminated




(F) The lexer couldn’t find the interior delimiter of a tr/// or tr[][]
or y/// or y[][] construct.  Missing the leading $ from variables
$tr or $y may cause this error.





Transliteration replacement not terminated




(F) The lexer couldn’t find the final delimiter of a tr///, tr[][],
y/// or y[][] construct.





’%s’ trapped by operation mask




(F) You tried to use an operator from a Safe compartment in which it’s
disallowed.  See (Safe).





truncate not implemented




(F) Your machine doesn’t implement a file truncation mechanism that
Configure knows about.





Type of arg %d to &CORE::%s must be %s




(F) The subroutine in question in the CORE package requires its argument
to be a hard reference to data of the specified type.  Overloading is
ignored, so a reference to an object that is not the specified type, but
nonetheless has overloading to handle it, will still not be accepted.





Type of arg %d to %s must be %s (not %s)




(F) This function requires the argument in that position to be of a
certain type.  Arrays must be @NAME or @{EXPR}.  Hashes must be
%NAME or %{EXPR}.  No implicit dereferencing is allowed–use the
{EXPR} forms as an explicit dereference.  See (perlref).





Type of argument to %s must be unblessed hashref or arrayref




(F) You called keys, values or each with a scalar argument that
was not a reference to an unblessed hash or array.





umask not implemented




(F) Your machine doesn’t implement the umask function and you tried to
use it to restrict permissions for yourself (EXPR & 0700).





Unable to create sub named "%s"




(F) You attempted to create or access a subroutine with an illegal name.





Unbalanced context: %d more PUSHes than POPs




(W internal) The exit code detected an internal inconsistency in how
many execution contexts were entered and left.





Unbalanced saves: %d more saves than restores




(W internal) The exit code detected an internal inconsistency in how
many values were temporarily localized.





Unbalanced scopes: %d more ENTERs than LEAVEs




(W internal) The exit code detected an internal inconsistency in how
many blocks were entered and left.





Unbalanced string table refcount: (%d) for "%s"




(W internal) On exit, Perl found some strings remaining in the shared
string table used for copy on write and for hash keys.  The entries
should have been freed, so this indicates a bug somewhere.





Unbalanced tmps: %d more allocs than frees




(W internal) The exit code detected an internal inconsistency in how
many mortal scalars were allocated and freed.





Undefined format "%s" called




(F) The format indicated doesn’t seem to exist.  Perhaps it’s really in
another package?  See (perlform).





Undefined sort subroutine "%s" called




(F) The sort comparison routine specified doesn’t seem to exist.
Perhaps it’s in a different package?  See (perlfunc)sort.





Undefined subroutine &%s called




(F) The subroutine indicated hasn’t been defined, or if it was, it has
since been undefined.





Undefined subroutine called




(F) The anonymous subroutine you’re trying to call hasn’t been defined,
or if it was, it has since been undefined.





Undefined subroutine in sort




(F) The sort comparison routine specified is declared but doesn’t seem
to have been defined yet.  See (perlfunc)sort.





Undefined top format "%s" called




(F) The format indicated doesn’t seem to exist.  Perhaps it’s really in
another package?  See (perlform).





Undefined value assigned to typeglob




(W misc) An undefined value was assigned to a typeglob, a la
*foo = undef.  This does nothing.  It’s possible that you really mean
undef *foo.





%s: Undefined variable




(A) You’ve accidentally run your script through csh instead of Perl.
Check the #! line, or manually feed your script into Perl yourself.





unexec of %s into %s failed!




(F) The unexec() routine failed for some reason.  See your local FSF
representative, who probably put it there in the first place.





Unexpected constant lvalue entersub entry via type/targ %d:%d




(P) When compiling a subroutine call in lvalue context, Perl failed an
internal consistency check.  It encountered a malformed op tree.





Unicode non-character U+%X is illegal for open interchange




(W utf8, nonchar) Certain codepoints, such as U+FFFE and U+FFFF, are
defined by the Unicode standard to be non-characters.  Those are
legal codepoints, but are reserved for internal use; so, applications
shouldn’t attempt to exchange them.  If you know what you are doing
you can turn off this warning by no warnings 'nonchar';.





Unicode surrogate U+%X is illegal in UTF-8




(W utf8, surrogate) You had a UTF-16 surrogate in a context where they are
not considered acceptable.  These code points, between U+D800 and
U+DFFF (inclusive), are used by Unicode only for UTF-16.  However, Perl
internally allows all unsigned integer code points (up to the size limit
available on your platform), including surrogates.  But these can cause
problems when being input or output, which is likely where this message
came from.  If you really really know what you are doing you can turn
off this warning by no warnings 'surrogate';.





Unknown BYTEORDER




(F) There are no byte-swapping functions for a machine with this byte
order.





Unknown error




(P) Perl was about to print an error message in $@, but the $@ variable
did not exist, even after an attempt to create it.





Unknown open() mode ’%s’




(F) The second argument of 3-argument open() is not among the list
of valid modes: <, >, >>, +<,
+>, +>>, -|, |-, <&, >&.





Unknown PerlIO layer "%s"




(W layer) An attempt was made to push an unknown layer onto the Perl I/O
system.  (Layers take care of transforming data between external and
internal representations.)  Note that some layers, such as mmap,
are not supported in all environments.  If your program didn’t
explicitly request the failing operation, it may be the result of the
value of the environment variable PERLIO.





Unknown process %x sent message to prime_env_iter: %s




(P) An error peculiar to VMS.  Perl was reading values for %ENV before
iterating over it, and someone else stuck a message in the stream of
data Perl expected.  Someone’s very confused, or perhaps trying to
subvert Perl’s population of %ENV for nefarious purposes.





Unknown "re" subpragma ’%s’ (known ones are: %s)




(W) You tried to use an unknown subpragma of the "re" pragma.





Unknown switch condition (?(%s in regex; marked by <– HERE in m/%s/




(F) The condition part of a (?(condition)if-clause|else-clause) construct
is not known.  The condition must be one of the following:



 (1) (2) ...        true if 1st, 2nd, etc., capture matched
 (<NAME>) ('NAME')  true if named capture matched
 (?=...) (?<=...)   true if subpattern matches
 (?!...) (?<!...)   true if subpattern fails to match
 (?{ CODE })        true if code returns a true value
 (R)                true if evaluating inside recursion
 (R1) (R2) ...      true if directly inside capture group 1, 2, etc.
 (R&NAME)           true if directly inside named capture
 (DEFINE)           always false; for defining named subpatterns



The <– HERE shows in the regular expression about where the problem was
discovered.  See (perlre).





Unknown Unicode option letter ’%c’




(F) You specified an unknown Unicode option.  See perlrun NAME documentation
of the -C switch for the list of known options.





Unknown Unicode option value %x




(F) You specified an unknown Unicode option.  See perlrun NAME documentation
of the -C switch for the list of known options.





Unknown verb pattern ’%s’ in regex; marked by <– HERE in m/%s/




(F) You either made a typo or have incorrectly put a * quantifier
after an open brace in your pattern.  Check the pattern and review
(perlre) for details on legal verb patterns.





Unknown warnings category ’%s’




(F) An error issued by the warnings pragma.  You specified a warnings
category that is unknown to perl at this point.



Note that if you want to enable a warnings category registered by a
module (e.g. use warnings 'File::Find'), you must have loaded this
module first.





unmatched [ in regex; marked by <– HERE in m/%s/




(F) The brackets around a character class must match.  If you wish to
include a closing bracket in a character class, backslash it or put it
first.  The <– HERE shows in the regular expression about where the
problem was discovered.  See (perlre).





unmatched ( in regex; marked by <– HERE in m/%s/




(F) Unbackslashed parentheses must always be balanced in regular
expressions.  If you’re a vi user, the % key is valuable for finding
the matching parenthesis.  The <– HERE shows in the regular expression
about where the problem was discovered.  See (perlre).





Unmatched right %s bracket




(F) The lexer counted more closing curly or square brackets than opening
ones, so you’re probably missing a matching opening bracket.  As a
general rule, you’ll find the missing one (so to speak) near the place
you were last editing.





Unquoted string "%s" may clash with future reserved word




(W reserved) You used a bareword that might someday be claimed as a
reserved word.  It’s best to put such a word in quotes, or capitalize it
somehow, or insert an underbar into it.  You might also declare it as a
subroutine.





Unrecognized character %s; marked by <– HERE after %s near column %d




(F) The Perl parser has no idea what to do with the specified character
in your Perl script (or eval) near the specified column.  Perhaps you tried 
to run a compressed script, a binary program, or a directory as a Perl program.





Unrecognized escape \%c in character class passed through in regex; marked by <– HERE in m/%s/




(W regexp) You used a backslash-character combination which is not
recognized by Perl inside character classes.  The character was
understood literally, but this may change in a future version of Perl.
The <– HERE shows in the regular expression about where the
escape was discovered.





Unrecognized escape \%c passed through




(W misc) You used a backslash-character combination which is not
recognized by Perl.  The character was understood literally, but this may
change in a future version of Perl.





Unrecognized escape \%s passed through in regex; marked by <– HERE in m/%s/




(W regexp) You used a backslash-character combination which is not
recognized by Perl.  The character(s) were understood literally, but
this may change in a future version of Perl.  The <– HERE shows in
the regular expression about where the escape was discovered.





Unrecognized signal name "%s"




(F) You specified a signal name to the kill() function that was not
recognized.  Say kill -l in your shell to see the valid signal names
on your system.





Unrecognized switch: -%s  (-h will show valid options)




(F) You specified an illegal option to Perl.  Don’t do that.  (If you
think you didn’t do that, check the #! line to see if it’s supplying the
bad switch on your behalf.)





Unsuccessful %s on filename containing newline




(W newline) A file operation was attempted on a filename, and that
operation failed, PROBABLY because the filename contained a newline,
PROBABLY because you forgot to chomp() it off.  See (perlfunc)chomp.





Unsupported directory function "%s" called




(F) Your machine doesn’t support opendir() and readdir().





Unsupported function %s




(F) This machine doesn’t implement the indicated function, apparently.
At least, Configure doesn’t think so.





Unsupported function fork




(F) Your version of executable does not support forking.



Note that under some systems, like OS/2, there may be different flavors
of Perl executables, some of which may support fork, some not.  Try
changing the name you call Perl by to perl_, perl__, and so on.





Unsupported script encoding %s




(F) Your program file begins with a Unicode Byte Order Mark (BOM) which
declares it to be in a Unicode encoding that Perl cannot read.





Unsupported socket function "%s" called




(F) Your machine doesn’t support the Berkeley socket mechanism, or at
least that’s what Configure thought.





Unterminated attribute list




(F) The lexer found something other than a simple identifier at the
start of an attribute, and it wasn’t a semicolon or the start of a
block.  Perhaps you terminated the parameter list of the previous
attribute too soon.  See (attributes).





Unterminated attribute parameter in attribute list




(F) The lexer saw an opening (left) parenthesis character while parsing
an attribute list, but the matching closing (right) parenthesis
character was not found.  You may need to add (or remove) a backslash
character to get your parentheses to balance.  See (attributes).





Unterminated compressed integer




(F) An argument to unpack("w",...) was incompatible with the BER
compressed integer format and could not be converted to an integer.
See (perlfunc)pack.





Unterminated \g{...} pattern in regex; marked by <– HERE in m/%s/




(F) You missed a close brace on a \g{..} pattern (group reference) in
a regular expression.  Fix the pattern and retry.





Unterminated <> operator




(F) The lexer saw a left angle bracket in a place where it was expecting
a term, so it’s looking for the corresponding right angle bracket, and
not finding it.  Chances are you left some needed parentheses out
earlier in the line, and you really meant a "less than".





Unterminated verb pattern argument in regex; marked by <– HERE in m/%s/




(F) You used a pattern of the form (*VERB:ARG) but did not terminate
the pattern with a ).  Fix the pattern and retry.





Unterminated verb pattern in regex; marked by <– HERE in m/%s/




(F) You used a pattern of the form (*VERB) but did not terminate
the pattern with a ).  Fix the pattern and retry.





untie attempted while %d inner references still exist




(W untie) A copy of the object returned from tie (or tied) was
still valid when untie was called.





Usage: POSIX::%s(%s)




(F) You called a POSIX function with incorrect arguments.
See (POSIX)FUNCTIONS for more information.





Usage: Win32::%s(%s)




(F) You called a Win32 function with incorrect arguments.
See (Win32) for more information.





$[ used in %s (did you mean $] ?)




(W syntax) You used $[ in a comparison, such as:



    if ($[ > 5.006) {
        ...
    }



You probably meant to use $] instead.  $[ is the base for indexing
arrays.  $] is the Perl version number in decimal.





Useless assignment to a temporary




(W misc) You assigned to an lvalue subroutine, but what
the subroutine returned was a temporary scalar about to
be discarded, so the assignment had no effect.





Useless (?-%s) - don’t use /%s modifier in regex; marked by <– HERE in m/%s/




(W regexp) You have used an internal modifier such as (?-o) that has no
meaning unless removed from the entire regexp:



    if ($string =~ /(?-o)$pattern/o) { ... }



must be written as



    if ($string =~ /$pattern/) { ... }



The <– HERE shows in the regular expression about
where the problem was discovered.  See (perlre).





Useless localization of %s




(W syntax) The localization of lvalues such as local($x=10) is legal,
but in fact the local() currently has no effect.  This may change at
some point in the future, but in the meantime such code is discouraged.





Useless (?%s) - use /%s modifier in regex; marked by <– HERE in m/%s/




(W regexp) You have used an internal modifier such as (?o) that has no
meaning unless applied to the entire regexp:



    if ($string =~ /(?o)$pattern/) { ... }



must be written as



    if ($string =~ /$pattern/o) { ... }



The <– HERE shows in the regular expression about
where the problem was discovered.  See (perlre).





Useless use of /d modifier in transliteration operator




(W misc) You have used the /d modifier where the searchlist has the
same length as the replacelist.  See (perlop) for more information
about the /d modifier.





Useless use of \E




(W misc) You have a \E in a double-quotish string without a \U,
\L or \Q preceding it.





Useless use of %s in void context




(W void) You did something without a side effect in a context that does
nothing with the return value, such as a statement that doesn’t return a
value from a block, or the left side of a scalar comma operator.  Very
often this points not to stupidity on your part, but a failure of Perl
to parse your program the way you thought it would.  For example, you’d
get this if you mixed up your C precedence with Python precedence and
said



    $one, $two = 1, 2;



when you meant to say



    ($one, $two) = (1, 2);



Another common error is to use ordinary parentheses to construct a list
reference when you should be using square or curly brackets, for
example, if you say



    $array = (1,2);



when you should have said



    $array = [1,2];



The square brackets explicitly turn a list value into a scalar value,
while parentheses do not.  So when a parenthesized list is evaluated in
a scalar context, the comma is treated like C’s comma operator, which
throws away the left argument, which is not what you want.  See
(perlref) for more on this.



This warning will not be issued for numerical constants equal to 0 or 1
since they are often used in statements like



    1 while sub_with_side_effects();



String constants that would normally evaluate to 0 or 1 are warned
about.





Useless use of "re" pragma




(W) You did use re; without any arguments.  That isn’t very useful.





Useless use of sort in scalar context




(W void) You used sort in scalar context, as in :



    my $x = sort @y;



This is not very useful, and perl currently optimizes this away.





Useless use of %s with no values




(W syntax) You used the push() or unshift() function with no arguments
apart from the array, like push(@x) or unshift(@foo).  That won’t
usually have any effect on the array, so is completely useless.  It’s
possible in principle that push(@tied_array) could have some effect
if the array is tied to a class which implements a PUSH method.  If so,
you can write it as push(@tied_array,()) to avoid this warning.





"use" not allowed in expression




(F) The "use" keyword is recognized and executed at compile time, and
returns no useful value.  See perlmod NAME.





Use of assignment to $[ is deprecated




(D deprecated) The $[ variable (index of the first element in an array)
is deprecated.  See perlvar $[.





Use of bare << to mean <<"" is deprecated




(D deprecated) You are now encouraged to use the explicitly quoted
form if you wish to use an empty line as the terminator of the here-document.





Use of comma-less variable list is deprecated




(D deprecated) The values you give to a format should be
separated by commas, not just aligned on a line.





Use of chdir(”) or chdir(undef) as chdir() deprecated




(D deprecated) chdir() with no arguments is documented to change to
$ENV{HOME} or $ENV{LOGDIR}.  chdir(undef) and chdir(”) share this
behavior, but that has been deprecated.  In future versions they
will simply fail.



Be careful to check that what you pass to chdir() is defined and not
blank, else you might find yourself in your home directory.





Use of /c modifier is meaningless in s///




(W regexp) You used the /c modifier in a substitution.  The /c
modifier is not presently meaningful in substitutions.





Use of /c modifier is meaningless without /g




(W regexp) You used the /c modifier with a regex operand, but didn’t
use the /g modifier.  Currently, /c is meaningful only when /g is
used.  (This may change in the future.)





Use of := for an empty attribute list is not allowed




(F) The construction my $x := 42 used to parse as equivalent to
my $x : = 42 (applying an empty attribute list to $x).
This construct was deprecated in 5.12.0, and has now been made a syntax
error, so := can be reclaimed as a new operator in the future.



If you need an empty attribute list, for example in a code generator, add
a space before the =.





Use of freed value in iteration




(F) Perhaps you modified the iterated array within the loop?
This error is typically caused by code like the following:



    @a = (3,4);
    @a = () for (1,2,@a);



You are not supposed to modify arrays while they are being iterated over.
For speed and efficiency reasons, Perl internally does not do full
reference-counting of iterated items, hence deleting such an item in the
middle of an iteration causes Perl to see a freed value.





Use of *glob{FILEHANDLE} is deprecated




(D deprecated) You are now encouraged to use the shorter *glob{IO} form
to access the filehandle slot within a typeglob.





Use of /g modifier is meaningless in split




(W regexp) You used the /g modifier on the pattern for a split
operator.  Since split always tries to match the pattern
repeatedly, the /g has no effect.





Use of "goto" to jump into a construct is deprecated




(D deprecated) Using goto to jump from an outer scope into an inner
scope is deprecated and should be avoided.





Use of inherited AUTOLOAD for non-method %s() is deprecated




(D deprecated) As an (ahem) accidental feature, AUTOLOAD
subroutines are looked up as methods (using the @ISA hierarchy)
even when the subroutines to be autoloaded were called as plain
functions (e.g. Foo::bar()), not as methods (e.g. Foo->bar() or
$obj->bar()).



This bug will be rectified in future by using method lookup only for
methods’ AUTOLOADs.  However, there is a significant base of existing
code that may be using the old behavior.  So, as an interim step, Perl
currently issues an optional warning when non-methods use inherited
AUTOLOADs.



The simple rule is:  Inheritance will not work when autoloading
non-methods.  The simple fix for old code is:  In any module that used
to depend on inheriting AUTOLOAD for non-methods from a base class
named BaseClass, execute *AUTOLOAD = \&BaseClass::AUTOLOAD during
startup.



In code that currently says use AutoLoader; @ISA = qw(AutoLoader);
you should remove AutoLoader from @ISA and change use AutoLoader; to
use AutoLoader 'AUTOLOAD';.





Use of %s in printf format not supported




(F) You attempted to use a feature of printf that is accessible from
only C.  This usually means there’s a better way to do it in Perl.





Use of %s is deprecated




(D deprecated) The construct indicated is no longer recommended for use,
generally because there’s a better way to do it, and also because the
old way has bad side effects.





Use of -l on filehandle %s




(W io) A filehandle represents an opened file, and when you opened the file
it already went past any symlink you are presumably trying to look for.
The operation returned undef.  Use a filename instead.





Use of %s on a handle without * is deprecated




(D deprecated) You used tie, tied or untie on a scalar but that scalar
happens to hold a typeglob, which means its filehandle will be tied.  If
you mean to tie a handle, use an explicit * as in tie *$handle.



This was a long-standing bug that was removed in Perl 5.16, as there was
no way to tie the scalar itself when it held a typeglob, and no way to
untie a scalar that had had a typeglob assigned to it.  If you see this
message, you must be using an older version.





Use of ?PATTERN? without explicit operator is deprecated




(D deprecated) You have written something like ?\w?, for a regular
expression that matches only once.  Starting this term directly with
the question mark delimiter is now deprecated, so that the question mark
will be available for use in new operators in the future.  Write m?\w?
instead, explicitly using the m operator: the question mark delimiter
still invokes match-once behaviour.





Use of qw(...) as parentheses is deprecated




(D deprecated) You have something like foreach $x qw(a b c) {...},
using a qw(...) list literal where a parenthesised expression is
expected.  Historically the parser fooled itself into thinking that
qw(...) literals were always enclosed in parentheses, and as a result
you could sometimes omit parentheses around them.  (You could never do
the foreach qw(a b c) {...} that you might have expected, though.)
The parser no longer lies to itself in this way.  Wrap the list literal
in parentheses, like foreach $x (qw(a b c)) {...}.





Use of reference "%s" as array index




(W misc) You tried to use a reference as an array index; this probably
isn’t what you mean, because references in numerical context tend
to be huge numbers, and so usually indicates programmer error.



If you really do mean it, explicitly numify your reference, like so:
$array[0+$ref].  This warning is not given for overloaded objects,
however, because you can overload the numification and stringification
operators and then you presumably know what you are doing.





Use of reserved word "%s" is deprecated




(D deprecated) The indicated bareword is a reserved word.  Future
versions of perl may use it as a keyword, so you’re better off either
explicitly quoting the word in a manner appropriate for its context of
use, or using a different name altogether.  The warning can be
suppressed for subroutine names by either adding a & prefix, or using
a package qualifier, e.g. &our(), or Foo::our().





Use of tainted arguments in %s is deprecated




(W taint, deprecated) You have supplied system() or exec() with multiple
arguments and at least one of them is tainted.  This used to be allowed
but will become a fatal error in a future version of perl.  Untaint your
arguments.  See perlsec NAME.





Use of uninitialized value%s




(W uninitialized) An undefined value was used as if it were already
defined.  It was interpreted as a "" or a 0, but maybe it was a mistake.
To suppress this warning assign a defined value to your variables.



To help you figure out what was undefined, perl will try to tell you
the name of the variable (if any) that was undefined.  In some cases
it cannot do this, so it also tells you what operation you used the
undefined value in.  Note, however, that perl optimizes your program
anid the operation displayed in the warning may not necessarily appear
literally in your program.  For example, "that $foo" is usually
optimized into "that " . $foo, and the warning will refer to the
concatenation (.) operator, even though there is no . in
your program.





Using a hash as a reference is deprecated




(D deprecated) You tried to use a hash as a reference, as in
%foo->{"bar"} or %$ref->{"hello"}.  Versions of perl <= 5.6.1
used to allow this syntax, but shouldn’t have.   It is now
deprecated, and will be removed in a future version.





Using an array as a reference is deprecated




(D deprecated) You tried to use an array as a reference, as in
@foo->[23] or @$ref->[99].  Versions of perl <= 5.6.1 used to
allow this syntax, but shouldn’t have.  It is now deprecated,
and will be removed in a future version.





Using just the first character returned by \N{} in character class




(W) A charnames handler may return a sequence of more than one character.
Currently all but the first one are discarded when used in a regular
expression pattern bracketed character class.





Using !~ with %s doesn’t make sense




(F) Using the !~ operator with s///r, tr///r or y///r is
currently reserved for future use, as the exact behaviour has not
been decided.  (Simply returning the boolean opposite of the
modified string is usually not particularly useful.)





UTF-16 surrogate U+%X




(W utf8, surrogate) You had a UTF-16 surrogate in a context where they are
not considered acceptable.  These code points, between U+D800 and
U+DFFF (inclusive), are used by Unicode only for UTF-16.  However, Perl
internally allows all unsigned integer code points (up to the size limit
available on your platform), including surrogates.  But these can cause
problems when being input or output, which is likely where this message
came from.  If you really really know what you are doing you can turn
off this warning by no warnings 'surrogate';.





Value of %s can be "0"; test with defined()




(W misc) In a conditional expression, you used <HANDLE>, <*> (glob),
each(), or readdir() as a boolean value.  Each of these constructs
can return a value of "0"; that would make the conditional expression
false, which is probably not what you intended.  When using these
constructs in conditional expressions, test their values with the
defined operator.





Value of CLI symbol "%s" too long




(W misc) A warning peculiar to VMS.  Perl tried to read the value of an
%ENV element from a CLI symbol table, and found a resultant string
longer than 1024 characters.  The return value has been truncated to
1024 characters.





Variable "%s" is not available




(W closure) During compilation, an inner named subroutine or eval is
attempting to capture an outer lexical that is not currently available.
This can happen for one of two reasons.  First, the outer lexical may be
declared in an outer anonymous subroutine that has not yet been created.
(Remember that named subs are created at compile time, while anonymous
subs are created at run-time.)  For example,



    sub { my $a; sub f { $a } }



At the time that f is created, it can’t capture the current value of $a,
since the anonymous subroutine hasn’t been created yet.  Conversely,
the following won’t give a warning since the anonymous subroutine has by
now been created and is live:



    sub { my $a; eval 'sub f { $a }' }->();



The second situation is caused by an eval accessing a variable that has
gone out of scope, for example,



    sub f {
        my $a;
        sub { eval '$a' }
    }
    f()->();



Here, when the ’$a’ in the eval is being compiled, f() is not currently being
executed, so its $a is not available for capture.





Variable "%s" is not imported%s




(W misc) With "use strict" in effect, you referred to a global variable
that you apparently thought was imported from another module, because
something else of the same name (usually a subroutine) is exported by
that module.  It usually means you put the wrong funny character on the
front of your variable.





Variable length lookbehind not implemented in m/%s/




(F) Lookbehind is allowed only for subexpressions whose length is fixed and
known at compile time.  See (perlre).





"%s" variable %s masks earlier declaration in same %s




(W misc) A "my", "our" or "state" variable has been redeclared in the
current scope or statement, effectively eliminating all access to the
previous instance.  This is almost always a typographical error.  Note
that the earlier variable will still exist until the end of the scope
or until all closure referents to it are destroyed.





Variable syntax




(A) You’ve accidentally run your script through csh instead
of Perl.  Check the #! line, or manually feed your script into
Perl yourself.





Variable "%s" will not stay shared




(W closure) An inner (nested) named subroutine is referencing a
lexical variable defined in an outer named subroutine.



When the inner subroutine is called, it will see the value of
the outer subroutine’s variable as it was before and during the *first*
call to the outer subroutine; in this case, after the first call to the
outer subroutine is complete, the inner and outer subroutines will no
longer share a common value for the variable.  In other words, the
variable will no longer be shared.



This problem can usually be solved by making the inner subroutine
anonymous, using the sub {} syntax.  When inner anonymous subs that
reference variables in outer subroutines are created, they
are automatically rebound to the current values of such variables.





vector argument not supported with alpha versions




(W internal) The %vd (s)printf format does not support version objects
with alpha parts.





Verb pattern ’%s’ has a mandatory argument in regex; marked by <– HERE in m/%s/




(F) You used a verb pattern that requires an argument.  Supply an
argument or check that you are using the right verb.





Verb pattern ’%s’ may not have an argument in regex; marked by <– HERE in m/%s/




(F) You used a verb pattern that is not allowed an argument.  Remove the 
argument or check that you are using the right verb.





Version number must be a constant number




(P) The attempt to translate a use Module n.n LIST statement into
its equivalent BEGIN block found an internal inconsistency with
the version number.





Version string ’%s’ contains invalid data; ignoring: ’%s’




(W misc) The version string contains invalid characters at the end, which
are being ignored.





Warning: something’s wrong




(W) You passed warn() an empty string (the equivalent of warn "") or
you called it with no args and $@ was empty.





Warning: unable to close filehandle %s properly




(S) The implicit close() done by an open() got an error indication on
the close().  This usually indicates your file system ran out of disk
space.





Warning: Use of "%s" without parentheses is ambiguous




(S ambiguous) You wrote a unary operator followed by something that
looks like a binary operator that could also have been interpreted as a
term or unary operator.  For instance, if you know that the rand
function has a default argument of 1.0, and you write



    rand + 5;



you may THINK you wrote the same thing as



    rand() + 5;



but in actual fact, you got



    rand(+5);



So put in parentheses to say what you really mean.





Wide character in %s




(S utf8) Perl met a wide character (>255) when it wasn’t expecting
one.  This warning is by default on for I/O (like print).  The easiest
way to quiet this warning is simply to add the :utf8 layer to the
output, e.g. binmode STDOUT, ':utf8'.  Another way to turn off the
warning is to add no warnings 'utf8'; but that is often closer to
cheating.  In general, you are supposed to explicitly mark the
filehandle with an encoding, see (open) and (perlfunc)binmode.





Within []-length ’%c’ not allowed




(F) The count in the (un)pack template may be replaced by [TEMPLATE]
only if TEMPLATE always matches the same amount of packed bytes that
can be determined from the template alone.  This is not possible if
it contains any of the codes @, /, U, u, w or a *-length.  Redesign
the template.





write() on closed filehandle %s




(W closed) The filehandle you’re writing to got itself closed sometime
before now.  Check your control flow.





%s "\x%X" does not map to Unicode




(F) When reading in different encodings Perl tries to map everything
into Unicode characters.  The bytes you read in are not legal in
this encoding, for example



    utf8 "\xE4" does not map to Unicode



if you try to read in the a-diaereses Latin-1 as UTF-8.





’X’ outside of string




(F) You had a (un)pack template that specified a relative position before
the beginning of the string being (un)packed.  See (perlfunc)pack.





’x’ outside of string in unpack




(F) You had a pack template that specified a relative position after
the end of the string being unpacked.  See (perlfunc)pack.





YOU HAVEN’T DISABLED SET-ID SCRIPTS IN THE KERNEL YET!




(F) And you probably never will, because you probably don’t have the
sources to your kernel, and your vendor probably doesn’t give a rip
about what you want.  Your best bet is to put a setuid C wrapper around
your script.





You need to quote "%s"




(W syntax) You assigned a bareword as a signal handler name.
Unfortunately, you already have a subroutine of that name declared,
which means that Perl 5 will try to call the subroutine when the
assignment is executed, which is probably not what you want.  (If it IS
what you want, put an & in front.)





Your random numbers are not that random




(F) When trying to initialise the random seed for hashes, Perl could
not get any randomness out of your system.  This usually indicates
Something Very Wrong.














Previous: perldiag DESCRIPTION, Up: perldiag   [Contents]





16.3 SEE ALSO



(warnings), (perllexwarn), (diagnostics).









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17 NAME data structure complex data structure struct





• NAME data structure complex data structure struct NAME:  



• NAME data structure complex data structure struct DESCRIPTION:  



• NAME data structure complex data structure struct REFERENCES:  



• NAME data structure complex data structure struct COMMON MISTAKES:  



• NAME data structure complex data structure struct CAVEAT ON PRECEDENCE:  



• NAME data structure complex data structure struct WHY YOU SHOULD ALWAYS use strict:  



• NAME data structure complex data structure struct DEBUGGING:  



• NAME data structure complex data structure struct CODE EXAMPLES:  



• NAME data structure complex data structure struct ARRAYS OF ARRAYS:  



• NAME data structure complex data structure struct HASHES OF ARRAYS:  



• NAME data structure complex data structure struct ARRAYS OF HASHES:  



• NAME data structure complex data structure struct HASHES OF HASHES:  



• NAME data structure complex data structure struct MORE ELABORATE RECORDS:  



• NAME data structure complex data structure struct Database Ties:  



• NAME data structure complex data structure struct SEE ALSO:  



• NAME data structure complex data structure struct AUTHOR:  












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17.1 NAME






perldsc - Perl Data Structures Cookbook









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17.2 DESCRIPTION



The single feature most sorely lacking in the Perl programming language
prior to its 5.0 release was complex data structures.  Even without direct
language support, some valiant programmers did manage to emulate them, but
it was hard work and not for the faint of heart.  You could occasionally
get away with the $m{$AoA,$b} notation borrowed from awk in which the
keys are actually more like a single concatenated string "$AoA$b", but
traversal and sorting were difficult.  More desperate programmers even
hacked Perl’s internal symbol table directly, a strategy that proved hard
to develop and maintain–to put it mildly.



The 5.0 release of Perl let us have complex data structures.  You
may now write something like this and all of a sudden, you’d have an array
with three dimensions!



    for $x (1 .. 10) {
        for $y (1 .. 10) {
            for $z (1 .. 10) {
                $AoA[$x][$y][$z] =
                    $x ** $y + $z;
            }
        }
    }



Alas, however simple this may appear, underneath it’s a much more
elaborate construct than meets the eye!



How do you print it out?  Why can’t you say just print @AoA?  How do
you sort it?  How can you pass it to a function or get one of these back
from a function?  Is it an object?  Can you save it to disk to read
back later?  How do you access whole rows or columns of that matrix?  Do
all the values have to be numeric?



As you see, it’s quite easy to become confused.  While some small portion
of the blame for this can be attributed to the reference-based
implementation, it’s really more due to a lack of existing documentation with
examples designed for the beginner.



This document is meant to be a detailed but understandable treatment of the
many different sorts of data structures you might want to develop.  It
should also serve as a cookbook of examples.  That way, when you need to
create one of these complex data structures, you can just pinch, pilfer, or
purloin a drop-in example from here.



Let’s look at each of these possible constructs in detail.  There are separate
sections on each of the following:





But for now, let’s look at general issues common to all
these types of data structures.









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17.3 REFERENCES









The most important thing to understand about all data structures in
Perl–including multidimensional arrays–is that even though they might
appear otherwise, Perl @ARRAYs and %HASHes are all internally
one-dimensional.  They can hold only scalar values (meaning a string,
number, or a reference).  They cannot directly contain other arrays or
hashes, but instead contain references to other arrays or hashes.




You can’t use a reference to an array or hash in quite the same way that you
would a real array or hash.  For C or C++ programmers unused to
distinguishing between arrays and pointers to the same, this can be
confusing.  If so, just think of it as the difference between a structure
and a pointer to a structure.



You can (and should) read more about references in (perlref).
Briefly, references are rather like pointers that know what they
point to.  (Objects are also a kind of reference, but we won’t be needing
them right away–if ever.)  This means that when you have something which
looks to you like an access to a two-or-more-dimensional array and/or hash,
what’s really going on is that the base type is
merely a one-dimensional entity that contains references to the next
level.  It’s just that you can use it as though it were a
two-dimensional one.  This is actually the way almost all C
multidimensional arrays work as well.



    $array[7][12]                       # array of arrays
    $array[7]{string}                   # array of hashes
    $hash{string}[7]                    # hash of arrays
    $hash{string}{'another string'}     # hash of hashes



Now, because the top level contains only references, if you try to print
out your array in with a simple print() function, you’ll get something
that doesn’t look very nice, like this:



    @AoA = ( [2, 3], [4, 5, 7], [0] );
    print $AoA[1][2];
  7
    print @AoA;
  ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)



That’s because Perl doesn’t (ever) implicitly dereference your variables.
If you want to get at the thing a reference is referring to, then you have
to do this yourself using either prefix typing indicators, like
${$blah}, @{$blah}, @{$blah[$i]}, or else postfix pointer arrows,
like $a->[3], $h->{fred}, or even $ob->method()->[3].









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17.4 COMMON MISTAKES



The two most common mistakes made in constructing something like
an array of arrays is either accidentally counting the number of
elements or else taking a reference to the same memory location
repeatedly.  Here’s the case where you just get the count instead
of a nested array:



    for $i (1..10) {
        @array = somefunc($i);
        $AoA[$i] = @array;      # WRONG!
    }



That’s just the simple case of assigning an array to a scalar and getting
its element count.  If that’s what you really and truly want, then you
might do well to consider being a tad more explicit about it, like this:



    for $i (1..10) {
        @array = somefunc($i);
        $counts[$i] = scalar @array;
    }



Here’s the case of taking a reference to the same memory location
again and again:



    for $i (1..10) {
        @array = somefunc($i);
        $AoA[$i] = \@array;     # WRONG!
    }



So, what’s the big problem with that?  It looks right, doesn’t it?
After all, I just told you that you need an array of references, so by
golly, you’ve made me one!



Unfortunately, while this is true, it’s still broken.  All the references
in @AoA refer to the very same place, and they will therefore all hold
whatever was last in @array!  It’s similar to the problem demonstrated in
the following C program:



    #include <pwd.h>
    main() {
        struct passwd *getpwnam(), *rp, *dp;
        rp = getpwnam("root");
        dp = getpwnam("daemon");

        printf("daemon name is %s\nroot name is %s\n",
                dp->pw_name, rp->pw_name);
    }



Which will print



    daemon name is daemon
    root name is daemon





The problem is that both rp and dp are pointers to the same location
in memory!  In C, you’d have to remember to malloc() yourself some new
memory.  In Perl, you’ll want to use the array constructor [] or the
hash constructor {} instead.   Here’s the right way to do the preceding
broken code fragments:




    for $i (1..10) {
        @array = somefunc($i);
        $AoA[$i] = [ @array ];
    }



The square brackets make a reference to a new array with a copy
of what’s in @array at the time of the assignment.  This is what
you want.



Note that this will produce something similar, but it’s
much harder to read:



    for $i (1..10) {
        @array = 0 .. $i;
        @{$AoA[$i]} = @array;
    }



Is it the same?  Well, maybe so–and maybe not.  The subtle difference
is that when you assign something in square brackets, you know for sure
it’s always a brand new reference with a new copy of the data.
Something else could be going on in this new case with the @{$AoA[$i]}
dereference on the left-hand-side of the assignment.  It all depends on
whether $AoA[$i] had been undefined to start with, or whether it
already contained a reference.  If you had already populated @AoA with
references, as in



    $AoA[3] = \@another_array;



Then the assignment with the indirection on the left-hand-side would
use the existing reference that was already there:



    @{$AoA[3]} = @array;



Of course, this would have the "interesting" effect of clobbering
@another_array.  (Have you ever noticed how when a programmer says
something is "interesting", that rather than meaning "intriguing",
they’re disturbingly more apt to mean that it’s "annoying",
"difficult", or both?  :-)



So just remember always to use the array or hash constructors with []
or {}, and you’ll be fine, although it’s not always optimally
efficient.



Surprisingly, the following dangerous-looking construct will
actually work out fine:



    for $i (1..10) {
        my @array = somefunc($i);
        $AoA[$i] = \@array;
    }



That’s because my() is more of a run-time statement than it is a
compile-time declaration per se.  This means that the my() variable is
remade afresh each time through the loop.  So even though it looks as
though you stored the same variable reference each time, you actually did
not!  This is a subtle distinction that can produce more efficient code at
the risk of misleading all but the most experienced of programmers.  So I
usually advise against teaching it to beginners.  In fact, except for
passing arguments to functions, I seldom like to see the gimme-a-reference
operator (backslash) used much at all in code.  Instead, I advise
beginners that they (and most of the rest of us) should try to use the
much more easily understood constructors [] and {} instead of
relying upon lexical (or dynamic) scoping and hidden reference-counting to
do the right thing behind the scenes.



In summary:



    $AoA[$i] = [ @array ];      # usually best
    $AoA[$i] = \@array;         # perilous; just how my() was that array?
    @{ $AoA[$i] } = @array;     # way too tricky for most programmers









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17.5 CAVEAT ON PRECEDENCE






Speaking of things like @{$AoA[$i]}, the following are actually the
same thing:




    $aref->[2][2]       # clear
    $$aref[2][2]        # confusing



That’s because Perl’s precedence rules on its five prefix dereferencers
(which look like someone swearing: $ @ * % &) make them bind more
tightly than the postfix subscripting brackets or braces!  This will no
doubt come as a great shock to the C or C++ programmer, who is quite
accustomed to using *a[i] to mean what’s pointed to by the i’th
element of a.  That is, they first take the subscript, and only then
dereference the thing at that subscript.  That’s fine in C, but this isn’t C.



The seemingly equivalent construct in Perl, $$aref[$i] first does
the deref of $aref, making it take $aref as a reference to an
array, and then dereference that, and finally tell you the i’th value
of the array pointed to by $AoA. If you wanted the C notion, you’d have to
write ${$AoA[$i]} to force the $AoA[$i] to get evaluated first
before the leading $ dereferencer.









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17.6 WHY YOU SHOULD ALWAYS use strict



If this is starting to sound scarier than it’s worth, relax.  Perl has
some features to help you avoid its most common pitfalls.  The best
way to avoid getting confused is to start every program like this:



    #!/usr/bin/perl -w
    use strict;



This way, you’ll be forced to declare all your variables with my() and
also disallow accidental "symbolic dereferencing".  Therefore if you’d done
this:



    my $aref = [
        [ "fred", "barney", "pebbles", "bambam", "dino", ],
        [ "homer", "bart", "marge", "maggie", ],
        [ "george", "jane", "elroy", "judy", ],
    ];

    print $aref[2][2];



The compiler would immediately flag that as an error at compile time,
because you were accidentally accessing @aref, an undeclared
variable, and it would thereby remind you to write instead:



    print $aref->[2][2]









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17.7 DEBUGGING













Before version 5.002, the standard Perl debugger didn’t do a very nice job of
printing out complex data structures.  With 5.002 or above, the
debugger includes several new features, including command line editing as
well as the x command to dump out complex data structures.  For
example, given the assignment to $AoA above, here’s the debugger output:



    DB<1> x $AoA
    $AoA = ARRAY(0x13b5a0)
       0  ARRAY(0x1f0a24)
          0  'fred'
          1  'barney'
          2  'pebbles'
          3  'bambam'
          4  'dino'
       1  ARRAY(0x13b558)
          0  'homer'
          1  'bart'
          2  'marge'
          3  'maggie'
       2  ARRAY(0x13b540)
          0  'george'
          1  'jane'
          2  'elroy'
          3  'judy'









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17.8 CODE EXAMPLES



Presented with little comment (these will get their own manpages someday)
here are short code examples illustrating access of various
types of data structures.









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17.9 ARRAYS OF ARRAYS







• NAME data structure complex data structure struct Declaration of an ARRAY OF ARRAYS:  



• NAME data structure complex data structure struct Generation of an ARRAY OF ARRAYS:  



• NAME data structure complex data structure struct Access and Printing of an ARRAY OF ARRAYS:  












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17.9.1 Declaration of an ARRAY OF ARRAYS



 @AoA = (
        [ "fred", "barney" ],
        [ "george", "jane", "elroy" ],
        [ "homer", "marge", "bart" ],
      );









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17.9.2 Generation of an ARRAY OF ARRAYS



 # reading from file
 while ( <> ) {
     push @AoA, [ split ];
 }

 # calling a function
 for $i ( 1 .. 10 ) {
     $AoA[$i] = [ somefunc($i) ];
 }

 # using temp vars
 for $i ( 1 .. 10 ) {
     @tmp = somefunc($i);
     $AoA[$i] = [ @tmp ];
 }

 # add to an existing row
 push @{ $AoA[0] }, "wilma", "betty";









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17.9.3 Access and Printing of an ARRAY OF ARRAYS



 # one element
 $AoA[0][0] = "Fred";

 # another element
 $AoA[1][1] =~ s/(\w)/\u$1/;

 # print the whole thing with refs
 for $aref ( @AoA ) {
     print "\t [ @$aref ],\n";
 }

 # print the whole thing with indices
 for $i ( 0 .. $#AoA ) {
     print "\t [ @{$AoA[$i]} ],\n";
 }

 # print the whole thing one at a time
 for $i ( 0 .. $#AoA ) {
     for $j ( 0 .. $#{ $AoA[$i] } ) {
         print "elt $i $j is $AoA[$i][$j]\n";
     }
 }









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17.10 HASHES OF ARRAYS







• NAME data structure complex data structure struct Declaration of a HASH OF ARRAYS:  



• NAME data structure complex data structure struct Generation of a HASH OF ARRAYS:  



• NAME data structure complex data structure struct Access and Printing of a HASH OF ARRAYS:  












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17.10.1 Declaration of a HASH OF ARRAYS



 %HoA = (
        flintstones        => [ "fred", "barney" ],
        jetsons            => [ "george", "jane", "elroy" ],
        simpsons           => [ "homer", "marge", "bart" ],
      );









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17.10.2 Generation of a HASH OF ARRAYS



 # reading from file
 # flintstones: fred barney wilma dino
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $HoA{$1} = [ split ];
 }

 # reading from file; more temps
 # flintstones: fred barney wilma dino
 while ( $line = <> ) {
     ($who, $rest) = split /:\s*/, $line, 2;
     @fields = split ' ', $rest;
     $HoA{$who} = [ @fields ];
 }

 # calling a function that returns a list
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     $HoA{$group} = [ get_family($group) ];
 }

 # likewise, but using temps
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     @members = get_family($group);
     $HoA{$group} = [ @members ];
 }

 # append new members to an existing family
 push @{ $HoA{"flintstones"} }, "wilma", "betty";









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17.10.3 Access and Printing of a HASH OF ARRAYS



 # one element
 $HoA{flintstones}[0] = "Fred";

 # another element
 $HoA{simpsons}[1] =~ s/(\w)/\u$1/;

 # print the whole thing
 foreach $family ( keys %HoA ) {
     print "$family: @{ $HoA{$family} }\n"
 }

 # print the whole thing with indices
 foreach $family ( keys %HoA ) {
     print "family: ";
     foreach $i ( 0 .. $#{ $HoA{$family} } ) {
         print " $i = $HoA{$family}[$i]";
     }
     print "\n";
 }

 # print the whole thing sorted by number of members
 foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
     print "$family: @{ $HoA{$family} }\n"
 }

 # print the whole thing sorted by number of members and name
 foreach $family ( sort {
                            @{$HoA{$b}} <=> @{$HoA{$a}}
                                        ||
                                    $a cmp $b
            } keys %HoA )
 {
     print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
 }









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17.11 ARRAYS OF HASHES







• NAME data structure complex data structure struct Declaration of an ARRAY OF HASHES:  



• NAME data structure complex data structure struct Generation of an ARRAY OF HASHES:  



• NAME data structure complex data structure struct Access and Printing of an ARRAY OF HASHES:  












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17.11.1 Declaration of an ARRAY OF HASHES



 @AoH = (
        {
            Lead     => "fred",
            Friend   => "barney",
        },
        {
            Lead     => "george",
            Wife     => "jane",
            Son      => "elroy",
        },
        {
            Lead     => "homer",
            Wife     => "marge",
            Son      => "bart",
        }
  );









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17.11.2 Generation of an ARRAY OF HASHES



 # reading from file
 # format: LEAD=fred FRIEND=barney
 while ( <> ) {
     $rec = {};
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $rec->{$key} = $value;
     }
     push @AoH, $rec;
 }


 # reading from file
 # format: LEAD=fred FRIEND=barney
 # no temp
 while ( <> ) {
     push @AoH, { split /[\s+=]/ };
 }

 # calling a function  that returns a key/value pair list, like
 # "lead","fred","daughter","pebbles"
 while ( %fields = getnextpairset() ) {
     push @AoH, { %fields };
 }

 # likewise, but using no temp vars
 while (<>) {
     push @AoH, { parsepairs($_) };
 }

 # add key/value to an element
 $AoH[0]{pet} = "dino";
 $AoH[2]{pet} = "santa's little helper";









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17.11.3 Access and Printing of an ARRAY OF HASHES



 # one element
 $AoH[0]{lead} = "fred";

 # another element
 $AoH[1]{lead} =~ s/(\w)/\u$1/;

 # print the whole thing with refs
 for $href ( @AoH ) {
     print "{ ";
     for $role ( keys %$href ) {
         print "$role=$href->{$role} ";
     }
     print "}\n";
 }

 # print the whole thing with indices
 for $i ( 0 .. $#AoH ) {
     print "$i is { ";
     for $role ( keys %{ $AoH[$i] } ) {
         print "$role=$AoH[$i]{$role} ";
     }
     print "}\n";
 }

 # print the whole thing one at a time
 for $i ( 0 .. $#AoH ) {
     for $role ( keys %{ $AoH[$i] } ) {
         print "elt $i $role is $AoH[$i]{$role}\n";
     }
 }









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17.12 HASHES OF HASHES







• NAME data structure complex data structure struct Declaration of a HASH OF HASHES:  



• NAME data structure complex data structure struct Generation of a HASH OF HASHES:  



• NAME data structure complex data structure struct Access and Printing of a HASH OF HASHES:  












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17.12.1 Declaration of a HASH OF HASHES



 %HoH = (
        flintstones => {
                lead      => "fred",
                pal       => "barney",
        },
        jetsons     => {
                lead      => "george",
                wife      => "jane",
                "his boy" => "elroy",
        },
        simpsons    => {
                lead      => "homer",
                wife      => "marge",
                kid       => "bart",
        },
 );









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17.12.2 Generation of a HASH OF HASHES



 # reading from file
 # flintstones: lead=fred pal=barney wife=wilma pet=dino
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $who = $1;
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $HoH{$who}{$key} = $value;
     }


 # reading from file; more temps
 while ( <> ) {
     next unless s/^(.*?):\s*//;
     $who = $1;
     $rec = {};
     $HoH{$who} = $rec;
     for $field ( split ) {
         ($key, $value) = split /=/, $field;
         $rec->{$key} = $value;
     }
 }

 # calling a function  that returns a key,value hash
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     $HoH{$group} = { get_family($group) };
 }

 # likewise, but using temps
 for $group ( "simpsons", "jetsons", "flintstones" ) {
     %members = get_family($group);
     $HoH{$group} = { %members };
 }

 # append new members to an existing family
 %new_folks = (
     wife => "wilma",
     pet  => "dino",
 );

 for $what (keys %new_folks) {
     $HoH{flintstones}{$what} = $new_folks{$what};
 }









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17.12.3 Access and Printing of a HASH OF HASHES



 # one element
 $HoH{flintstones}{wife} = "wilma";

 # another element
 $HoH{simpsons}{lead} =~ s/(\w)/\u$1/;

 # print the whole thing
 foreach $family ( keys %HoH ) {
     print "$family: { ";
     for $role ( keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

 # print the whole thing  somewhat sorted
 foreach $family ( sort keys %HoH ) {
     print "$family: { ";
     for $role ( sort keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }


 # print the whole thing sorted by number of members
 foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
     print "$family: { ";
     for $role ( sort keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }

 # establish a sort order (rank) for each role
 $i = 0;
 for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }

 # now print the whole thing sorted by number of members
 foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) {
     print "$family: { ";
     # and print these according to rank order
     for $role ( sort { $rank{$a} <=> $rank{$b} }  keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
     }
     print "}\n";
 }









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17.13 MORE ELABORATE RECORDS








• NAME data structure complex data structure struct Declaration of MORE ELABORATE RECORDS:  



• NAME data structure complex data structure struct Declaration of a HASH OF COMPLEX RECORDS:  



• NAME data structure complex data structure struct Generation of a HASH OF COMPLEX RECORDS:  












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17.13.1 Declaration of MORE ELABORATE RECORDS



Here’s a sample showing how to create and use a record whose fields are of
many different sorts:



     $rec = {
         TEXT      => $string,
         SEQUENCE  => [ @old_values ],
         LOOKUP    => { %some_table },
         THATCODE  => \&some_function,
         THISCODE  => sub { $_[0] ** $_[1] },
         HANDLE    => \*STDOUT,
     };

     print $rec->{TEXT};

     print $rec->{SEQUENCE}[0];
     $last = pop @ { $rec->{SEQUENCE} };

     print $rec->{LOOKUP}{"key"};
     ($first_k, $first_v) = each %{ $rec->{LOOKUP} };

     $answer = $rec->{THATCODE}->($arg);
     $answer = $rec->{THISCODE}->($arg1, $arg2);

     # careful of extra block braces on fh ref
     print { $rec->{HANDLE} } "a string\n";

     use FileHandle;
     $rec->{HANDLE}->autoflush(1);
     $rec->{HANDLE}->print(" a string\n");









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17.13.2 Declaration of a HASH OF COMPLEX RECORDS



     %TV = (
        flintstones => {
            series   => "flintstones",
            nights   => [ qw(monday thursday friday) ],
            members  => [
                { name => "fred",    role => "lead", age  => 36, },
                { name => "wilma",   role => "wife", age  => 31, },
                { name => "pebbles", role => "kid",  age  =>  4, },
            ],
        },

        jetsons     => {
            series   => "jetsons",
            nights   => [ qw(wednesday saturday) ],
            members  => [
                { name => "george",  role => "lead", age  => 41, },
                { name => "jane",    role => "wife", age  => 39, },
                { name => "elroy",   role => "kid",  age  =>  9, },
            ],
         },

        simpsons    => {
            series   => "simpsons",
            nights   => [ qw(monday) ],
            members  => [
                { name => "homer", role => "lead", age  => 34, },
                { name => "marge", role => "wife", age => 37, },
                { name => "bart",  role => "kid",  age  =>  11, },
            ],
         },
      );









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17.13.3 Generation of a HASH OF COMPLEX RECORDS



     # reading from file
     # this is most easily done by having the file itself be
     # in the raw data format as shown above.  perl is happy
     # to parse complex data structures if declared as data, so
     # sometimes it's easiest to do that

     # here's a piece by piece build up
     $rec = {};
     $rec->{series} = "flintstones";
     $rec->{nights} = [ find_days() ];

     @members = ();
     # assume this file in field=value syntax
     while (<>) {
         %fields = split /[\s=]+/;
         push @members, { %fields };
     }
     $rec->{members} = [ @members ];

     # now remember the whole thing
     $TV{ $rec->{series} } = $rec;

     ###########################################################
     # now, you might want to make interesting extra fields that
     # include pointers back into the same data structure so if
     # change one piece, it changes everywhere, like for example
     # if you wanted a {kids} field that was a reference
     # to an array of the kids' records without having duplicate
     # records and thus update problems.
     ###########################################################
     foreach $family (keys %TV) {
         $rec = $TV{$family}; # temp pointer
         @kids = ();
         for $person ( @{ $rec->{members} } ) {
             if ($person->{role} =~ /kid|son|daughter/) {
                 push @kids, $person;
             }
         }
         # REMEMBER: $rec and $TV{$family} point to same data!!
         $rec->{kids} = [ @kids ];
     }

     # you copied the array, but the array itself contains pointers
     # to uncopied objects. this means that if you make bart get
     # older via

     $TV{simpsons}{kids}[0]{age}++;

     # then this would also change in
     print $TV{simpsons}{members}[2]{age};

     # because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
     # both point to the same underlying anonymous hash table

     # print the whole thing
     foreach $family ( keys %TV ) {
         print "the $family";
         print " is on during @{ $TV{$family}{nights} }\n";
         print "its members are:\n";
         for $who ( @{ $TV{$family}{members} } ) {
             print " $who->{name} ($who->{role}), age $who->{age}\n";
         }
         print "it turns out that $TV{$family}{lead} has ";
         print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
         print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
         print "\n";
     }









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17.14 Database Ties



You cannot easily tie a multilevel data structure (such as a hash of
hashes) to a dbm file.  The first problem is that all but GDBM and
Berkeley DB have size limitations, but beyond that, you also have problems
with how references are to be represented on disk.  One experimental
module that does partially attempt to address this need is the MLDBM
module.  Check your nearest CPAN site as described in (perlmodlib) for
source code to MLDBM.









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17.15 SEE ALSO



(perlref), perllol NAME, perldata NAME, (perlobj)









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17.16 AUTHOR



Tom Christiansen <tchrist@perl.com>



Last update:
Wed Oct 23 04:57:50 MET DST 1996









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18 perldtrace





• perldtrace NAME:  



• perldtrace SYNOPSIS:  



• perldtrace DESCRIPTION:  



• perldtrace HISTORY:  



• perldtrace PROBES:  



• perldtrace EXAMPLES:  



• perldtrace REFERENCES:  



• perldtrace AUTHORS:  












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18.1 NAME



perldtrace - Perl’s support for DTrace









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18.2 SYNOPSIS



    # dtrace -Zn 'perl::sub-entry, perl::sub-return { trace(copyinstr(arg0)) }'
    dtrace: description 'perl::sub-entry, perl::sub-return ' matched 10 probes

    # perl -E 'sub outer { inner(@_) } sub inner { say shift } outer("hello")'
    hello

    (dtrace output)
    CPU     ID                    FUNCTION:NAME
      0  75915       Perl_pp_entersub:sub-entry   BEGIN
      0  75915       Perl_pp_entersub:sub-entry   import
      0  75922      Perl_pp_leavesub:sub-return   import
      0  75922      Perl_pp_leavesub:sub-return   BEGIN
      0  75915       Perl_pp_entersub:sub-entry   outer
      0  75915       Perl_pp_entersub:sub-entry   inner
      0  75922      Perl_pp_leavesub:sub-return   inner
      0  75922      Perl_pp_leavesub:sub-return   outer









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18.3 DESCRIPTION



DTrace is a framework for comprehensive system- and application-level
tracing. Perl is a DTrace provider, meaning it exposes several
probes for instrumentation. You can use these in conjunction
with kernel-level probes, as well as probes from other providers
such as MySQL, in order to diagnose software defects, or even just
your application’s bottlenecks.



Perl must be compiled with the -Dusedtrace option in order to
make use of the provided probes. While DTrace aims to have no
overhead when its instrumentation is not active, Perl’s support
itself cannot uphold that guarantee, so it is built without DTrace
probes under most systems. One notable exception is that Mac OS X
ships a /usr/bin/perl with DTrace support enabled.









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18.4 HISTORY





5.10.1




Perl’s initial DTrace support was added, providing sub-entry and
sub-return probes.





5.14.0




The sub-entry and sub-return probes gain a fourth argument: the
package name of the function.





5.16.0




The phase-change probe was added.














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18.5 PROBES





sub-entry(SUBNAME, FILE, LINE, PACKAGE)




Traces the entry of any subroutine. Note that all of the variables
refer to the subroutine that is being invoked; there is currently
no way to get ahold of any information about the subroutine’s
caller from a DTrace action.



    :*perl*::sub-entry {
        printf("%s::%s entered at %s line %d\n",
               copyinstr(arg3), copyinstr(arg0), copyinstr(arg1), arg0);
    }





sub-return(SUBNAME, FILE, LINE, PACKAGE)




Traces the exit of any subroutine. Note that all of the variables
refer to the subroutine that is returning; there is currently no
way to get ahold of any information about the subroutine’s caller
from a DTrace action.



    :*perl*::sub-return {
        printf("%s::%s returned at %s line %d\n",
               copyinstr(arg3), copyinstr(arg0), copyinstr(arg1), arg0);
    }





phase-change(NEWPHASE, OLDPHASE)




Traces changes to Perl’s interpreter state. You can internalize this
as tracing changes to Perl’s ${^GLOBAL_PHASE} variable, especially
since the values for NEWPHASE and OLDPHASE are the strings that
${^GLOBAL_PHASE} reports.



    :*perl*::phase-change {
        printf("Phase changed from %s to %s\n",
            copyinstr(arg1), copyinstr(arg0));
    }














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18.6 EXAMPLES





Most frequently called functions




    # dtrace -qZn 'sub-entry { @[strjoin(strjoin(copyinstr(arg3),"::"),copyinstr(arg0))] = count() } END {trunc(@, 10)}'

    Class::MOP::Attribute::slots                                    400
    Try::Tiny::catch                                                411
    Try::Tiny::try                                                  411
    Class::MOP::Instance::inline_slot_access                        451
    Class::MOP::Class::Immutable::Trait:::around                    472
    Class::MOP::Mixin::AttributeCore::has_initializer               496
    Class::MOP::Method::Wrapped::__ANON__                           544
    Class::MOP::Package::_package_stash                             737
    Class::MOP::Class::initialize                                  1128
    Class::MOP::get_metaclass_by_name                              1204





Trace function calls




    # dtrace -qFZn 'sub-entry, sub-return { trace(copyinstr(arg0)) }'

    0  -> Perl_pp_entersub                        BEGIN
    0  <- Perl_pp_leavesub                        BEGIN
    0  -> Perl_pp_entersub                        BEGIN
    0    -> Perl_pp_entersub                      import
    0    <- Perl_pp_leavesub                      import
    0  <- Perl_pp_leavesub                        BEGIN
    0  -> Perl_pp_entersub                        BEGIN
    0    -> Perl_pp_entersub                      dress
    0    <- Perl_pp_leavesub                      dress
    0    -> Perl_pp_entersub                      dirty
    0    <- Perl_pp_leavesub                      dirty
    0    -> Perl_pp_entersub                      whiten
    0    <- Perl_pp_leavesub                      whiten
    0  <- Perl_dounwind                           BEGIN





Function calls during interpreter cleanup




    # dtrace -Zn 'phase-change /copyinstr(arg0) == "END"/ { self->ending = 1 } sub-entry /self->ending/ { trace(copyinstr(arg0)) }'

    CPU     ID                    FUNCTION:NAME
      1  77214       Perl_pp_entersub:sub-entry   END
      1  77214       Perl_pp_entersub:sub-entry   END
      1  77214       Perl_pp_entersub:sub-entry   cleanup
      1  77214       Perl_pp_entersub:sub-entry   _force_writable
      1  77214       Perl_pp_entersub:sub-entry   _force_writable





System calls at compile time




    # dtrace -qZn 'phase-change /copyinstr(arg0) == "START"/ { self->interesting = 1 } phase-change /copyinstr(arg0) == "RUN"/ { self->interesting = 0 } syscall::: /self->interesting/ { @[probefunc] = count() } END { trunc(@, 3) }'

    lseek                                                           310
    read                                                            374
    stat64                                                         1056














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18.7 REFERENCES





DTrace User Guide




http://download.oracle.com/docs/cd/E19082-01/819-3620/index.html





DTrace: Dynamic Tracing in Oracle Solaris, Mac OS X and FreeBSD




http://www.amazon.com/DTrace-Dynamic-Tracing-Solaris-FreeBSD/dp/0132091518/














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18.8 AUTHORS



Shawn M Moore sartak@gmail.com









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19 perlebcdic





• perlebcdic NAME:  



• perlebcdic DESCRIPTION:  



• perlebcdic COMMON CHARACTER CODE SETS:  



• perlebcdic SINGLE OCTET TABLES:  



• perlebcdic IDENTIFYING CHARACTER CODE SETS:  



• perlebcdic CONVERSIONS:  



• perlebcdic OPERATOR DIFFERENCES:  



• perlebcdic FUNCTION DIFFERENCES:  



• perlebcdic REGULAR EXPRESSION DIFFERENCES:  



• perlebcdic SOCKETS:  



• perlebcdic SORTING:  



• perlebcdic TRANSFORMATION FORMATS:  



• perlebcdic Hashing order and checksums:  



• perlebcdic I18N AND L10N:  



• perlebcdic MULTI-OCTET CHARACTER SETS:  



• perlebcdic OS ISSUES:  



• perlebcdic BUGS:  



• perlebcdic SEE ALSO:  



• perlebcdic REFERENCES:  



• perlebcdic HISTORY:  



• perlebcdic AUTHOR:  












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19.1 NAME



perlebcdic - Considerations for running Perl on EBCDIC platforms









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19.2 DESCRIPTION



An exploration of some of the issues facing Perl programmers
on EBCDIC based computers.  We do not cover localization, 
internationalization, or multi-byte character set issues other
than some discussion of UTF-8 and UTF-EBCDIC.



Portions that are still incomplete are marked with XXX.



Perl used to work on EBCDIC machines, but there are now areas of the code where
it doesn’t.  If you want to use Perl on an EBCDIC machine, please let us know
by sending mail to perlbug@perl.org









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19.3 COMMON CHARACTER CODE SETS





• perlebcdic ASCII:  



• perlebcdic ISO 8859:  



• perlebcdic Latin 1 (ISO 8859-1):  



• perlebcdic EBCDIC:  



• perlebcdic The 13 variant characters:  



• perlebcdic 0037:  



• perlebcdic 1047:  



• perlebcdic POSIX-BC:  



• perlebcdic Unicode code points versus EBCDIC code points:  



• perlebcdic Remaining Perl Unicode problems in EBCDIC:  



• perlebcdic Unicode and UTF:  



• perlebcdic Using Encode:  












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19.3.1 ASCII



The American Standard Code for Information Interchange (ASCII or US-ASCII) is a
set of
integers running from 0 to 127 (decimal) that imply character 
interpretation by the display and other systems of computers.  
The range 0..127 can be covered by setting the bits in a 7-bit binary 
digit, hence the set is sometimes referred to as "7-bit ASCII".  
ASCII was described by the American National Standards Institute 
document ANSI X3.4-1986.  It was also described by ISO 646:1991 
(with localization for currency symbols).  The full ASCII set is 
given in the table below as the first 128 elements.  Languages that 
can be written adequately with the characters in ASCII include 
English, Hawaiian, Indonesian, Swahili and some Native American 
languages.



There are many character sets that extend the range of integers
from 0..2**7-1 up to 2**8-1, or 8 bit bytes (octets if you prefer).
One common one is the ISO 8859-1 character set.









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19.3.2 ISO 8859



The ISO 8859-$n are a collection of character code sets from the 
International Organization for Standardization (ISO) each of which 
adds characters to the ASCII set that are typically found in European 
languages many of which are based on the Roman, or Latin, alphabet.









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19.3.3 Latin 1 (ISO 8859-1)



A particular 8-bit extension to ASCII that includes grave and acute 
accented Latin characters.  Languages that can employ ISO 8859-1 
include all the languages covered by ASCII as well as Afrikaans, 
Albanian, Basque, Catalan, Danish, Faroese, Finnish, Norwegian, 
Portuguese, Spanish, and Swedish.  Dutch is covered albeit without 
the ij ligature.  French is covered too but without the oe ligature. 
German can use ISO 8859-1 but must do so without German-style
quotation marks.  This set is based on Western European extensions 
to ASCII and is commonly encountered in world wide web work.
In IBM character code set identification terminology ISO 8859-1 is
also known as CCSID 819 (or sometimes 0819 or even 00819).









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19.3.4 EBCDIC



The Extended Binary Coded Decimal Interchange Code refers to a 
large collection of single- and multi-byte coded character sets that are
different from ASCII or ISO 8859-1 and are all slightly different from each
other; they typically run on host computers.  The EBCDIC encodings derive from
8-bit byte extensions of Hollerith punched card encodings.  The layout on the
cards was such that high bits were set for the upper and lower case alphabet
characters [a-z] and [A-Z], but there were gaps within each Latin alphabet
range.



Some IBM EBCDIC character sets may be known by character code set 
identification numbers (CCSID numbers) or code page numbers.



Perl can be compiled on platforms that run any of three commonly used EBCDIC
character sets, listed below.









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19.3.5 The 13 variant characters



Among IBM EBCDIC character code sets there are 13 characters that
are often mapped to different integer values.  Those characters
are known as the 13 "variant" characters and are:



    \ [ ] { } ^ ~ ! # | $ @ ` 



When Perl is compiled for a platform, it looks at some of these characters to
guess which EBCDIC character set the platform uses, and adapts itself
accordingly to that platform.  If the platform uses a character set that is not
one of the three Perl knows about, Perl will either fail to compile, or
mistakenly and silently choose one of the three.
They are:









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19.3.6 0037



Character code set ID 0037 is a mapping of the ASCII plus Latin-1 
characters (i.e. ISO 8859-1) to an EBCDIC set.  0037 is used 
in North American English locales on the OS/400 operating system 
that runs on AS/400 computers.  CCSID 0037 differs from ISO 8859-1 
in 237 places, in other words they agree on only 19 code point values.









Next: , Previous: perlebcdic 0037, Up: perlebcdic COMMON CHARACTER CODE SETS   [Contents]





19.3.7 1047



Character code set ID 1047 is also a mapping of the ASCII plus 
Latin-1 characters (i.e. ISO 8859-1) to an EBCDIC set.  1047 is 
used under Unix System Services for OS/390 or z/OS, and OpenEdition 
for VM/ESA.  CCSID 1047 differs from CCSID 0037 in eight places.









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19.3.8 POSIX-BC



The EBCDIC code page in use on Siemens’ BS2000 system is distinct from
1047 and 0037.  It is identified below as the POSIX-BC set.









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19.3.9 Unicode code points versus EBCDIC code points



In Unicode terminology a code point is the number assigned to a
character: for example, in EBCDIC the character "A" is usually assigned
the number 193.  In Unicode the character "A" is assigned the number 65.
This causes a problem with the semantics of the pack/unpack "U", which
are supposed to pack Unicode code points to characters and back to numbers.
The problem is: which code points to use for code points less than 256?
(for 256 and over there’s no problem: Unicode code points are used)
In EBCDIC, for the low 256 the EBCDIC code points are used.  This
means that the equivalences



    pack("U", ord($character)) eq $character
    unpack("U", $character) == ord $character



will hold.  (If Unicode code points were applied consistently over
all the possible code points, pack("U",ord("A")) would in EBCDIC
equal A with acute or chr(101), and unpack("U", "A") would equal
65, or non-breaking space, not 193, or ord "A".)









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19.3.10 Remaining Perl Unicode problems in EBCDIC











Next: , Previous: perlebcdic Remaining Perl Unicode problems in EBCDIC, Up: perlebcdic COMMON CHARACTER CODE SETS   [Contents]





19.3.11 Unicode and UTF



UTF stands for Unicode Transformation Format.
UTF-8 is an encoding of Unicode into a sequence of 8-bit byte chunks, based on
ASCII and Latin-1.
The length of a sequence required to represent a Unicode code point
depends on the ordinal number of that code point,
with larger numbers requiring more bytes.
UTF-EBCDIC is like UTF-8, but based on EBCDIC.



You may see the term invariant character or code point.
This simply means that the character has the same numeric
value when encoded as when not.
(Note that this is a very different concept from The 13 variant characters
mentioned above.)
For example, the ordinal value of ’A’ is 193 in most EBCDIC code pages,
and also is 193 when encoded in UTF-EBCDIC.
All variant code points occupy at least two bytes when encoded.
In UTF-8, the code points corresponding to the lowest 128
ordinal numbers (0 - 127: the ASCII characters) are invariant.
In UTF-EBCDIC, there are 160 invariant characters.
(If you care, the EBCDIC invariants are those characters
which have ASCII equivalents, plus those that correspond to
the C1 controls (80..9f on ASCII platforms).)



A string encoded in UTF-EBCDIC may be longer (but never shorter) than
one encoded in UTF-8.









Previous: perlebcdic Unicode and UTF, Up: perlebcdic COMMON CHARACTER CODE SETS   [Contents]





19.3.12 Using Encode



Starting from Perl 5.8 you can use the standard new module Encode
to translate from EBCDIC to Latin-1 code points.
Encode knows about more EBCDIC character sets than Perl can currently
be compiled to run on.



   use Encode 'from_to';

   my %ebcdic = ( 176 => 'cp37', 95 => 'cp1047', 106 => 'posix-bc' );

   # $a is in EBCDIC code points
   from_to($a, $ebcdic{ord '^'}, 'latin1');
   # $a is ISO 8859-1 code points



and from Latin-1 code points to EBCDIC code points



   use Encode 'from_to';

   my %ebcdic = ( 176 => 'cp37', 95 => 'cp1047', 106 => 'posix-bc' );

   # $a is ISO 8859-1 code points
   from_to($a, 'latin1', $ebcdic{ord '^'});
   # $a is in EBCDIC code points



For doing I/O it is suggested that you use the autotranslating features
of PerlIO, see perluniintro NAME.



Since version 5.8 Perl uses the new PerlIO I/O library.  This enables
you to use different encodings per IO channel.  For example you may use



    use Encode;
    open($f, ">:encoding(ascii)", "test.ascii");
    print $f "Hello World!\n";
    open($f, ">:encoding(cp37)", "test.ebcdic");
    print $f "Hello World!\n";
    open($f, ">:encoding(latin1)", "test.latin1");
    print $f "Hello World!\n";
    open($f, ">:encoding(utf8)", "test.utf8");
    print $f "Hello World!\n";



to get four files containing "Hello World!\n" in ASCII, CP 0037 EBCDIC,
ISO 8859-1 (Latin-1) (in this example identical to ASCII since only ASCII
characters were printed), and 
UTF-EBCDIC (in this example identical to normal EBCDIC since only characters
that don’t differ between EBCDIC and UTF-EBCDIC were printed).  See the
documentation of Encode::PerlIO for details.



As the PerlIO layer uses raw IO (bytes) internally, all this totally
ignores things like the type of your filesystem (ASCII or EBCDIC).









Next: , Previous: perlebcdic COMMON CHARACTER CODE SETS, Up: perlebcdic   [Contents]





19.4 SINGLE OCTET TABLES



The following tables list the ASCII and Latin 1 ordered sets including
the subsets: C0 controls (0..31), ASCII graphics (32..7e), delete (7f),
C1 controls (80..9f), and Latin-1 (a.k.a. ISO 8859-1) (a0..ff).  In the 
table non-printing control character names as well as the Latin 1 
extensions to ASCII have been labelled with character names roughly 
corresponding to The Unicode Standard, Version 3.0 albeit with 
substitutions such as s/LATIN// and s/VULGAR// in all cases, 
s/CAPITAL LETTER// in some cases, and s/SMALL LETTER ([A-Z])/\l$1/ 
in some other cases.  The "names" of the controls listed here are 
the Unicode Version 1 names, except for the few that don’t have names, in which
case the names in the Wikipedia article were used
(http://en.wikipedia.org/wiki/C0_and_C1_control_codes).
The differences between the 0037 and 1047 sets are 
flagged with ***.  The differences between the 1047 and POSIX-BC sets 
are flagged with ###.  All ord() numbers listed are decimal.  If you 
would rather see this table listing octal values then run the table 
(that is, the pod version of this document since this recipe may not 
work with a pod2_other_format translation) through:





recipe 0









    perl -ne 'if(/(.{43})(\d+)\s+(\d+)\s+(\d+)\s+(\d+)/)' \
     -e '{printf("%s%-9.03o%-9.03o%-9.03o%.03o\n",$1,$2,$3,$4,$5)}' \
     perlebcdic.pod



If you want to retain the UTF-x code points then in script form you
might want to write:





recipe 1









 open(FH,"<perlebcdic.pod") or die "Could not open perlebcdic.pod: $!";
 while (<FH>) {
     if (/(.{43})(\d+)\s+(\d+)\s+(\d+)\s+(\d+)\s+(\d+)\.?(\d*)\s+(\d+)\.?(\d*)/)
     {
         if ($7 ne '' && $9 ne '') {
             printf(
                "%s%-9.03o%-9.03o%-9.03o%-9.03o%-3o.%-5o%-3o.%.03o\n",
                                            $1,$2,$3,$4,$5,$6,$7,$8,$9);
         }
         elsif ($7 ne '') {
             printf("%s%-9.03o%-9.03o%-9.03o%-9.03o%-3o.%-5o%.03o\n",
                                           $1,$2,$3,$4,$5,$6,$7,$8);
         }
         else {
             printf("%s%-9.03o%-9.03o%-9.03o%-9.03o%-9.03o%.03o\n",
                                                $1,$2,$3,$4,$5,$6,$8);
         }
     }
 }



If you would rather see this table listing hexadecimal values then
run the table through:





recipe 2









    perl -ne 'if(/(.{43})(\d+)\s+(\d+)\s+(\d+)\s+(\d+)/)' \
     -e '{printf("%s%-9.02X%-9.02X%-9.02X%.02X\n",$1,$2,$3,$4,$5)}' \
     perlebcdic.pod



Or, in order to retain the UTF-x code points in hexadecimal:





recipe 3









 open(FH,"<perlebcdic.pod") or die "Could not open perlebcdic.pod: $!";
 while (<FH>) {
     if (/(.{43})(\d+)\s+(\d+)\s+(\d+)\s+(\d+)\s+(\d+)\.?(\d*)\s+(\d+)\.?(\d*)/)
     {
         if ($7 ne '' && $9 ne '') {
             printf(
                "%s%-9.02X%-9.02X%-9.02X%-9.02X%-2X.%-6.02X%02X.%02X\n",
                                           $1,$2,$3,$4,$5,$6,$7,$8,$9);
         }
         elsif ($7 ne '') {
             printf("%s%-9.02X%-9.02X%-9.02X%-9.02X%-2X.%-6.02X%02X\n",
                                              $1,$2,$3,$4,$5,$6,$7,$8);
         }
         else {
             printf("%s%-9.02X%-9.02X%-9.02X%-9.02X%-9.02X%02X\n",
                                                  $1,$2,$3,$4,$5,$6,$8);
         }
     }
 }


                                      ISO 8859-1  CCSID    CCSID                    CCSID 1047
 chr                                  CCSID 0819  0037     1047    POSIX-BC  UTF-8  UTF-EBCDIC
 ----------------------------------------------------------------------------------------------
 <NULL>                                    0        0        0        0        0        0 
 <START OF HEADING>                        1        1        1        1        1        1
 <START OF TEXT>                           2        2        2        2        2        2
 <END OF TEXT>                             3        3        3        3        3        3
 <END OF TRANSMISSION>                     4        55       55       55       4        55 
 <ENQUIRY>                                 5        45       45       45       5        45 
 <ACKNOWLEDGE>                             6        46       46       46       6        46 
 <BELL>                                    7        47       47       47       7        47 
 <BACKSPACE>                               8        22       22       22       8        22 
 <HORIZONTAL TABULATION>                   9        5        5        5        9        5 
 <LINE FEED>                               10       37       21       21       10       21       ***
 <VERTICAL TABULATION>                     11       11       11       11       11       11
 <FORM FEED>                               12       12       12       12       12       12
 <CARRIAGE RETURN>                         13       13       13       13       13       13
 <SHIFT OUT>                               14       14       14       14       14       14
 <SHIFT IN>                                15       15       15       15       15       15
 <DATA LINK ESCAPE>                        16       16       16       16       16       16
 <DEVICE CONTROL ONE>                      17       17       17       17       17       17
 <DEVICE CONTROL TWO>                      18       18       18       18       18       18
 <DEVICE CONTROL THREE>                    19       19       19       19       19       19
 <DEVICE CONTROL FOUR>                     20       60       60       60       20       60
 <NEGATIVE ACKNOWLEDGE>                    21       61       61       61       21       61
 <SYNCHRONOUS IDLE>                        22       50       50       50       22       50
 <END OF TRANSMISSION BLOCK>               23       38       38       38       23       38
 <CANCEL>                                  24       24       24       24       24       24
 <END OF MEDIUM>                           25       25       25       25       25       25
 <SUBSTITUTE>                              26       63       63       63       26       63
 <ESCAPE>                                  27       39       39       39       27       39
 <FILE SEPARATOR>                          28       28       28       28       28       28
 <GROUP SEPARATOR>                         29       29       29       29       29       29
 <RECORD SEPARATOR>                        30       30       30       30       30       30
 <UNIT SEPARATOR>                          31       31       31       31       31       31
 <SPACE>                                   32       64       64       64       32       64
 !                                         33       90       90       90       33       90
 "                                         34       127      127      127      34       127
 #                                         35       123      123      123      35       123
 $                                         36       91       91       91       36       91
 %                                         37       108      108      108      37       108
 &                                         38       80       80       80       38       80
 '                                         39       125      125      125      39       125
 (                                         40       77       77       77       40       77
 )                                         41       93       93       93       41       93
 *                                         42       92       92       92       42       92
 +                                         43       78       78       78       43       78
 ,                                         44       107      107      107      44       107
 -                                         45       96       96       96       45       96
 .                                         46       75       75       75       46       75
 /                                         47       97       97       97       47       97
 0                                         48       240      240      240      48       240
 1                                         49       241      241      241      49       241
 2                                         50       242      242      242      50       242
 3                                         51       243      243      243      51       243
 4                                         52       244      244      244      52       244
 5                                         53       245      245      245      53       245
 6                                         54       246      246      246      54       246
 7                                         55       247      247      247      55       247
 8                                         56       248      248      248      56       248
 9                                         57       249      249      249      57       249
 :                                         58       122      122      122      58       122
 ;                                         59       94       94       94       59       94
 <                                         60       76       76       76       60       76
 =                                         61       126      126      126      61       126
 >                                         62       110      110      110      62       110
 ?                                         63       111      111      111      63       111
 @                                         64       124      124      124      64       124
 A                                         65       193      193      193      65       193
 B                                         66       194      194      194      66       194
 C                                         67       195      195      195      67       195
 D                                         68       196      196      196      68       196
 E                                         69       197      197      197      69       197
 F                                         70       198      198      198      70       198
 G                                         71       199      199      199      71       199
 H                                         72       200      200      200      72       200
 I                                         73       201      201      201      73       201
 J                                         74       209      209      209      74       209
 K                                         75       210      210      210      75       210
 L                                         76       211      211      211      76       211
 M                                         77       212      212      212      77       212
 N                                         78       213      213      213      78       213
 O                                         79       214      214      214      79       214
 P                                         80       215      215      215      80       215
 Q                                         81       216      216      216      81       216
 R                                         82       217      217      217      82       217
 S                                         83       226      226      226      83       226
 T                                         84       227      227      227      84       227
 U                                         85       228      228      228      85       228
 V                                         86       229      229      229      86       229
 W                                         87       230      230      230      87       230
 X                                         88       231      231      231      88       231
 Y                                         89       232      232      232      89       232
 Z                                         90       233      233      233      90       233
 [                                         91       186      173      187      91       173      *** ###
 \                                         92       224      224      188      92       224      ### 
 ]                                         93       187      189      189      93       189      ***
 ^                                         94       176      95       106      94       95       *** ###
 _                                         95       109      109      109      95       109
 `                                         96       121      121      74       96       121      ###
 a                                         97       129      129      129      97       129
 b                                         98       130      130      130      98       130
 c                                         99       131      131      131      99       131
 d                                         100      132      132      132      100      132
 e                                         101      133      133      133      101      133
 f                                         102      134      134      134      102      134
 g                                         103      135      135      135      103      135
 h                                         104      136      136      136      104      136
 i                                         105      137      137      137      105      137
 j                                         106      145      145      145      106      145
 k                                         107      146      146      146      107      146
 l                                         108      147      147      147      108      147
 m                                         109      148      148      148      109      148
 n                                         110      149      149      149      110      149
 o                                         111      150      150      150      111      150
 p                                         112      151      151      151      112      151
 q                                         113      152      152      152      113      152
 r                                         114      153      153      153      114      153
 s                                         115      162      162      162      115      162
 t                                         116      163      163      163      116      163
 u                                         117      164      164      164      117      164
 v                                         118      165      165      165      118      165
 w                                         119      166      166      166      119      166
 x                                         120      167      167      167      120      167
 y                                         121      168      168      168      121      168
 z                                         122      169      169      169      122      169
 {                                         123      192      192      251      123      192      ###
 |                                         124      79       79       79       124      79
 }                                         125      208      208      253      125      208      ###
 ~                                         126      161      161      255      126      161      ###
 <DELETE>                                  127      7        7        7        127      7
 <PADDING CHARACTER>                       128      32       32       32       194.128  32
 <HIGH OCTET PRESET>                       129      33       33       33       194.129  33
 <BREAK PERMITTED HERE>                    130      34       34       34       194.130  34
 <NO BREAK HERE>                           131      35       35       35       194.131  35
 <INDEX>                                   132      36       36       36       194.132  36
 <NEXT LINE>                               133      21       37       37       194.133  37       ***
 <START OF SELECTED AREA>                  134      6        6        6        194.134  6
 <END OF SELECTED AREA>                    135      23       23       23       194.135  23
 <CHARACTER TABULATION SET>                136      40       40       40       194.136  40
 <CHARACTER TABULATION WITH JUSTIFICATION> 137      41       41       41       194.137  41
 <LINE TABULATION SET>                     138      42       42       42       194.138  42
 <PARTIAL LINE FORWARD>                    139      43       43       43       194.139  43
 <PARTIAL LINE BACKWARD>                   140      44       44       44       194.140  44
 <REVERSE LINE FEED>                       141      9        9        9        194.141  9
 <SINGLE SHIFT TWO>                        142      10       10       10       194.142  10
 <SINGLE SHIFT THREE>                      143      27       27       27       194.143  27
 <DEVICE CONTROL STRING>                   144      48       48       48       194.144  48
 <PRIVATE USE ONE>                         145      49       49       49       194.145  49
 <PRIVATE USE TWO>                         146      26       26       26       194.146  26
 <SET TRANSMIT STATE>                      147      51       51       51       194.147  51
 <CANCEL CHARACTER>                        148      52       52       52       194.148  52
 <MESSAGE WAITING>                         149      53       53       53       194.149  53
 <START OF GUARDED AREA>                   150      54       54       54       194.150  54
 <END OF GUARDED AREA>                     151      8        8        8        194.151  8
 <START OF STRING>                         152      56       56       56       194.152  56
 <SINGLE GRAPHIC CHARACTER INTRODUCER>     153      57       57       57       194.153  57
 <SINGLE CHARACTER INTRODUCER>             154      58       58       58       194.154  58
 <CONTROL SEQUENCE INTRODUCER>             155      59       59       59       194.155  59
 <STRING TERMINATOR>                       156      4        4        4        194.156  4
 <OPERATING SYSTEM COMMAND>                157      20       20       20       194.157  20
 <PRIVACY MESSAGE>                         158      62       62       62       194.158  62
 <APPLICATION PROGRAM COMMAND>             159      255      255      95       194.159  255      ###
 <NON-BREAKING SPACE>                      160      65       65       65       194.160  128.65
 <INVERTED EXCLAMATION MARK>               161      170      170      170      194.161  128.66
 <CENT SIGN>                               162      74       74       176      194.162  128.67   ###
 <POUND SIGN>                              163      177      177      177      194.163  128.68
 <CURRENCY SIGN>                           164      159      159      159      194.164  128.69
 <YEN SIGN>                                165      178      178      178      194.165  128.70
 <BROKEN BAR>                              166      106      106      208      194.166  128.71   ###
 <SECTION SIGN>                            167      181      181      181      194.167  128.72
 <DIAERESIS>                               168      189      187      121      194.168  128.73   *** ###
 <COPYRIGHT SIGN>                          169      180      180      180      194.169  128.74
 <FEMININE ORDINAL INDICATOR>              170      154      154      154      194.170  128.81
 <LEFT POINTING GUILLEMET>                 171      138      138      138      194.171  128.82
 <NOT SIGN>                                172      95       176      186      194.172  128.83   *** ###
 <SOFT HYPHEN>                             173      202      202      202      194.173  128.84
 <REGISTERED TRADE MARK SIGN>              174      175      175      175      194.174  128.85
 <MACRON>                                  175      188      188      161      194.175  128.86   ###
 <DEGREE SIGN>                             176      144      144      144      194.176  128.87
 <PLUS-OR-MINUS SIGN>                      177      143      143      143      194.177  128.88
 <SUPERSCRIPT TWO>                         178      234      234      234      194.178  128.89
 <SUPERSCRIPT THREE>                       179      250      250      250      194.179  128.98
 <ACUTE ACCENT>                            180      190      190      190      194.180  128.99
 <MICRO SIGN>                              181      160      160      160      194.181  128.100
 <PARAGRAPH SIGN>                          182      182      182      182      194.182  128.101
 <MIDDLE DOT>                              183      179      179      179      194.183  128.102
 <CEDILLA>                                 184      157      157      157      194.184  128.103
 <SUPERSCRIPT ONE>                         185      218      218      218      194.185  128.104
 <MASC. ORDINAL INDICATOR>                 186      155      155      155      194.186  128.105
 <RIGHT POINTING GUILLEMET>                187      139      139      139      194.187  128.106
 <FRACTION ONE QUARTER>                    188      183      183      183      194.188  128.112
 <FRACTION ONE HALF>                       189      184      184      184      194.189  128.113
 <FRACTION THREE QUARTERS>                 190      185      185      185      194.190  128.114
 <INVERTED QUESTION MARK>                  191      171      171      171      194.191  128.115
 <A WITH GRAVE>                            192      100      100      100      195.128  138.65
 <A WITH ACUTE>                            193      101      101      101      195.129  138.66
 <A WITH CIRCUMFLEX>                       194      98       98       98       195.130  138.67
 <A WITH TILDE>                            195      102      102      102      195.131  138.68
 <A WITH DIAERESIS>                        196      99       99       99       195.132  138.69
 <A WITH RING ABOVE>                       197      103      103      103      195.133  138.70
 <CAPITAL LIGATURE AE>                     198      158      158      158      195.134  138.71
 <C WITH CEDILLA>                          199      104      104      104      195.135  138.72
 <E WITH GRAVE>                            200      116      116      116      195.136  138.73
 <E WITH ACUTE>                            201      113      113      113      195.137  138.74
 <E WITH CIRCUMFLEX>                       202      114      114      114      195.138  138.81
 <E WITH DIAERESIS>                        203      115      115      115      195.139  138.82
 <I WITH GRAVE>                            204      120      120      120      195.140  138.83
 <I WITH ACUTE>                            205      117      117      117      195.141  138.84
 <I WITH CIRCUMFLEX>                       206      118      118      118      195.142  138.85
 <I WITH DIAERESIS>                        207      119      119      119      195.143  138.86
 <CAPITAL LETTER ETH>                      208      172      172      172      195.144  138.87
 <N WITH TILDE>                            209      105      105      105      195.145  138.88
 <O WITH GRAVE>                            210      237      237      237      195.146  138.89
 <O WITH ACUTE>                            211      238      238      238      195.147  138.98
 <O WITH CIRCUMFLEX>                       212      235      235      235      195.148  138.99
 <O WITH TILDE>                            213      239      239      239      195.149  138.100
 <O WITH DIAERESIS>                        214      236      236      236      195.150  138.101
 <MULTIPLICATION SIGN>                     215      191      191      191      195.151  138.102
 <O WITH STROKE>                           216      128      128      128      195.152  138.103
 <U WITH GRAVE>                            217      253      253      224      195.153  138.104  ###
 <U WITH ACUTE>                            218      254      254      254      195.154  138.105
 <U WITH CIRCUMFLEX>                       219      251      251      221      195.155  138.106  ###
 <U WITH DIAERESIS>                        220      252      252      252      195.156  138.112
 <Y WITH ACUTE>                            221      173      186      173      195.157  138.113  *** ###
 <CAPITAL LETTER THORN>                    222      174      174      174      195.158  138.114
 <SMALL LETTER SHARP S>                    223      89       89       89       195.159  138.115
 <a WITH GRAVE>                            224      68       68       68       195.160  139.65
 <a WITH ACUTE>                            225      69       69       69       195.161  139.66
 <a WITH CIRCUMFLEX>                       226      66       66       66       195.162  139.67
 <a WITH TILDE>                            227      70       70       70       195.163  139.68
 <a WITH DIAERESIS>                        228      67       67       67       195.164  139.69
 <a WITH RING ABOVE>                       229      71       71       71       195.165  139.70
 <SMALL LIGATURE ae>                       230      156      156      156      195.166  139.71
 <c WITH CEDILLA>                          231      72       72       72       195.167  139.72
 <e WITH GRAVE>                            232      84       84       84       195.168  139.73
 <e WITH ACUTE>                            233      81       81       81       195.169  139.74
 <e WITH CIRCUMFLEX>                       234      82       82       82       195.170  139.81
 <e WITH DIAERESIS>                        235      83       83       83       195.171  139.82
 <i WITH GRAVE>                            236      88       88       88       195.172  139.83
 <i WITH ACUTE>                            237      85       85       85       195.173  139.84
 <i WITH CIRCUMFLEX>                       238      86       86       86       195.174  139.85
 <i WITH DIAERESIS>                        239      87       87       87       195.175  139.86
 <SMALL LETTER eth>                        240      140      140      140      195.176  139.87
 <n WITH TILDE>                            241      73       73       73       195.177  139.88
 <o WITH GRAVE>                            242      205      205      205      195.178  139.89
 <o WITH ACUTE>                            243      206      206      206      195.179  139.98
 <o WITH CIRCUMFLEX>                       244      203      203      203      195.180  139.99
 <o WITH TILDE>                            245      207      207      207      195.181  139.100
 <o WITH DIAERESIS>                        246      204      204      204      195.182  139.101
 <DIVISION SIGN>                           247      225      225      225      195.183  139.102
 <o WITH STROKE>                           248      112      112      112      195.184  139.103
 <u WITH GRAVE>                            249      221      221      192      195.185  139.104  ###
 <u WITH ACUTE>                            250      222      222      222      195.186  139.105
 <u WITH CIRCUMFLEX>                       251      219      219      219      195.187  139.106
 <u WITH DIAERESIS>                        252      220      220      220      195.188  139.112
 <y WITH ACUTE>                            253      141      141      141      195.189  139.113
 <SMALL LETTER thorn>                      254      142      142      142      195.190  139.114
 <y WITH DIAERESIS>                        255      223      223      223      195.191  139.115



If you would rather see the above table in CCSID 0037 order rather than
ASCII + Latin-1 order then run the table through:





recipe 4









 perl \
    -ne 'if(/.{43}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}/)'\
     -e '{push(@l,$_)}' \
     -e 'END{print map{$_->[0]}' \
     -e '          sort{$a->[1] <=> $b->[1]}' \
     -e '          map{[$_,substr($_,52,3)]}@l;}' perlebcdic.pod



If you would rather see it in CCSID 1047 order then change the number
52 in the last line to 61, like this:





recipe 5









 perl \
    -ne 'if(/.{43}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}/)'\
    -e '{push(@l,$_)}' \
    -e 'END{print map{$_->[0]}' \
    -e '          sort{$a->[1] <=> $b->[1]}' \
    -e '          map{[$_,substr($_,61,3)]}@l;}' perlebcdic.pod



If you would rather see it in POSIX-BC order then change the number
61 in the last line to 70, like this:





recipe 6









 perl \
    -ne 'if(/.{43}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}\s{6,8}\d{1,3}/)'\
     -e '{push(@l,$_)}' \
     -e 'END{print map{$_->[0]}' \
     -e '          sort{$a->[1] <=> $b->[1]}' \
     -e '          map{[$_,substr($_,70,3)]}@l;}' perlebcdic.pod









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19.5 IDENTIFYING CHARACTER CODE SETS



To determine the character set you are running under from perl one 
could use the return value of ord() or chr() to test one or more 
character values.  For example:



    $is_ascii  = "A" eq chr(65);
    $is_ebcdic = "A" eq chr(193);



Also, "\t" is a HORIZONTAL TABULATION character so that:



    $is_ascii  = ord("\t") == 9;
    $is_ebcdic = ord("\t") == 5;



To distinguish EBCDIC code pages try looking at one or more of
the characters that differ between them.  For example:



    $is_ebcdic_37   = "\n" eq chr(37);
    $is_ebcdic_1047 = "\n" eq chr(21);



Or better still choose a character that is uniquely encoded in any
of the code sets, e.g.:



    $is_ascii           = ord('[') == 91;
    $is_ebcdic_37       = ord('[') == 186;
    $is_ebcdic_1047     = ord('[') == 173;
    $is_ebcdic_POSIX_BC = ord('[') == 187;



However, it would be unwise to write tests such as:



    $is_ascii = "\r" ne chr(13);  #  WRONG
    $is_ascii = "\n" ne chr(10);  #  ILL ADVISED



Obviously the first of these will fail to distinguish most ASCII platforms
from either a CCSID 0037, a 1047, or a POSIX-BC EBCDIC platform since "\r" eq 
chr(13) under all of those coded character sets.  But note too that 
because "\n" is chr(13) and "\r" is chr(10) on the Macintosh (which is an 
ASCII platform) the second $is_ascii test will lead to trouble there.



To determine whether or not perl was built under an EBCDIC 
code page you can use the Config module like so:



    use Config;
    $is_ebcdic = $Config{'ebcdic'} eq 'define';









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19.6 CONVERSIONS





• perlebcdic tr///:  



• perlebcdic iconv:  



• perlebcdic C RTL:  












Next: , Up: perlebcdic CONVERSIONS   [Contents]





19.6.1 tr///



In order to convert a string of characters from one character set to 
another a simple list of numbers, such as in the right columns in the
above table, along with perl’s tr/// operator is all that is needed.  
The data in the table are in ASCII/Latin1 order, hence the EBCDIC columns
provide easy-to-use ASCII/Latin1 to EBCDIC operations that are also easily 
reversed.



For example, to convert ASCII/Latin1 to code page 037 take the output of the
second numbers column from the output of recipe 2 (modified to add ’\’
characters) and use it in tr/// like so:



    $cp_037 = 
    '\x00\x01\x02\x03\x37\x2D\x2E\x2F\x16\x05\x25\x0B\x0C\x0D\x0E\x0F' .
    '\x10\x11\x12\x13\x3C\x3D\x32\x26\x18\x19\x3F\x27\x1C\x1D\x1E\x1F' .
    '\x40\x5A\x7F\x7B\x5B\x6C\x50\x7D\x4D\x5D\x5C\x4E\x6B\x60\x4B\x61' .
    '\xF0\xF1\xF2\xF3\xF4\xF5\xF6\xF7\xF8\xF9\x7A\x5E\x4C\x7E\x6E\x6F' .
    '\x7C\xC1\xC2\xC3\xC4\xC5\xC6\xC7\xC8\xC9\xD1\xD2\xD3\xD4\xD5\xD6' .
    '\xD7\xD8\xD9\xE2\xE3\xE4\xE5\xE6\xE7\xE8\xE9\xBA\xE0\xBB\xB0\x6D' .
    '\x79\x81\x82\x83\x84\x85\x86\x87\x88\x89\x91\x92\x93\x94\x95\x96' .
    '\x97\x98\x99\xA2\xA3\xA4\xA5\xA6\xA7\xA8\xA9\xC0\x4F\xD0\xA1\x07' .
    '\x20\x21\x22\x23\x24\x15\x06\x17\x28\x29\x2A\x2B\x2C\x09\x0A\x1B' .
    '\x30\x31\x1A\x33\x34\x35\x36\x08\x38\x39\x3A\x3B\x04\x14\x3E\xFF' .
    '\x41\xAA\x4A\xB1\x9F\xB2\x6A\xB5\xBD\xB4\x9A\x8A\x5F\xCA\xAF\xBC' .
    '\x90\x8F\xEA\xFA\xBE\xA0\xB6\xB3\x9D\xDA\x9B\x8B\xB7\xB8\xB9\xAB' .
    '\x64\x65\x62\x66\x63\x67\x9E\x68\x74\x71\x72\x73\x78\x75\x76\x77' .
    '\xAC\x69\xED\xEE\xEB\xEF\xEC\xBF\x80\xFD\xFE\xFB\xFC\xAD\xAE\x59' .
    '\x44\x45\x42\x46\x43\x47\x9C\x48\x54\x51\x52\x53\x58\x55\x56\x57' .
    '\x8C\x49\xCD\xCE\xCB\xCF\xCC\xE1\x70\xDD\xDE\xDB\xDC\x8D\x8E\xDF';

    my $ebcdic_string = $ascii_string;
    eval '$ebcdic_string =~ tr/\000-\377/' . $cp_037 . '/';



To convert from EBCDIC 037 to ASCII just reverse the order of the tr///
arguments like so:



    my $ascii_string = $ebcdic_string;
    eval '$ascii_string =~ tr/' . $cp_037 . '/\000-\377/';



Similarly one could take the output of the third numbers column from recipe 2
to obtain a $cp_1047 table.  The fourth numbers column of the output from
recipe 2 could provide a $cp_posix_bc table suitable for transcoding as
well.



If you wanted to see the inverse tables, you would first have to sort on the
desired numbers column as in recipes 4, 5 or 6, then take the output of the
first numbers column.









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19.6.2 iconv



XPG operability often implies the presence of an iconv utility
available from the shell or from the C library.  Consult your system’s
documentation for information on iconv.



On OS/390 or z/OS see the iconv(1) manpage.  One way to invoke the iconv 
shell utility from within perl would be to:



    # OS/390 or z/OS example
    $ascii_data = `echo '$ebcdic_data'| iconv -f IBM-1047 -t ISO8859-1`



or the inverse map:



    # OS/390 or z/OS example
    $ebcdic_data = `echo '$ascii_data'| iconv -f ISO8859-1 -t IBM-1047`



For other perl-based conversion options see the Convert::* modules on CPAN.









Previous: perlebcdic iconv, Up: perlebcdic CONVERSIONS   [Contents]





19.6.3 C RTL



The OS/390 and z/OS C run-time libraries provide _atoe() and _etoa() functions.









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19.7 OPERATOR DIFFERENCES



The .. range operator treats certain character ranges with 
care on EBCDIC platforms.  For example the following array
will have twenty six elements on either an EBCDIC platform
or an ASCII platform:



    @alphabet = ('A'..'Z');   #  $#alphabet == 25



The bitwise operators such as & ^ | may return different results
when operating on string or character data in a perl program running 
on an EBCDIC platform than when run on an ASCII platform.  Here is
an example adapted from the one in (perlop):



    # EBCDIC-based examples
    print "j p \n" ^ " a h";                      # prints "JAPH\n"
    print "JA" | "  ph\n";                        # prints "japh\n" 
    print "JAPH\nJunk" & "\277\277\277\277\277";  # prints "japh\n";
    print 'p N$' ^ " E<H\n";                      # prints "Perl\n";



An interesting property of the 32 C0 control characters
in the ASCII table is that they can "literally" be constructed
as control characters in perl, e.g. (chr(0) eq \c@)>
(chr(1) eq \cA)>, and so on.  Perl on EBCDIC platforms has been
ported to take \c@ to chr(0) and \cA to chr(1), etc. as well, but the
thirty three characters that result depend on which code page you are
using.  The table below uses the standard acronyms for the controls.
The POSIX-BC and 1047 sets are
identical throughout this range and differ from the 0037 set at only 
one spot (21 decimal).  Note that the LINE FEED character
may be generated by \cJ on ASCII platforms but by \cU on 1047 or POSIX-BC 
platforms and cannot be generated as a "\c.letter." control character on 
0037 platforms.  Note also that \c\ cannot be the final element in a string
or regex, as it will absorb the terminator.   But \c\X is a FILE
SEPARATOR concatenated with X for all X.



 chr   ord   8859-1    0037    1047 && POSIX-BC     
 -----------------------------------------------------------------------
 \c?   127   <DEL>       "            "    
 \c@     0   <NUL>     <NUL>        <NUL>
 \cA     1   <SOH>     <SOH>        <SOH> 
 \cB     2   <STX>     <STX>        <STX>
 \cC     3   <ETX>     <ETX>        <ETX>
 \cD     4   <EOT>     <ST>         <ST>    
 \cE     5   <ENQ>     <HT>         <HT>    
 \cF     6   <ACK>     <SSA>        <SSA>    
 \cG     7   <BEL>     <DEL>        <DEL>   
 \cH     8   <BS>      <EPA>        <EPA>  
 \cI     9   <HT>      <RI>         <RI>   
 \cJ    10   <LF>      <SS2>        <SS2>  
 \cK    11   <VT>      <VT>         <VT>
 \cL    12   <FF>      <FF>         <FF>    
 \cM    13   <CR>      <CR>         <CR> 
 \cN    14   <SO>      <SO>         <SO>
 \cO    15   <SI>      <SI>         <SI>
 \cP    16   <DLE>     <DLE>        <DLE> 
 \cQ    17   <DC1>     <DC1>        <DC1>
 \cR    18   <DC2>     <DC2>        <DC2>
 \cS    19   <DC3>     <DC3>        <DC3> 
 \cT    20   <DC4>     <OSC>        <OSC>   
 \cU    21   <NAK>     <NEL>        <LF>              ***
 \cV    22   <SYN>     <BS>         <BS>
 \cW    23   <ETB>     <ESA>        <ESA> 
 \cX    24   <CAN>     <CAN>        <CAN>
 \cY    25   <EOM>     <EOM>        <EOM>
 \cZ    26   <SUB>     <PU2>        <PU2>  
 \c[    27   <ESC>     <SS3>        <SS3>  
 \c\X   28   <FS>X     <FS>X        <FS>X
 \c]    29   <GS>      <GS>         <GS>
 \c^    30   <RS>      <RS>         <RS>
 \c_    31   <US>      <US>         <US>









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19.8 FUNCTION DIFFERENCES





chr()




chr() must be given an EBCDIC code number argument to yield a desired 
character return value on an EBCDIC platform.  For example:



    $CAPITAL_LETTER_A = chr(193);





ord()




ord() will return EBCDIC code number values on an EBCDIC platform.
For example:



    $the_number_193 = ord("A");





pack()




The c and C templates for pack() are dependent upon character set 
encoding.  Examples of usage on EBCDIC include:



    $foo = pack("CCCC",193,194,195,196);
    # $foo eq "ABCD"
    $foo = pack("C4",193,194,195,196);
    # same thing

    $foo = pack("ccxxcc",193,194,195,196);
    # $foo eq "AB\0\0CD"





print()




One must be careful with scalars and strings that are passed to
print that contain ASCII encodings.  One common place
for this to occur is in the output of the MIME type header for
CGI script writing.  For example, many perl programming guides 
recommend something similar to:



    print "Content-type:\ttext/html\015\012\015\012"; 
    # this may be wrong on EBCDIC



Under the IBM OS/390 USS Web Server or WebSphere on z/OS for example 
you should instead write that as:



    print "Content-type:\ttext/html\r\n\r\n"; # OK for DGW et al



That is because the translation from EBCDIC to ASCII is done
by the web server in this case (such code will not be appropriate for
the Macintosh however).  Consult your web server’s documentation for 
further details.





printf()




The formats that can convert characters to numbers and vice versa
will be different from their ASCII counterparts when executed
on an EBCDIC platform.  Examples include:



    printf("%c%c%c",193,194,195);  # prints ABC





sort()




EBCDIC sort results may differ from ASCII sort results especially for 
mixed case strings.  This is discussed in more detail below.





sprintf()




See the discussion of printf() above.  An example of the use
of sprintf would be:



    $CAPITAL_LETTER_A = sprintf("%c",193);





unpack()




See the discussion of pack() above.














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19.9 REGULAR EXPRESSION DIFFERENCES



As of perl 5.005_03 the letter range regular expressions such as 
[A-Z] and [a-z] have been especially coded to not pick up gap 
characters.  For example, characters such as ô o WITH CIRCUMFLEX 
that lie between I and J would not be matched by the 
regular expression range /[H-K]/.  This works in
the other direction, too, if either of the range end points is
explicitly numeric: [\x89-\x91] will match \x8e, even
though \x89 is i and \x91  is j, and \x8e
is a gap character from the alphabetic viewpoint.



If you do want to match the alphabet gap characters in a single octet 
regular expression try matching the hex or octal code such 
as /\313/ on EBCDIC or /\364/ on ASCII platforms to 
have your regular expression match o WITH CIRCUMFLEX.



Another construct to be wary of is the inappropriate use of hex or
octal constants in regular expressions.  Consider the following
set of subs:



    sub is_c0 {
        my $char = substr(shift,0,1);
        $char =~ /[\000-\037]/;
    }

    sub is_print_ascii {
        my $char = substr(shift,0,1);
        $char =~ /[\040-\176]/;
    }

    sub is_delete {
        my $char = substr(shift,0,1);
        $char eq "\177";
    }

    sub is_c1 {
        my $char = substr(shift,0,1);
        $char =~ /[\200-\237]/;
    }

    sub is_latin_1 {
        my $char = substr(shift,0,1);
        $char =~ /[\240-\377]/;
    }



The above would be adequate if the concern was only with numeric code points.
However, the concern may be with characters rather than code points 
and on an EBCDIC platform it may be desirable for constructs such as 
if (is_print_ascii("A")) {print "A is a printable character\n";} to print
out the expected message.  One way to represent the above collection
of character classification subs that is capable of working across the
four coded character sets discussed in this document is as follows:



    sub Is_c0 {
        my $char = substr(shift,0,1);
        if (ord('^')==94)  { # ascii
            return $char =~ /[\000-\037]/;
        } 
        if (ord('^')==176) { # 0037
            return $char =~ /[\000-\003\067\055-\057\026\005\045\013-\023\074\075\062\046\030\031\077\047\034-\037]/;
        }
        if (ord('^')==95 || ord('^')==106) { # 1047 || posix-bc
            return $char =~ /[\000-\003\067\055-\057\026\005\025\013-\023\074\075\062\046\030\031\077\047\034-\037]/;
        }
    }

    sub Is_print_ascii {
        my $char = substr(shift,0,1);
        $char =~ /[ !"\#\$%&'()*+,\-.\/0-9:;<=>?\@A-Z[\\\]^_`a-z{|}~]/;
    }

    sub Is_delete {
        my $char = substr(shift,0,1);
        if (ord('^')==94)  { # ascii
            return $char eq "\177";
        }
        else  {              # ebcdic
            return $char eq "\007";
        }
    }

    sub Is_c1 {
        my $char = substr(shift,0,1);
        if (ord('^')==94)  { # ascii
            return $char =~ /[\200-\237]/;
        }
        if (ord('^')==176) { # 0037
            return $char =~ /[\040-\044\025\006\027\050-\054\011\012\033\060\061\032\063-\066\010\070-\073\040\024\076\377]/;
        }
        if (ord('^')==95)  { # 1047
            return $char =~ /[\040-\045\006\027\050-\054\011\012\033\060\061\032\063-\066\010\070-\073\040\024\076\377]/;
        }
        if (ord('^')==106) { # posix-bc
            return $char =~ 
              /[\040-\045\006\027\050-\054\011\012\033\060\061\032\063-\066\010\070-\073\040\024\076\137]/;
        }
    }

    sub Is_latin_1 {
        my $char = substr(shift,0,1);
        if (ord('^')==94)  { # ascii
            return $char =~ /[\240-\377]/;
        }
        if (ord('^')==176) { # 0037
            return $char =~ 
              /[\101\252\112\261\237\262\152\265\275\264\232\212\137\312\257\274\220\217\352\372\276\240\266\263\235\332\233\213\267\270\271\253\144\145\142\146\143\147\236\150\164\161-\163\170\165-\167\254\151\355\356\353\357\354\277\200\375\376\373\374\255\256\131\104\105\102\106\103\107\234\110\124\121-\123\130\125-\127\214\111\315\316\313\317\314\341\160\335\336\333\334\215\216\337]/;
        }
        if (ord('^')==95)  { # 1047
            return $char =~
              /[\101\252\112\261\237\262\152\265\273\264\232\212\260\312\257\274\220\217\352\372\276\240\266\263\235\332\233\213\267\270\271\253\144\145\142\146\143\147\236\150\164\161-\163\170\165-\167\254\151\355\356\353\357\354\277\200\375\376\373\374\272\256\131\104\105\102\106\103\107\234\110\124\121-\123\130\125-\127\214\111\315\316\313\317\314\341\160\335\336\333\334\215\216\337]/; 
        }
        if (ord('^')==106) { # posix-bc
            return $char =~ 
              /[\101\252\260\261\237\262\320\265\171\264\232\212\272\312\257\241\220\217\352\372\276\240\266\263\235\332\233\213\267\270\271\253\144\145\142\146\143\147\236\150\164\161-\163\170\165-\167\254\151\355\356\353\357\354\277\200\340\376\335\374\255\256\131\104\105\102\106\103\107\234\110\124\121-\123\130\125-\127\214\111\315\316\313\317\314\341\160\300\336\333\334\215\216\337]/;
        }
    }



Note however that only the Is_ascii_print() sub is really independent 
of coded character set.  Another way to write Is_latin_1() would be 
to use the characters in the range explicitly:



    sub Is_latin_1 {
        my $char = substr(shift,0,1);
        $char =~ /[ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ]/;
    }



Although that form may run into trouble in network transit (due to the 
presence of 8 bit characters) or on non ISO-Latin character sets.









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19.10 SOCKETS



Most socket programming assumes ASCII character encodings in network
byte order.  Exceptions can include CGI script writing under a
host web server where the server may take care of translation for you.
Most host web servers convert EBCDIC data to ISO-8859-1 or Unicode on
output.









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19.11 SORTING



One big difference between ASCII-based character sets and EBCDIC ones
are the relative positions of upper and lower case letters and the
letters compared to the digits.  If sorted on an ASCII-based platform the
two-letter abbreviation for a physician comes before the two letter
abbreviation for drive; that is:



 @sorted = sort(qw(Dr. dr.));  # @sorted holds ('Dr.','dr.') on ASCII,
                                  # but ('dr.','Dr.') on EBCDIC



The property of lowercase before uppercase letters in EBCDIC is
even carried to the Latin 1 EBCDIC pages such as 0037 and 1047.
An example would be that Ë E WITH DIAERESIS (203) comes 
before ë e WITH DIAERESIS (235) on an ASCII platform, but 
the latter (83) comes before the former (115) on an EBCDIC platform.  
(Astute readers will note that the uppercase version of ß 
SMALL LETTER SHARP S is simply "SS" and that the upper case version of 
ÿ y WITH DIAERESIS is not in the 0..255 range but it is 
at U+x0178 in Unicode, or "\x{178}" in a Unicode enabled Perl).



The sort order will cause differences between results obtained on
ASCII platforms versus EBCDIC platforms.  What follows are some suggestions
on how to deal with these differences.





• perlebcdic Ignore ASCII vs. EBCDIC sort differences.:  



• perlebcdic MONO CASE then sort data.:  



• perlebcdic Convert, sort data, then re convert.:  



• perlebcdic Perform sorting on one type of platform only.:  












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19.11.1 Ignore ASCII vs. EBCDIC sort differences.



This is the least computationally expensive strategy.  It may require
some user education.









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19.11.2 MONO CASE then sort data.



In order to minimize the expense of mono casing mixed-case text, try to
tr/// towards the character set case most employed within the data.
If the data are primarily UPPERCASE non Latin 1 then apply tr/[a-z]/[A-Z]/
then sort().  If the data are primarily lowercase non Latin 1 then
apply tr/[A-Z]/[a-z]/ before sorting.  If the data are primarily UPPERCASE
and include Latin-1 characters then apply:  



    tr/[a-z]/[A-Z]/;
    tr/[àáâãäåæçèéêëìíîïðñòóôõöøùúûüýþ]/[ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞ/;
    s/ß/SS/g;



then sort().  Do note however that such Latin-1 manipulation does not 
address the ÿ y WITH DIAERESIS character that will remain at 
code point 255 on ASCII platforms, but 223 on most EBCDIC platforms 
where it will sort to a place less than the EBCDIC numerals.  With a 
Unicode-enabled Perl you might try:



    tr/^?/\x{178}/;



The strategy of mono casing data before sorting does not preserve the case 
of the data and may not be acceptable for that reason.









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19.11.3 Convert, sort data, then re convert.



This is the most expensive proposition that does not employ a network
connection.









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19.11.4 Perform sorting on one type of platform only.



This strategy can employ a network connection.  As such
it would be computationally expensive.









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19.12 TRANSFORMATION FORMATS



There are a variety of ways of transforming data with an intra character set 
mapping that serve a variety of purposes.  Sorting was discussed in the 
previous section and a few of the other more popular mapping techniques are 
discussed next.





• perlebcdic URL decoding and encoding:  



• perlebcdic uu encoding and decoding:  



• perlebcdic Quoted-Printable encoding and decoding:  



• perlebcdic Caesarean ciphers:  












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19.12.1 URL decoding and encoding



Note that some URLs have hexadecimal ASCII code points in them in an
attempt to overcome character or protocol limitation issues.  For example 
the tilde character is not on every keyboard hence a URL of the form:



    http://www.pvhp.com/~pvhp/



may also be expressed as either of:



    http://www.pvhp.com/%7Epvhp/

    http://www.pvhp.com/%7epvhp/



where 7E is the hexadecimal ASCII code point for ’~’.  Here is an example
of decoding such a URL under CCSID 1047:



    $url = 'http://www.pvhp.com/%7Epvhp/';
    # this array assumes code page 1047
    my @a2e_1047 = (
          0,  1,  2,  3, 55, 45, 46, 47, 22,  5, 21, 11, 12, 13, 14, 15,
         16, 17, 18, 19, 60, 61, 50, 38, 24, 25, 63, 39, 28, 29, 30, 31,
         64, 90,127,123, 91,108, 80,125, 77, 93, 92, 78,107, 96, 75, 97,
        240,241,242,243,244,245,246,247,248,249,122, 94, 76,126,110,111,
        124,193,194,195,196,197,198,199,200,201,209,210,211,212,213,214,
        215,216,217,226,227,228,229,230,231,232,233,173,224,189, 95,109,
        121,129,130,131,132,133,134,135,136,137,145,146,147,148,149,150,
        151,152,153,162,163,164,165,166,167,168,169,192, 79,208,161,  7,
         32, 33, 34, 35, 36, 37,  6, 23, 40, 41, 42, 43, 44,  9, 10, 27,
         48, 49, 26, 51, 52, 53, 54,  8, 56, 57, 58, 59,  4, 20, 62,255,
         65,170, 74,177,159,178,106,181,187,180,154,138,176,202,175,188,
        144,143,234,250,190,160,182,179,157,218,155,139,183,184,185,171,
        100,101, 98,102, 99,103,158,104,116,113,114,115,120,117,118,119,
        172,105,237,238,235,239,236,191,128,253,254,251,252,186,174, 89,
         68, 69, 66, 70, 67, 71,156, 72, 84, 81, 82, 83, 88, 85, 86, 87,
        140, 73,205,206,203,207,204,225,112,221,222,219,220,141,142,223
    );
    $url =~ s/%([0-9a-fA-F]{2})/pack("c",$a2e_1047[hex($1)])/ge;



Conversely, here is a partial solution for the task of encoding such 
a URL under the 1047 code page:



    $url = 'http://www.pvhp.com/~pvhp/';
    # this array assumes code page 1047
    my @e2a_1047 = (
          0,  1,  2,  3,156,  9,134,127,151,141,142, 11, 12, 13, 14, 15,
         16, 17, 18, 19,157, 10,  8,135, 24, 25,146,143, 28, 29, 30, 31,
        128,129,130,131,132,133, 23, 27,136,137,138,139,140,  5,  6,  7,
        144,145, 22,147,148,149,150,  4,152,153,154,155, 20, 21,158, 26,
         32,160,226,228,224,225,227,229,231,241,162, 46, 60, 40, 43,124,
         38,233,234,235,232,237,238,239,236,223, 33, 36, 42, 41, 59, 94,
         45, 47,194,196,192,193,195,197,199,209,166, 44, 37, 95, 62, 63,
        248,201,202,203,200,205,206,207,204, 96, 58, 35, 64, 39, 61, 34,
        216, 97, 98, 99,100,101,102,103,104,105,171,187,240,253,254,177,
        176,106,107,108,109,110,111,112,113,114,170,186,230,184,198,164,
        181,126,115,116,117,118,119,120,121,122,161,191,208, 91,222,174,
        172,163,165,183,169,167,182,188,189,190,221,168,175, 93,180,215,
        123, 65, 66, 67, 68, 69, 70, 71, 72, 73,173,244,246,242,243,245,
        125, 74, 75, 76, 77, 78, 79, 80, 81, 82,185,251,252,249,250,255,
         92,247, 83, 84, 85, 86, 87, 88, 89, 90,178,212,214,210,211,213,
         48, 49, 50, 51, 52, 53, 54, 55, 56, 57,179,219,220,217,218,159
    );
    # The following regular expression does not address the 
    # mappings for: ('.' => '%2E', '/' => '%2F', ':' => '%3A') 
    $url =~ s/([\t "#%&\(\),;<=>\?\@\[\\\]^`{|}~])/sprintf("%%%02X",$e2a_1047[ord($1)])/ge;



where a more complete solution would split the URL into components 
and apply a full s/// substitution only to the appropriate parts.



In the remaining examples a @e2a or @a2e array may be employed
but the assignment will not be shown explicitly.  For code page 1047
you could use the @a2e_1047 or @e2a_1047 arrays just shown.









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19.12.2 uu encoding and decoding



The u template to pack() or unpack() will render EBCDIC data in EBCDIC 
characters equivalent to their ASCII counterparts.  For example, the 
following will print "Yes indeed\n" on either an ASCII or EBCDIC computer:



    $all_byte_chrs = '';
    for (0..255) { $all_byte_chrs .= chr($_); }
    $uuencode_byte_chrs = pack('u', $all_byte_chrs);
    ($uu = <<'ENDOFHEREDOC') =~ s/^\s*//gm;
    M``$"`P0%!@<("0H+#`T.#Q`1$A,4%187&!D:&QP='A\@(2(C)"4F)R@I*BLL
    M+2XO,#$R,S0U-C<X.3H[/#T^/T!!0D-$149'2$E*2TQ-3D]045)35%565UA9
    M6EM<75Y?8&%B8V1E9F=H:6IK;&UN;W!Q<G-T=79W>'EZ>WQ]?G^`@8*#A(6&
    MAXB)BHN,C8Z/D)&2DY25EI>8F9J;G)V>GZ"AHJ.DI::GJ*FJJZRMKJ^PL;*S
    MM+6VM[BYNKN\O;Z_P,'"P\3%QL?(R<K+S,W.S]#1TM/4U=;7V-G:V]S=WM_@
    ?X>+CY.7FY^CIZNOL[>[O\/'R\_3U]O?X^?K[_/W^_P``
    ENDOFHEREDOC
    if ($uuencode_byte_chrs eq $uu) {
        print "Yes ";
    }
    $uudecode_byte_chrs = unpack('u', $uuencode_byte_chrs);
    if ($uudecode_byte_chrs eq $all_byte_chrs) {
        print "indeed\n";
    }



Here is a very spartan uudecoder that will work on EBCDIC provided
that the @e2a array is filled in appropriately:



    #!/usr/local/bin/perl
    @e2a = ( # this must be filled in
           );
    $_ = <> until ($mode,$file) = /^begin\s*(\d*)\s*(\S*)/;
    open(OUT, "> $file") if $file ne "";
    while(<>) {
        last if /^end/;
        next if /[a-z]/;
        next unless int(((($e2a[ord()] - 32 ) & 077) + 2) / 3) ==
            int(length() / 4);
        print OUT unpack("u", $_);
    }
    close(OUT);
    chmod oct($mode), $file;









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19.12.3 Quoted-Printable encoding and decoding



On ASCII-encoded platforms it is possible to strip characters outside of
the printable set using:



    # This QP encoder works on ASCII only
    $qp_string =~ s/([=\x00-\x1F\x80-\xFF])/sprintf("=%02X",ord($1))/ge;



Whereas a QP encoder that works on both ASCII and EBCDIC platforms 
would look somewhat like the following (where the EBCDIC branch @e2a 
array is omitted for brevity):



    if (ord('A') == 65) {    # ASCII
        $delete = "\x7F";    # ASCII
        @e2a = (0 .. 255)    # ASCII to ASCII identity map
    }
    else {                   # EBCDIC
        $delete = "\x07";    # EBCDIC
        @e2a =               # EBCDIC to ASCII map (as shown above)
    }
    $qp_string =~
      s/([^ !"\#\$%&'()*+,\-.\/0-9:;<>?\@A-Z[\\\]^_`a-z{|}~$delete])/sprintf("=%02X",$e2a[ord($1)])/ge;



(although in production code the substitutions might be done
in the EBCDIC branch with the @e2a array and separately in the 
ASCII branch without the expense of the identity map).



Such QP strings can be decoded with:



    # This QP decoder is limited to ASCII only
    $string =~ s/=([0-9A-Fa-f][0-9A-Fa-f])/chr hex $1/ge;
    $string =~ s/=[\n\r]+$//;



Whereas a QP decoder that works on both ASCII and EBCDIC platforms 
would look somewhat like the following (where the @a2e array is
omitted for brevity):



    $string =~ s/=([0-9A-Fa-f][0-9A-Fa-f])/chr $a2e[hex $1]/ge;
    $string =~ s/=[\n\r]+$//;









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19.12.4 Caesarean ciphers



The practice of shifting an alphabet one or more characters for encipherment
dates back thousands of years and was explicitly detailed by Gaius Julius
Caesar in his Gallic Wars text.  A single alphabet shift is sometimes 
referred to as a rotation and the shift amount is given as a number $n after
the string ’rot’ or "rot$n".  Rot0 and rot26 would designate identity maps 
on the 26-letter English version of the Latin alphabet.  Rot13 has the 
interesting property that alternate subsequent invocations are identity maps 
(thus rot13 is its own non-trivial inverse in the group of 26 alphabet 
rotations).  Hence the following is a rot13 encoder and decoder that will 
work on ASCII and EBCDIC platforms:



    #!/usr/local/bin/perl

    while(<>){
        tr/n-za-mN-ZA-M/a-zA-Z/;
        print;
    }



In one-liner form:



    perl -ne 'tr/n-za-mN-ZA-M/a-zA-Z/;print'









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19.13 Hashing order and checksums



To the extent that it is possible to write code that depends on 
hashing order there may be differences between hashes as stored
on an ASCII-based platform and hashes stored on an EBCDIC-based platform.
XXX









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19.14 I18N AND L10N



Internationalization (I18N) and localization (L10N) are supported at least 
in principle even on EBCDIC platforms.  The details are system-dependent 
and discussed under the perlebcdic OS ISSUES section below.









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19.15 MULTI-OCTET CHARACTER SETS



Perl may work with an internal UTF-EBCDIC encoding form for wide characters 
on EBCDIC platforms in a manner analogous to the way that it works with 
the UTF-8 internal encoding form on ASCII based platforms.



Legacy multi byte EBCDIC code pages XXX.









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19.16 OS ISSUES



There may be a few system-dependent issues 
of concern to EBCDIC Perl programmers.





• perlebcdic OS/400:  



• perlebcdic OS/390, z/OS:  



• perlebcdic VM/ESA?:  



• perlebcdic POSIX-BC?:  












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19.16.1 OS/400





PASE




The PASE environment is a runtime environment for OS/400 that can run
executables built for PowerPC AIX in OS/400; see (perlos400).  PASE
is ASCII-based, not EBCDIC-based as the ILE.





IFS access




XXX.














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19.16.2 OS/390, z/OS



Perl runs under Unix Systems Services or USS.





chcp




chcp is supported as a shell utility for displaying and changing 
one’s code page.  See also chcp(1).





dataset access




For sequential data set access try:



    my @ds_records = `cat //DSNAME`;



or:



    my @ds_records = `cat //'HLQ.DSNAME'`;



See also the OS390::Stdio module on CPAN.





OS/390, z/OS iconv




iconv is supported as both a shell utility and a C RTL routine.
See also the iconv(1) and iconv(3) manual pages.





locales




On OS/390 or z/OS see (locale) for information on locales.  The L10N files
are in /usr/nls/locale.  $Config{d_setlocale} is ’define’ on OS/390
or z/OS.














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19.16.3 VM/ESA?



XXX.









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19.16.4 POSIX-BC?



XXX.









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19.17 BUGS



This pod document contains literal Latin 1 characters and may encounter 
translation difficulties.  In particular one popular nroff implementation 
was known to strip accented characters to their unaccented counterparts 
while attempting to view this document through the pod2man program 
(for example, you may see a plain y rather than one with a diaeresis 
as in ÿ).  Another nroff truncated the resultant manpage at
the first occurrence of 8 bit characters.



Not all shells will allow multiple -e string arguments to perl to
be concatenated together properly as recipes 0, 2, 4, 5, and 6 might 
seem to imply.









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19.18 SEE ALSO



perllocale NAME, (perlfunc), perlunicode NAME, (utf8).









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19.19 REFERENCES



http://anubis.dkuug.dk/i18n/charmaps



http://www.unicode.org/



http://www.unicode.org/unicode/reports/tr16/



http://www.wps.com/projects/codes/
ASCII: American Standard Code for Information Infiltration Tom Jennings,
September 1999.



The Unicode Standard, Version 3.0 The Unicode Consortium, Lisa Moore ed., 
ISBN 0-201-61633-5, Addison Wesley Developers Press, February 2000. 



CDRA: IBM - Character Data Representation Architecture - 
Reference and Registry, IBM SC09-2190-00, December 1996. 



"Demystifying Character Sets", Andrea Vine, Multilingual Computing 
& Technology, #26 Vol. 10 Issue 4, August/September 1999;
ISSN 1523-0309; Multilingual Computing Inc. Sandpoint ID, USA.



Codes, Ciphers, and Other Cryptic and Clandestine Communication
Fred B. Wrixon, ISBN 1-57912-040-7, Black Dog & Leventhal Publishers,
1998.



http://www.bobbemer.com/P-BIT.HTM
IBM - EBCDIC and the P-bit; The biggest Computer Goof Ever Robert Bemer.









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19.20 HISTORY



15 April 2001: added UTF-8 and UTF-EBCDIC to main table, pvhp.









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19.21 AUTHOR



Peter Prymmer pvhp@best.com wrote this in 1999 and 2000 
with CCSID 0819 and 0037 help from Chris Leach and 
André Pirard A.Pirard@ulg.ac.be as well as POSIX-BC 
help from Thomas Dorner Thomas.Dorner@start.de.
Thanks also to Vickie Cooper, Philip Newton, William Raffloer, and 
Joe Smith.  Trademarks, registered trademarks, service marks and 
registered service marks used in this document are the property of 
their respective owners.









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20 perlembed





• perlembed NAME:  



• perlembed DESCRIPTION:  



• perlembed Hiding Perl_:  



• perlembed MORAL:  



• perlembed AUTHOR:  



• perlembed COPYRIGHT:  












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20.1 NAME



perlembed - how to embed perl in your C program









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20.2 DESCRIPTION





• perlembed PREAMBLE:  



• perlembed ROADMAP:  



• perlembed Compiling your C program:  



• perlembed Adding a Perl interpreter to your C program:  



• perlembed Calling a Perl subroutine from your C program:  



• perlembed Evaluating a Perl statement from your C program:  



• perlembed Performing Perl pattern matches and substitutions from your C program:  



• perlembed Fiddling with the Perl stack from your C program:  



• perlembed Maintaining a persistent interpreter:  



• perlembed Execution of END blocks:  



• perlembed $0 assignments:  



• perlembed Maintaining multiple interpreter instances:  



• perlembed Using Perl modules, which themselves use C libraries, from your C program:  












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20.2.1 PREAMBLE



Do you want to:





Use C from Perl?




Read (perlxstut), (perlxs), (h2xs), perlguts NAME, and (perlapi).





Use a Unix program from Perl?




Read about back-quotes and about system and exec in (perlfunc).





Use Perl from Perl?




Read about (perlfunc)do and (perlfunc)eval and (perlfunc)require 
and (perlfunc)use.





Use C from C?




Rethink your design.





Use Perl from C?




Read on...














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20.2.2 ROADMAP











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20.2.3 Compiling your C program



If you have trouble compiling the scripts in this documentation,
you’re not alone.  The cardinal rule: COMPILE THE PROGRAMS IN EXACTLY
THE SAME WAY THAT YOUR PERL WAS COMPILED.  (Sorry for yelling.)



Also, every C program that uses Perl must link in the perl library.
What’s that, you ask?  Perl is itself written in C; the perl library
is the collection of compiled C programs that were used to create your
perl executable (/usr/bin/perl or equivalent).  (Corollary: you
can’t use Perl from your C program unless Perl has been compiled on
your machine, or installed properly–that’s why you shouldn’t blithely
copy Perl executables from machine to machine without also copying the
lib directory.)



When you use Perl from C, your C program will–usually–allocate,
"run", and deallocate a PerlInterpreter object, which is defined by
the perl library.



If your copy of Perl is recent enough to contain this documentation
(version 5.002 or later), then the perl library (and EXTERN.h and
perl.h, which you’ll also need) will reside in a directory
that looks like this:



    /usr/local/lib/perl5/your_architecture_here/CORE



or perhaps just



    /usr/local/lib/perl5/CORE



or maybe something like



    /usr/opt/perl5/CORE



Execute this statement for a hint about where to find CORE:



    perl -MConfig -e 'print $Config{archlib}'



Here’s how you’d compile the example in the next section,
Adding a Perl interpreter to your C program, on my Linux box:



    % gcc -O2 -Dbool=char -DHAS_BOOL -I/usr/local/include
    -I/usr/local/lib/perl5/i586-linux/5.003/CORE
    -L/usr/local/lib/perl5/i586-linux/5.003/CORE
    -o interp interp.c -lperl -lm



(That’s all one line.)  On my DEC Alpha running old 5.003_05, the 
incantation is a bit different:



    % cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
    -I/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE
    -L/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE -L/usr/local/lib
    -D__LANGUAGE_C__ -D_NO_PROTO -o interp interp.c -lperl -lm



How can you figure out what to add?  Assuming your Perl is post-5.001,
execute a perl -V command and pay special attention to the "cc" and
"ccflags" information.



You’ll have to choose the appropriate compiler (cc, gcc, et al.) for
your machine: perl -MConfig -e 'print $Config{cc}' will tell you what
to use.



You’ll also have to choose the appropriate library directory
(/usr/local/lib/...) for your machine.  If your compiler complains
that certain functions are undefined, or that it can’t locate
-lperl, then you need to change the path following the -L.  If it
complains that it can’t find EXTERN.h and perl.h, you need to
change the path following the -I.



You may have to add extra libraries as well.  Which ones?
Perhaps those printed by



   perl -MConfig -e 'print $Config{libs}'



Provided your perl binary was properly configured and installed the
ExtUtils::Embed module will determine all of this information for
you:



   % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`



If the ExtUtils::Embed module isn’t part of your Perl distribution,
you can retrieve it from
http://www.perl.com/perl/CPAN/modules/by-module/ExtUtils/
(If this documentation came from your Perl distribution, then you’re
running 5.004 or better and you already have it.)



The ExtUtils::Embed kit on CPAN also contains all source code for
the examples in this document, tests, additional examples and other
information you may find useful.









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20.2.4 Adding a Perl interpreter to your C program



In a sense, perl (the C program) is a good example of embedding Perl
(the language), so I’ll demonstrate embedding with miniperlmain.c,
included in the source distribution.  Here’s a bastardized, non-portable
version of miniperlmain.c containing the essentials of embedding:



    #include <EXTERN.h>               /* from the Perl distribution     */
    #include <perl.h>                 /* from the Perl distribution     */

    static PerlInterpreter *my_perl;  /***    The Perl interpreter    ***/

    int main(int argc, char **argv, char **env)
    {
        PERL_SYS_INIT3(&argc,&argv,&env);
        my_perl = perl_alloc();
        perl_construct(my_perl);
        PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
        perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
        perl_run(my_perl);
        perl_destruct(my_perl);
        perl_free(my_perl);
        PERL_SYS_TERM();
    }



Notice that we don’t use the env pointer.  Normally handed to
perl_parse as its final argument, env here is replaced by
NULL, which means that the current environment will be used.



The macros PERL_SYS_INIT3() and PERL_SYS_TERM() provide system-specific
tune up of the C runtime environment necessary to run Perl interpreters;
they should only be called once regardless of how many interpreters you
create or destroy. Call PERL_SYS_INIT3() before you create your first
interpreter, and PERL_SYS_TERM() after you free your last interpreter.



Since PERL_SYS_INIT3() may change env, it may be more appropriate to
provide env as an argument to perl_parse().



Also notice that no matter what arguments you pass to perl_parse(),
PERL_SYS_INIT3() must be invoked on the C main() argc, argv and env and
only once.



Now compile this program (I’ll call it interp.c) into an executable:



    % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`



After a successful compilation, you’ll be able to use interp just
like perl itself:



    % interp
    print "Pretty Good Perl \n";
    print "10890 - 9801 is ", 10890 - 9801;
    <CTRL-D>
    Pretty Good Perl
    10890 - 9801 is 1089



or



    % interp -e 'printf("%x", 3735928559)'
    deadbeef



You can also read and execute Perl statements from a file while in the
midst of your C program, by placing the filename in argv[1] before
calling perl_run.









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20.2.5 Calling a Perl subroutine from your C program



To call individual Perl subroutines, you can use any of the call_*
functions documented in perlcall NAME.
In this example we’ll use call_argv.



That’s shown below, in a program I’ll call showtime.c.



    #include <EXTERN.h>
    #include <perl.h>

    static PerlInterpreter *my_perl;

    int main(int argc, char **argv, char **env)
    {
        char *args[] = { NULL };
        PERL_SYS_INIT3(&argc,&argv,&env);
        my_perl = perl_alloc();
        perl_construct(my_perl);

        perl_parse(my_perl, NULL, argc, argv, NULL);
        PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

        /*** skipping perl_run() ***/

        call_argv("showtime", G_DISCARD | G_NOARGS, args);

        perl_destruct(my_perl);
        perl_free(my_perl);
        PERL_SYS_TERM();
    }



where showtime is a Perl subroutine that takes no arguments (that’s the
G_NOARGS) and for which I’ll ignore the return value (that’s the
G_DISCARD).  Those flags, and others, are discussed in perlcall NAME.



I’ll define the showtime subroutine in a file called showtime.pl:



    print "I shan't be printed.";

    sub showtime {
        print time;
    }



Simple enough.  Now compile and run:



    % cc -o showtime showtime.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

    % showtime showtime.pl
    818284590



yielding the number of seconds that elapsed between January 1, 1970
(the beginning of the Unix epoch), and the moment I began writing this
sentence.



In this particular case we don’t have to call perl_run, as we set 
the PL_exit_flag PERL_EXIT_DESTRUCT_END which executes END blocks in
perl_destruct.



If you want to pass arguments to the Perl subroutine, you can add
strings to the NULL-terminated args list passed to
call_argv.  For other data types, or to examine return values,
you’ll need to manipulate the Perl stack.  That’s demonstrated in
Fiddling with the Perl stack from your C program.









Next: , Previous: perlembed Calling a Perl subroutine from your C program, Up: perlembed DESCRIPTION   [Contents]





20.2.6 Evaluating a Perl statement from your C program



Perl provides two API functions to evaluate pieces of Perl code.
These are (perlapi)eval_sv and (perlapi)eval_pv.



Arguably, these are the only routines you’ll ever need to execute
snippets of Perl code from within your C program.  Your code can be as
long as you wish; it can contain multiple statements; it can employ
(perlfunc)use, (perlfunc)require, and (perlfunc)do to
include external Perl files.



eval_pv lets us evaluate individual Perl strings, and then
extract variables for coercion into C types.  The following program,
string.c, executes three Perl strings, extracting an int from
the first, a float from the second, and a char * from the third.



   #include <EXTERN.h>
   #include <perl.h>

   static PerlInterpreter *my_perl;

   main (int argc, char **argv, char **env)
   {
       char *embedding[] = { "", "-e", "0" };

       PERL_SYS_INIT3(&argc,&argv,&env);
       my_perl = perl_alloc();
       perl_construct( my_perl );

       perl_parse(my_perl, NULL, 3, embedding, NULL);
       PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
       perl_run(my_perl);

       /** Treat $a as an integer **/
       eval_pv("$a = 3; $a **= 2", TRUE);
       printf("a = %d\n", SvIV(get_sv("a", 0)));

       /** Treat $a as a float **/
       eval_pv("$a = 3.14; $a **= 2", TRUE);
       printf("a = %f\n", SvNV(get_sv("a", 0)));

       /** Treat $a as a string **/
       eval_pv("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
       printf("a = %s\n", SvPV_nolen(get_sv("a", 0)));

       perl_destruct(my_perl);
       perl_free(my_perl);
       PERL_SYS_TERM();
   }



All of those strange functions with sv in their names help convert Perl scalars to C types.  They’re described in perlguts NAME and (perlapi).



If you compile and run string.c, you’ll see the results of using
SvIV() to create an int, SvNV() to create a float, and
SvPV() to create a string:



   a = 9
   a = 9.859600
   a = Just Another Perl Hacker



In the example above, we’ve created a global variable to temporarily
store the computed value of our eval’ed expression.  It is also
possible and in most cases a better strategy to fetch the return value
from eval_pv() instead.  Example:



   ...
   SV *val = eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
   printf("%s\n", SvPV_nolen(val));
   ...



This way, we avoid namespace pollution by not creating global
variables and we’ve simplified our code as well.









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20.2.7 Performing Perl pattern matches and substitutions from your C program



The eval_sv() function lets us evaluate strings of Perl code, so we can
define some functions that use it to "specialize" in matches and
substitutions: match(), substitute(), and matches().



   I32 match(SV *string, char *pattern);



Given a string and a pattern (e.g., m/clasp/ or /\b\w*\b/, which
in your C program might appear as "/\\b\\w*\\b/"), match()
returns 1 if the string matches the pattern and 0 otherwise.



   int substitute(SV **string, char *pattern);



Given a pointer to an SV and an =~ operation (e.g.,
s/bob/robert/g or tr[A-Z][a-z]), substitute() modifies the string
within the SV as according to the operation, returning the number of substitutions
made.



   int matches(SV *string, char *pattern, AV **matches);



Given an SV, a pattern, and a pointer to an empty AV,
matches() evaluates $string =~ $pattern in a list context, and
fills in matches with the array elements, returning the number of matches found.



Here’s a sample program, match.c, that uses all three (long lines have
been wrapped here):



 #include <EXTERN.h>
 #include <perl.h>

 static PerlInterpreter *my_perl;

 /** my_eval_sv(code, error_check)
 ** kinda like eval_sv(), 
 ** but we pop the return value off the stack 
 **/
 SV* my_eval_sv(SV *sv, I32 croak_on_error)
 {
     dSP;
     SV* retval;


     PUSHMARK(SP);
     eval_sv(sv, G_SCALAR);

     SPAGAIN;
     retval = POPs;
     PUTBACK;

     if (croak_on_error && SvTRUE(ERRSV))
        croak(SvPVx_nolen(ERRSV));

     return retval;
 }

 /** match(string, pattern)
 **
 ** Used for matches in a scalar context.
 **
 ** Returns 1 if the match was successful; 0 otherwise.
 **/

 I32 match(SV *string, char *pattern)
 {
     SV *command = newSV(0), *retval;

     sv_setpvf(command, "my $string = '%s'; $string =~ %s",
              SvPV_nolen(string), pattern);

     retval = my_eval_sv(command, TRUE);
     SvREFCNT_dec(command);

     return SvIV(retval);
 }

 /** substitute(string, pattern)
 **
 ** Used for =~ operations that modify their left-hand side (s/// and tr///)
 **
 ** Returns the number of successful matches, and
 ** modifies the input string if there were any.
 **/

 I32 substitute(SV **string, char *pattern)
 {
     SV *command = newSV(0), *retval;

     sv_setpvf(command, "$string = '%s'; ($string =~ %s)",
              SvPV_nolen(*string), pattern);

     retval = my_eval_sv(command, TRUE);
     SvREFCNT_dec(command);

     *string = get_sv("string", 0);
     return SvIV(retval);
 }

 /** matches(string, pattern, matches)
 **
 ** Used for matches in a list context.
 **
 ** Returns the number of matches,
 ** and fills in **matches with the matching substrings
 **/

 I32 matches(SV *string, char *pattern, AV **match_list)
 {
     SV *command = newSV(0);
     I32 num_matches;

     sv_setpvf(command, "my $string = '%s'; @array = ($string =~ %s)",
              SvPV_nolen(string), pattern);

     my_eval_sv(command, TRUE);
     SvREFCNT_dec(command);

     *match_list = get_av("array", 0);
     num_matches = av_len(*match_list) + 1;

     return num_matches;
 }

 main (int argc, char **argv, char **env)
 {
     char *embedding[] = { "", "-e", "0" };
     AV *match_list;
     I32 num_matches, i;
     SV *text;

     PERL_SYS_INIT3(&argc,&argv,&env);
     my_perl = perl_alloc();
     perl_construct(my_perl);
     perl_parse(my_perl, NULL, 3, embedding, NULL);
     PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

     text = newSV(0);
     sv_setpv(text, "When he is at a convenience store and the "
        "bill comes to some amount like 76 cents, Maynard is "
        "aware that there is something he *should* do, something "
        "that will enable him to get back a quarter, but he has "
        "no idea *what*.  He fumbles through his red squeezey "
        "changepurse and gives the boy three extra pennies with "
        "his dollar, hoping that he might luck into the correct "
        "amount.  The boy gives him back two of his own pennies "
        "and then the big shiny quarter that is his prize. "
        "-RICHH");

     if (match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
        printf("match: Text contains the word 'quarter'.\n\n");
     else
        printf("match: Text doesn't contain the word 'quarter'.\n\n");

     if (match(text, "m/eighth/")) /** Does text contain 'eighth'? **/
        printf("match: Text contains the word 'eighth'.\n\n");
     else
        printf("match: Text doesn't contain the word 'eighth'.\n\n");

     /** Match all occurrences of /wi../ **/
     num_matches = matches(text, "m/(wi..)/g", &match_list);
     printf("matches: m/(wi..)/g found %d matches...\n", num_matches);

     for (i = 0; i < num_matches; i++)
        printf("match: %s\n", SvPV_nolen(*av_fetch(match_list, i, FALSE)));
     printf("\n");

     /** Remove all vowels from text **/
     num_matches = substitute(&text, "s/[aeiou]//gi");
     if (num_matches) {
        printf("substitute: s/[aeiou]//gi...%d substitutions made.\n",
               num_matches);
        printf("Now text is: %s\n\n", SvPV_nolen(text));
     }

     /** Attempt a substitution **/
     if (!substitute(&text, "s/Perl/C/")) {
        printf("substitute: s/Perl/C...No substitution made.\n\n");
     }

     SvREFCNT_dec(text);
     PL_perl_destruct_level = 1;
     perl_destruct(my_perl);
     perl_free(my_perl);
     PERL_SYS_TERM();
 }



which produces the output (again, long lines have been wrapped here)



   match: Text contains the word 'quarter'.

   match: Text doesn't contain the word 'eighth'.

   matches: m/(wi..)/g found 2 matches...
   match: will
   match: with

   substitute: s/[aeiou]//gi...139 substitutions made.
   Now text is: Whn h s t  cnvnnc str nd th bll cms t sm mnt lk 76 cnts,
   Mynrd s wr tht thr s smthng h *shld* d, smthng tht wll nbl hm t gt bck
   qrtr, bt h hs n d *wht*.  H fmbls thrgh hs rd sqzy chngprs nd gvs th by
   thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt.  Th by gvs
   hm bck tw f hs wn pnns nd thn th bg shny qrtr tht s hs prz. -RCHH

   substitute: s/Perl/C...No substitution made.









Next: , Previous: perlembed Performing Perl pattern matches and substitutions from your C program, Up: perlembed DESCRIPTION   [Contents]





20.2.8 Fiddling with the Perl stack from your C program



When trying to explain stacks, most computer science textbooks mumble
something about spring-loaded columns of cafeteria plates: the last
thing you pushed on the stack is the first thing you pop off.  That’ll
do for our purposes: your C program will push some arguments onto "the Perl
stack", shut its eyes while some magic happens, and then pop the
results–the return value of your Perl subroutine–off the stack.



First you’ll need to know how to convert between C types and Perl
types, with newSViv() and sv_setnv() and newAV() and all their
friends.  They’re described in perlguts NAME and (perlapi).



Then you’ll need to know how to manipulate the Perl stack.  That’s
described in perlcall NAME.



Once you’ve understood those, embedding Perl in C is easy.



Because C has no builtin function for integer exponentiation, let’s
make Perl’s ** operator available to it (this is less useful than it
sounds, because Perl implements ** with C’s pow() function).  First
I’ll create a stub exponentiation function in power.pl:



    sub expo {
        my ($a, $b) = @_;
        return $a ** $b;
    }



Now I’ll create a C program, power.c, with a function
PerlPower() that contains all the perlguts necessary to push the
two arguments into expo() and to pop the return value out.  Take a
deep breath...



    #include <EXTERN.h>
    #include <perl.h>

    static PerlInterpreter *my_perl;

    static void
    PerlPower(int a, int b)
    {
      dSP;                            /* initialize stack pointer      */
      ENTER;                          /* everything created after here */
      SAVETMPS;                       /* ...is a temporary variable.   */
      PUSHMARK(SP);                   /* remember the stack pointer    */
      XPUSHs(sv_2mortal(newSViv(a))); /* push the base onto the stack  */
      XPUSHs(sv_2mortal(newSViv(b))); /* push the exponent onto stack  */
      PUTBACK;                      /* make local stack pointer global */
      call_pv("expo", G_SCALAR);      /* call the function             */
      SPAGAIN;                        /* refresh stack pointer         */
                                    /* pop the return value from stack */
      printf ("%d to the %dth power is %d.\n", a, b, POPi);
      PUTBACK;
      FREETMPS;                       /* free that return value        */
      LEAVE;                       /* ...and the XPUSHed "mortal" args.*/
    }

    int main (int argc, char **argv, char **env)
    {
      char *my_argv[] = { "", "power.pl" };

      PERL_SYS_INIT3(&argc,&argv,&env);
      my_perl = perl_alloc();
      perl_construct( my_perl );

      perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
      PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
      perl_run(my_perl);

      PerlPower(3, 4);                      /*** Compute 3 ** 4 ***/

      perl_destruct(my_perl);
      perl_free(my_perl);
      PERL_SYS_TERM();
    }



Compile and run:



    % cc -o power power.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

    % power
    3 to the 4th power is 81.









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20.2.9 Maintaining a persistent interpreter



When developing interactive and/or potentially long-running
applications, it’s a good idea to maintain a persistent interpreter
rather than allocating and constructing a new interpreter multiple
times.  The major reason is speed: since Perl will only be loaded into
memory once.



However, you have to be more cautious with namespace and variable
scoping when using a persistent interpreter.  In previous examples
we’ve been using global variables in the default package main.  We
knew exactly what code would be run, and assumed we could avoid
variable collisions and outrageous symbol table growth.



Let’s say your application is a server that will occasionally run Perl
code from some arbitrary file.  Your server has no way of knowing what
code it’s going to run.  Very dangerous.



If the file is pulled in by perl_parse(), compiled into a newly
constructed interpreter, and subsequently cleaned out with
perl_destruct() afterwards, you’re shielded from most namespace
troubles.



One way to avoid namespace collisions in this scenario is to translate
the filename into a guaranteed-unique package name, and then compile
the code into that package using (perlfunc)eval.  In the example
below, each file will only be compiled once.  Or, the application
might choose to clean out the symbol table associated with the file
after it’s no longer needed.  Using (perlapi)call_argv, We’ll
call the subroutine Embed::Persistent::eval_file which lives in the
file persistent.pl and pass the filename and boolean cleanup/cache
flag as arguments.



Note that the process will continue to grow for each file that it
uses.  In addition, there might be AUTOLOADed subroutines and other
conditions that cause Perl’s symbol table to grow.  You might want to
add some logic that keeps track of the process size, or restarts
itself after a certain number of requests, to ensure that memory
consumption is minimized.  You’ll also want to scope your variables
with (perlfunc)my whenever possible.



 package Embed::Persistent;
 #persistent.pl

 use strict;
 our %Cache;
 use Symbol qw(delete_package);

 sub valid_package_name {
     my($string) = @_;
     $string =~ s/([^A-Za-z0-9\/])/sprintf("_%2x",unpack("C",$1))/eg;
     # second pass only for words starting with a digit
     $string =~ s|/(\d)|sprintf("/_%2x",unpack("C",$1))|eg;

     # Dress it up as a real package name
     $string =~ s|/|::|g;
     return "Embed" . $string;
 }

 sub eval_file {
     my($filename, $delete) = @_;
     my $package = valid_package_name($filename);
     my $mtime = -M $filename;
     if(defined $Cache{$package}{mtime}
        &&
        $Cache{$package}{mtime} <= $mtime)
     {
        # we have compiled this subroutine already,
        # it has not been updated on disk, nothing left to do
        print STDERR "already compiled $package->handler\n";
     }
     else {
        local *FH;
        open FH, $filename or die "open '$filename' $!";
        local($/) = undef;
        my $sub = <FH>;
        close FH;

        #wrap the code into a subroutine inside our unique package
        my $eval = qq{package $package; sub handler { $sub; }};
        {
            # hide our variables within this block
            my($filename,$mtime,$package,$sub);
            eval $eval;
        }
        die $@ if $@;

        #cache it unless we're cleaning out each time
        $Cache{$package}{mtime} = $mtime unless $delete;
     }

     eval {$package->handler;};
     die $@ if $@;

     delete_package($package) if $delete;

     #take a look if you want
     #print Devel::Symdump->rnew($package)->as_string, $/;
 }

 1;

 __END__

 /* persistent.c */
 #include <EXTERN.h>
 #include <perl.h>

 /* 1 = clean out filename's symbol table after each request, 0 = don't */
 #ifndef DO_CLEAN
 #define DO_CLEAN 0
 #endif

 #define BUFFER_SIZE 1024

 static PerlInterpreter *my_perl = NULL;

 int
 main(int argc, char **argv, char **env)
 {
     char *embedding[] = { "", "persistent.pl" };
     char *args[] = { "", DO_CLEAN, NULL };
     char filename[BUFFER_SIZE];
     int exitstatus = 0;

     PERL_SYS_INIT3(&argc,&argv,&env);
     if((my_perl = perl_alloc()) == NULL) {
        fprintf(stderr, "no memory!");
        exit(1);
     }
     perl_construct(my_perl);

     PL_origalen = 1; /* don't let $0 assignment update the proctitle or embedding[0] */
     exitstatus = perl_parse(my_perl, NULL, 2, embedding, NULL);
     PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
     if(!exitstatus) {
        exitstatus = perl_run(my_perl);

        while(printf("Enter file name: ") &&
              fgets(filename, BUFFER_SIZE, stdin)) {

            filename[strlen(filename)-1] = '\0'; /* strip \n */
            /* call the subroutine, passing it the filename as an argument */
            args[0] = filename;
            call_argv("Embed::Persistent::eval_file",
                           G_DISCARD | G_EVAL, args);

            /* check $@ */
            if(SvTRUE(ERRSV))
                fprintf(stderr, "eval error: %s\n", SvPV_nolen(ERRSV));
        }
     }

     PL_perl_destruct_level = 0;
     perl_destruct(my_perl);
     perl_free(my_perl);
     PERL_SYS_TERM();
     exit(exitstatus);
 }



Now compile:



 % cc -o persistent persistent.c `perl -MExtUtils::Embed -e ccopts -e ldopts`



Here’s an example script file:



 #test.pl
 my $string = "hello";
 foo($string);

 sub foo {
     print "foo says: @_\n";
 }



Now run:



 % persistent
 Enter file name: test.pl
 foo says: hello
 Enter file name: test.pl
 already compiled Embed::test_2epl->handler
 foo says: hello
 Enter file name: ^C









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20.2.10 Execution of END blocks



Traditionally END blocks have been executed at the end of the perl_run.
This causes problems for applications that never call perl_run. Since
perl 5.7.2 you can specify PL_exit_flags |= PERL_EXIT_DESTRUCT_END
to get the new behaviour. This also enables the running of END blocks if
the perl_parse fails and perl_destruct will return the exit value.









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20.2.11 $0 assignments



When a perl script assigns a value to $0 then the perl runtime will
try to make this value show up as the program name reported by "ps" by
updating the memory pointed to by the argv passed to perl_parse() and
also calling API functions like setproctitle() where available.  This
behaviour might not be appropriate when embedding perl and can be
disabled by assigning the value 1 to the variable PL_origalen
before perl_parse() is called.



The persistent.c example above is for instance likely to segfault
when $0 is assigned to if the PL_origalen = 1; assignment is
removed.  This because perl will try to write to the read only memory
of the embedding[] strings.









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20.2.12 Maintaining multiple interpreter instances



Some rare applications will need to create more than one interpreter
during a session.  Such an application might sporadically decide to
release any resources associated with the interpreter.



The program must take care to ensure that this takes place before
the next interpreter is constructed.  By default, when perl is not
built with any special options, the global variable
PL_perl_destruct_level is set to 0, since extra cleaning isn’t
usually needed when a program only ever creates a single interpreter
in its entire lifetime.



Setting PL_perl_destruct_level to 1 makes everything squeaky clean:



 while(1) {
     ...
     /* reset global variables here with PL_perl_destruct_level = 1 */
     PL_perl_destruct_level = 1;
     perl_construct(my_perl);
     ...
     /* clean and reset _everything_ during perl_destruct */
     PL_perl_destruct_level = 1;
     perl_destruct(my_perl);
     perl_free(my_perl);
     ...
     /* let's go do it again! */
 }



When perl_destruct() is called, the interpreter’s syntax parse tree
and symbol tables are cleaned up, and global variables are reset.  The
second assignment to PL_perl_destruct_level is needed because
perl_construct resets it to 0.



Now suppose we have more than one interpreter instance running at the
same time.  This is feasible, but only if you used the Configure option
-Dusemultiplicity or the options -Dusethreads -Duseithreads when
building perl.  By default, enabling one of these Configure options
sets the per-interpreter global variable PL_perl_destruct_level to
1, so that thorough cleaning is automatic and interpreter variables
are initialized correctly.  Even if you don’t intend to run two or
more interpreters at the same time, but to run them sequentially, like
in the above example, it is recommended to build perl with the
-Dusemultiplicity option otherwise some interpreter variables may
not be initialized correctly between consecutive runs and your
application may crash.



See also (perlxs)Thread-aware system interfaces.



Using -Dusethreads -Duseithreads rather than -Dusemultiplicity
is more appropriate if you intend to run multiple interpreters
concurrently in different threads, because it enables support for
linking in the thread libraries of your system with the interpreter.



Let’s give it a try:



 #include <EXTERN.h>
 #include <perl.h>

 /* we're going to embed two interpreters */

 #define SAY_HELLO "-e", "print qq(Hi, I'm $^X\n)"

 int main(int argc, char **argv, char **env)
 {
     PerlInterpreter *one_perl, *two_perl;
     char *one_args[] = { "one_perl", SAY_HELLO };
     char *two_args[] = { "two_perl", SAY_HELLO };

     PERL_SYS_INIT3(&argc,&argv,&env);
     one_perl = perl_alloc();
     two_perl = perl_alloc();

     PERL_SET_CONTEXT(one_perl);
     perl_construct(one_perl);
     PERL_SET_CONTEXT(two_perl);
     perl_construct(two_perl);

     PERL_SET_CONTEXT(one_perl);
     perl_parse(one_perl, NULL, 3, one_args, (char **)NULL);
     PERL_SET_CONTEXT(two_perl);
     perl_parse(two_perl, NULL, 3, two_args, (char **)NULL);

     PERL_SET_CONTEXT(one_perl);
     perl_run(one_perl);
     PERL_SET_CONTEXT(two_perl);
     perl_run(two_perl);

     PERL_SET_CONTEXT(one_perl);
     perl_destruct(one_perl);
     PERL_SET_CONTEXT(two_perl);
     perl_destruct(two_perl);

     PERL_SET_CONTEXT(one_perl);
     perl_free(one_perl);
     PERL_SET_CONTEXT(two_perl);
     perl_free(two_perl);
     PERL_SYS_TERM();
 }



Note the calls to PERL_SET_CONTEXT().  These are necessary to initialize
the global state that tracks which interpreter is the "current" one on
the particular process or thread that may be running it.  It should
always be used if you have more than one interpreter and are making
perl API calls on both interpreters in an interleaved fashion.



PERL_SET_CONTEXT(interp) should also be called whenever interp is
used by a thread that did not create it (using either perl_alloc(), or
the more esoteric perl_clone()).



Compile as usual:



 % cc -o multiplicity multiplicity.c `perl -MExtUtils::Embed -e ccopts -e ldopts`



Run it, Run it:



 % multiplicity
 Hi, I'm one_perl
 Hi, I'm two_perl









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20.2.13 Using Perl modules, which themselves use C libraries, from your C program



If you’ve played with the examples above and tried to embed a script
that use()s a Perl module (such as Socket) which itself uses a C or C++ library,
this probably happened:



 Can't load module Socket, dynamic loading not available in this perl.
  (You may need to build a new perl executable which either supports
  dynamic loading or has the Socket module statically linked into it.)



What’s wrong?



Your interpreter doesn’t know how to communicate with these extensions
on its own.  A little glue will help.  Up until now you’ve been
calling perl_parse(), handing it NULL for the second argument:



 perl_parse(my_perl, NULL, argc, my_argv, NULL);



That’s where the glue code can be inserted to create the initial contact between
Perl and linked C/C++ routines.  Let’s take a look some pieces of perlmain.c
to see how Perl does this:



 static void xs_init (pTHX);

 EXTERN_C void boot_DynaLoader (pTHX_ CV* cv);
 EXTERN_C void boot_Socket (pTHX_ CV* cv);


 EXTERN_C void
 xs_init(pTHX)
 {
        char *file = __FILE__;
        /* DynaLoader is a special case */
        newXS("DynaLoader::boot_DynaLoader", boot_DynaLoader, file);
        newXS("Socket::bootstrap", boot_Socket, file);
 }



Simply put: for each extension linked with your Perl executable
(determined during its initial configuration on your
computer or when adding a new extension),
a Perl subroutine is created to incorporate the extension’s
routines.  Normally, that subroutine is named
Module::bootstrap() and is invoked when you say use Module.  In
turn, this hooks into an XSUB, boot_Module, which creates a Perl
counterpart for each of the extension’s XSUBs.  Don’t worry about this
part; leave that to the xsubpp and extension authors.  If your
extension is dynamically loaded, DynaLoader creates Module::bootstrap()
for you on the fly.  In fact, if you have a working DynaLoader then there
is rarely any need to link in any other extensions statically.



Once you have this code, slap it into the second argument of perl_parse():



 perl_parse(my_perl, xs_init, argc, my_argv, NULL);



Then compile:



 % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

 % interp
   use Socket;
   use SomeDynamicallyLoadedModule;

   print "Now I can use extensions!\n"'



ExtUtils::Embed can also automate writing the xs_init glue code.



 % perl -MExtUtils::Embed -e xsinit -- -o perlxsi.c
 % cc -c perlxsi.c `perl -MExtUtils::Embed -e ccopts`
 % cc -c interp.c  `perl -MExtUtils::Embed -e ccopts`
 % cc -o interp perlxsi.o interp.o `perl -MExtUtils::Embed -e ldopts`



Consult (perlxs), perlguts NAME, and (perlapi) for more details.









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20.3 Hiding Perl_



If you completely hide the short forms of the Perl public API,
add -DPERL_NO_SHORT_NAMES to the compilation flags.  This means that
for example instead of writing



    warn("%d bottles of beer on the wall", bottlecount);



you will have to write the explicit full form



    Perl_warn(aTHX_ "%d bottles of beer on the wall", bottlecount);



(See perlguts Background and PERL_IMPLICIT_CONTEXT for the explanation
of the aTHX_. )  Hiding the short forms is very useful for avoiding
all sorts of nasty (C preprocessor or otherwise) conflicts with other
software packages (Perl defines about 2400 APIs with these short names,
take or leave few hundred, so there certainly is room for conflict.)









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20.4 MORAL



You can sometimes write faster code in C, but
you can always write code faster in Perl.  Because you can use
each from the other, combine them as you wish.









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20.5 AUTHOR



Jon Orwant <orwant@media.mit.edu> and Doug MacEachern
<dougm@covalent.net>, with small contributions from Tim Bunce, Tom
Christiansen, Guy Decoux, Hallvard Furuseth, Dov Grobgeld, and Ilya
Zakharevich.



Doug MacEachern has an article on embedding in Volume 1, Issue 4 of
The Perl Journal ( http://www.tpj.com/ ).  Doug is also the developer of the
most widely-used Perl embedding: the mod_perl system
(perl.apache.org), which embeds Perl in the Apache web server.
Oracle, Binary Evolution, ActiveState, and Ben Sugars’s nsapi_perl
have used this model for Oracle, Netscape and Internet Information
Server Perl plugins.









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20.6 COPYRIGHT



Copyright (C) 1995, 1996, 1997, 1998 Doug MacEachern and Jon Orwant.  All
Rights Reserved.



This document may be distributed under the same terms as Perl itself.









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21 perlexperiment





• perlexperiment NAME:  



• perlexperiment DESCRIPTION:  



• perlexperiment AUTHORS:  



• perlexperiment COPYRIGHT:  



• perlexperiment LICENSE:  












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21.1 NAME



perlexperiment - A listing of experimental features in Perl









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21.2 DESCRIPTION



This document lists the current and past experimental features in the perl
core. Although all of these are documented with their appropriate topics,
this succinct listing gives you an overview and basic facts about their
status.



So far I’ve merely tried to find and list the experimental features and infer
their inception, versions, etc. There’s a lot of speculation here.





• perlexperiment Current experiments:  



• perlexperiment Accepted features:  



• perlexperiment Removed features:  












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21.2.1 Current experiments





fork() emulation




Introduced in Perl 5.6.1



See also perlfork NAME





Weak references




Introduced in Perl 5.6.0





Internal file glob




Introduced in Perl 5.6.0



Accepted in XXX





64-bit support




Introduced in Perl 5.005



Accepted in XXX





die accepts a reference




Introduced in Perl 5.005



Accepted in Perl XXX





Unicode support




Introduced in Perl 5.6.0



Accepted in Perl 5.8.0 XXX





-Dusemultiplicity -Dusethreads




Introduced in Perl 5.6.0





Long Doubles Still Don’t Work In Solaris




Introduced in Perl 5.7.0





GetOpt::Long Options can now take multiple values at once (experimental)




Getopt::Long upgraded to version 2.35



Removed in Perl 5.8.8





5.005-style threading




Introduced in Perl 5.005



Removed in Perl 5.10 XXX





Test::Harness::Straps




Removed in Perl 5.10.1





perlcc




Introduced in Perl 5.005



Removed in Perl 5.9.0





our can now have an experimental optional attribute unique




Introduced in Perl 5.8.0



Deprecated in Perl 5.10.0





Assertions




The -A command line switch



Introduced in Perl 5.9.0



Removed in Perl 5.9.5





Linux abstract Unix domain sockets




Introduced in Perl 5.9.2



See also (Socket)





(Pod-HTML2Pod)Pod::HTML2Pod






(Pod-PXML)Pod::PXML






threads






The <:pop> IO pseudolayer




See also perlrun NAME





The <:win32> IO pseudolayer




See also perlrun NAME





MLDBM




See also (perldsc)





internal functions with M flag




See also perlguts NAME





lex_start API




Introduced in Perl 5.13.7





internal API for %H




Introduced in Perl 5.13.7



See also cophh_ in (perlapi).





av_create_and_push






av_create_and_unshift_one






av_create_and_unshift_one






PL_keyword_plugin






hv_iternext_flags






lex_bufutf8






lex_discard_to






lex_grow_linestr






lex_next_chunk






lex_peek_unichar






lex_read_space






lex_read_to






lex_read_unichar






lex_stuff_pv






lex_stuff_pvn






lex_stuff_pvs






lex_stuff_sv






lex_unstuff






parse_fullstmt






parse_stmtseq






PL_parser->bufend






PL_parser->bufptr






PL_parser->linestart






PL_parser->linestr






Perl_signbit






pad_findmy






sv_utf8_decode






sv_utf8_downgrade






bytes_from_utf8






bytes_to_utf8






utf8_to_bytes






DB module




Introduced in Perl 5.6.0



See also (perldebug), perldebtut NAME





The pseudo-hash data type




Introduced in Perl 5.6.0





Lvalue subroutines




Introduced in Perl 5.6.0



See also (perlsub)





There is an installhtml target in the Makefile.






Unicode in Perl on EBCDIC






(?{code})




See also (perlre)





(??{ code })




See also (perlre)





Backtracking control verbs




(*ACCEPT)



Introduced in: Perl 5.10



See also: (perlre)Special Backtracking Control Verbs





Code expressions, conditional expressions, and independent expressions in regexes






The \N regex character class




The \N character class, not to be confused with the named character
sequence \N{NAME}, denotes any non-newline character in a regular
expression.



Introduced in: Perl 5.12



See also: 





gv_try_downgrade




See also (perlintern)





Experimental Support for Sun Studio Compilers for Linux OS




See also (perllinux)





Pluggable keywords




See (perlapi)PL_keyword_plugin for the mechanism.



Introduced in: Perl 5.11.2














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21.2.2 Accepted features



These features were so wildly successful and played so well with others that
we decided to remove their experimental status and admit them as full, stable
features in the world of Perl, lavishing all the benefits and luxuries thereof. 
They are also awarded +5 Stability and +3 Charisma.





(none yet identified)















Previous: perlexperiment Accepted features, Up: perlexperiment DESCRIPTION   [Contents]





21.2.3 Removed features



These features are no longer considered experimental and their functionality
has disappeared. It’s your own fault if you wrote production programs using
these features after we explicitly told you not to (see perlpolicy NAME).





legacy




The experimental legacy pragma was swallowed by the feature pragma.



Introduced in: 5.11.2



Removed in: 5.11.3














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21.3 AUTHORS



brian d foy <brian.d.foy@gmail.com>









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21.4 COPYRIGHT



Copyright 2010, brian d foy <brian.d.foy@gmail.com>









Previous: perlexperiment COPYRIGHT, Up: perlexperiment   [Contents]





21.5 LICENSE



You can use and redistribute this document under the same terms as Perl
itself.









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22 perlfilter





• perlfilter NAME:  



• perlfilter DESCRIPTION:  



• perlfilter CONCEPTS:  



• perlfilter USING FILTERS:  



• perlfilter WRITING A SOURCE FILTER:  



• perlfilter WRITING A SOURCE FILTER IN C:  



• perlfilter CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE:  



• perlfilter WRITING A SOURCE FILTER IN PERL:  



• perlfilter USING CONTEXT: THE DEBUG FILTER:  



• perlfilter CONCLUSION:  



• perlfilter THINGS TO LOOK OUT FOR:  



• perlfilter REQUIREMENTS:  



• perlfilter AUTHOR:  



• perlfilter Copyrights:  












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22.1 NAME



perlfilter - Source Filters









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22.2 DESCRIPTION



This article is about a little-known feature of Perl called
source filters. Source filters alter the program text of a module
before Perl sees it, much as a C preprocessor alters the source text of
a C program before the compiler sees it. This article tells you more
about what source filters are, how they work, and how to write your
own.



The original purpose of source filters was to let you encrypt your
program source to prevent casual piracy. This isn’t all they can do, as
you’ll soon learn. But first, the basics.









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22.3 CONCEPTS



Before the Perl interpreter can execute a Perl script, it must first
read it from a file into memory for parsing and compilation. If that
script itself includes other scripts with a use or require
statement, then each of those scripts will have to be read from their
respective files as well.



Now think of each logical connection between the Perl parser and an
individual file as a source stream. A source stream is created when
the Perl parser opens a file, it continues to exist as the source code
is read into memory, and it is destroyed when Perl is finished parsing
the file. If the parser encounters a require or use statement in
a source stream, a new and distinct stream is created just for that
file.



The diagram below represents a single source stream, with the flow of
source from a Perl script file on the left into the Perl parser on the
right. This is how Perl normally operates.



    file -------> parser



There are two important points to remember:



    

  1.  Although there can be any number of source streams in existence at any
given time, only one will be active.


  2.  Every source stream is associated with only one file.





A source filter is a special kind of Perl module that intercepts and
modifies a source stream before it reaches the parser. A source filter
changes our diagram like this:



    file ----> filter ----> parser



If that doesn’t make much sense, consider the analogy of a command
pipeline. Say you have a shell script stored in the compressed file
trial.gz. The simple pipeline command below runs the script without
needing to create a temporary file to hold the uncompressed file.



    gunzip -c trial.gz | sh



In this case, the data flow from the pipeline can be represented as follows:



    trial.gz ----> gunzip ----> sh



With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl’s parser:



     compressed           gunzip
    Perl program ---> source filter ---> parser









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22.4 USING FILTERS



So how do you use a source filter in a Perl script? Above, I said that
a source filter is just a special kind of module. Like all Perl
modules, a source filter is invoked with a use statement.



Say you want to pass your Perl source through the C preprocessor before
execution. As it happens, the source filters distribution comes with a C
preprocessor filter module called Filter::cpp.



Below is an example program, cpp_test, which makes use of this filter.
Line numbers have been added to allow specific lines to be referenced
easily.



    1: use Filter::cpp;
    2: #define TRUE 1
    3: $a = TRUE;
    4: print "a = $a\n";



When you execute this script, Perl creates a source stream for the
file. Before the parser processes any of the lines from the file, the
source stream looks like this:



    cpp_test ---------> parser



Line 1, use Filter::cpp, includes and installs the cpp filter
module. All source filters work this way. The use statement is compiled
and executed at compile time, before any more of the file is read, and
it attaches the cpp filter to the source stream behind the scenes. Now
the data flow looks like this:



    cpp_test ----> cpp filter ----> parser



As the parser reads the second and subsequent lines from the source
stream, it feeds those lines through the cpp source filter before
processing them. The cpp filter simply passes each line through the
real C preprocessor. The output from the C preprocessor is then
inserted back into the source stream by the filter.



                  .-> cpp --.
                  |         |
                  |         |
                  |       <-'
   cpp_test ----> cpp filter ----> parser



The parser then sees the following code:



    use Filter::cpp;
    $a = 1;
    print "a = $a\n";



Let’s consider what happens when the filtered code includes another
module with use:



    1: use Filter::cpp;
    2: #define TRUE 1
    3: use Fred;
    4: $a = TRUE;
    5: print "a = $a\n";



The cpp filter does not apply to the text of the Fred module, only
to the text of the file that used it (cpp_test). Although the use
statement on line 3 will pass through the cpp filter, the module that
gets included (Fred) will not. The source streams look like this
after line 3 has been parsed and before line 4 is parsed:



    cpp_test ---> cpp filter ---> parser (INACTIVE)

    Fred.pm ----> parser



As you can see, a new stream has been created for reading the source
from Fred.pm. This stream will remain active until all of Fred.pm
has been parsed. The source stream for cpp_test will still exist,
but is inactive. Once the parser has finished reading Fred.pm, the
source stream associated with it will be destroyed. The source stream
for cpp_test then becomes active again and the parser reads line 4
and subsequent lines from cpp_test.



You can use more than one source filter on a single file. Similarly,
you can reuse the same filter in as many files as you like.



For example, if you have a uuencoded and compressed source file, it is
possible to stack a uudecode filter and an uncompression filter like
this:



    use Filter::uudecode; use Filter::uncompress;
    M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
    M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
    ...



Once the first line has been processed, the flow will look like this:



    file ---> uudecode ---> uncompress ---> parser
               filter         filter



Data flows through filters in the same order they appear in the source
file. The uudecode filter appeared before the uncompress filter, so the
source file will be uudecoded before it’s uncompressed.









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22.5 WRITING A SOURCE FILTER



There are three ways to write your own source filter. You can write it
in C, use an external program as a filter, or write the filter in Perl.
I won’t cover the first two in any great detail, so I’ll get them out
of the way first. Writing the filter in Perl is most convenient, so
I’ll devote the most space to it.









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22.6 WRITING A SOURCE FILTER IN C



The first of the three available techniques is to write the filter
completely in C. The external module you create interfaces directly
with the source filter hooks provided by Perl.



The advantage of this technique is that you have complete control over
the implementation of your filter. The big disadvantage is the
increased complexity required to write the filter - not only do you
need to understand the source filter hooks, but you also need a
reasonable knowledge of Perl guts. One of the few times it is worth
going to this trouble is when writing a source scrambler. The
decrypt filter (which unscrambles the source before Perl parses it)
included with the source filter distribution is an example of a C
source filter (see Decryption Filters, below).





Decryption Filters




All decryption filters work on the principle of "security through
obscurity." Regardless of how well you write a decryption filter and
how strong your encryption algorithm is, anyone determined enough can
retrieve the original source code. The reason is quite simple - once
the decryption filter has decrypted the source back to its original
form, fragments of it will be stored in the computer’s memory as Perl
parses it. The source might only be in memory for a short period of
time, but anyone possessing a debugger, skill, and lots of patience can
eventually reconstruct your program.



That said, there are a number of steps that can be taken to make life
difficult for the potential cracker. The most important: Write your
decryption filter in C and statically link the decryption module into
the Perl binary. For further tips to make life difficult for the
potential cracker, see the file decrypt.pm in the source filters
distribution.














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22.7 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE



An alternative to writing the filter in C is to create a separate
executable in the language of your choice. The separate executable
reads from standard input, does whatever processing is necessary, and
writes the filtered data to standard output. Filter::cpp is an
example of a source filter implemented as a separate executable - the
executable is the C preprocessor bundled with your C compiler.



The source filter distribution includes two modules that simplify this
task: Filter::exec and Filter::sh. Both allow you to run any
external executable. Both use a coprocess to control the flow of data
into and out of the external executable. (For details on coprocesses,
see Stephens, W.R., "Advanced Programming in the UNIX Environment."
Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference
between them is that Filter::exec spawns the external command
directly, while Filter::sh spawns a shell to execute the external
command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning
a shell allows you to make use of the shell metacharacters and
redirection facilities.



Here is an example script that uses Filter::sh:



    use Filter::sh 'tr XYZ PQR';
    $a = 1;
    print "XYZ a = $a\n";



The output you’ll get when the script is executed:



    PQR a = 1



Writing a source filter as a separate executable works fine, but a
small performance penalty is incurred. For example, if you execute the
small example above, a separate subprocess will be created to run the
Unix tr command. Each use of the filter requires its own subprocess.
If creating subprocesses is expensive on your system, you might want to
consider one of the other options for creating source filters.









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22.8 WRITING A SOURCE FILTER IN PERL



The easiest and most portable option available for creating your own
source filter is to write it completely in Perl. To distinguish this
from the previous two techniques, I’ll call it a Perl source filter.



To help understand how to write a Perl source filter we need an example
to study. Here is a complete source filter that performs rot13
decoding. (Rot13 is a very simple encryption scheme used in Usenet
postings to hide the contents of offensive posts. It moves every letter
forward thirteen places, so that A becomes N, B becomes O, and Z
becomes M.)



   package Rot13;

   use Filter::Util::Call;

   sub import {
      my ($type) = @_;
      my ($ref) = [];
      filter_add(bless $ref);
   }

   sub filter {
      my ($self) = @_;
      my ($status);

      tr/n-za-mN-ZA-M/a-zA-Z/
         if ($status = filter_read()) > 0;
      $status;
   }

   1;



All Perl source filters are implemented as Perl classes and have the
same basic structure as the example above.



First, we include the Filter::Util::Call module, which exports a
number of functions into your filter’s namespace. The filter shown
above uses two of these functions, filter_add() and
filter_read().



Next, we create the filter object and associate it with the source
stream by defining the import function. If you know Perl well
enough, you know that import is called automatically every time a
module is included with a use statement. This makes import the ideal
place to both create and install a filter object.



In the example filter, the object ($ref) is blessed just like any
other Perl object. Our example uses an anonymous array, but this isn’t
a requirement. Because this example doesn’t need to store any context
information, we could have used a scalar or hash reference just as
well. The next section demonstrates context data.



The association between the filter object and the source stream is made
with the filter_add() function. This takes a filter object as a
parameter ($ref in this case) and installs it in the source stream.



Finally, there is the code that actually does the filtering. For this
type of Perl source filter, all the filtering is done in a method
called filter(). (It is also possible to write a Perl source filter
using a closure. See the Filter::Util::Call manual page for more
details.) It’s called every time the Perl parser needs another line of
source to process. The filter() method, in turn, reads lines from
the source stream using the filter_read() function.



If a line was available from the source stream, filter_read()
returns a status value greater than zero and appends the line to $_.
A status value of zero indicates end-of-file, less than zero means an
error. The filter function itself is expected to return its status in
the same way, and put the filtered line it wants written to the source
stream in $_. The use of $_ accounts for the brevity of most Perl
source filters.



In order to make use of the rot13 filter we need some way of encoding
the source file in rot13 format. The script below, mkrot13, does
just that.



    die "usage mkrot13 filename\n" unless @ARGV;
    my $in = $ARGV[0];
    my $out = "$in.tmp";
    open(IN, "<$in") or die "Cannot open file $in: $!\n";
    open(OUT, ">$out") or die "Cannot open file $out: $!\n";

    print OUT "use Rot13;\n";
    while (<IN>) {
       tr/a-zA-Z/n-za-mN-ZA-M/;
       print OUT;
    }

    close IN;
    close OUT;
    unlink $in;
    rename $out, $in;



If we encrypt this with mkrot13:



    print " hello fred \n";



the result will be this:



    use Rot13;
    cevag "uryyb serq\a";



Running it produces this output:



    hello fred









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22.9 USING CONTEXT: THE DEBUG FILTER



The rot13 example was a trivial example. Here’s another demonstration
that shows off a few more features.



Say you wanted to include a lot of debugging code in your Perl script
during development, but you didn’t want it available in the released
product. Source filters offer a solution. In order to keep the example
simple, let’s say you wanted the debugging output to be controlled by
an environment variable, DEBUG. Debugging code is enabled if the
variable exists, otherwise it is disabled.



Two special marker lines will bracket debugging code, like this:



    ## DEBUG_BEGIN
    if ($year > 1999) {
       warn "Debug: millennium bug in year $year\n";
    }
    ## DEBUG_END



The filter ensures that Perl parses the code between the <DEBUG_BEGIN>
and DEBUG_END markers only when the DEBUG environment variable
exists. That means that when DEBUG does exist, the code above
should be passed through the filter unchanged. The marker lines can
also be passed through as-is, because the Perl parser will see them as
comment lines. When DEBUG isn’t set, we need a way to disable the
debug code. A simple way to achieve that is to convert the lines
between the two markers into comments:



    ## DEBUG_BEGIN
    #if ($year > 1999) {
    #     warn "Debug: millennium bug in year $year\n";
    #}
    ## DEBUG_END



Here is the complete Debug filter:



    package Debug;

    use strict;
    use warnings;
    use Filter::Util::Call;

    use constant TRUE => 1;
    use constant FALSE => 0;

    sub import {
       my ($type) = @_;
       my (%context) = (
         Enabled => defined $ENV{DEBUG},
         InTraceBlock => FALSE,
         Filename => (caller)[1],
         LineNo => 0,
         LastBegin => 0,
       );
       filter_add(bless \%context);
    }

    sub Die {
       my ($self) = shift;
       my ($message) = shift;
       my ($line_no) = shift || $self->{LastBegin};
       die "$message at $self->{Filename} line $line_no.\n"
    }

    sub filter {
       my ($self) = @_;
       my ($status);
       $status = filter_read();
       ++ $self->{LineNo};

       # deal with EOF/error first
       if ($status <= 0) {
           $self->Die("DEBUG_BEGIN has no DEBUG_END")
               if $self->{InTraceBlock};
           return $status;
       }

       if ($self->{InTraceBlock}) {
          if (/^\s*##\s*DEBUG_BEGIN/ ) {
              $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
          } elsif (/^\s*##\s*DEBUG_END/) {
              $self->{InTraceBlock} = FALSE;
          }

          # comment out the debug lines when the filter is disabled
          s/^/#/ if ! $self->{Enabled};
       } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
          $self->{InTraceBlock} = TRUE;
          $self->{LastBegin} = $self->{LineNo};
       } elsif ( /^\s*##\s*DEBUG_END/ ) {
          $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
       }
       return $status;
    }

    1;



The big difference between this filter and the previous example is the
use of context data in the filter object. The filter object is based on
a hash reference, and is used to keep various pieces of context
information between calls to the filter function. All but two of the
hash fields are used for error reporting. The first of those two,
Enabled, is used by the filter to determine whether the debugging code
should be given to the Perl parser. The second, InTraceBlock, is true
when the filter has encountered a DEBUG_BEGIN line, but has not yet
encountered the following DEBUG_END line.



If you ignore all the error checking that most of the code does, the
essence of the filter is as follows:



    sub filter {
       my ($self) = @_;
       my ($status);
       $status = filter_read();

       # deal with EOF/error first
       return $status if $status <= 0;
       if ($self->{InTraceBlock}) {
          if (/^\s*##\s*DEBUG_END/) {
             $self->{InTraceBlock} = FALSE
          }

          # comment out debug lines when the filter is disabled
          s/^/#/ if ! $self->{Enabled};
       } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
          $self->{InTraceBlock} = TRUE;
       }
       return $status;
    }



Be warned: just as the C-preprocessor doesn’t know C, the Debug filter
doesn’t know Perl. It can be fooled quite easily:



    print <<EOM;
    ##DEBUG_BEGIN
    EOM



Such things aside, you can see that a lot can be achieved with a modest
amount of code.









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22.10 CONCLUSION



You now have better understanding of what a source filter is, and you
might even have a possible use for them. If you feel like playing with
source filters but need a bit of inspiration, here are some extra
features you could add to the Debug filter.



First, an easy one. Rather than having debugging code that is
all-or-nothing, it would be much more useful to be able to control
which specific blocks of debugging code get included. Try extending the
syntax for debug blocks to allow each to be identified. The contents of
the DEBUG environment variable can then be used to control which
blocks get included.



Once you can identify individual blocks, try allowing them to be
nested. That isn’t difficult either.



Here is an interesting idea that doesn’t involve the Debug filter.
Currently Perl subroutines have fairly limited support for formal
parameter lists. You can specify the number of parameters and their
type, but you still have to manually take them out of the @_ array
yourself. Write a source filter that allows you to have a named
parameter list. Such a filter would turn this:



    sub MySub ($first, $second, @rest) { ... }



into this:



    sub MySub($$@) {
       my ($first) = shift;
       my ($second) = shift;
       my (@rest) = @_;
       ...
    }



Finally, if you feel like a real challenge, have a go at writing a
full-blown Perl macro preprocessor as a source filter. Borrow the
useful features from the C preprocessor and any other macro processors
you know. The tricky bit will be choosing how much knowledge of Perl’s
syntax you want your filter to have.









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22.11 THINGS TO LOOK OUT FOR





Some Filters Clobber the DATA Handle




Some source filters use the DATA handle to read the calling program.
When using these source filters you cannot rely on this handle, nor expect
any particular kind of behavior when operating on it.  Filters based on
Filter::Util::Call (and therefore Filter::Simple) do not alter the DATA
filehandle.














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22.12 REQUIREMENTS



The Source Filters distribution is available on CPAN, in 



    CPAN/modules/by-module/Filter



Starting from Perl 5.8 Filter::Util::Call (the core part of the
Source Filters distribution) is part of the standard Perl distribution.
Also included is a friendlier interface called Filter::Simple, by
Damian Conway.









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22.13 AUTHOR



Paul Marquess <Paul.Marquess@btinternet.com>









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22.14 Copyrights



This article originally appeared in The Perl Journal #11, and is
copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and
The Perl Journal.  This document may be distributed under the same terms
as Perl itself.









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23 perlfork





• perlfork NAME:  



• perlfork SYNOPSIS:  



• perlfork DESCRIPTION:  



• perlfork CAVEATS AND LIMITATIONS:  



• perlfork PORTABILITY CAVEATS:  



• perlfork BUGS:  



• perlfork AUTHOR:  



• perlfork SEE ALSO:  












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23.1 NAME



perlfork - Perl’s fork() emulation









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23.2 SYNOPSIS



    NOTE:  As of the 5.8.0 release, fork() emulation has considerably
    matured.  However, there are still a few known bugs and differences
    from real fork() that might affect you.  See the "BUGS" and
    "CAVEATS AND LIMITATIONS" sections below.



Perl provides a fork() keyword that corresponds to the Unix system call
of the same name.  On most Unix-like platforms where the fork() system
call is available, Perl’s fork() simply calls it.



On some platforms such as Windows where the fork() system call is not
available, Perl can be built to emulate fork() at the interpreter level.
While the emulation is designed to be as compatible as possible with the
real fork() at the level of the Perl program, there are certain
important differences that stem from the fact that all the pseudo child
"processes" created this way live in the same real process as far as the
operating system is concerned.



This document provides a general overview of the capabilities and
limitations of the fork() emulation.  Note that the issues discussed here
are not applicable to platforms where a real fork() is available and Perl
has been configured to use it.









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23.3 DESCRIPTION



The fork() emulation is implemented at the level of the Perl interpreter.
What this means in general is that running fork() will actually clone the
running interpreter and all its state, and run the cloned interpreter in
a separate thread, beginning execution in the new thread just after the
point where the fork() was called in the parent.  We will refer to the
thread that implements this child "process" as the pseudo-process.



To the Perl program that called fork(), all this is designed to be
transparent.  The parent returns from the fork() with a pseudo-process
ID that can be subsequently used in any process-manipulation functions;
the child returns from the fork() with a value of 0 to signify that
it is the child pseudo-process.





• perlfork Behavior of other Perl features in forked pseudo-processes:  



• perlfork Resource limits:  



• perlfork Killing the parent process:  



• perlfork Lifetime of the parent process and pseudo-processes:  












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23.3.1 Behavior of other Perl features in forked pseudo-processes



Most Perl features behave in a natural way within pseudo-processes.





$$ or $PROCESS_ID




This special variable is correctly set to the pseudo-process ID.
It can be used to identify pseudo-processes within a particular
session.  Note that this value is subject to recycling if any
pseudo-processes are launched after others have been wait()-ed on.





%ENV




Each pseudo-process maintains its own virtual environment.  Modifications
to %ENV affect the virtual environment, and are only visible within that
pseudo-process, and in any processes (or pseudo-processes) launched from
it.





chdir() and all other builtins that accept filenames




Each pseudo-process maintains its own virtual idea of the current directory.
Modifications to the current directory using chdir() are only visible within
that pseudo-process, and in any processes (or pseudo-processes) launched from
it.  All file and directory accesses from the pseudo-process will correctly
map the virtual working directory to the real working directory appropriately.





wait() and waitpid()




wait() and waitpid() can be passed a pseudo-process ID returned by fork().
These calls will properly wait for the termination of the pseudo-process
and return its status.





kill()




kill('KILL', ...) can be used to terminate a pseudo-process by
passing it the ID returned by fork(). The outcome of kill on a pseudo-process
is unpredictable and it should not be used except
under dire circumstances, because the operating system may not
guarantee integrity of the process resources when a running thread is
terminated.  The process which implements the pseudo-processes can be blocked
and the Perl interpreter hangs. Note that using kill('KILL', ...) on a
pseudo-process() may typically cause memory leaks, because the thread
that implements the pseudo-process does not get a chance to clean up
its resources.



kill('TERM', ...) can also be used on pseudo-processes, but the
signal will not be delivered while the pseudo-process is blocked by a
system call, e.g. waiting for a socket to connect, or trying to read
from a socket with no data available.  Starting in Perl 5.14 the
parent process will not wait for children to exit once they have been
signalled with kill('TERM', ...) to avoid deadlock during process
exit.  You will have to explicitly call waitpid() to make sure the
child has time to clean-up itself, but you are then also responsible
that the child is not blocking on I/O either.





exec()




Calling exec() within a pseudo-process actually spawns the requested
executable in a separate process and waits for it to complete before
exiting with the same exit status as that process.  This means that the
process ID reported within the running executable will be different from
what the earlier Perl fork() might have returned.  Similarly, any process
manipulation functions applied to the ID returned by fork() will affect the
waiting pseudo-process that called exec(), not the real process it is
waiting for after the exec().



When exec() is called inside a pseudo-process then DESTROY methods and
END blocks will still be called after the external process returns.





exit()




exit() always exits just the executing pseudo-process, after automatically
wait()-ing for any outstanding child pseudo-processes.  Note that this means
that the process as a whole will not exit unless all running pseudo-processes
have exited.  See below for some limitations with open filehandles.





Open handles to files, directories and network sockets




All open handles are dup()-ed in pseudo-processes, so that closing
any handles in one process does not affect the others.  See below for
some limitations.














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23.3.2 Resource limits



In the eyes of the operating system, pseudo-processes created via the fork()
emulation are simply threads in the same process.  This means that any
process-level limits imposed by the operating system apply to all
pseudo-processes taken together.  This includes any limits imposed by the
operating system on the number of open file, directory and socket handles,
limits on disk space usage, limits on memory size, limits on CPU utilization
etc.









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23.3.3 Killing the parent process



If the parent process is killed (either using Perl’s kill() builtin, or
using some external means) all the pseudo-processes are killed as well,
and the whole process exits.









Previous: perlfork Killing the parent process, Up: perlfork DESCRIPTION   [Contents]





23.3.4 Lifetime of the parent process and pseudo-processes



During the normal course of events, the parent process and every
pseudo-process started by it will wait for their respective pseudo-children
to complete before they exit.  This means that the parent and every
pseudo-child created by it that is also a pseudo-parent will only exit
after their pseudo-children have exited.



Starting with Perl 5.14 a parent will not wait() automatically
for any child that has been signalled with sig('TERM', ...)
to avoid a deadlock in case the child is blocking on I/O and
never receives the signal.









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23.4 CAVEATS AND LIMITATIONS





BEGIN blocks




The fork() emulation will not work entirely correctly when called from
within a BEGIN block.  The forked copy will run the contents of the
BEGIN block, but will not continue parsing the source stream after the
BEGIN block.  For example, consider the following code:



    BEGIN {
        fork and exit;          # fork child and exit the parent
        print "inner\n";
    }
    print "outer\n";



This will print:



    inner



rather than the expected:



    inner
    outer



This limitation arises from fundamental technical difficulties in
cloning and restarting the stacks used by the Perl parser in the
middle of a parse.





Open filehandles




Any filehandles open at the time of the fork() will be dup()-ed.  Thus,
the files can be closed independently in the parent and child, but beware
that the dup()-ed handles will still share the same seek pointer.  Changing
the seek position in the parent will change it in the child and vice-versa.
One can avoid this by opening files that need distinct seek pointers
separately in the child.



On some operating systems, notably Solaris and Unixware, calling exit()
from a child process will flush and close open filehandles in the parent,
thereby corrupting the filehandles.  On these systems, calling _exit()
is suggested instead.  _exit() is available in Perl through the 
POSIX module.  Please consult your system’s manpages for more information
on this.





Open directory handles




Perl will completely read from all open directory handles until they
reach the end of the stream.  It will then seekdir() back to the
original location and all future readdir() requests will be fulfilled
from the cache buffer.  That means that neither the directory handle held
by the parent process nor the one held by the child process will see
any changes made to the directory after the fork() call.



Note that rewinddir() has a similar limitation on Windows and will not
force readdir() to read the directory again either.  Only a newly
opened directory handle will reflect changes to the directory.





Forking pipe open() not yet implemented




The open(FOO, "|-") and open(BAR, "-|") constructs are not yet
implemented.  This limitation can be easily worked around in new code
by creating a pipe explicitly.  The following example shows how to
write to a forked child:



    # simulate open(FOO, "|-")
    sub pipe_to_fork ($) {
        my $parent = shift;
        pipe my $child, $parent or die;
        my $pid = fork();
        die "fork() failed: $!" unless defined $pid;
        if ($pid) {
            close $child;
        }
        else {
            close $parent;
            open(STDIN, "<&=" . fileno($child)) or die;
        }
        $pid;
    }

    if (pipe_to_fork('FOO')) {
        # parent
        print FOO "pipe_to_fork\n";
        close FOO;
    }
    else {
        # child
        while (<STDIN>) { print; }
        exit(0);
    }



And this one reads from the child:



    # simulate open(FOO, "-|")
    sub pipe_from_fork ($) {
        my $parent = shift;
        pipe $parent, my $child or die;
        my $pid = fork();
        die "fork() failed: $!" unless defined $pid;
        if ($pid) {
            close $child;
        }
        else {
            close $parent;
            open(STDOUT, ">&=" . fileno($child)) or die;
        }
        $pid;
    }

    if (pipe_from_fork('BAR')) {
        # parent
        while (<BAR>) { print; }
        close BAR;
    }
    else {
        # child
        print "pipe_from_fork\n";
        exit(0);
    }



Forking pipe open() constructs will be supported in future.





Global state maintained by XSUBs




External subroutines (XSUBs) that maintain their own global state may
not work correctly.  Such XSUBs will either need to maintain locks to
protect simultaneous access to global data from different pseudo-processes,
or maintain all their state on the Perl symbol table, which is copied
naturally when fork() is called.  A callback mechanism that provides
extensions an opportunity to clone their state will be provided in the
near future.





Interpreter embedded in larger application




The fork() emulation may not behave as expected when it is executed in an
application which embeds a Perl interpreter and calls Perl APIs that can
evaluate bits of Perl code.  This stems from the fact that the emulation
only has knowledge about the Perl interpreter’s own data structures and
knows nothing about the containing application’s state.  For example, any
state carried on the application’s own call stack is out of reach.





Thread-safety of extensions




Since the fork() emulation runs code in multiple threads, extensions
calling into non-thread-safe libraries may not work reliably when
calling fork().  As Perl’s threading support gradually becomes more
widely adopted even on platforms with a native fork(), such extensions
are expected to be fixed for thread-safety.














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23.5 PORTABILITY CAVEATS



In portable Perl code, kill(9, $child) must not be used on forked processes.
Killing a forked process is unsafe and has unpredictable results.
See kill(), above.









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23.6 BUGS











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23.7 AUTHOR



Support for concurrent interpreters and the fork() emulation was implemented
by ActiveState, with funding from Microsoft Corporation.



This document is authored and maintained by Gurusamy Sarathy
<gsar@activestate.com>.









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23.8 SEE ALSO



(perlfunc)fork, perlipc NAME









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24 NAME format report chart





• NAME format report chart NAME:  



• NAME format report chart DESCRIPTION:  



• NAME format report chart NOTES:  



• NAME format report chart WARNINGS:  












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24.1 NAME






perlform - Perl formats









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24.2 DESCRIPTION




Perl has a mechanism to help you generate simple reports and charts.  To
facilitate this, Perl helps you code up your output page close to how it
will look when it’s printed.  It can keep track of things like how many
lines are on a page, what page you’re on, when to print page headers,
etc.  Keywords are borrowed from FORTRAN: format() to declare and write()
to execute; see their entries in (perlfunc).  Fortunately, the layout is
much more legible, more like BASIC’s PRINT USING statement.  Think of it
as a poor man’s nroff(1).




Formats, like packages and subroutines, are declared rather than
executed, so they may occur at any point in your program.  (Usually it’s
best to keep them all together though.) They have their own namespace
apart from all the other "types" in Perl.  This means that if you have a
function named "Foo", it is not the same thing as having a format named
"Foo".  However, the default name for the format associated with a given
filehandle is the same as the name of the filehandle.  Thus, the default
format for STDOUT is named "STDOUT", and the default format for filehandle
TEMP is named "TEMP".  They just look the same.  They aren’t.



Output record formats are declared as follows:



    format NAME =
    FORMLIST
    .



If the name is omitted, format "STDOUT" is defined. A single "." in 
column 1 is used to terminate a format.  FORMLIST consists of a sequence 
of lines, each of which may be one of three types:



    

  1.  A comment, indicated by putting a ’#’ in the first column.


  2.  A "picture" line giving the format for one output line.


  3.  An argument line supplying values to plug into the previous picture line.



















Picture lines contain output field definitions, intermingled with
literal text. These lines do not undergo any kind of variable interpolation.
Field definitions are made up from a set of characters, for starting and
extending a field to its desired width. This is the complete set of
characters for field definitions:




   @    start of regular field
   ^    start of special field
   <    pad character for left justification
   |    pad character for centering
   >    pad character for right justification
   #    pad character for a right-justified numeric field
   0    instead of first #: pad number with leading zeroes
   .    decimal point within a numeric field
   ...  terminate a text field, show "..." as truncation evidence
   @*   variable width field for a multi-line value
   ^*   variable width field for next line of a multi-line value
   ~    suppress line with all fields empty
   ~~   repeat line until all fields are exhausted



Each field in a picture line starts with either "@" (at) or "^" (caret),
indicating what we’ll call, respectively, a "regular" or "special" field.
The choice of pad characters determines whether a field is textual or
numeric. The tilde operators are not part of a field.  Let’s look at
the various possibilities in detail.





• NAME format report chart Text Fields:  



• NAME format report chart Numeric Fields:  



• NAME format report chart The Field @* for Variable-Width Multi-Line Text:  



• NAME format report chart The Field ^* for Variable-Width One-line-at-a-time Text:  



• NAME format report chart Specifying Values:  



• NAME format report chart Using Fill Mode:  



• NAME format report chart Suppressing Lines Where All Fields Are Void:  



• NAME format report chart Repeating Format Lines:  



• NAME format report chart Top of Form Processing:  



• NAME format report chart Format Variables:  












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24.2.1 Text Fields




The length of the field is supplied by padding out the field with multiple 
"<", ">", or "|" characters to specify a non-numeric field with,
respectively, left justification, right justification, or centering. 
For a regular field, the value (up to the first newline) is taken and
printed according to the selected justification, truncating excess characters.
If you terminate a text field with "...", three dots will be shown if
the value is truncated. A special text field may be used to do rudimentary 
multi-line text block filling; see Using Fill Mode for details.



   Example:
      format STDOUT =
      @<<<<<<   @||||||   @>>>>>>
      "left",   "middle", "right"
      .
   Output:
      left      middle    right









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24.2.2 Numeric Fields





Using "#" as a padding character specifies a numeric field, with
right justification. An optional "." defines the position of the
decimal point. With a "0" (zero) instead of the first "#", the
formatted number will be padded with leading zeroes if necessary.
A special numeric field is blanked out if the value is undefined.
If the resulting value would exceed the width specified the field is
filled with "#" as overflow evidence.



   Example:
      format STDOUT =
      @###   @.###   @##.###  @###   @###   ^####
       42,   3.1415,  undef,    0, 10000,   undef
      .
   Output:
        42   3.142     0.000     0   ####









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24.2.3 The Field @* for Variable-Width Multi-Line Text




The field "@*" can be used for printing multi-line, nontruncated
values; it should (but need not) appear by itself on a line. A final
line feed is chomped off, but all other characters are emitted verbatim.









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24.2.4 The Field ^* for Variable-Width One-line-at-a-time Text




Like "@*", this is a variable-width field. The value supplied must be a 
scalar variable. Perl puts the first line (up to the first "\n") of the 
text into the field, and then chops off the front of the string so that 
the next time the variable is referenced, more of the text can be printed. 
The variable will not be restored.



   Example:
      $text = "line 1\nline 2\nline 3";
      format STDOUT =
      Text: ^*
            $text
      ~~    ^*
            $text
      .
   Output:
      Text: line 1
            line 2
            line 3









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24.2.5 Specifying Values




The values are specified on the following format line in the same order as
the picture fields.  The expressions providing the values must be
separated by commas.  They are all evaluated in a list context
before the line is processed, so a single list expression could produce
multiple list elements.  The expressions may be spread out to more than
one line if enclosed in braces.  If so, the opening brace must be the first
token on the first line.  If an expression evaluates to a number with a
decimal part, and if the corresponding picture specifies that the decimal
part should appear in the output (that is, any picture except multiple "#"
characters without an embedded "."), the character used for the decimal
point is determined by the current LC_NUMERIC locale if use locale is in
effect.  This means that, if, for example, the run-time environment happens
to specify a German locale, "," will be used instead of the default ".".  See
perllocale NAME and WARNINGS for more information.









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24.2.6 Using Fill Mode




On text fields the caret enables a kind of fill mode.  Instead of an
arbitrary expression, the value supplied must be a scalar variable
that contains a text string.  Perl puts the next portion of the text into
the field, and then chops off the front of the string so that the next time
the variable is referenced, more of the text can be printed.  (Yes, this
means that the variable itself is altered during execution of the write()
call, and is not restored.)  The next portion of text is determined by
a crude line-breaking algorithm. You may use the carriage return character
(\r) to force a line break. You can change which characters are legal 
to break on by changing the variable $: (that’s 
$FORMAT_LINE_BREAK_CHARACTERS if you’re using the English module) to a 
list of the desired characters.



Normally you would use a sequence of fields in a vertical stack associated 
with the same scalar variable to print out a block of text. You might wish 
to end the final field with the text "...", which will appear in the output 
if the text was too long to appear in its entirety.  









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24.2.7 Suppressing Lines Where All Fields Are Void




Using caret fields can produce lines where all fields are blank. You can
suppress such lines by putting a "~" (tilde) character anywhere in the
line.  The tilde will be translated to a space upon output.









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24.2.8 Repeating Format Lines




If you put two contiguous tilde characters "~~" anywhere into a line,
the line will be repeated until all the fields on the line are exhausted,
i.e. undefined. For special (caret) text fields this will occur sooner or
later, but if you use a text field of the at variety, the  expression you
supply had better not give the same value every time forever! (shift(@f)
is a simple example that would work.)  Don’t use a regular (at) numeric 
field in such lines, because it will never go blank.









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24.2.9 Top of Form Processing






Top-of-form processing is by default handled by a format with the
same name as the current filehandle with "_TOP" concatenated to it.
It’s triggered at the top of each page.  See (perlfunc)write.



Examples:



 # a report on the /etc/passwd file
 format STDOUT_TOP =
                         Passwd File
 Name                Login    Office   Uid   Gid Home
 ------------------------------------------------------------------
 .
 format STDOUT =
 @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
 $name,              $login,  $office,$uid,$gid, $home
 .


 # a report from a bug report form
 format STDOUT_TOP =
                         Bug Reports
 @<<<<<<<<<<<<<<<<<<<<<<<     @|||         @>>>>>>>>>>>>>>>>>>>>>>>
 $system,                      $%,         $date
 ------------------------------------------------------------------
 .
 format STDOUT =
 Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
          $subject
 Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        $index,                       $description
 Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
           $priority,        $date,   $description
 From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
       $from,                         $description
 Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
              $programmer,            $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      $description
 ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
                                      $description
 .



It is possible to intermix print()s with write()s on the same output
channel, but you’ll have to handle $- ($FORMAT_LINES_LEFT)
yourself.









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24.2.10 Format Variables





The current format name is stored in the variable $~ ($FORMAT_NAME),
and the current top of form format name is in $^ ($FORMAT_TOP_NAME).
The current output page number is stored in $% ($FORMAT_PAGE_NUMBER),
and the number of lines on the page is in $= ($FORMAT_LINES_PER_PAGE).
Whether to autoflush output on this handle is stored in $|
($OUTPUT_AUTOFLUSH).  The string output before each top of page (except
the first) is stored in $^L ($FORMAT_FORMFEED).  These variables are
set on a per-filehandle basis, so you’ll need to select() into a different
one to affect them:



    select((select(OUTF),
            $~ = "My_Other_Format",
            $^ = "My_Top_Format"
           )[0]);



Pretty ugly, eh?  It’s a common idiom though, so don’t be too surprised
when you see it.  You can at least use a temporary variable to hold
the previous filehandle: (this is a much better approach in general,
because not only does legibility improve, you now have an intermediary
stage in the expression to single-step the debugger through):



    $ofh = select(OUTF);
    $~ = "My_Other_Format";
    $^ = "My_Top_Format";
    select($ofh);



If you use the English module, you can even read the variable names:



    use English '-no_match_vars';
    $ofh = select(OUTF);
    $FORMAT_NAME     = "My_Other_Format";
    $FORMAT_TOP_NAME = "My_Top_Format";
    select($ofh);



But you still have those funny select()s.  So just use the FileHandle
module.  Now, you can access these special variables using lowercase
method names instead:



    use FileHandle;
    format_name     OUTF "My_Other_Format";
    format_top_name OUTF "My_Top_Format";



Much better!









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24.3 NOTES



Because the values line may contain arbitrary expressions (for at fields,
not caret fields), you can farm out more sophisticated processing
to other functions, like sprintf() or one of your own.  For example:



    format Ident =
        @<<<<<<<<<<<<<<<
        &commify($n)
    .



To get a real at or caret into the field, do this:



    format Ident =
    I have an @ here.
            "@"
    .



To center a whole line of text, do something like this:



    format Ident =
    @|||||||||||||||||||||||||||||||||||||||||||||||
            "Some text line"
    .



There is no builtin way to say "float this to the right hand side
of the page, however wide it is."  You have to specify where it goes.
The truly desperate can generate their own format on the fly, based
on the current number of columns, and then eval() it:



    $format  = "format STDOUT = \n"
             . '^' . '<' x $cols . "\n"
             . '$entry' . "\n"
             . "\t^" . "<" x ($cols-8) . "~~\n"
             . '$entry' . "\n"
             . ".\n";
    print $format if $Debugging;
    eval $format;
    die $@ if $@;



Which would generate a format looking something like this:



 format STDOUT =
 ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 $entry
         ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
 $entry
 .



Here’s a little program that’s somewhat like fmt(1):



 format =
 ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
 $_

 .

 $/ = '';
 while (<>) {
     s/\s*\n\s*/ /g;
     write;
 }





• NAME format report chart Footers:  



• NAME format report chart Accessing Formatting Internals:  












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24.3.1 Footers





While $FORMAT_TOP_NAME contains the name of the current header format,
there is no corresponding mechanism to automatically do the same thing
for a footer.  Not knowing how big a format is going to be until you
evaluate it is one of the major problems.  It’s on the TODO list.



Here’s one strategy:  If you have a fixed-size footer, you can get footers
by checking $FORMAT_LINES_LEFT before each write() and print the footer
yourself if necessary.



Here’s another strategy: Open a pipe to yourself, using open(MYSELF, "|-")
(see (perlfunc)open) and always write() to MYSELF instead of STDOUT.
Have your child process massage its STDIN to rearrange headers and footers
however you like.  Not very convenient, but doable.









Previous: NAME format report chart Footers, Up: NAME format report chart NOTES   [Contents]





24.3.2 Accessing Formatting Internals




For low-level access to the formatting mechanism.  you may use formline()
and access $^A (the $ACCUMULATOR variable) directly.



For example:



    $str = formline <<'END', 1,2,3;
    @<<<  @|||  @>>>
    END

    print "Wow, I just stored '$^A' in the accumulator!\n";



Or to make an swrite() subroutine, which is to write() what sprintf()
is to printf(), do this:



    use Carp;
    sub swrite {
        croak "usage: swrite PICTURE ARGS" unless @_;
        my $format = shift;
        $^A = "";
        formline($format,@_);
        return $^A;
    }

    $string = swrite(<<'END', 1, 2, 3);
 Check me out
 @<<<  @|||  @>>>
 END
    print $string;









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24.4 WARNINGS



The lone dot that ends a format can also prematurely end a mail
message passing through a misconfigured Internet mailer (and based on
experience, such misconfiguration is the rule, not the exception).  So
when sending format code through mail, you should indent it so that
the format-ending dot is not on the left margin; this will prevent
SMTP cutoff.



Lexical variables (declared with "my") are not visible within a
format unless the format is declared within the scope of the lexical
variable.  (They weren’t visible at all before version 5.001.)



If a program’s environment specifies an LC_NUMERIC locale and use
locale is in effect when the format is declared, the locale is used
to specify the decimal point character in formatted output.  Formatted
output cannot be controlled by use locale at the time when write()
is called. See perllocale NAME for further discussion of locale handling.



Within strings that are to be displayed in a fixed-length text field,
each control character is substituted by a space. (But remember the
special meaning of \r when using fill mode.) This is done to avoid
misalignment when control characters "disappear" on some output media.









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25 NAME function





• NAME function NAME:  



• NAME function DESCRIPTION:  












Next: , Up: NAME function   [Contents]





25.1 NAME




perlfunc - Perl builtin functions









Previous: NAME function NAME, Up: NAME function   [Contents]





25.2 DESCRIPTION



The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators.  These differ in their precedence relationship with a
following comma.  (See the precedence table in (perlop).)  List
operators take more than one argument, while unary operators can never
take more than one argument.  Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator.  A unary operator generally provides scalar context to its
argument, while a list operator may provide either scalar or list
contexts for its arguments.  If it does both, scalar arguments 
come first and list argument follow, and there can only ever
be one such list argument.  For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.



In the syntax descriptions that follow, list operators that expect a
list (and provide list context for elements of the list) are shown
with LIST as an argument.  Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Commas should separate literal elements of the LIST.



Any function in the list below may be used either with or without
parentheses around its arguments.  (The syntax descriptions omit the
parentheses.)  If you use parentheses, the simple but occasionally 
surprising rule is this: It looks like a function, therefore it is a
function, and precedence doesn’t matter.  Otherwise it’s a list
operator or unary operator, and precedence does matter.  Whitespace
between the function and left parenthesis doesn’t count, so sometimes
you need to be careful:



    print 1+2+4;      # Prints 7.
    print(1+2) + 4;   # Prints 3.
    print (1+2)+4;    # Also prints 3!
    print +(1+2)+4;   # Prints 7.
    print ((1+2)+4);  # Prints 7.



If you run Perl with the -w switch it can warn you about this.  For
example, the third line above produces:



    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.



A few functions take no arguments at all, and therefore work as neither
unary nor list operators.  These include such functions as time
and endpwent.  For example, time+86_400 always means
time() + 86_400.



For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in scalar context by
returning the undefined value, and in list context by returning the
empty list.




Remember the following important rule: There is no rule that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa.  It might do two totally different things.
Each operator and function decides which sort of value would be most
appropriate to return in scalar context.  Some operators return the
length of the list that would have been returned in list context.  Some
operators return the first value in the list.  Some operators return the
last value in the list.  Some operators return a count of successful
operations.  In general, they do what you want, unless you want
consistency.




A named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context.  You can’t get a list
like (1,2,3) into being in scalar context, because the compiler knows
the context at compile time.  It would generate the scalar comma operator
there, not the list construction version of the comma.  That means it
was never a list to start with.



In general, functions in Perl that serve as wrappers for system calls ("syscalls")
of the same name (like chown(2), fork(2), closedir(2), etc.) return
true when they succeed and undef otherwise, as is usually mentioned
in the descriptions below.  This is different from the C interfaces,
which return -1 on failure.  Exceptions to this rule include wait,
waitpid, and syscall.  System calls also set the special $!
variable on failure.  Other functions do not, except accidentally.



Extension modules can also hook into the Perl parser to define new
kinds of keyword-headed expression.  These may look like functions, but
may also look completely different.  The syntax following the keyword
is defined entirely by the extension.  If you are an implementor, see
(perlapi)PL_keyword_plugin for the mechanism.  If you are using such
a module, see the module’s documentation for details of the syntax that
it defines.





• NAME function Perl Functions by Category:  



• NAME function Portability:  



• NAME function Alphabetical Listing of Perl Functions:  



• NAME function Non-function Keywords by Cross-reference:  












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25.2.1 Perl Functions by Category




Here are Perl’s functions (including things that look like
functions, like some keywords and named operators)
arranged by category.  Some functions appear in more
than one place.





Functions for SCALARs or strings







chomp, chop, chr, crypt, fc, hex, index, lc,
lcfirst, length, oct, ord, pack, q//, qq//, reverse,
rindex, sprintf, substr, tr///, uc, ucfirst, y///



fc is available only if the "fc" feature is enabled or if it is
prefixed with CORE::.  The "fc" feature is enabled automatically
with a use v5.16 (or higher) declaration in the current scope.





Regular expressions and pattern matching







m//, pos, qr//, quotemeta, s///, split, study





Numeric functions








abs, atan2, cos, exp, hex, int, log, oct, rand,
sin, sqrt, srand





Functions for real @ARRAYs





each, keys, pop, push, shift, splice, unshift, values





Functions for list data





grep, join, map, qw//, reverse, sort, unpack





Functions for real %HASHes





delete, each, exists, keys, values





Input and output functions








binmode, close, closedir, dbmclose, dbmopen, die, eof,
fileno, flock, format, getc, print, printf, read,
readdir, readline rewinddir, say, seek, seekdir, select,
syscall, sysread, sysseek, syswrite, tell, telldir,
truncate, warn, write



say is available only if the "say" feature is enabled or if it is
prefixed with CORE::.  The "say" feature is enabled automatically
with a use v5.10 (or higher) declaration in the current scope.





Functions for fixed-length data or records




pack, read, syscall, sysread, sysseek, syswrite, unpack,
vec





Functions for filehandles, files, or directories










-X, chdir, chmod, chown, chroot, fcntl, glob,
ioctl, link, lstat, mkdir, open, opendir,
readlink, rename, rmdir, stat, symlink, sysopen,
umask, unlink, utime





Keywords related to the control flow of your Perl program





break, caller, continue, die, do,
dump, eval, evalbytes exit,
__FILE__, goto, last, __LINE__, next, __PACKAGE__,
redo, return, sub, __SUB__, wantarray



break is available only if you enable the experimental "switch"
feature or use the CORE:: prefix. The "switch" feature also enables
the default, given and when statements, which are documented in
(perlsyn)Switch Statements. The "switch" feature is enabled
automatically with a use v5.10 (or higher) declaration in the current
scope. In Perl v5.14 and earlier, continue required the "switch"
feature, like the other keywords.



evalbytes is only available with with the "evalbytes" feature (see
(feature)) or if prefixed with CORE::.  __SUB__ is only available
with with the "current_sub" feature or if prefixed with CORE::. Both
the "evalbytes" and "current_sub" features are enabled automatically
with a use v5.16 (or higher) declaration in the current scope.





Keywords related to scoping




caller, import, local, my, our, package, state, use



state is available only if the "state" feature is enabled or if it is
prefixed with CORE::.  The "state" feature is enabled automatically
with a use v5.10 (or higher) declaration in the current scope.





Miscellaneous functions




defined, formline, lock, prototype, reset, scalar, undef





Functions for processes and process groups







alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx//, readpipe, setpgrp,
setpriority, sleep, system,
times, wait, waitpid





Keywords related to Perl modules





do, import, no, package, require, use





Keywords related to classes and object-orientation







bless, dbmclose, dbmopen, package, ref, tie, tied,
untie, use





Low-level socket functions






accept, bind, connect, getpeername, getsockname,
getsockopt, listen, recv, send, setsockopt, shutdown,
socket, socketpair





System V interprocess communication functions










msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
shmctl, shmget, shmread, shmwrite





Fetching user and group info











endgrent, endhostent, endnetent, endpwent, getgrent,
getgrgid, getgrnam, getlogin, getpwent, getpwnam,
getpwuid, setgrent, setpwent





Fetching network info











endprotoent, endservent, gethostbyaddr, gethostbyname,
gethostent, getnetbyaddr, getnetbyname, getnetent,
getprotobyname, getprotobynumber, getprotoent,
getservbyname, getservbyport, getservent, sethostent,
setnetent, setprotoent, setservent





Time-related functions






gmtime, localtime, time, times





Non-function keywords




and, AUTOLOAD, BEGIN, CHECK, cmp, CORE, __DATA__,
default, DESTROY, else, elseif, elsif, END, __END__,
eq, for, foreach, ge, given, gt, if, INIT, le,
lt, ne, not, or, UNITCHECK, unless, until, when,
while, x, xor














Next: , Previous: NAME function Perl Functions by Category, Up: NAME function DESCRIPTION   [Contents]





25.2.2 Portability






Perl was born in Unix and can therefore access all common Unix
system calls.  In non-Unix environments, the functionality of some
Unix system calls may not be available or details of the available
functionality may differ slightly.  The Perl functions affected
by this are:



-X, binmode, chmod, chown, chroot, crypt,
dbmclose, dbmopen, dump, endgrent, endhostent,
endnetent, endprotoent, endpwent, endservent, exec,
fcntl, flock, fork, getgrent, getgrgid, gethostbyname,
gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent,
getppid, getpgrp, getpriority, getprotobynumber,
getprotoent, getpwent, getpwnam, getpwuid,
getservbyport, getservent, getsockopt, glob, ioctl,
kill, link, lstat, msgctl, msgget, msgrcv,
msgsnd, open, pipe, readlink, rename, select, semctl,
semget, semop, setgrent, sethostent, setnetent,
setpgrp, setpriority, setprotoent, setpwent,
setservent, setsockopt, shmctl, shmget, shmread,
shmwrite, socket, socketpair,
stat, symlink, syscall, sysopen, system,
times, truncate, umask, unlink,
utime, wait, waitpid



For more information about the portability of these functions, see
perlport NAME and other available platform-specific documentation.









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25.2.3 Alphabetical Listing of Perl Functions





-X FILEHANDLE

































-X EXPR






-X DIRHANDLE






-X




A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename, a filehandle, or a dirhandle, 
and tests the associated file to see if something is true about it.  If the
argument is omitted, tests $_, except for -t, which tests STDIN.
Unless otherwise documented, it returns 1 for true and '' for false, or
the undefined value if the file doesn’t exist.  Despite the funny
names, precedence is the same as any other named unary operator.  The
operator may be any of:



    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size (is empty).
    -s  File has nonzero size (returns size in bytes).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII text file (heuristic guess).
    -B  File is a "binary" file (opposite of -T).

    -M  Script start time minus file modification time, in days.
    -A  Same for access time.
    -C  Same for inode change time (Unix, may differ for other platforms)



Example:



    while (<>) {
        chomp;
        next unless -f $_;  # ignore specials
        #...
    }



Note that -s/a/b/ does not do a negated substitution.  Saying
-exp($foo) still works as expected, however: only single letters
following a minus are interpreted as file tests.



These operators are exempt from the "looks like a function rule" described
above.  That is, an opening parenthesis after the operator does not affect
how much of the following code constitutes the argument.  Put the opening
parentheses before the operator to separate it from code that follows (this
applies only to operators with higher precedence than unary operators, of
course):



    -s($file) + 1024   # probably wrong; same as -s($file + 1024)
    (-s $file) + 1024  # correct



The interpretation of the file permission operators -r, -R,
-w, -W, -x, and -X is by default based solely on the mode
of the file and the uids and gids of the user.  There may be other
reasons you can’t actually read, write, or execute the file: for
example network filesystem access controls, ACLs (access control lists),
read-only filesystems, and unrecognized executable formats.  Note
that the use of these six specific operators to verify if some operation
is possible is usually a mistake, because it may be open to race
conditions.



Also note that, for the superuser on the local filesystems, the -r,
-R, -w, and -W tests always return 1, and -x and -X return 1
if any execute bit is set in the mode.  Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
or temporarily set their effective uid to something else.



If you are using ACLs, there is a pragma called filetest that may
produce more accurate results than the bare stat() mode bits.
When under use filetest 'access' the above-mentioned filetests
test whether the permission can(not) be granted using the
access(2) family of system calls.  Also note that the -x and -X may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls’ definitions.  Note also that, due to
the implementation of use filetest 'access', the _ special
filehandle won’t cache the results of the file tests when this pragma is
in effect.  Read the documentation for the filetest pragma for more
information.



The -T and -B switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many strange characters (>30%)
are found, it’s a -B file; otherwise it’s a -T file.  Also, any file
containing a zero byte in the first block is considered a binary file.  If -T
or -B is used on a filehandle, the current IO buffer is examined
rather than the first block.  Both -T and -B return true on an empty
file, or a file at EOF when testing a filehandle.  Because you have to
read a file to do the -T test, on most occasions you want to use a -f
against the file first, as in next unless -f $file && -T $file.



If any of the file tests (or either the stat or lstat operator) is given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn’t work with -t, and you need to remember
that lstat() and -l leave values in the stat structure for the
symbolic link, not the real file.)  (Also, if the stat buffer was filled by
an lstat call, -T and -B will reset it with the results of stat _).
Example:



    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;



As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
test operators, in a way that -f -w -x $file is equivalent to
-x $file && -w _ && -f _.  (This is only fancy fancy: if you use
the return value of -f $file as an argument to another filetest
operator, no special magic will happen.)



Portability issues: perlport -X.



To avoid confusing would-be users of your code with mysterious
syntax errors, put something like this at the top of your script:



    use 5.010;  # so filetest ops can stack





abs VALUE








abs




Returns the absolute value of its argument.
If VALUE is omitted, uses $_.





accept NEWSOCKET,GENERICSOCKET





Accepts an incoming socket connect, just as accept(2) 
does.  Returns the packed address if it succeeded, false otherwise.
See the example in perlipc Sockets: Client/Server Communication.



On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor, as determined by the
value of $^F.  See perlvar $^F.





alarm SECONDS









alarm




Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds has elapsed.  If SECONDS is not
specified, the value stored in $_ is used.  (On some machines,
unfortunately, the elapsed time may be up to one second less or more
than you specified because of how seconds are counted, and process
scheduling may delay the delivery of the signal even further.)



Only one timer may be counting at once.  Each call disables the
previous timer, and an argument of 0 may be supplied to cancel the
previous timer without starting a new one.  The returned value is the
amount of time remaining on the previous timer.



For delays of finer granularity than one second, the Time::HiRes module
(from CPAN, and starting from Perl 5.8 part of the standard
distribution) provides ualarm().  You may also use Perl’s four-argument
version of select() leaving the first three arguments undefined, or you
might be able to use the syscall interface to access setitimer(2) if
your system supports it.  See (perlfaq8) for details.



It is usually a mistake to intermix alarm and sleep calls, because
sleep may be internally implemented on your system with alarm.



If you want to use alarm to time out a system call you need to use an
eval/die pair.  You can’t rely on the alarm causing the system call to
fail with $! set to EINTR because Perl sets up signal handlers to
restart system calls on some systems.  Using eval/die always works,
modulo the caveats given in perlipc Signals.



    eval {
        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "alarm\n";   # propagate unexpected errors
        # timed out
    }
    else {
        # didn't
    }



For more information see perlipc NAME.



Portability issues: perlport alarm.





atan2 Y,X








Returns the arctangent of Y/X in the range -PI to PI.



For the tangent operation, you may use the Math::Trig::tan
function, or use the familiar relation:



    sub tan { sin($_[0]) / cos($_[0])  }



The return value for atan2(0,0) is implementation-defined; consult
your atan2(3) manpage for more information.



Portability issues: perlport atan2.





bind SOCKET,NAME





Binds a network address to a socket, just as bind(2)
does.  Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
perlipc Sockets: Client/Server Communication.





binmode FILEHANDLE, LAYER











binmode FILEHANDLE




Arranges for FILEHANDLE to be read or written in "binary" or "text"
mode on systems where the run-time libraries distinguish between
binary and text files.  If FILEHANDLE is an expression, the value is
taken as the name of the filehandle.  Returns true on success,
otherwise it returns undef and sets $! (errno).



On some systems (in general, DOS- and Windows-based systems) binmode()
is necessary when you’re not working with a text file.  For the sake
of portability it is a good idea always to use it when appropriate,
and never to use it when it isn’t appropriate.  Also, people can
set their I/O to be by default UTF8-encoded Unicode, not bytes.



In other words: regardless of platform, use binmode() on binary data,
like images, for example.



If LAYER is present it is a single string, but may contain multiple
directives.  The directives alter the behaviour of the filehandle.
When LAYER is present, using binmode on a text file makes sense.



If LAYER is omitted or specified as :raw the filehandle is made
suitable for passing binary data.  This includes turning off possible CRLF
translation and marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in "Programming Perl" (the
Camel, 3rd edition) or elsewhere, :raw is not simply the inverse of :crlf.
Other layers that would affect the binary nature of the stream are
also disabled.  See (PerlIO), perlrun NAME, and the discussion about the
PERLIO environment variable.



The :bytes, :crlf, :utf8, and any other directives of the
form :..., are called I/O layers.  The open pragma can be used to
establish default I/O layers.  See (open).



The LAYER parameter of the binmode() function is described as "DISCIPLINE"
in "Programming Perl, 3rd Edition".  However, since the publishing of this
book, by many known as "Camel III", the consensus of the naming of this
functionality has moved from "discipline" to "layer".  All documentation
of this version of Perl therefore refers to "layers" rather than to
"disciplines".  Now back to the regularly scheduled documentation...



To mark FILEHANDLE as UTF-8, use :utf8 or :encoding(UTF-8).
:utf8 just marks the data as UTF-8 without further checking,
while :encoding(UTF-8) checks the data for actually being valid
UTF-8.  More details can be found in (PerlIO-encoding).



In general, binmode() should be called after open() but before any I/O
is done on the filehandle.  Calling binmode() normally flushes any
pending buffered output data (and perhaps pending input data) on the
handle.  An exception to this is the :encoding layer that
changes the default character encoding of the handle; see ‘open’.
The :encoding layer sometimes needs to be called in
mid-stream, and it doesn’t flush the stream.  The :encoding
also implicitly pushes on top of itself the :utf8 layer because
internally Perl operates on UTF8-encoded Unicode characters.



The operating system, device drivers, C libraries, and Perl run-time
system all conspire to let the programmer treat a single
character (\n) as the line terminator, irrespective of external
representation.  On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of \n is made up of more than
one character.



All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
a single character to end each line in the external representation of text
(even though that single character is CARRIAGE RETURN on old, pre-Darwin
flavors of Mac OS, and is LINE FEED on Unix and most VMS files).  In other
systems like OS/2, DOS, and the various flavors of MS-Windows, your program
sees a \n as a simple \cJ, but what’s stored in text files are the
two characters \cM\cJ.  That means that if you don’t use binmode() on
these systems, \cM\cJ sequences on disk will be converted to \n on
input, and any \n in your program will be converted back to \cM\cJ on
output.  This is what you want for text files, but it can be disastrous for
binary files.



Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that, if your binary
data contain \cZ, the I/O subsystem will regard it as the end of
the file, unless you use binmode().



binmode() is important not only for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see perlport NAME for more details).  See the $/ and $\ variables
in perlvar NAME for how to manually set your input and output
line-termination sequences.



Portability issues: perlport binmode.





bless REF,CLASSNAME







bless REF




This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package.  If CLASSNAME is omitted, the current package
is used.  Because a bless is often the last thing in a constructor,
it returns the reference for convenience.  Always use the two-argument
version if a derived class might inherit the function doing the blessing.
See(perlobj) for more about the blessing (and blessings) of objects.



Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names.  To prevent
confusion, you may wish to avoid such package names as well.  Make sure
that CLASSNAME is a true value.



See perlmod Perl Modules.





break




Break out of a given() block.



This keyword is enabled by the "switch" feature: see
(feature) for more information.  You can also access it by
prefixing it with CORE::.  Alternately, include a use
v5.10 or later to the current scope.





caller EXPR










caller




Returns the context of the current subroutine call.  In scalar context,
returns the caller’s package name if there is a caller (that is, if
we’re in a subroutine or eval or require) and the undefined value
otherwise.  In list context, returns



    # 0         1          2
    ($package, $filename, $line) = caller;



With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.



    #  0         1          2      3            4
    ($package, $filename, $line, $subroutine, $hasargs,

    #  5          6          7            8       9         10
    $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
     = caller($i);



Here $subroutine may be (eval) if the frame is not a subroutine
call, but an eval.  In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the frame is created by a
require or use statement, $evaltext contains the text of the
eval EXPR statement.  In particular, for an eval BLOCK statement,
$subroutine is (eval), but $evaltext is undefined.  (Note also that
each use statement creates a require frame inside an eval EXPR
frame.)  $subroutine may also be (unknown) if this particular
subroutine happens to have been deleted from the symbol table.
$hasargs is true if a new instance of @_ was set up for the frame.
$hints and $bitmask contain pragmatic hints that the caller was
compiled with.  The $hints and $bitmask values are subject to change
between versions of Perl, and are not meant for external use.



$hinthash is a reference to a hash containing the value of %^H when the
caller was compiled, or undef if %^H was empty.  Do not modify the values
of this hash, as they are the actual values stored in the optree.



Furthermore, when called from within the DB package in
list context, and with an argument, caller returns more
detailed information: it sets the list variable @DB::args to be the
arguments with which the subroutine was invoked.



Be aware that the optimizer might have optimized call frames away before
caller had a chance to get the information.  That means that caller(N)
might not return information about the call frame you expect it to, for
N > 1.  In particular, @DB::args might have information from the
previous time caller was called.



Be aware that setting @DB::args is best effort, intended for
debugging or generating backtraces, and should not be relied upon.  In
particular, as @_ contains aliases to the caller’s arguments, Perl does
not take a copy of @_, so @DB::args will contain modifications the
subroutine makes to @_ or its contents, not the original values at call
time.  @DB::args, like @_, does not hold explicit references to its
elements, so under certain cases its elements may have become freed and
reallocated for other variables or temporary values.  Finally, a side effect
of the current implementation is that the effects of shift @_ can
normally be undone (but not pop @_ or other splicing, and not if a
reference to @_ has been taken, and subject to the caveat about reallocated
elements), so @DB::args is actually a hybrid of the current state and
initial state of @_.  Buyer beware.





chdir EXPR









chdir FILEHANDLE






chdir DIRHANDLE






chdir




Changes the working directory to EXPR, if possible.  If EXPR is omitted,
changes to the directory specified by $ENV{HOME}, if set; if not,
changes to the directory specified by $ENV{LOGDIR}.  (Under VMS, the
variable $ENV{SYS$LOGIN} is also checked, and used if it is set.)  If
neither is set, chdir does nothing.  It returns true on success,
false otherwise.  See the example under die.



On systems that support fchdir(2), you may pass a filehandle or
directory handle as the argument.  On systems that don’t support fchdir(2),
passing handles raises an exception.





chmod LIST







Changes the permissions of a list of files.  The first element of the
list must be the numeric mode, which should probably be an octal
number, and which definitely should not be a string of octal digits:
0644 is okay, but "0644" is not.  Returns the number of files
successfully changed.  See also oct if all you have is a string.



    $cnt = chmod 0755, "foo", "bar";
    chmod 0755, @executables;
    $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
                                             # --w----r-T
    $mode = "0644"; chmod oct($mode), "foo"; # this is better
    $mode = 0644;   chmod $mode, "foo";      # this is best



On systems that support fchmod(2), you may pass filehandles among the
files.  On systems that don’t support fchmod(2), passing filehandles raises
an exception.  Filehandles must be passed as globs or glob references to be
recognized; barewords are considered filenames.



    open(my $fh, "<", "foo");
    my $perm = (stat $fh)[2] & 07777;
    chmod($perm | 0600, $fh);



You can also import the symbolic S_I* constants from the Fcntl
module:



    use Fcntl qw( :mode );
    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
    # Identical to the chmod 0755 of the example above.



Portability issues: perlport chmod.





chomp VARIABLE











chomp( LIST )






chomp




This safer version of chop removes any trailing string
that corresponds to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in the English module).  It returns the total
number of characters removed from all its arguments.  It’s often used to
remove the newline from the end of an input record when you’re worried
that the final record may be missing its newline.  When in paragraph
mode ($/ = ""), it removes all trailing newlines from the string.
When in slurp mode ($/ = undef) or fixed-length record mode ($/ is
a reference to an integer or the like; see perlvar NAME) chomp() won’t
remove anything.
If VARIABLE is omitted, it chomps $_.  Example:



    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        # ...
    }



If VARIABLE is a hash, it chomps the hash’s values, but not its keys.



You can actually chomp anything that’s an lvalue, including an assignment:



    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);



If you chomp a list, each element is chomped, and the total number of
characters removed is returned.



Note that parentheses are necessary when you’re chomping anything
that is not a simple variable.  This is because chomp $cwd = `pwd`;
is interpreted as (chomp $cwd) = `pwd`;, rather than as
chomp( $cwd = `pwd` ) which you might expect.  Similarly,
chomp $a, $b is interpreted as chomp($a), $b rather than
as chomp($a, $b).





chop VARIABLE







chop( LIST )






chop




Chops off the last character of a string and returns the character
chopped.  It is much more efficient than s/.$//s because it neither
scans nor copies the string.  If VARIABLE is omitted, chops $_.
If VARIABLE is a hash, it chops the hash’s values, but not its keys.



You can actually chop anything that’s an lvalue, including an assignment.



If you chop a list, each element is chopped.  Only the value of the
last chop is returned.



Note that chop returns the last character.  To return all but the last
character, use substr($string, 0, -1).



See also chomp.





chown LIST








Changes the owner (and group) of a list of files.  The first two
elements of the list must be the numeric uid and gid, in that
order.  A value of -1 in either position is interpreted by most
systems to leave that value unchanged.  Returns the number of files
successfully changed.



    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;



On systems that support fchown(2), you may pass filehandles among the
files.  On systems that don’t support fchown(2), passing filehandles raises
an exception.  Filehandles must be passed as globs or glob references to be
recognized; barewords are considered filenames.



Here’s an example that looks up nonnumeric uids in the passwd file:



    print "User: ";
    chomp($user = <STDIN>);
    print "Files: ";
    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = glob($pattern);  # expand filenames
    chown $uid, $gid, @ary;



On most systems, you are not allowed to change the ownership of the
file unless you’re the superuser, although you should be able to change
the group to any of your secondary groups.  On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
On POSIX systems, you can detect this condition this way:



    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);



Portability issues: perlport chmod.





chr NUMBER










chr




Returns the character represented by that NUMBER in the character set.
For example, chr(65) is "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face.  



Negative values give the Unicode replacement character (chr(0xfffd)),
except under the (bytes) pragma, where the low eight bits of the value
(truncated to an integer) are used.



If NUMBER is omitted, uses $_.



For the reverse, use ord.



Note that characters from 128 to 255 (inclusive) are by default
internally not encoded as UTF-8 for backward compatibility reasons.



See perlunicode NAME for more about Unicode.





chroot FILENAME








chroot




This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a / by your process and all its children.  (It doesn’t
change your current working directory, which is unaffected.)  For security
reasons, this call is restricted to the superuser.  If FILENAME is
omitted, does a chroot to $_.



Portability issues: perlport chroot.





close FILEHANDLE







close




Closes the file or pipe associated with the filehandle, flushes the IO
buffers, and closes the system file descriptor.  Returns true if those
operations succeed and if no error was reported by any PerlIO
layer.  Closes the currently selected filehandle if the argument is
omitted.



You don’t have to close FILEHANDLE if you are immediately going to do
another open on it, because open closes it for you.  (See
open.)  However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open does not.



If the filehandle came from a piped open, close returns false if one of
the other syscalls involved fails or if its program exits with non-zero
status.  If the only problem was that the program exited non-zero, $!
will be set to 0.  Closing a pipe also waits for the process executing
on the pipe to exit–in case you wish to look at the output of the pipe
afterwards–and implicitly puts the exit status value of that command into
$? and ${^CHILD_ERROR_NATIVE}.



If there are multiple threads running, close on a filehandle from a
piped open returns true without waiting for the child process to terminate,
if the filehandle is still open in another thread.



Closing the read end of a pipe before the process writing to it at the
other end is done writing results in the writer receiving a SIGPIPE.  If
the other end can’t handle that, be sure to read all the data before
closing the pipe.



Example:



    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...                        # print stuff to output
    close OUTPUT                # wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')          # get sort's results
        or die "Can't open 'foo' for input: $!";



FILEHANDLE may be an expression whose value can be used as an indirect
filehandle, usually the real filehandle name or an autovivified handle.





closedir DIRHANDLE





Closes a directory opened by opendir and returns the success of that
system call.





connect SOCKET,NAME





Attempts to connect to a remote socket, just like connect(2).
Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
perlipc Sockets: Client/Server Communication.





continue BLOCK







continue




When followed by a BLOCK, continue is actually a
flow control statement rather than a function.  If
there is a continue BLOCK attached to a BLOCK (typically in a while or
foreach), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a for loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the next statement (which is similar to the C continue
statement).



last, next, or redo may appear within a continue
block; last and redo behave as if they had been executed within
the main block.  So will next, but since it will execute a continue
block, it may be more entertaining.



    while (EXPR) {
        ### redo always comes here
        do_something;
    } continue {
        ### next always comes here
        do_something_else;
        # then back the top to re-check EXPR
    }
    ### last always comes here



Omitting the continue section is equivalent to using an
empty one, logically enough, so next goes directly back
to check the condition at the top of the loop.



When there is no BLOCK, continue is a function that
falls through the current when or default block instead of iterating
a dynamically enclosing foreach or exiting a lexically enclosing given.
In Perl 5.14 and earlier, this form of continue was
only available when the "switch" feature was enabled.
See (feature) and (perlsyn)Switch Statements for more
information.





cos EXPR










cos




Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted,
takes the cosine of $_.



For the inverse cosine operation, you may use the Math::Trig::acos()
function, or use this relation:



    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }





crypt PLAINTEXT,SALT














Creates a digest string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has not
been extirpated as a potential munition).



crypt() is a one-way hash function.  The PLAINTEXT and SALT are turned
into a short string, called a digest, which is returned.  The same
PLAINTEXT and SALT will always return the same string, but there is no
(known) way to get the original PLAINTEXT from the hash.  Small
changes in the PLAINTEXT or SALT will result in large changes in the
digest.



There is no decrypt function.  This function isn’t all that useful for
cryptography (for that, look for Crypt modules on your nearby CPAN
mirror) and the name "crypt" is a bit of a misnomer.  Instead it is
primarily used to check if two pieces of text are the same without
having to transmit or store the text itself.  An example is checking
if a correct password is given.  The digest of the password is stored,
not the password itself.  The user types in a password that is
crypt()’d with the same salt as the stored digest.  If the two digests
match, the password is correct.



When verifying an existing digest string you should use the digest as
the salt (like crypt($plain, $digest) eq $digest).  The SALT used
to create the digest is visible as part of the digest.  This ensures
crypt() will hash the new string with the same salt as the digest.
This allows your code to work with the standard ‘crypt’ and
with more exotic implementations.  In other words, assume
nothing about the returned string itself nor about how many bytes 
of SALT may matter.



Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set [./0-9A-Za-z], and only
the first eight bytes of PLAINTEXT mattered.  But alternative
hashing schemes (like MD5), higher level security schemes (like C2),
and implementations on non-Unix platforms may produce different
strings.



When choosing a new salt create a random two character string whose
characters come from the set [./0-9A-Za-z] (like join '', ('.',
'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]).  This set of
characters is just a recommendation; the characters allowed in
the salt depend solely on your system’s crypt library, and Perl can’t
restrict what salts crypt() accepts.



Here’s an example that makes sure that whoever runs this program knows
their password:



    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }



Of course, typing in your own password to whoever asks you
for it is unwise.



The ‘crypt’ function is unsuitable for hashing large quantities
of data, not least of all because you can’t get the information
back.  Look at the (Digest) module for more robust algorithms.



If using crypt() on a Unicode string (which potentially has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of)
the string back to an eight-bit byte string before calling crypt()
(on that copy).  If that works, good.  If not, crypt() dies with
Wide character in crypt.



Portability issues: perlport crypt.





dbmclose HASH





[This function has been largely superseded by the untie function.]



Breaks the binding between a DBM file and a hash.



Portability issues: perlport dbmclose.





dbmopen HASH,DBNAME,MASK









[This function has been largely superseded by the
tie function.]



This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash.  HASH is the name of the hash.  (Unlike normal open, the first
argument is not a filehandle, even though it looks like one).  DBNAME
is the name of the database (without the .dir or .pag extension if
any).  If the database does not exist, it is created with protection
specified by MASK (as modified by the umask).  To prevent creation of
the database if it doesn’t exist, you may specify a MODE
of 0, and the function will return a false value if it
can’t find an existing database.  If your system supports
only the older DBM functions, you may make only one dbmopen call in your
program.  In older versions of Perl, if your system had neither DBM nor
ndbm, calling dbmopen produced a fatal error; it now falls back to
sdbm(3).



If you don’t have write access to the DBM file, you can only read hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an eval 
to trap the error.



Note that functions such as keys and values may return huge lists
when used on large DBM files.  You may prefer to use the each
function to iterate over large DBM files.  Example:



    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);



See also (AnyDBM_File) for a more general description of the pros and
cons of the various dbm approaches, as well as (DB_File) for a particularly
rich implementation.



You can control which DBM library you use by loading that library
before you call dbmopen():



    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";



Portability issues: perlport dbmopen.





defined EXPR









defined




Returns a Boolean value telling whether EXPR has a value other than
the undefined value undef.  If EXPR is not present, $_ is
checked.



Many operations return undef to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish undef from
other values.  (A simple Boolean test will not distinguish among
undef, zero, the empty string, and "0", which are all equally
false.)  Note that since undef is a valid scalar, its presence
doesn’t necessarily indicate an exceptional condition: pop
returns undef when its argument is an empty array, or when the
element to return happens to be undef.



You may also use defined(&func) to check whether subroutine &func
has ever been defined.  The return value is unaffected by any forward
declarations of &func.  A subroutine that is not defined
may still be callable: its package may have an AUTOLOAD method that
makes it spring into existence the first time that it is called; see
(perlsub).



Use of defined on aggregates (hashes and arrays) is deprecated.  It
used to report whether memory for that aggregate had ever been
allocated.  This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:



    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }



When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use ‘exists’ for the latter
purpose.



Examples:



    print if defined $switch{D};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
        unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;



Note:  Many folks tend to overuse defined and are then surprised to
discover that the number 0 and "" (the zero-length string) are, in fact,
defined values.  For example, if you say



    "ab" =~ /a(.*)b/;



The pattern match succeeds and $1 is defined, although it
matched "nothing".  It didn’t really fail to match anything.  Rather, it
matched something that happened to be zero characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it’s an admission that it couldn’t give you an honest answer.  So you
should use defined only when questioning the integrity of what
you’re trying to do.  At other times, a simple comparison to 0 or "" is
what you want.



See also undef, ‘exists’, ref.





delete EXPR





Given an expression that specifies an element or slice of a hash, delete
deletes the specified elements from that hash so that exists() on that element
no longer returns true.  Setting a hash element to the undefined value does
not remove its key, but deleting it does; see ‘exists’.



In list context, returns the value or values deleted, or the last such
element in scalar context.  The return list’s length always matches that of
the argument list: deleting non-existent elements returns the undefined value
in their corresponding positions.



delete() may also be used on arrays and array slices, but its behavior is less
straightforward.  Although exists() will return false for deleted entries,
deleting array elements never changes indices of existing values; use shift()
or splice() for that.  However, if all deleted elements fall at the end of an
array, the array’s size shrinks to the position of the highest element that
still tests true for exists(), or to 0 if none do.



WARNING: Calling delete on array values is deprecated and likely to
be removed in a future version of Perl.



Deleting from %ENV modifies the environment.  Deleting from a hash tied to
a DBM file deletes the entry from the DBM file.  Deleting from a tied hash
or array may not necessarily return anything; it depends on the implementation
of the tied package’s DELETE method, which may do whatever it pleases.



The delete local EXPR construct localizes the deletion to the current
block at run time.  Until the block exits, elements locally deleted
temporarily no longer exist.  See (perlsub)Localized deletion of elements
of composite types.



    %hash = (foo => 11, bar => 22, baz => 33);
    $scalar = delete $hash{foo};             # $scalar is 11
    $scalar = delete @hash{qw(foo bar)};     # $scalar is 22
    @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)



The following (inefficiently) deletes all the values of %HASH and @ARRAY:



    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

    foreach $index (0 .. $#ARRAY) {
        delete $ARRAY[$index];
    }



And so do these:



    delete @HASH{keys %HASH};

    delete @ARRAY[0 .. $#ARRAY];



But both are slower than assigning the empty list
or undefining %HASH or @ARRAY, which is the customary 
way to empty out an aggregate:



    %HASH = ();     # completely empty %HASH
    undef %HASH;    # forget %HASH ever existed

    @ARRAY = ();    # completely empty @ARRAY
    undef @ARRAY;   # forget @ARRAY ever existed



The EXPR can be arbitrarily complicated provided its
final operation is an element or slice of an aggregate:



    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

    delete $ref->[$x][$y][$index];
    delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];





die LIST










die raises an exception.  Inside an eval the error message is stuffed
into $@ and the eval is terminated with the undefined value.
If the exception is outside of all enclosing evals, then the uncaught
exception prints LIST to STDERR and exits with a non-zero value.  If you
need to exit the process with a specific exit code, see exit.



Equivalent examples:



    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"



If the last element of LIST does not end in a newline, the current
script line number and input line number (if any) are also printed,
and a newline is supplied.  Note that the "input line number" (also
known as "chunk") is subject to whatever notion of "line" happens to
be currently in effect, and is also available as the special variable
$..  See perlvar $/ and perlvar $..



Hint: sometimes appending ", stopped" to your message will cause it
to make better sense when the string "at foo line 123" is appended.
Suppose you are running script "canasta".



    die "/etc/games is no good";
    die "/etc/games is no good, stopped";



produce, respectively



    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.



If the output is empty and $@ already contains a value (typically from a
previous eval) that value is reused after appending "\t...propagated".
This is useful for propagating exceptions:



    eval { ... };
    die unless $@ =~ /Expected exception/;



If the output is empty and $@ contains an object reference that has a
PROPAGATE method, that method will be called with additional file
and line number parameters.  The return value replaces the value in
$@;  i.e., as if $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };
were called.



If $@ is empty then the string "Died" is used.



If an uncaught exception results in interpreter exit, the exit code is
determined from the values of $! and $? with this pseudocode:



    exit $! if $!;              # errno
    exit $? >> 8 if $? >> 8;    # child exit status
    exit 255;                   # last resort



The intent is to squeeze as much possible information about the likely cause
into the limited space of the system exit
code.  However, as $! is the value
of C’s errno, which can be set by any system call, this means that the value
of the exit code used by die can be non-predictable, so should not be relied
upon, other than to be non-zero.



You can also call die with a reference argument, and if this is trapped
within an eval, $@ contains that reference.  This permits more
elaborate exception handling using objects that maintain arbitrary state
about the exception.  Such a scheme is sometimes preferable to matching
particular string values of $@ with regular expressions.  Because $@ 
is a global variable and eval may be used within object implementations,
be careful that analyzing the error object doesn’t replace the reference in
the global variable.  It’s easiest to make a local copy of the reference
before any manipulations.  Here’s an example:



    use Scalar::Util "blessed";

    eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
    if (my $ev_err = $@) {
        if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
            # handle Some::Module::Exception
        }
        else {
            # handle all other possible exceptions
        }
    }



Because Perl stringifies uncaught exception messages before display,
you’ll probably want to overload stringification operations on
exception objects.  See (overload) for details about that.



You can arrange for a callback to be run just before the die
does its deed, by setting the $SIG{__DIE__} hook.  The associated
handler is called with the error text and can change the error
message, if it sees fit, by calling die again.  See
perlvar %SIG for details on setting %SIG entries, and
eval BLOCK for some examples.  Although this feature was 
to be run only right before your program was to exit, this is not
currently so: the $SIG{__DIE__} hook is currently called
even inside eval()ed blocks/strings!  If one wants the hook to do
nothing in such situations, put



    die @_ if $^S;



as the first line of the handler (see perlvar $^S).  Because
this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.



See also exit(), warn(), and the Carp module.





do BLOCK






Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by the while or
until loop modifier, executes the BLOCK once before testing the loop
condition.  (On other statements the loop modifiers test the conditional
first.)



do BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block.
See (perlsyn) for alternative strategies.





do SUBROUTINE(LIST)





This form of subroutine call is deprecated.  SUBROUTINE can be a bareword,
a scalar variable or a subroutine beginning with &.





do EXPR





Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.



    do 'stat.pl';



is just like



    eval `cat stat.pl`;



except that it’s more efficient and concise, keeps track of the current
filename for error messages, searches the @INC directories, and updates
%INC if the file is found.  See perlvar @INC and perlvar %INC for
these variables.  It also differs in that code evaluated with do FILENAME
cannot see lexicals in the enclosing scope; eval STRING does.  It’s the
same, however, in that it does reparse the file every time you call it,
so you probably don’t want to do this inside a loop.



If do can read the file but cannot compile it, it returns undef and sets
an error message in $@.  If do cannot read the file, it returns undef
and sets $! to the error.  Always check $@ first, as compilation
could fail in a way that also sets $!.  If the file is successfully
compiled, do returns the value of the last expression evaluated.



Inclusion of library modules is better done with the
use and require operators, which also do automatic error checking
and raise an exception if there’s a problem.



You might like to use do to read in a program configuration
file.  Manual error checking can be done this way:



    # read in config files: system first, then user
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc")
    {
        unless ($return = do $file) {
            warn "couldn't parse $file: $@" if $@;
            warn "couldn't do $file: $!"    unless defined $return;
            warn "couldn't run $file"       unless $return;
        }
    }





dump LABEL









dump




This function causes an immediate core dump.  See also the -u
command-line switch in perlrun NAME, which does the same thing.
Primarily this is so that you can use the undump program (not
supplied) to turn your core dump into an executable binary after
having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing
a goto LABEL (with all the restrictions that goto suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If LABEL is omitted, restarts the program from the top.



WARNING: Any files opened at the time of the dump will not
be open any more when the program is reincarnated, with possible
resulting confusion by Perl.



This function is now largely obsolete, mostly because it’s very hard to
convert a core file into an executable.  That’s why you should now invoke
it as CORE::dump(), if you don’t want to be warned against a possible
typo.



Portability issues: perlport dump.





each HASH








each ARRAY







each EXPR




When called on a hash in list context, returns a 2-element list
consisting of the key and value for the next element of a hash.  In Perl
5.12 and later only, it will also return the index and value for the next
element of an array so that you can iterate over it; older Perls consider
this a syntax error.  When called in scalar context, returns only the key
(not the value) in a hash, or the index in an array.



Hash entries are returned in an apparently random order.  The actual random
order is subject to change in future versions of Perl, but it is
guaranteed to be in the same order as either the keys or values
function would produce on the same (unmodified) hash.  Since Perl
5.8.2 the ordering can be different even between different runs of Perl
for security reasons (see perlsec Algorithmic Complexity Attacks).



After each has returned all entries from the hash or array, the next
call to each returns the empty list in list context and undef in
scalar context; the next call following that one restarts iteration.
Each hash or array has its own internal iterator, accessed by each,
keys, and values.  The iterator is implicitly reset when each has
reached the end as just described; it can be explicitly reset by calling
keys or values on the hash or array.  If you add or delete a hash’s
elements while iterating over it, entries may be skipped or duplicated–so
don’t do that.  Exception: In the current implementation, it is always safe
to delete the item most recently returned by each(), so the following
code works properly:



        while (($key, $value) = each %hash) {
          print $key, "\n";
          delete $hash{$key};   # This is safe
        }



This prints out your environment like the printenv(1) program,
but in a different order:



    while (($key,$value) = each %ENV) {
        print "$key=$value\n";
    }



Starting with Perl 5.14, each can take a scalar EXPR, which must hold
reference to an unblessed hash or array.  The argument will be dereferenced
automatically.  This aspect of each is considered highly experimental.
The exact behaviour may change in a future version of Perl.



    while (($key,$value) = each $hashref) { ... }



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.012;  # so keys/values/each work on arrays
    use 5.014;  # so keys/values/each work on scalars (experimental)



See also keys, values, and sort.





eof FILEHANDLE









eof ()






eof




Returns 1 if the next read on FILEHANDLE will return end of file or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle.  (Note that this function actually
reads a character and then ungetcs it, so isn’t useful in an
interactive context.)  Do not read from a terminal file (or call
eof(FILEHANDLE) on it) after end-of-file is reached.  File types such
as terminals may lose the end-of-file condition if you do.



An eof without an argument uses the last file read.  Using eof()
with empty parentheses is different.  It refers to the pseudo file
formed from the files listed on the command line and accessed via the
<> operator.  Since <> isn’t explicitly opened,
as a normal filehandle is, an eof() before <> has been
used will cause @ARGV to be examined to determine if input is
available.   Similarly, an eof() after <> has returned
end-of-file will assume you are processing another @ARGV list,
and if you haven’t set @ARGV, will read input from STDIN;
see (perlop)I/O Operators.



In a while (<>) loop, eof or eof(ARGV) can be used to
detect the end of each file, whereas eof() will detect the end 
of the very last file only.  Examples:



    # reset line numbering on each input file
    while (<>) {
        next if /^\s*#/;  # skip comments
        print "$.\t$_";
    } continue {
        close ARGV if eof;  # Not eof()!
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {  # check for end of last file
            print "--------------\n";
        }
        print;
        last if eof();      # needed if we're reading from a terminal
    }



Practical hint: you almost never need to use eof in Perl, because the
input operators typically return undef when they run out of data or 
encounter an error.





eval EXPR














eval BLOCK






eval




In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within scalar context) is first parsed, and if there were no
errors, executed as a block within the lexical context of the current Perl
program.  This means, that in particular, any outer lexical variables are
visible to it, and any package variable settings or subroutine and format
definitions remain afterwards.



Note that the value is parsed every time the eval executes.
If EXPR is omitted, evaluates $_.  This form is typically used to
delay parsing and subsequent execution of the text of EXPR until run time.



If the unicode_eval feature is enabled (which is the default under a
use 5.16 or higher declaration), EXPR or $_ is treated as a string of
characters, so use utf8 declarations have no effect, and source filters
are forbidden.  In the absence of the unicode_eval feature, the string
will sometimes be treated as characters and sometimes as bytes, depending
on the internal encoding, and source filters activated within the eval
exhibit the erratic, but historical, behaviour of affecting some outer file
scope that is still compiling.  See also the evalbytes keyword, which
always treats its input as a byte stream and works properly with source
filters, and the (feature) pragma.



In the second form, the code within the BLOCK is parsed only once–at the
same time the code surrounding the eval itself was parsed–and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.



The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.



In both forms, the value returned is the value of the last expression
evaluated inside the mini-program; a return statement may be also used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar, or list context, depending on the context of the eval 
itself.  See wantarray for more on how the evaluation context can be 
determined.



If there is a syntax error or runtime error, or a die statement is
executed, eval returns undef in scalar context
or an empty list in list context, and $@ is set to the error
message.  (Prior to 5.16, a bug caused undef to be returned
in list context for syntax errors, but not for runtime errors.)
If there was no error, $@ is set to the empty string.  A
control flow operator like last or goto can bypass the setting of
$@.  Beware that using eval neither silences Perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into $@.
To do either of those, you have to use the $SIG{__WARN__} facility, or
turn off warnings inside the BLOCK or EXPR using no warnings 'all'.
See ‘warn’, perlvar NAME, (warnings) and (perllexwarn).



Note that, because eval traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket or symlink)
is implemented.  It is also Perl’s exception-trapping mechanism, where
the die operator is used to raise exceptions.



If you want to trap errors when loading an XS module, some problems with
the binary interface (such as Perl version skew) may be fatal even with
eval unless $ENV{PERL_DL_NONLAZY} is set.  See perlrun NAME.



If the code to be executed doesn’t vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in $@.
Examples:



    # make divide-by-zero nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = }; # WRONG

    # a run-time error
    eval '$answer =';   # sets $@



Using the eval{} form as an exception trap in libraries does have some
issues.  Due to the current arguably broken state of __DIE__ hooks, you
may wish not to trigger any __DIE__ hooks that user code may have installed.
You can use the local $SIG{__DIE__} construct for this purpose,
as this example shows:



    # a private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
    warn $@ if $@;



This is especially significant, given that __DIE__ hooks can call
die again, which has the effect of changing their error messages:



    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} =
              sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }



Because this promotes action at a distance, this counterintuitive behavior
may be fixed in a future release.



With an eval, you should be especially careful to remember what’s
being looked at when:



    eval $x;        # CASE 1
    eval "$x";      # CASE 2

    eval '$x';      # CASE 3
    eval { $x };    # CASE 4

    eval "\$$x++";  # CASE 5
    $$x++;          # CASE 6



Cases 1 and 2 above behave identically: they run the code contained in
the variable $x.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code '$x', which
does nothing but return the value of $x.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you would like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.



Before Perl 5.14, the assignment to $@ occurred before restoration 
of localized variables, which means that for your code to run on older
versions, a temporary is required if you want to mask some but not all
errors:



    # alter $@ on nefarious repugnancy only
    {
       my $e;
       {
          local $@; # protect existing $@
          eval { test_repugnancy() };
          # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
          $@ =~ /nefarious/ and $e = $@;
       }
       die $e if defined $e
    }



eval BLOCK does not count as a loop, so the loop control statements
next, last, or redo cannot be used to leave or restart the block.



An eval '' executed within the DB package doesn’t see the usual
surrounding lexical scope, but rather the scope of the first non-DB piece
of code that called it.  You don’t normally need to worry about this unless
you are writing a Perl debugger.





evalbytes EXPR







evalbytes




This function is like eval with a string argument, except it always
parses its argument, or $_ if EXPR is omitted, as a string of bytes.  A
string containing characters whose ordinal value exceeds 255 results in an
error.  Source filters activated within the evaluated code apply to the
code itself.



This function is only available under the evalbytes feature, a
use v5.16 (or higher) declaration, or with a CORE:: prefix.  See
(feature) for more information.





exec LIST








exec PROGRAM LIST




The exec function executes a system command and never returns;
use system instead of exec if you want it to return.  It fails and
returns false only if the command does not exist and it is executed
directly instead of via your system’s command shell (see below).



Since it’s a common mistake to use exec instead of system, Perl
warns you if exec is called in void context and if there is a following
statement that isn’t die, warn, or exit (if -w is set–but
you always do that, right?).  If you really want to follow an exec
with some other statement, you can use one of these styles to avoid the warning:



    exec ('foo')   or print STDERR "couldn't exec foo: $!";
    { exec ('foo') }; print STDERR "couldn't exec foo: $!";



If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in LIST.
If there is only one scalar argument or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system’s command shell for parsing
(this is /bin/sh -c on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to execvp, which is more efficient.
Examples:



    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";



If you don’t really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multivalued list, even if there is only a single scalar in
the list.)  Example:



    $shell = '/bin/csh';
    exec $shell '-sh';    # pretend it's a login shell



or, more directly,



    exec {'/bin/csh'} '-sh';  # pretend it's a login shell



When the arguments get executed via the system shell, results are
subject to its quirks and capabilities.  See (perlop)`STRING`
for details.



Using an indirect object with exec or system is also more
secure.  This usage (which also works fine with system()) forces
interpretation of the arguments as a multivalued list, even if the
list had just one argument.  That way you’re safe from the shell
expanding wildcards or splitting up words with whitespace in them.



    @args = ( "echo surprise" );

    exec @args;               # subject to shell escapes
                                # if @args == 1
    exec { $args[0] } @args;  # safe even with one-arg list



The first version, the one without the indirect object, ran the echo
program, passing it "surprise" an argument.  The second version didn’t;
it tried to run a program named "echo surprise", didn’t find it, and set
$? to a non-zero value indicating failure.



Beginning with v5.6.0, Perl attempts to flush all files opened for
output before the exec, but this may not be supported on some platforms
(see perlport NAME).  To be safe, you may need to set $| ($AUTOFLUSH
in English) or call the autoflush() method of IO::Handle on any
open handles to avoid lost output.



Note that exec will not call your END blocks, nor will it invoke
DESTROY methods on your objects.



Portability issues: perlport exec.





exists EXPR






Given an expression that specifies an element of a hash, returns true if the
specified element in the hash has ever been initialized, even if the
corresponding value is undefined.



    print "Exists\n"    if exists $hash{$key};
    print "Defined\n"   if defined $hash{$key};
    print "True\n"      if $hash{$key};



exists may also be called on array elements, but its behavior is much less
obvious and is strongly tied to the use of ‘delete’ on arrays.  Be aware
that calling exists on array values is deprecated and likely to be removed in
a future version of Perl.



    print "Exists\n"    if exists $array[$index];
    print "Defined\n"   if defined $array[$index];
    print "True\n"      if $array[$index];



A hash or array element can be true only if it’s defined and defined only if
it exists, but the reverse doesn’t necessarily hold true.



Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been declared, even
if it is undefined.  Mentioning a subroutine name for exists or defined
does not count as declaring it.  Note that a subroutine that does not
exist may still be callable: its package may have an AUTOLOAD
method that makes it spring into existence the first time that it is
called; see (perlsub).



    print "Exists\n"  if exists &subroutine;
    print "Defined\n" if defined &subroutine;



Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash or array key lookup or subroutine name:



    if (exists $ref->{A}->{B}->{$key})  { }
    if (exists $hash{A}{B}{$key})       { }

    if (exists $ref->{A}->{B}->[$ix])   { }
    if (exists $hash{A}{B}[$ix])        { }

    if (exists &{$ref->{A}{B}{$key}})   { }



Although the most deeply nested array or hash element will not spring into
existence just because its existence was tested, any intervening ones will.
Thus $ref->{"A"} and $ref->{"A"}->{"B"} will spring
into existence due to the existence test for the $key element above.
This happens anywhere the arrow operator is used, including even here:



    undef $ref;
    if (exists $ref->{"Some key"})    { }
    print $ref;  # prints HASH(0x80d3d5c)



This surprising autovivification in what does not at first–or even
second–glance appear to be an lvalue context may be fixed in a future
release.



Use of a subroutine call, rather than a subroutine name, as an argument
to exists() is an error.



    exists &sub;    # OK
    exists &sub();  # Error





exit EXPR









exit




Evaluates EXPR and exits immediately with that value.    Example:



    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;



See also die.  If EXPR is omitted, exits with 0 status.  The only
universally recognized values for EXPR are 0 for success and 1
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running.  For example, exiting
69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause
the mailer to return the item undelivered, but that’s not true everywhere.



Don’t use exit to abort a subroutine if there’s any chance that
someone might want to trap whatever error happened.  Use die instead,
which can be trapped by an eval.



The exit() function does not always exit immediately.  It calls any
defined END routines first, but these END routines may not
themselves abort the exit.  Likewise any object destructors that need to
be called are called before the real exit.  END routines and destructors
can change the exit status by modifying $?.  If this is a problem, you
can call POSIX::_exit($status) to avoid END and destructor processing.
See perlmod NAME for details.



Portability issues: perlport exit.





exp EXPR











exp




Returns e (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives exp($_).





fc EXPR











fc




Returns the casefolded version of EXPR.  This is the internal function
implementing the \F escape in double-quoted strings.



Casefolding is the process of mapping strings to a form where case
differences are erased; comparing two strings in their casefolded
form is effectively a way of asking if two strings are equal,
regardless of case.



Roughly, if you ever found yourself writing this



    lc($this) eq lc($that)  # Wrong!
        # or
    uc($this) eq uc($that)  # Also wrong!
        # or
    $this =~ /\Q$that/i     # Right!



Now you can write



    fc($this) eq fc($that)



And get the correct results.



Perl only implements the full form of casefolding.
For further information on casefolding, refer to
the Unicode Standard, specifically sections 3.13 Default Case Operations,
4.2 Case-Normative, and 5.18 Case Mappings,
available at http://www.unicode.org/versions/latest/, as well as the
Case Charts available at http://www.unicode.org/charts/case/.



If EXPR is omitted, uses $_.



This function behaves the same way under various pragma, such as in a locale,
as lc does.



While the Unicode Standard defines two additional forms of casefolding,
one for Turkic languages and one that never maps one character into multiple
characters, these are not provided by the Perl core; However, the CPAN module
Unicode::Casing may be used to provide an implementation.



This keyword is available only when the "fc" feature is enabled,
or when prefixed with CORE::; See (feature). Alternately,
include a use v5.16 or later to the current scope.





fcntl FILEHANDLE,FUNCTION,SCALAR





Implements the fcntl(2) function.  You’ll probably have to say



    use Fcntl;



first to get the correct constant definitions.  Argument processing and
value returned work just like ioctl below.
For example:



    use Fcntl;
    fcntl($filehandle, F_GETFL, $packed_return_buffer)
        or die "can't fcntl F_GETFL: $!";



You don’t have to check for defined on the return from fcntl.
Like ioctl, it maps a 0 return from the system call into
"0 but true" in Perl.  This string is true in boolean context and 0
in numeric context.  It is also exempt from the normal -w warnings
on improper numeric conversions.



Note that fcntl raises an exception if used on a machine that
doesn’t implement fcntl(2).  See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.



Here’s an example of setting a filehandle named REMOTE to be
non-blocking at the system level.  You’ll have to negotiate $|
on your own, though.



    use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

    $flags = fcntl(REMOTE, F_GETFL, 0)
                or die "Can't get flags for the socket: $!\n";

    $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
                or die "Can't set flags for the socket: $!\n";



Portability issues: perlport fcntl.





__FILE__





A special token that returns the name of the file in which it occurs.





fileno FILEHANDLE





Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open.  If there is no real file descriptor at the OS
level, as can happen with filehandles connected to memory objects via
open with a reference for the third argument, -1 is returned.



This is mainly useful for constructing
bitmaps for select and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.



You can use this to find out whether two handles refer to the
same underlying descriptor:



    if (fileno(THIS) == fileno(THAT)) {
        print "THIS and THAT are dups\n";
    }





flock FILEHANDLE,OPERATION







Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true
for success, false on failure.  Produces a fatal error if used on a
machine that doesn’t implement flock(2), fcntl(2) locking, or lockf(3).
flock is Perl’s portable file-locking interface, although it locks
entire files only, not records.



Two potentially non-obvious but traditional flock semantics are
that it waits indefinitely until the lock is granted, and that its locks
are merely advisory.  Such discretionary locks are more flexible, but
offer fewer guarantees.  This means that programs that do not also use
flock may modify files locked with flock.  See perlport NAME, 
your port’s specific documentation, and your system-specific local manpages
for details.  It’s best to assume traditional behavior if you’re writing
portable programs.  (But if you’re not, you should as always feel perfectly
free to write for your own system’s idiosyncrasies (sometimes called
"features").  Slavish adherence to portability concerns shouldn’t get
in the way of your getting your job done.)



OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB.  These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if you import them from the (Fcntl) module,
either individually, or as a group using the :flock tag.  LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock.  If LOCK_NB is bitwise-or’ed with
LOCK_SH or LOCK_EX, then flock returns immediately rather than blocking
waiting for the lock; check the return status to see if you got it.



To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.



Note that the emulation built with lockf(3) doesn’t provide shared
locks, and it requires that FILEHANDLE be open with write intent.  These
are the semantics that lockf(3) implements.  Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn’t bite too many people.



Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
be open with read intent to use LOCK_SH and requires that it be open
with write intent to use LOCK_EX.



Note also that some versions of flock cannot lock things over the
network; you would need to use the more system-specific fcntl for
that.  If you like you can force Perl to ignore your system’s flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch -Ud_flock to the Configure program when you configure
and build a new Perl.



Here’s a mailbox appender for BSD systems.



    use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants

    sub lock {
        my ($fh) = @_;
        flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";

        # and, in case someone appended while we were waiting...
        seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
    }

    sub unlock {
        my ($fh) = @_;
        flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
    }

    open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
        or die "Can't open mailbox: $!";

    lock($mbox);
    print $mbox $msg,"\n\n";
    unlock($mbox);



On systems that support a real flock(2), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl(2)
function lose their locks, making it seriously harder to write servers.



See also (DB_File) for other flock() examples.



Portability issues: perlport flock.





fork







Does a fork(2) system call to create a new process running the
same program at the same point.  It returns the child pid to the
parent process, 0 to the child process, or undef if the fork is
unsuccessful.  File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied.  On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.



Beginning with v5.6.0, Perl attempts to flush all files opened for
output before forking the child process, but this may not be supported
on some platforms (see perlport NAME).  To be safe, you may need to set
$| ($AUTOFLUSH in English) or call the autoflush() method of
IO::Handle on any open handles to avoid duplicate output.



If you fork without ever waiting on your children, you will
accumulate zombies.  On some systems, you can avoid this by setting
$SIG{CHLD} to "IGNORE".  See also perlipc NAME for more examples of
forking and reaping moribund children.



Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
backgrounded job launched from a remote shell) won’t think you’re done.
You should reopen those to /dev/null if it’s any issue.



On some platforms such as Windows, where the fork() system call is not available,
Perl can be built to emulate fork() in the Perl interpreter.
The emulation is designed, at the level of the Perl program,
to be as compatible as possible with the "Unix" fork().
However it has limitations that have to be considered in code intended to be portable.
See perlfork NAME for more details.



Portability issues: perlport fork.





format





Declare a picture format for use by the write function.  For
example:



    format Something =
        Test: @<<<<<<<< @||||| @>>>>>
              $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantity;
    $~ = 'Something';
    write;



See (perlform) for many details and examples.





formline PICTURE,LIST





This is an internal function used by formats, though you may call it,
too.  It formats (see (perlform)) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, $^A (or $ACCUMULATOR in English).
Eventually, when a write is done, the contents of
$^A are written to some filehandle.  You could also read $^A
and then set $^A back to "".  Note that a format typically
does one formline per line of form, but the formline function itself
doesn’t care how many newlines are embedded in the PICTURE.  This means
that the ~ and ~~ tokens treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.



Be careful if you put double quotes around the picture, because an @
character may be taken to mean the beginning of an array name.
formline always returns true.  See (perlform) for other examples.



If you are trying to use this instead of write to capture the output,
you may find it easier to open a filehandle to a scalar
(open $fh, ">", \$output) and write to that instead.





getc FILEHANDLE










getc




Returns the next character from the input file attached to FILEHANDLE,
or the undefined value at end of file or if there was an error (in
the latter case $! is set).  If FILEHANDLE is omitted, reads from
STDIN.  This is not particularly efficient.  However, it cannot be
used by itself to fetch single characters without waiting for the user
to hit enter.  For that, try something more like:



    if ($BSD_STYLE) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon', 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
    }
    print "\n";



Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.



The POSIX::getattr function can do this more portably on
systems purporting POSIX compliance.  See also the Term::ReadKey
module from your nearest CPAN site; details on CPAN can be found under
(perlmodlib)CPAN.





getlogin






This implements the C library function of the same name, which on most
systems returns the current login from /etc/utmp, if any.  If it
returns the empty string, use getpwuid.



    $login = getlogin || getpwuid($<) || "Kilroy";



Do not consider getlogin for authentication: it is not as
secure as getpwuid.



Portability issues: perlport getlogin.





getpeername SOCKET






Returns the packed sockaddr address of the other end of the SOCKET
connection.



    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);





getpgrp PID






Returns the current process group for the specified PID.  Use
a PID of 0 to get the current process group for the
current process.  Will raise an exception if used on a machine that
doesn’t implement getpgrp(2).  If PID is omitted, returns the process
group of the current process.  Note that the POSIX version of getpgrp
does not accept a PID argument, so only PID==0 is truly portable.



Portability issues: perlport getpgrp.





getppid







Returns the process id of the parent process.



Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
around non-POSIX thread semantics the minority of Linux systems (and
Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
has since been removed. See the documentation for $$ for
details.



Portability issues: perlport getppid.





getpriority WHICH,WHO







Returns the current priority for a process, a process group, or a user.
(See getpriority(2).)  Will raise a fatal exception if used on a
machine that doesn’t implement getpriority(2).



Portability issues: perlport getpriority.





getpwnam NAME




































getgrnam NAME






gethostbyname NAME






getnetbyname NAME






getprotobyname NAME






getpwuid UID






getgrgid GID






getservbyname NAME,PROTO






gethostbyaddr ADDR,ADDRTYPE






getnetbyaddr ADDR,ADDRTYPE






getprotobynumber NUMBER






getservbyport PORT,PROTO






getpwent






getgrent






gethostent






getnetent






getprotoent






getservent






setpwent






setgrent






sethostent STAYOPEN






setnetent STAYOPEN






setprotoent STAYOPEN






setservent STAYOPEN






endpwent






endgrent






endhostent






endnetent






endprotoent






endservent




These routines are the same as their counterparts in the
system C library.  In list context, the return values from the
various get routines are as follows:



    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*



(If the entry doesn’t exist you get an empty list.)



The exact meaning of the $gcos field varies but it usually contains
the real name of the user (as opposed to the login name) and other
information pertaining to the user.  Beware, however, that in many
system users are able to change this information and therefore it
cannot be trusted and therefore the $gcos is tainted (see
perlsec NAME).  The $passwd and $shell, user’s encrypted password and
login shell, are also tainted, for the same reason.



In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn’t exist you get the undefined value.)  For example:



    $uid   = getpwnam($name);
    $name  = getpwuid($num);
    $name  = getpwent();
    $gid   = getgrnam($name);
    $name  = getgrgid($num);
    $name  = getgrent();
    #etc.



In getpw*() the fields $quota, $comment, and $expire are special
in that they are unsupported on many systems.  If the
$quota is unsupported, it is an empty scalar.  If it is supported, it
usually encodes the disk quota.  If the $comment field is unsupported,
it is an empty scalar.  If it is supported it usually encodes some
administrative comment about the user.  In some systems the $quota
field may be $change or $age, fields that have to do with password
aging.  In some systems the $comment field may be $class.  The $expire
field, if present, encodes the expiration period of the account or the
password.  For the availability and the exact meaning of these fields
in your system, please consult getpwnam(3) and your system’s 
pwd.h file.  You can also find out from within Perl what your
$quota and $comment fields mean and whether you have the $expire field
by using the Config module and the values d_pwquota, d_pwage,
d_pwchange, d_pwcomment, and d_pwexpire.  Shadow password
files are supported only if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines gets the
shadow versions if you’re running under privilege or if there exists
the shadow(3) functions as found in System V (this includes Solaris
and Linux).  Those systems that implement a proprietary shadow password
facility are unlikely to be supported.



The $members value returned by getgr*() is a space-separated list of
the login names of the members of the group.



For the gethost*() functions, if the h_errno variable is supported in
C, it will be returned to you via $? if the function call fails.  The
@addrs value returned by a successful call is a list of raw
addresses returned by the corresponding library call.  In the
Internet domain, each address is four bytes long; you can unpack it
by saying something like:



    ($a,$b,$c,$d) = unpack('W4',$addr[0]);



The Socket library makes this slightly easier:



    use Socket;
    $iaddr = inet_aton("127.1"); # or whatever address
    $name  = gethostbyaddr($iaddr, AF_INET);

    # or going the other way
    $straddr = inet_ntoa($iaddr);



In the opposite way, to resolve a hostname to the IP address
you can write this:



    use Socket;
    $packed_ip = gethostbyname("www.perl.org");
    if (defined $packed_ip) {
        $ip_address = inet_ntoa($packed_ip);
    }



Make sure gethostbyname() is called in SCALAR context and that
its return value is checked for definedness.



The getprotobynumber function, even though it only takes one argument,
has the precedence of a list operator, so beware:



    getprotobynumber $number eq 'icmp'   # WRONG
    getprotobynumber($number eq 'icmp')  # actually means this
    getprotobynumber($number) eq 'icmp'  # better this way



If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided
in standard modules: File::stat, Net::hostent, Net::netent,
Net::protoent, Net::servent, Time::gmtime, Time::localtime,
and User::grent.  These override the normal built-ins, supplying
versions that return objects with the appropriate names
for each field.  For example:



   use File::stat;
   use User::pwent;
   $is_his = (stat($filename)->uid == pwent($whoever)->uid);



Even though it looks as though they’re the same method calls (uid),
they aren’t, because a File::stat object is different from
a User::pwent object.



Portability issues: perlport getpwnam to perlport endservent.





getsockname SOCKET





Returns the packed sockaddr address of this end of the SOCKET connection,
in case you don’t know the address because you have several different
IPs that the connection might have come in on.



    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = sockaddr_in($mysockaddr);
    printf "Connect to %s [%s]\n",
       scalar gethostbyaddr($myaddr, AF_INET),
       inet_ntoa($myaddr);





getsockopt SOCKET,LEVEL,OPTNAME





Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
Options may exist at multiple protocol levels depending on the socket
type, but at least the uppermost socket level SOL_SOCKET (defined in the
Socket module) will exist.  To query options at another level the
protocol number of the appropriate protocol controlling the option
should be supplied.  For example, to indicate that an option is to be
interpreted by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using getprotobyname.



The function returns a packed string representing the requested socket
option, or undef on error, with the reason for the error placed in
$!.  Just what is in the packed string depends on LEVEL and OPTNAME;
consult getsockopt(2) for details.  A common case is that the option is an
integer, in which case the result is a packed integer, which you can decode
using unpack with the i (or I) format.



Here’s an example to test whether Nagle’s algorithm is enabled on a socket:



    use Socket qw(:all);

    defined(my $tcp = getprotobyname("tcp"))
        or die "Could not determine the protocol number for tcp";
    # my $tcp = IPPROTO_TCP; # Alternative
    my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
        or die "getsockopt TCP_NODELAY: $!";
    my $nodelay = unpack("I", $packed);
    print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";



Portability issues: ‘perlport getsockopt’.





glob EXPR










glob




In list context, returns a (possibly empty) list of filename expansions on
the value of EXPR such as the standard Unix shell /bin/csh would do.  In
scalar context, glob iterates through such filename expansions, returning
undef when the list is exhausted.  This is the internal function
implementing the <*.c> operator, but you can use it directly.  If
EXPR is omitted, $_ is used.  The <*.c> operator is discussed in
more detail in (perlop)I/O Operators.



Note that glob splits its arguments on whitespace and treats
each segment as separate pattern.  As such, glob("*.c *.h") 
matches all files with a .c or .h extension.  The expression
glob(".* *") matches all files in the current working directory.
If you want to glob filenames that might contain whitespace, you’ll
have to use extra quotes around the spacey filename to protect it.
For example, to glob filenames that have an e followed by a space
followed by an f, use either of:



    @spacies = <"*e f*">;
    @spacies = glob '"*e f*"';
    @spacies = glob q("*e f*");



If you had to get a variable through, you could do this:



    @spacies = glob "'*${var}e f*'";
    @spacies = glob qq("*${var}e f*");



If non-empty braces are the only wildcard characters used in the
glob, no filenames are matched, but potentially many strings
are returned.  For example, this produces nine strings, one for
each pairing of fruits and colors:



    @many =  glob "{apple,tomato,cherry}={green,yellow,red}";



Beginning with v5.6.0, this operator is implemented using the standard
File::Glob extension.  See (File-Glob) for details, including
bsd_glob which does not treat whitespace as a pattern separator.



Portability issues: perlport glob.





gmtime EXPR









gmtime




Works just like localtime but the returned values are
localized for the standard Greenwich time zone.



Note: When called in list context, $isdst, the last value
returned by gmtime, is always 0.  There is no
Daylight Saving Time in GMT.



Portability issues: perlport gmtime.





goto LABEL









goto EXPR






goto &NAME




The goto-LABEL form finds the statement labeled with LABEL and
resumes execution there.  It can’t be used to get out of a block or
subroutine given to sort.  It can be used to go almost anywhere
else within the dynamic scope, including out of subroutines, but it’s
usually better to use some other construct such as last or die.
The author of Perl has never felt the need to use this form of goto
(in Perl, that is; C is another matter).  (The difference is that C
does not offer named loops combined with loop control.  Perl does, and
this replaces most structured uses of goto in other languages.)



The goto-EXPR form expects a label name, whose scope will be resolved
dynamically.  This allows for computed gotos per FORTRAN, but isn’t
necessarily recommended if you’re optimizing for maintainability:



    goto ("FOO", "BAR", "GLARCH")[$i];



As shown in this example, goto-EXPR is exempt from the "looks like a
function" rule.  A pair of parentheses following it does not (necessarily)
delimit its argument.  goto("NE")."XT" is equivalent to goto NEXT.



Use of goto-LABEL or goto-EXPR to jump into a construct is
deprecated and will issue a warning.  Even then, it may not be used to
go into any construct that requires initialization, such as a
subroutine or a foreach loop.  It also can’t be used to go into a
construct that is optimized away.



The goto-&NAME form is quite different from the other forms of
goto.  In fact, it isn’t a goto in the normal sense at all, and
doesn’t have the stigma associated with other gotos.  Instead, it
exits the current subroutine (losing any changes set by local()) and
immediately calls in its place the named subroutine using the current
value of @_.  This is used by AUTOLOAD subroutines that wish to
load another subroutine and then pretend that the other subroutine had
been called in the first place (except that any modifications to @_
in the current subroutine are propagated to the other subroutine.)
After the goto, not even caller will be able to tell that this
routine was called first.



NAME needn’t be the name of a subroutine; it can be a scalar variable
containing a code reference or a block that evaluates to a code
reference.





grep BLOCK LIST







grep EXPR,LIST




This is similar in spirit to, but not the same as, grep(1) and its
relatives.  In particular, it is not limited to using regular expressions.



Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to true.  In scalar
context, returns the number of times the expression was true.



    @foo = grep(!/^#/, @bar);    # weed out comments



or equivalently,



    @foo = grep {!/^#/} @bar;    # weed out comments



Note that $_ is an alias to the list value, so it can be used to
modify the elements of the LIST.  While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Similarly, grep returns aliases into the original list, much as a for
loop’s index variable aliases the list elements.  That is, modifying an
element of a list returned by grep (for example, in a foreach, map
or another grep) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.



If $_ is lexical in the scope where the grep appears (because it has
been declared with my $_) then, in addition to being locally aliased to
the list elements, $_ keeps being lexical inside the block; i.e., it
can’t be seen from the outside, avoiding any potential side-effects.



See also ‘map’ for a list composed of the results of the BLOCK or EXPR.





hex EXPR








hex




Interprets EXPR as a hex string and returns the corresponding value.
(To convert strings that might start with either 0, 0x, or 0b, see
oct.)  If EXPR is omitted, uses $_.



    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same



Hex strings may only represent integers.  Strings that would cause
integer overflow trigger a warning.  Leading whitespace is not stripped,
unlike oct().  To present something as hex, look into printf,
‘sprintf’, and ‘unpack’.





import LIST





There is no builtin import function.  It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The use function calls the import method
for the package used.  See also ‘use’, perlmod NAME, and (Exporter).





index STR,SUBSTR,POSITION









index STR,SUBSTR




The index function searches for one string within another, but without
the wildcard-like behavior of a full regular-expression pattern match.
It returns the position of the first occurrence of SUBSTR in STR at
or after POSITION.  If POSITION is omitted, starts searching from the
beginning of the string.  POSITION before the beginning of the string
or after its end is treated as if it were the beginning or the end,
respectively.  POSITION and the return value are based at zero.
If the substring is not found, index returns -1.





int EXPR











int




Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.
You should not use this function for rounding: one because it truncates
towards 0, and two because machine representations of floating-point
numbers can sometimes produce counterintuitive results.  For example,
int(-6.725/0.025) produces -268 rather than the correct -269; that’s
because it’s really more like -268.99999999999994315658 instead.  Usually,
the sprintf, printf, or the POSIX::floor and POSIX::ceil
functions will serve you better than will int().





ioctl FILEHANDLE,FUNCTION,SCALAR





Implements the ioctl(2) function.  You’ll probably first have to say



    require "sys/ioctl.ph";  # probably in $Config{archlib}/sys/ioctl.ph



to get the correct function definitions.  If sys/ioctl.ph doesn’t
exist or doesn’t have the correct definitions you’ll have to roll your
own, based on your C header files such as <sys/ioctl.h>.
(There is a Perl script called h2ph that comes with the Perl kit that
may help you in this, but it’s nontrivial.)  SCALAR will be read and/or
written depending on the FUNCTION; a C pointer to the string value of SCALAR
will be passed as the third argument of the actual ioctl call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
true, add a 0 to the scalar before using it.)  The pack and unpack
functions may be needed to manipulate the values of structures used by
ioctl.



The return value of ioctl (and fcntl) is as follows:



    if OS returns:      then Perl returns:
        -1               undefined value
         0              string "0 but true"
    anything else           that number



Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:



    $retval = ioctl(...) || -1;
    printf "System returned %d\n", $retval;



The special string "0 but true" is exempt from -w complaints
about improper numeric conversions.



Portability issues: ‘perlport ioctl’.





join EXPR,LIST





Joins the separate strings of LIST into a single string with fields
separated by the value of EXPR, and returns that new string.  Example:



    $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);



Beware that unlike split, join doesn’t take a pattern as its
first argument.  Compare split.





keys HASH








keys ARRAY






keys EXPR




Called in list context, returns a list consisting of all the keys of the
named hash, or in Perl 5.12 or later only, the indices of an array.  Perl
releases prior to 5.12 will produce a syntax error if you try to use an
array argument.  In scalar context, returns the number of keys or indices.



The keys of a hash are returned in an apparently random order.  The actual
random order is subject to change in future versions of Perl, but it
is guaranteed to be the same order as either the values or each
function produces (given that the hash has not been modified).  Since
Perl 5.8.1 the ordering can be different even between different runs of
Perl for security reasons (see perlsec Algorithmic Complexity Attacks).



As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
(see ‘each’).  In particular, calling keys() in void context resets
the iterator with no other overhead.



Here is yet another way to print your environment:



    @keys = keys %ENV;
    @values = values %ENV;
    while (@keys) {
        print pop(@keys), '=', pop(@values), "\n";
    }



or how about sorted by key:



    foreach $key (sort(keys %ENV)) {
        print $key, '=', $ENV{$key}, "\n";
    }



The returned values are copies of the original keys in the hash, so
modifying them will not affect the original hash.  Compare ‘values’.



To sort a hash by value, you’ll need to use a sort function.
Here’s a descending numeric sort of a hash by its values:



    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
        printf "%4d %s\n", $hash{$key}, $key;
    }



Used as an lvalue, keys allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say



    keys %hash = 200;



then %hash will have at least 200 buckets allocated for it–256 of them,
in fact, since it rounds up to the next power of two.  These
buckets will be retained even if you do %hash = (), use undef
%hash if you want to free the storage while %hash is still in scope.
You can’t shrink the number of buckets allocated for the hash using
keys in this way (but you needn’t worry about doing this by accident,
as trying has no effect).  keys @array in an lvalue context is a syntax
error.



Starting with Perl 5.14, keys can take a scalar EXPR, which must contain
a reference to an unblessed hash or array.  The argument will be
dereferenced automatically.  This aspect of keys is considered highly
experimental.  The exact behaviour may change in a future version of Perl.



    for (keys $hashref) { ... }
    for (keys $obj->get_arrayref) { ... }



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.012;  # so keys/values/each work on arrays
    use 5.014;  # so keys/values/each work on scalars (experimental)



See also each, values, and sort.





kill SIGNAL, LIST






kill SIGNAL






Sends a signal to a list of processes.  Returns the number of
processes successfully signaled (which is not necessarily the
same as the number actually killed).



    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;



If SIGNAL is zero, no signal is sent to the process, but kill
checks whether it’s possible to send a signal to it (that
means, to be brief, that the process is owned by the same user, or we are
the super-user).  This is useful to check that a child process is still
alive (even if only as a zombie) and hasn’t changed its UID.  See
perlport NAME for notes on the portability of this construct.



Unlike in the shell, if SIGNAL is negative, it kills process groups instead
of processes.  That means you usually
want to use positive not negative signals.
You may also use a signal name in quotes.



The behavior of kill when a PROCESS number is zero or negative depends on
the operating system.  For example, on POSIX-conforming systems, zero will
signal the current process group and -1 will signal all processes.



See perlipc Signals for more details.



On some platforms such as Windows where the fork() system call is not available.
Perl can be built to emulate fork() at the interpreter level.
This emulation has limitations related to kill that have to be considered,
for code running on Windows and in code intended to be portable.



See perlfork NAME for more details.



If there is no LIST of processes, no signal is sent, and the return
value is 0.  This form is sometimes used, however, because it causes
tainting checks to be run.  But see
perlsec Laundering and Detecting Tainted Data.



Portability issues: perlport kill.





last LABEL








last




The last command is like the break statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
continue block, if any, is not executed:



    LINE: while (<STDIN>) {
        last LINE if /^$/;  # exit when done with header
        #...
    }



last cannot be used to exit a block that returns a value such as
eval {}, sub {}, or do {}, and should not be used to exit
a grep() or map() operation.



Note that a block by itself is semantically identical to a loop
that executes once.  Thus last can be used to effect an early
exit out of such a block.



See also continue for an illustration of how last, next, and
redo work.





lc EXPR








lc




Returns a lowercased version of EXPR.  This is the internal function
implementing the \L escape in double-quoted strings.



If EXPR is omitted, uses $_.



What gets returned depends on several factors:





If use bytes is in effect:






On EBCDIC platforms




The results are what the C language system call tolower() returns.





On ASCII platforms




The results follow ASCII semantics.  Only characters A-Z change, to a-z
respectively.










Otherwise, if use locale (but not use locale ':not_characters') is in effect:




Respects current LC_CTYPE locale for code points < 256; and uses Unicode
semantics for the remaining code points (this last can only happen if
the UTF8 flag is also set).  See perllocale NAME.



A deficiency in this is that case changes that cross the 255/256
boundary are not well-defined.  For example, the lower case of LATIN CAPITAL
LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
platforms).   But under use locale, the lower case of U+1E9E is
itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
current locale, and Perl has no way of knowing if that character even
exists in the locale, much less what code point it is.  Perl returns
the input character unchanged, for all instances (and there aren’t
many) where the 255/256 boundary would otherwise be crossed.





Otherwise, If EXPR has the UTF8 flag set:




Unicode semantics are used for the case change.





Otherwise, if use feature 'unicode_strings' or use locale ':not_characters') is in effect:




Unicode semantics are used for the case change.





Otherwise:






On EBCDIC platforms




The results are what the C language system call tolower() returns.





On ASCII platforms




ASCII semantics are used for the case change.  The lowercase of any character
outside the ASCII range is the character itself.















lcfirst EXPR








lcfirst




Returns the value of EXPR with the first character lowercased.  This
is the internal function implementing the \l escape in
double-quoted strings.



If EXPR is omitted, uses $_.



This function behaves the same way under various pragmata, such as in a locale,
as lc does.





length EXPR








length




Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns the length of $_.  If EXPR is undefined, returns
undef.



This function cannot be used on an entire array or hash to find out how
many elements these have.  For that, use scalar @array and scalar keys
%hash, respectively.



Like all Perl character operations, length() normally deals in logical
characters, not physical bytes.  For how many bytes a string encoded as
UTF-8 would take up, use length(Encode::encode_utf8(EXPR)) (you’ll have
to use Encode first).  See (Encode) and perlunicode NAME.





__LINE__





A special token that compiles to the current line number.





link OLDFILE,NEWFILE





Creates a new filename linked to the old filename.  Returns true for
success, false otherwise.



Portability issues: perlport link.





listen SOCKET,QUEUESIZE





Does the same thing that the listen(2) system call does.  Returns true if
it succeeded, false otherwise.  See the example in
perlipc Sockets: Client/Server Communication.





local EXPR





You really probably want to be using my instead, because local isn’t
what most people think of as "local".  See
(perlsub)Private Variables via my() for details.



A local modifies the listed variables to be local to the enclosing
block, file, or eval.  If more than one value is listed, the list must
be placed in parentheses.  See (perlsub)Temporary Values via local()
for details, including issues with tied arrays and hashes.



The delete local EXPR construct can also be used to localize the deletion
of array/hash elements to the current block.
See (perlsub)Localized deletion of elements of composite types.





localtime EXPR








localtime




Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone.  Typically used as
follows:



    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                localtime(time);



All list elements are numeric and come straight out of the C ‘struct
tm’.  $sec, $min, and $hour are the seconds, minutes, and hours
of the specified time.



$mday is the day of the month and $mon the month in
the range 0..11, with 0 indicating January and 11 indicating December.
This makes it easy to get a month name from a list:



    my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
    print "$abbr[$mon] $mday";
    # $mon=9, $mday=18 gives "Oct 18"



$year contains the number of years since 1900.  To get a 4-digit
year write:



    $year += 1900;



To get the last two digits of the year (e.g., "01" in 2001) do:



    $year = sprintf("%02d", $year % 100);



$wday is the day of the week, with 0 indicating Sunday and 3 indicating
Wednesday.  $yday is the day of the year, in the range 0..364
(or 0..365 in leap years.)



$isdst is true if the specified time occurs during Daylight Saving
Time, false otherwise.



If EXPR is omitted, localtime() uses the current time (as returned
by time(3)).



In scalar context, localtime() returns the ctime(3) value:



    $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"



The format of this scalar value is not locale-dependent
but built into Perl.  For GMT instead of local
time use the gmtime builtin.  See also the
Time::Local module (for converting seconds, minutes, hours, and such back to
the integer value returned by time()), and the (POSIX) module’s strftime(3)
and mktime(3) functions.



To get somewhat similar but locale-dependent date strings, set up your
locale environment variables appropriately (please see perllocale NAME) and
try for example:



    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
    # or for GMT formatted appropriately for your locale:
    $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;



Note that the %a and %b, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.



The (Time-gmtime) and (Time-localtime) modules provide a convenient,
by-name access mechanism to the gmtime() and localtime() functions,
respectively.



For a comprehensive date and time representation look at the
(DateTime) module on CPAN.



Portability issues: perlport localtime.





lock THING





This function places an advisory lock on a shared variable or referenced
object contained in THING until the lock goes out of scope.



The value returned is the scalar itself, if the argument is a scalar, or a
reference, if the argument is a hash, array or subroutine.



lock() is a "weak keyword" : this means that if you’ve defined a function
by this name (before any calls to it), that function will be called
instead.  If you are not under use threads::shared this does nothing.
See (threads-shared).





log EXPR











log




Returns the natural logarithm (base e) of EXPR.  If EXPR is omitted,
returns the log of $_.  To get the
log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N.  For example:



    sub log10 {
        my $n = shift;
        return log($n)/log(10);
    }



See also exp for the inverse operation.





lstat FILEHANDLE







lstat EXPR






lstat DIRHANDLE






lstat




Does the same thing as the stat function (including setting the
special _ filehandle) but stats a symbolic link instead of the file
the symbolic link points to.  If symbolic links are unimplemented on
your system, a normal stat is done.  For much more detailed
information, please see the documentation for stat.



If EXPR is omitted, stats $_.



Portability issues: perlport lstat.





m//




The match operator.  See (perlop)Regexp Quote-Like Operators.





map BLOCK LIST







map EXPR,LIST




Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value composed of the
results of each such evaluation.  In scalar context, returns the
total number of elements so generated.  Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.



    @chars = map(chr, @numbers);



translates a list of numbers to the corresponding characters.



    my @squares = map { $_ * $_ } @numbers;



translates a list of numbers to their squared values.



    my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;



shows that number of returned elements can differ from the number of
input elements.  To omit an element, return an empty list ().
This could also be achieved by writing



    my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;



which makes the intention more clear.



Map always returns a list, which can be
assigned to a hash such that the elements
become key/value pairs.  See perldata NAME for more details.



    %hash = map { get_a_key_for($_) => $_ } @array;



is just a funny way to write



    %hash = ();
    foreach (@array) {
        $hash{get_a_key_for($_)} = $_;
    }



Note that $_ is an alias to the list value, so it can be used to
modify the elements of the LIST.  While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Using a regular foreach loop for this purpose would be clearer in
most cases.  See also ‘grep’ for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.



If $_ is lexical in the scope where the map appears (because it has
been declared with my $_), then, in addition to being locally aliased to
the list elements, $_ keeps being lexical inside the block; that is, it
can’t be seen from the outside, avoiding any potential side-effects.



{ starts both hash references and blocks, so map { ... could be either
the start of map BLOCK LIST or map EXPR, LIST.  Because Perl doesn’t look
ahead for the closing } it has to take a guess at which it’s dealing with
based on what it finds just after the
{.  Usually it gets it right, but if it
doesn’t it won’t realize something is wrong until it gets to the } and
encounters the missing (or unexpected) comma.  The syntax error will be
reported close to the }, but you’ll need to change something near the {
such as using a unary + to give Perl some help:



    %hash = map {  "\L$_" => 1  } @array  # perl guesses EXPR.  wrong
    %hash = map { +"\L$_" => 1  } @array  # perl guesses BLOCK. right
    %hash = map { ("\L$_" => 1) } @array  # this also works
    %hash = map {  lc($_) => 1  } @array  # as does this.
    %hash = map +( lc($_) => 1 ), @array  # this is EXPR and works!

    %hash = map  ( lc($_), 1 ),   @array  # evaluates to (1, @array)



or to force an anon hash constructor use +{:



   @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end



to get a list of anonymous hashes each with only one entry apiece.





mkdir FILENAME,MASK









mkdir FILENAME






mkdir




Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by umask).  If it succeeds it
returns true; otherwise it returns false and sets $! (errno).
MASK defaults to 0777 if omitted, and FILENAME defaults
to $_ if omitted.



In general, it is better to create directories with a permissive MASK
and let the user modify that with their umask than it is to supply
a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance).  The perlfunc(1) entry on
umask discusses the choice of MASK in more detail.



Note that according to the POSIX 1003.1-1996 the FILENAME may have any
number of trailing slashes.  Some operating and filesystems do not get
this right, so Perl automatically removes all trailing slashes to keep
everyone happy.



To recursively create a directory structure, look at
the mkpath function of the (File-Path) module.





msgctl ID,CMD,ARG





Calls the System V IPC function msgctl(2).  You’ll probably have to say



    use IPC::SysV;



first to get the correct constant definitions.  If CMD is IPC_STAT,
then ARG must be a variable that will hold the returned msqid_ds
structure.  Returns like ioctl: the undefined value for error,
"0 but true" for zero, or the actual return value otherwise.  See also
perlipc SysV IPC and the documentation for IPC::SysV and
IPC::Semaphore.



Portability issues: perlport msgctl.





msgget KEY,FLAGS





Calls the System V IPC function msgget(2).  Returns the message queue
id, or undef on error.  See also
perlipc SysV IPC and the documentation for IPC::SysV and
IPC::Msg.



Portability issues: perlport msgget.





msgrcv ID,VAR,SIZE,TYPE,FLAGS





Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that when a message is received, the message type as a
native long integer will be the first thing in VAR, followed by the
actual message.  This packing may be opened with unpack("l! a*").
Taints the variable.  Returns true if successful, false 
on error.  See also perlipc SysV IPC and the documentation for
IPC::SysV and IPC::SysV::Msg.



Portability issues: perlport msgrcv.





msgsnd ID,MSG,FLAGS





Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the native long integer message
type, be followed by the length of the actual message, and then finally
the message itself.  This kind of packing can be achieved with
pack("l! a*", $type, $message).  Returns true if successful,
false on error.  See also the IPC::SysV
and IPC::SysV::Msg documentation.



Portability issues: perlport msgsnd.





my EXPR







my TYPE EXPR






my EXPR : ATTRS






my TYPE EXPR : ATTRS




A my declares the listed variables to be local (lexically) to the
enclosing block, file, or eval.  If more than one value is listed,
the list must be placed in parentheses.



The exact semantics and interface of TYPE and ATTRS are still
evolving.  TYPE is currently bound to the use of the fields pragma,
and attributes are handled using the attributes pragma, or starting
from Perl 5.8.0 also via the Attribute::Handlers module.  See
(perlsub)Private Variables via my() for details, and (fields),
(attributes), and (Attribute-Handlers).





next LABEL








next




The next command is like the continue statement in C; it starts
the next iteration of the loop:



    LINE: while (<STDIN>) {
        next LINE if /^#/;  # discard comments
        #...
    }



Note that if there were a continue block on the above, it would get
executed even on discarded lines.  If LABEL is omitted, the command
refers to the innermost enclosing loop.



next cannot be used to exit a block which returns a value such as
eval {}, sub {}, or do {}, and should not be used to exit
a grep() or map() operation.



Note that a block by itself is semantically identical to a loop
that executes once.  Thus next will exit such a block early.



See also continue for an illustration of how last, next, and
redo work.





no MODULE VERSION LIST








no MODULE VERSION






no MODULE LIST






no MODULE






no VERSION




See the use function, of which no is the opposite.





oct EXPR












oct




Interprets EXPR as an octal string and returns the corresponding
value.  (If EXPR happens to start off with 0x, interprets it as a
hex string.  If EXPR starts off with 0b, it is interpreted as a
binary string.  Leading whitespace is ignored in all three cases.)
The following will handle decimal, binary, octal, and hex in standard
Perl notation:



    $val = oct($val) if $val =~ /^0/;



If EXPR is omitted, uses $_.   To go the other way (produce a number
in octal), use sprintf() or printf():



    $dec_perms = (stat("filename"))[2] & 07777;
    $oct_perm_str = sprintf "%o", $perms;



The oct() function is commonly used when a string such as 644 needs
to be converted into a file mode, for example.  Although Perl 
automatically converts strings into numbers as needed, this automatic
conversion assumes base 10.



Leading white space is ignored without warning, as too are any trailing 
non-digits, such as a decimal point (oct only handles non-negative
integers, not negative integers or floating point).





open FILEHANDLE,EXPR










open FILEHANDLE,MODE,EXPR






open FILEHANDLE,MODE,EXPR,LIST






open FILEHANDLE,MODE,REFERENCE






open FILEHANDLE




Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.



Simple examples to open a file for reading:



    open(my $fh, "<", "input.txt") 
        or die "cannot open < input.txt: $!";



and for writing:



    open(my $fh, ">", "output.txt") 
        or die "cannot open > output.txt: $!";



(The following is a comprehensive reference to open(): for a gentler
introduction you may consider perlopentut NAME.)



If FILEHANDLE is an undefined scalar variable (or array or hash element), a
new filehandle is autovivified, meaning that the variable is assigned a
reference to a newly allocated anonymous filehandle.  Otherwise if
FILEHANDLE is an expression, its value is the real filehandle.  (This is
considered a symbolic reference, so use strict "refs" should not be
in effect.)



If EXPR is omitted, the global (package) scalar variable of the same
name as the FILEHANDLE contains the filename.  (Note that lexical 
variables–those declared with my or state–will not work for this
purpose; so if you’re using my or state, specify EXPR in your
call to open.)



If three (or more) arguments are specified, the open mode (including
optional encoding) in the second argument are distinct from the filename in
the third.  If MODE is < or nothing, the file is opened for input.
If MODE is >, the file is opened for output, with existing files
first being truncated ("clobbered") and nonexisting files newly created.
If MODE is >>, the file is opened for appending, again being
created if necessary.



You can put a + in front of the > or < to
indicate that you want both read and write access to the file; thus
+< is almost always preferred for read/write updates–the 
+> mode would clobber the file first.  You can’t usually use
either read-write mode for updating textfiles, since they have
variable-length records.  See the -i switch in perlrun NAME for a
better approach.  The file is created with permissions of 0666
modified by the process’s umask value.



These various prefixes correspond to the fopen(3) modes of r,
r+, w, w+, a, and a+.



In the one- and two-argument forms of the call, the mode and filename
should be concatenated (in that order), preferably separated by white
space.  You can–but shouldn’t–omit the mode in these forms when that mode
is <.  It is always safe to use the two-argument form of open if
the filename argument is a known literal.



For three or more arguments if MODE is |-, the filename is
interpreted as a command to which output is to be piped, and if MODE
is -|, the filename is interpreted as a command that pipes
output to us.  In the two-argument (and one-argument) form, one should
replace dash (-) with the command.
See perlipc Using open() for IPC for more examples of this.
(You are not allowed to open to a command that pipes both in and
out, but see (IPC-Open2), (IPC-Open3), and
perlipc Bidirectional Communication with Another Process for
alternatives.)



In the form of pipe opens taking three or more arguments, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments
to the command invoked if the platform supports it.  The meaning of
open with more than three arguments for non-pipe modes is not yet
defined, but experimental "layers" may give extra LIST arguments
meaning.



In the two-argument (and one-argument) form, opening <- 
or - opens STDIN and opening >- opens STDOUT.



You may (and usually should) use the three-argument form of open to specify
I/O layers (sometimes referred to as "disciplines") to apply to the handle
that affect how the input and output are processed (see (open) and
(PerlIO) for more details).  For example:



  open(my $fh, "<:encoding(UTF-8)", "filename")
    || die "can't open UTF-8 encoded filename: $!";



opens the UTF8-encoded file containing Unicode characters;
see perluniintro NAME.  Note that if layers are specified in the
three-argument form, then default layers stored in ${^OPEN} (see perlvar NAME;
usually set by the open pragma or the switch -CioD) are ignored.
Those layers will also be ignored if you specifying a colon with no name
following it.  In that case the default layer for the operating system
(:raw on Unix, :crlf on Windows) is used.



Open returns nonzero on success, the undefined value otherwise.  If
the open involved a pipe, the return value happens to be the pid of
the subprocess.



If you’re running Perl on a system that distinguishes between text
files and binary files, then you should check out ‘binmode’ for tips
for dealing with this.  The key distinction between systems that need
binmode and those that don’t is their text file formats.  Systems
like Unix, Mac OS, and Plan 9, that end lines with a single
character and encode that character in C as "\n" do not
need binmode.  The rest need it.



When opening a file, it’s seldom a good idea to continue 
if the request failed, so open is frequently used with
die.  Even if die won’t do what you want (say, in a CGI script,
where you want to format a suitable error message (but there are
modules that can help with that problem)) always check
the return value from opening a file.  



As a special case the three-argument form with a read/write mode and the third
argument being undef:



    open(my $tmp, "+>", undef) or die ...



opens a filehandle to an anonymous temporary file.  Also using +<
works for symmetry, but you really should consider writing something
to the temporary file first.  You will need to seek() to do the
reading.



Since v5.8.0, Perl has built using PerlIO by default.  Unless you’ve
changed this (such as building Perl with Configure -Uuseperlio), you can
open filehandles directly to Perl scalars via:



    open($fh, ">", \$variable) || ..



To (re)open STDOUT or STDERR as an in-memory file, close it first:



    close STDOUT;
    open(STDOUT, ">", \$variable)
        or die "Can't open STDOUT: $!";



General examples:



    $ARTICLE = 100;
    open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, ">>/usr/spool/news/twitlog");  # (log is reserved)
    # if the open fails, output is discarded

    open(my $dbase, "+<", "dbase.mine")      # open for update
        or die "Can't open 'dbase.mine' for update: $!";

    open(my $dbase, "+<dbase.mine")          # ditto
        or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, "-|", "caesar <$article")  # decrypt article
        or die "Can't start caesar: $!";

    open(ARTICLE, "caesar <$article |")      # ditto
        or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >Tmp$$")            # $$ is our process id
        or die "Can't start sort: $!";

    # in-memory files
    open(MEMORY, ">", \$var)
        or die "Can't open memory file: $!";
    print MEMORY "foo!\n";                   # output will appear in $var

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, "fh00");
    }

    sub process {
        my($filename, $input) = @_;
        $input++;    # this is a string increment
        unless (open($input, "<", $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        local $_;
        while (<$input>) {    # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            #...          # whatever
        }
    }



See perliol NAME for detailed info on PerlIO.



You may also, in the Bourne shell tradition, specify an EXPR beginning
with >&, in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
duped (as dup(2)) and opened.  You may use & after >,
>>, <, +>, +>>, and +<.
The mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents
of IO buffers.)  If you use the three-argument
form, then you can pass either a
number, the name of a filehandle, or the normal "reference to a glob".



Here is a script that saves, redirects, and restores STDOUT and
STDERR using various methods:



    #!/usr/bin/perl
    open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
    open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";

    open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
    open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";

    select STDERR; $| = 1;  # make unbuffered
    select STDOUT; $| = 1;  # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
    open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";



If you specify '<&=X', where X is a file descriptor number
or a filehandle, then Perl will do an equivalent of C’s fdopen of
that file descriptor (and not call dup(2)); this is more
parsimonious of file descriptors.  For example:



    # open for input, reusing the fileno of $fd
    open(FILEHANDLE, "<&=$fd")



or



    open(FILEHANDLE, "<&=", $fd)



or



    # open for append, using the fileno of OLDFH
    open(FH, ">>&=", OLDFH)



or



    open(FH, ">>&=OLDFH")



Being parsimonious on filehandles is also useful (besides being
parsimonious) for example when something is dependent on file
descriptors, like for example locking using flock().  If you do just
open(A, ">>&B"), the filehandle A will not have the same file
descriptor as B, and therefore flock(A) will not flock(B) nor vice
versa.  But with open(A, ">>&=B"), the filehandles will share
the same underlying system file descriptor.



Note that under Perls older than 5.8.0, Perl uses the standard C library’s’
fdopen() to implement the = functionality.  On many Unix systems,
fdopen() fails when file descriptors exceed a certain value, typically 255.
For Perls 5.8.0 and later, PerlIO is (most often) the default.



You can see whether your Perl was built with PerlIO by running perl -V
and looking for the useperlio= line.  If useperlio is define, you
have PerlIO; otherwise you don’t.



If you open a pipe on the command - (that is, specify either |- or -|
with the one- or two-argument forms of open), 
an implicit fork is done, so open returns twice: in the parent
process it returns the pid
of the child process, and in the child process it returns (a defined) 0.
Use defined($pid) or // to determine whether the open was successful.



For example, use either



    $child_pid = open(FROM_KID, "-|")   // die "can't fork: $!";



or
    $child_pid = open(TO_KID,   "|-") 	// die "can’t fork: $!";



followed by 



    if ($child_pid) {
        # am the parent:
        # either write TO_KID or else read FROM_KID
        ...
        wait $child_pid;
    } else {
        # am the child; use STDIN/STDOUT normally
        ...
        exit;
    } 



The filehandle behaves normally for the parent, but I/O to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process, the filehandle isn’t opened–I/O happens from/to
the new STDOUT/STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when running setuid and
you don’t want to have to scan shell commands for metacharacters.



The following blocks are more or less equivalent:



    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
    open(FOO, "|-", "tr", '[a-z]', '[A-Z]');

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|", "cat -n '$file'");
    open(FOO, "-|") || exec "cat", "-n", $file;
    open(FOO, "-|", "cat", "-n", $file);



The last two examples in each block show the pipe as "list form", which is
not yet supported on all platforms.  A good rule of thumb is that if
your platform has a real fork() (in other words, if your platform is
Unix, including Linux and MacOS X), you can use the list form.  You would 
want to use the list form of the pipe so you can pass literal arguments
to the command without risk of the shell interpreting any shell metacharacters
in them.  However, this also bars you from opening pipes to commands
that intentionally contain shell metacharacters, such as:



    open(FOO, "|cat -n | expand -4 | lpr")
        // die "Can't open pipeline to lpr: $!";



See perlipc Safe Pipe Opens for more examples of this.



Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before any operation that may do a fork, but this may not be
supported on some platforms (see perlport NAME).  To be safe, you may need
to set $| ($AUTOFLUSH in English) or call the autoflush() method
of IO::Handle on any open handles.



On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor as determined by the value
of $^F.  See perlvar $^F.



Closing any piped filehandle causes the parent process to wait for the
child to finish, then returns the status value in $? and
${^CHILD_ERROR_NATIVE}.



The filename passed to the one- and two-argument forms of open() will
have leading and trailing whitespace deleted and normal
redirection characters honored.  This property, known as "magic open",
can often be used to good effect.  A user could specify a filename of
"rsh cat file |", or you could change certain filenames as needed:



    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(FH, $filename) or die "Can't open $filename: $!";



Use the three-argument form to open a file with arbitrary weird characters in it,



    open(FOO, "<", $file)
        || die "can't open < $file: $!";



otherwise it’s necessary to protect any leading and trailing whitespace:



    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0")
        || die "open failed: $!";



(this may not work on some bizarre filesystems).  One should
conscientiously choose between the magic and three-argument form
of open():



    open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";



will allow the user to specify an argument of the form "rsh cat file |",
but will not work on a filename that happens to have a trailing space, while



    open(IN, "<", $ARGV[0])
        || die "can't open < $ARGV[0]: $!";



will have exactly the opposite restrictions.



If you want a "real" C open (see open(2) on your system), then you
should use the sysopen function, which involves no such magic (but may
use subtly different filemodes than Perl open(), which is mapped to C
fopen()).  This is another way to protect your filenames from
interpretation.  For example:



    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
        or die "sysopen $path: $!";
    $oldfh = select(HANDLE); $| = 1; select($oldfh);
    print HANDLE "stuff $$\n";
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;



Using the constructor from the IO::Handle package (or one of its
subclasses, such as IO::File or IO::Socket), you can generate anonymous
filehandles that have the scope of the variables used to hold them, then
automatically (but silently) close once their reference counts become
zero, typically at scope exit:



    use IO::File;
    #...
    sub read_myfile_munged {
        my $ALL = shift;
        # or just leave it undef to autoviv
        my $handle = IO::File->new;
        open($handle, "<", "myfile") or die "myfile: $!";
        $first = <$handle>
            or return ();     # Automatically closed here.
        mung($first) or die "mung failed";  # Or here.
        return (first, <$handle>) if $ALL;  # Or here.
        return $first;                      # Or here.
    }



WARNING: The previous example has a bug because the automatic
close that happens when the refcount on handle does not
properly detect and report failures.  Always close the handle
yourself and inspect the return value.



    close($handle) 
        || warn "close failed: $!";



See ‘seek’ for some details about mixing reading and writing.



Portability issues: perlport open.





opendir DIRHANDLE,EXPR





Opens a directory named EXPR for processing by readdir, telldir,
seekdir, rewinddir, and closedir.  Returns true if successful.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.  If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle; that is, it’s autovivified.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.



See the example at readdir.





ord EXPR








ord




Returns the numeric value of the first character of EXPR.
If EXPR is an empty string, returns 0.  If EXPR is omitted, uses $_.
(Note character, not byte.)



For the reverse, see chr.
See perlunicode NAME for more about Unicode.





our EXPR








our TYPE EXPR






our EXPR : ATTRS






our TYPE EXPR : ATTRS




our associates a simple name with a package variable in the current
package for use within the current scope.  When use strict 'vars' is in
effect, our lets you use declared global variables without qualifying
them with package names, within the lexical scope of the our declaration.
In this way our differs from use vars, which is package-scoped.



Unlike my or state, which allocates storage for a variable and
associates a simple name with that storage for use within the current
scope, our associates a simple name with a package (read: global)
variable in the current package, for use within the current lexical scope.
In other words, our has the same scoping rules as my or state, but
does not necessarily create a variable.



If more than one value is listed, the list must be placed
in parentheses.



    our $foo;
    our($bar, $baz);



An our declaration declares a global variable that will be visible
across its entire lexical scope, even across package boundaries.  The
package in which the variable is entered is determined at the point
of the declaration, not at the point of use.  This means the following
behavior holds:



    package Foo;
    our $bar;      # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    print $bar;    # prints 20, as it refers to $Foo::bar



Multiple our declarations with the same name in the same lexical
scope are allowed if they are in different packages.  If they happen
to be in the same package, Perl will emit warnings if you have asked
for them, just like multiple my declarations.  Unlike a second
my declaration, which will bind the name to a fresh variable, a
second our declaration in the same package, in the same scope, is
merely redundant.



    use warnings;
    package Foo;
    our $bar;      # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    our $bar = 30; # declares $Bar::bar for rest of lexical scope
    print $bar;    # prints 30

    our $bar;      # emits warning but has no other effect
    print $bar;    # still prints 30



An our declaration may also have a list of attributes associated
with it.



The exact semantics and interface of TYPE and ATTRS are still
evolving.  TYPE is currently bound to the use of the fields pragma,
and attributes are handled using the attributes pragma, or, starting
from Perl 5.8.0, also via the Attribute::Handlers module.  See
(perlsub)Private Variables via my() for details, and (fields),
(attributes), and (Attribute-Handlers).





pack TEMPLATE,LIST





Takes a LIST of values and converts it into a string using the rules
given by the TEMPLATE.  The resulting string is the concatenation of
the converted values.  Typically, each converted value looks
like its machine-level representation.  For example, on 32-bit machines
an integer may be represented by a sequence of 4 bytes, which  will in
Perl be presented as a string that’s 4 characters long. 



See perlpacktut NAME for an introduction to this function.



The TEMPLATE is a sequence of characters that give the order and type
of values, as follows:



    a  A string with arbitrary binary data, will be null padded.
    A  A text (ASCII) string, will be space padded.
    Z  A null-terminated (ASCIZ) string, will be null padded.

    b  A bit string (ascending bit order inside each byte,
       like vec()).
    B  A bit string (descending bit order inside each byte).
    h  A hex string (low nybble first).
    H  A hex string (high nybble first).

    c  A signed char (8-bit) value.
    C  An unsigned char (octet) value.
    W  An unsigned char value (can be greater than 255).

    s  A signed short (16-bit) value.
    S  An unsigned short value.

    l  A signed long (32-bit) value.
    L  An unsigned long value.

    q  A signed quad (64-bit) value.
    Q  An unsigned quad value.
         (Quads are available only if your system supports 64-bit
          integer values _and_ if Perl has been compiled to support
          those.  Raises an exception otherwise.)

    i  A signed integer value.
    I  A unsigned integer value.
         (This 'integer' is _at_least_ 32 bits wide.  Its exact
          size depends on what a local C compiler calls 'int'.)

    n  An unsigned short (16-bit) in "network" (big-endian) order.
    N  An unsigned long (32-bit) in "network" (big-endian) order.
    v  An unsigned short (16-bit) in "VAX" (little-endian) order.
    V  An unsigned long (32-bit) in "VAX" (little-endian) order.

    j  A Perl internal signed integer value (IV).
    J  A Perl internal unsigned integer value (UV).

    f  A single-precision float in native format.
    d  A double-precision float in native format.

    F  A Perl internal floating-point value (NV) in native format
    D  A float of long-double precision in native format.
         (Long doubles are available only if your system supports
          long double values _and_ if Perl has been compiled to
          support those.  Raises an exception otherwise.)

    p  A pointer to a null-terminated string.
    P  A pointer to a structure (fixed-length string).

    u  A uuencoded string.
    U  A Unicode character number.  Encodes to a character in char-
       acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
       byte mode.

    w  A BER compressed integer (not an ASN.1 BER, see perlpacktut
       for details).  Its bytes represent an unsigned integer in
       base 128, most significant digit first, with as few digits
       as possible.  Bit eight (the high bit) is set on each byte
       except the last.

    x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
    X  Back up a byte.
    @  Null-fill or truncate to absolute position, counted from the
       start of the innermost ()-group.
    .  Null-fill or truncate to absolute position specified by
       the value.
    (  Start of a ()-group.



One or more modifiers below may optionally follow certain letters in the
TEMPLATE (the second column lists letters for which the modifier is valid):



    !   sSlLiI     Forces native (short, long, int) sizes instead
                   of fixed (16-/32-bit) sizes.

        xX         Make x and X act as alignment commands.

        nNvV       Treat integers as signed instead of unsigned.

        @.         Specify position as byte offset in the internal
                   representation of the packed string.  Efficient
                   but dangerous.

    >   sSiIlLqQ   Force big-endian byte-order on the type.
        jJfFdDpP   (The "big end" touches the construct.)

    <   sSiIlLqQ   Force little-endian byte-order on the type.
        jJfFdDpP   (The "little end" touches the construct.)



The > and < modifiers can also be used on () groups 
to force a particular byte-order on all components in that group, 
including all its subgroups.



The following rules apply:





Examples:



    $foo = pack("WWWW",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("W4",65,66,67,68);
    # same thing
    $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters.
    $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters.  You don't get the
    # UTF-8 bytes because the U at the start of the format caused
    # a switch to U0-mode, so the UTF-8 bytes get joined into
    # characters
    $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
    # This is the UTF-8 encoding of the string in the
    # previous example

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    # NOTE: The examples above featuring "W" and "c" are true
    # only on ASCII and ASCII-derived systems such as ISO Latin 1
    # and UTF-8.  On EBCDIC systems, the first example would be
    #      $foo = pack("WWWW",193,194,195,196);

    $foo = pack("s2",1,2);
    # "\001\000\002\000" on little-endian
    # "\000\001\000\002" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    $utmp_template = "Z8 Z8 Z16 L";
    $utmp = pack($utmp_template, @utmp1);
    # a struct utmp (BSDish)

    @utmp2 = unpack($utmp_template, $utmp);
    # "@utmp1" eq "@utmp2"

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

    $foo = pack('sx2l', 12, 34);
    # short 12, two zero bytes padding, long 34
    $bar = pack('s@4l', 12, 34);
    # short 12, zero fill to position 4, long 34
    # $foo eq $bar
    $baz = pack('s.l', 12, 4, 34);
    # short 12, zero fill to position 4, long 34

    $foo = pack('nN', 42, 4711);
    # pack big-endian 16- and 32-bit unsigned integers
    $foo = pack('S>L>', 42, 4711);
    # exactly the same
    $foo = pack('s<l<', -42, 4711);
    # pack little-endian 16- and 32-bit signed integers
    $foo = pack('(sl)<', -42, 4711);
    # exactly the same



The same template may generally also be used in unpack().





package NAMESPACE






package NAMESPACE VERSION










package NAMESPACE BLOCK






package NAMESPACE VERSION BLOCK








Declares the BLOCK or the rest of the compilation unit as being in the
given namespace.  The scope of the package declaration is either the
supplied code BLOCK or, in the absence of a BLOCK, from the declaration
itself through the end of current scope (the enclosing block, file, or
eval).  That is, the forms without a BLOCK are operative through the end
of the current scope, just like the my, state, and our operators.
All unqualified dynamic identifiers in this scope will be in the given
namespace, except where overridden by another package declaration or
when they’re one of the special identifiers that qualify into main::,
like STDOUT, ARGV, ENV, and the punctuation variables.



A package statement affects dynamic variables only, including those
you’ve used local on, but not lexical variables, which are created
with my, state, or our.  Typically it would be the first 
declaration in a file included by require or use.  You can switch into a
package in more than one place, since this only determines which default 
symbol table the compiler uses for the rest of that block.  You can refer to
identifiers in other packages than the current one by prefixing the identifier
with the package name and a double colon, as in $SomePack::var
or ThatPack::INPUT_HANDLE.  If package name is omitted, the main
package as assumed.  That is, $::sail is equivalent to
$main::sail (as well as to $main'sail, still seen in ancient
code, mostly from Perl 4).



If VERSION is provided, package sets the $VERSION variable in the given
namespace to a (version) object with the VERSION provided.  VERSION must be a
"strict" style version number as defined by the (version) module: a positive
decimal number (integer or decimal-fraction) without exponentiation or else a
dotted-decimal v-string with a leading ’v’ character and at least three
components.  You should set $VERSION only once per package.



See perlmod Packages for more information about packages, modules,
and classes.  See (perlsub) for other scoping issues.





__PACKAGE__





A special token that returns the name of the package in which it occurs.





pipe READHANDLE,WRITEHANDLE





Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful.  In addition, note that Perl’s pipes use
IO buffering, so you may need to set $| to flush your WRITEHANDLE
after each command, depending on the application.



See (IPC-Open2), (IPC-Open3), and
perlipc Bidirectional Communication with Another Process
for examples of such things.



On systems that support a close-on-exec flag on files, that flag is set
on all newly opened file descriptors whose filenos are higher than 
the current value of $^F (by default 2 for STDERR).  See perlvar $^F.





pop ARRAY








pop EXPR






pop




Pops and returns the last value of the array, shortening the array by
one element.



Returns the undefined value if the array is empty, although this may also
happen at other times.  If ARRAY is omitted, pops the @ARGV array in the
main program, but the @_ array in subroutines, just like shift.



Starting with Perl 5.14, pop can take a scalar EXPR, which must hold a
reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of pop is considered highly experimental.
The exact behaviour may change in a future version of Perl.



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.014;  # so push/pop/etc work on scalars (experimental)





pos SCALAR








pos




Returns the offset of where the last m//g search left off for the
variable in question ($_ is used when the variable is not
specified).  Note that 0 is a valid match offset.  undef indicates
that the search position is reset (usually due to match failure, but
can also be because no match has yet been run on the scalar).



pos directly accesses the location used by the regexp engine to
store the offset, so assigning to pos will change that offset, and
so will also influence the \G zero-width assertion in regular
expressions.  Both of these effects take place for the next match, so
you can’t affect the position with pos during the current match,
such as in (?{pos() = 5}) or s//pos() = 5/e.



Setting pos also resets the matched with zero-length flag, described
under (perlre)Repeated Patterns Matching a Zero-length Substring.



Because a failed m//gc match doesn’t reset the offset, the return
from pos won’t change either in this case.  See (perlre) and
(perlop).





print FILEHANDLE LIST







print FILEHANDLE






print LIST






print




Prints a string or a list of strings.  Returns true if successful.
FILEHANDLE may be a scalar variable containing the name of or a reference
to the filehandle, thus introducing one level of indirection.  (NOTE: If
FILEHANDLE is a variable and the next token is a term, it may be
misinterpreted as an operator unless you interpose a + or put
parentheses around the arguments.)  If FILEHANDLE is omitted, prints to the
last selected (see select) output handle.  If LIST is omitted, prints
$_ to the currently selected output handle.  To use FILEHANDLE alone to
print the content of $_ to it, you must use a real filehandle like
FH, not an indirect one like $fh.  To set the default output handle
to something other than STDOUT, use the select operation.



The current value of $, (if any) is printed between each LIST item.  The
current value of $\ (if any) is printed after the entire LIST has been
printed.  Because print takes a LIST, anything in the LIST is evaluated in
list context, including any subroutines whose return lists you pass to
print.  Be careful not to follow the print keyword with a left
parenthesis unless you want the corresponding right parenthesis to
terminate the arguments to the print; put parentheses around all arguments
(or interpose a +, but that doesn’t look as good).



If you’re storing handles in an array or hash, or in general whenever
you’re using any expression more complex than a bareword handle or a plain,
unsubscripted scalar variable to retrieve it, you will have to use a block
returning the filehandle value instead, in which case the LIST may not be
omitted:



    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";



Printing to a closed pipe or socket will generate a SIGPIPE signal.  See
perlipc NAME for more on signal handling.





printf FILEHANDLE FORMAT, LIST







printf FILEHANDLE






printf FORMAT, LIST






printf




Equivalent to print FILEHANDLE sprintf(FORMAT, LIST), except that $\
(the output record separator) is not appended.  The first argument of the
list will be interpreted as the printf format.  See
sprintf for an
explanation of the format argument.  If you omit the LIST, $_ is used;
to use FILEHANDLE without a LIST, you must use a real filehandle like
FH, not an indirect one like $fh.  If use locale (including
use locale ':not_characters') is in effect and
POSIX::setlocale() has been called, the character used for the decimal
separator in formatted floating-point numbers is affected by the LC_NUMERIC
locale setting.  See perllocale NAME and (POSIX).



Don’t fall into the trap of using a printf when a simple
print would do.  The print is more efficient and less
error prone.





prototype FUNCTION





Returns the prototype of a function as a string (or undef if the
function has no prototype).  FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.



If FUNCTION is a string starting with CORE::, the rest is taken as a
name for a Perl builtin.  If the builtin is not overridable (such as
qw//) or if its arguments cannot be adequately expressed by a prototype
(such as system), prototype() returns undef, because the builtin
does not really behave like a Perl function.  Otherwise, the string
describing the equivalent prototype is returned.





push ARRAY,LIST








push EXPR,LIST




Treats ARRAY as a stack by appending the values of LIST to the end of
ARRAY.  The length of ARRAY increases by the length of LIST.  Has the same
effect as



    for $value (LIST) {
        $ARRAY[++$#ARRAY] = $value;
    }



but is more efficient.  Returns the number of elements in the array following
the completed push.



Starting with Perl 5.14, push can take a scalar EXPR, which must hold a
reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of push is considered highly experimental.
The exact behaviour may change in a future version of Perl.



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.014;  # so push/pop/etc work on scalars (experimental)





q/STRING/






qq/STRING/






qw/STRING/






qx/STRING/




Generalized quotes.  See (perlop)Quote-Like Operators.





qr/STRING/




Regexp-like quote.  See (perlop)Regexp Quote-Like Operators.





quotemeta EXPR








quotemeta




Returns the value of EXPR with all the ASCII non-"word"
characters backslashed.  (That is, all ASCII characters not matching
/[A-Za-z_0-9]/ will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing
the \Q escape in double-quoted strings.
(See below for the behavior on non-ASCII code points.)



If EXPR is omitted, uses $_.



quotemeta (and \Q ... \E) are useful when interpolating strings into
regular expressions, because by default an interpolated variable will be
considered a mini-regular expression.  For example:



    my $sentence = 'The quick brown fox jumped over the lazy dog';
    my $substring = 'quick.*?fox';
    $sentence =~ s{$substring}{big bad wolf};



Will cause $sentence to become 'The big bad wolf jumped over...'.



On the other hand:



    my $sentence = 'The quick brown fox jumped over the lazy dog';
    my $substring = 'quick.*?fox';
    $sentence =~ s{\Q$substring\E}{big bad wolf};



Or:



    my $sentence = 'The quick brown fox jumped over the lazy dog';
    my $substring = 'quick.*?fox';
    my $quoted_substring = quotemeta($substring);
    $sentence =~ s{$quoted_substring}{big bad wolf};



Will both leave the sentence as is.
Normally, when accepting literal string
input from the user, quotemeta() or \Q must be used.



In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
strings, but not quoted in UTF-8 strings.



Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
quoting non-ASCII characters; the quoting of ASCII characters is
unchanged.



Also unchanged is the quoting of non-UTF-8 strings when outside the
scope of a use feature 'unicode_strings', which is to quote all
characters in the upper Latin1 range.  This provides complete backwards
compatibility for old programs which do not use Unicode.  (Note that
unicode_strings is automatically enabled within the scope of a
use v5.12 or greater.)



Within the scope of use locale, all non-ASCII Latin1 code points
are quoted whether the string is encoded as UTF-8 or not.  As mentioned
above, locale does not affect the quoting of ASCII-range characters.
This protects against those locales where characters such as "|" are
considered to be word characters.



Otherwise, Perl quotes non-ASCII characters using an adaptation from
Unicode (see http://www.unicode.org/reports/tr31/.)
The only code points that are quoted are those that have any of the
Unicode properties:  Pattern_Syntax, Pattern_White_Space, White_Space,
Default_Ignorable_Code_Point, or General_Category=Control.



Of these properties, the two important ones are Pattern_Syntax and
Pattern_White_Space.  They have been set up by Unicode for exactly this
purpose of deciding which characters in a regular expression pattern
should be quoted.  No character that can be in an identifier has these
properties.



Perl promises, that if we ever add regular expression pattern
metacharacters to the dozen already defined
(\ | ( ) [ { ^ $ * + ? .), that we will only use ones that have the
Pattern_Syntax property.  Perl also promises, that if we ever add
characters that are considered to be white space in regular expressions
(currently mostly affected by /x), they will all have the
Pattern_White_Space property.



Unicode promises that the set of code points that have these two
properties will never change, so something that is not quoted in v5.16
will never need to be quoted in any future Perl release.  (Not all the
code points that match Pattern_Syntax have actually had characters
assigned to them; so there is room to grow, but they are quoted
whether assigned or not.  Perl, of course, would never use an
unassigned code point as an actual metacharacter.)



Quoting characters that have the other 3 properties is done to enhance
the readability of the regular expression and not because they actually
need to be quoted for regular expression purposes (characters with the
White_Space property are likely to be indistinguishable on the page or
screen from those with the Pattern_White_Space property; and the other
two properties contain non-printing characters).





rand EXPR








rand




Returns a random fractional number greater than or equal to 0 and less
than the value of EXPR.  (EXPR should be positive.)  If EXPR is
omitted, the value 1 is used.  Currently EXPR with the value 0 is
also special-cased as 1 (this was undocumented before Perl 5.8.0
and is subject to change in future versions of Perl).  Automatically calls
srand unless srand has already been called.  See also srand.



Apply int() to the value returned by rand() if you want random
integers instead of random fractional numbers.  For example,



    int(rand(10))



returns a random integer between 0 and 9, inclusive.



(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.)



rand() is not cryptographically secure.  You should not rely
on it in security-sensitive situations.  As of this writing, a
number of third-party CPAN modules offer random number generators
intended by their authors to be cryptographically secure,
including: (Data-Entropy), (Crypt-Random), (Math-Random-Secure),
and (Math-TrulyRandom).





read FILEHANDLE,SCALAR,LENGTH,OFFSET








read FILEHANDLE,SCALAR,LENGTH




Attempts to read LENGTH characters of data into variable SCALAR
from the specified FILEHANDLE.  Returns the number of characters
actually read, 0 at end of file, or undef if there was an error (in
the latter case $! is also set).  SCALAR will be grown or shrunk 
so that the last character actually read is the last character of the
scalar after the read.



An OFFSET may be specified to place the read data at some place in the
string other than the beginning.  A negative OFFSET specifies
placement at that many characters counting backwards from the end of
the string.  A positive OFFSET greater than the length of SCALAR
results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.



The call is implemented in terms of either Perl’s or your system’s native
fread(3) library function.  To get a true read(2) system call, see
sysread.



Note the characters: depending on the status of the filehandle,
either (8-bit) bytes or characters are read.  By default, all
filehandles operate on bytes, but for example if the filehandle has
been opened with the :utf8 I/O layer (see ‘open’, and the open
pragma, (open)), the I/O will operate on UTF8-encoded Unicode
characters, not bytes.  Similarly for the :encoding pragma:
in that case pretty much any characters can be read.





readdir DIRHANDLE





Returns the next directory entry for a directory opened by opendir.
If used in list context, returns all the rest of the entries in the
directory.  If there are no more entries, returns the undefined value in
scalar context and the empty list in list context.



If you’re planning to filetest the return values out of a readdir, you’d
better prepend the directory in question.  Otherwise, because we didn’t
chdir there, it would have been testing the wrong file.



    opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
    closedir $dh;



As of Perl 5.11.2 you can use a bare readdir in a while loop,
which will set $_ on every iteration.



    opendir(my $dh, $some_dir) || die;
    while(readdir $dh) {
        print "$some_dir/$_\n";
    }
    closedir $dh;



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious failures, put this sort of thing at the
top of your file to signal that your code will work only on Perls of a
recent vintage:



    use 5.012; # so readdir assigns to $_ in a lone while test





readline EXPR






readline







Reads from the filehandle whose typeglob is contained in EXPR (or from
*ARGV if EXPR is not provided).  In scalar context, each call reads and
returns the next line until end-of-file is reached, whereupon the
subsequent call returns undef.  In list context, reads until end-of-file
is reached and returns a list of lines.  Note that the notion of "line"
used here is whatever you may have defined with $/ or
$INPUT_RECORD_SEPARATOR).  See perlvar $/.



When $/ is set to undef, when readline is in scalar
context (i.e., file slurp mode), and when an empty file is read, it
returns '' the first time, followed by undef subsequently.



This is the internal function implementing the <EXPR>
operator, but you can use it directly.  The <EXPR>
operator is discussed in more detail in (perlop)I/O Operators.



    $line = <STDIN>;
    $line = readline(*STDIN);    # same thing



If readline encounters an operating system error, $! will be set
with the corresponding error message.  It can be helpful to check
$! when you are reading from filehandles you don’t trust, such as a
tty or a socket.  The following example uses the operator form of
readline and dies if the result is not defined.



    while ( ! eof($fh) ) {
        defined( $_ = <$fh> ) or die "readline failed: $!";
        ...
    }



Note that you have can’t handle readline errors that way with the
ARGV filehandle.  In that case, you have to open each element of
@ARGV yourself since eof handles ARGV differently.



    foreach my $arg (@ARGV) {
        open(my $fh, $arg) or warn "Can't open $arg: $!";

        while ( ! eof($fh) ) {
            defined( $_ = <$fh> )
                or die "readline failed for $arg: $!";
            ...
        }
    }





readlink EXPR







readlink




Returns the value of a symbolic link, if symbolic links are
implemented.  If not, raises an exception.  If there is a system
error, returns the undefined value and sets $! (errno).  If EXPR is
omitted, uses $_.



Portability issues: perlport readlink.





readpipe EXPR






readpipe





EXPR is executed as a system command.
The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially
multi-line) string.  In list context, returns a list of lines
(however you’ve defined lines with $/ or $INPUT_RECORD_SEPARATOR).
This is the internal function implementing the qx/EXPR/
operator, but you can use it directly.  The qx/EXPR/
operator is discussed in more detail in (perlop)I/O Operators.
If EXPR is omitted, uses $_.





recv SOCKET,SCALAR,LENGTH,FLAGS





Receives a message on a socket.  Attempts to receive LENGTH characters
of data into variable SCALAR from the specified SOCKET filehandle.
SCALAR will be grown or shrunk to the length actually read.  Takes the
same flags as the system call of the same name.  Returns the address
of the sender if SOCKET’s protocol supports this; returns an empty
string otherwise.  If there’s an error, returns the undefined value.
This call is actually implemented in terms of recvfrom(2) system call.
See perlipc UDP: Message Passing for examples.



Note the characters: depending on the status of the socket, either
(8-bit) bytes or characters are received.  By default all sockets
operate on bytes, but for example if the socket has been changed using
binmode() to operate with the :encoding(utf8) I/O layer (see the
open pragma, (open)), the I/O will operate on UTF8-encoded Unicode
characters, not bytes.  Similarly for the :encoding pragma: in that
case pretty much any characters can be read.





redo LABEL







redo




The redo command restarts the loop block without evaluating the
conditional again.  The continue block, if any, is not executed.  If
the LABEL is omitted, the command refers to the innermost enclosing
loop.  Programs that want to lie to themselves about what was just input 
normally use this command:



    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            $front = $_;
            while (<STDIN>) {
                if (/}/) {  # end of comment?
                    s|^|$front\{|;
                    redo LINE;
                }
            }
        }
        print;
    }



redo cannot be used to retry a block that returns a value such as
eval {}, sub {}, or do {}, and should not be used to exit
a grep() or map() operation.



Note that a block by itself is semantically identical to a loop
that executes once.  Thus redo inside such a block will effectively
turn it into a looping construct.



See also continue for an illustration of how last, next, and
redo work.





ref EXPR








ref




Returns a non-empty string if EXPR is a reference, the empty
string otherwise.  If EXPR
is not specified, $_ will be used.  The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:



    SCALAR
    ARRAY
    HASH
    CODE
    REF
    GLOB
    LVALUE
    FORMAT
    IO
    VSTRING
    Regexp



If the referenced object has been blessed into a package, then that package
name is returned instead.  You can think of ref as a typeof operator.



    if (ref($r) eq "HASH") {
        print "r is a reference to a hash.\n";
    }
    unless (ref($r)) {
        print "r is not a reference at all.\n";
    }



The return value LVALUE indicates a reference to an lvalue that is not
a variable.  You get this from taking the reference of function calls like
pos() or substr().  VSTRING is returned if the reference points
to a version string.



The result Regexp indicates that the argument is a regular expression
resulting from qr//.



See also (perlref).





rename OLDNAME,NEWNAME








Changes the name of a file; an existing file NEWNAME will be
clobbered.  Returns true for success, false otherwise.



Behavior of this function varies wildly depending on your system
implementation.  For example, it will usually not work across file system
boundaries, even though the system mv command sometimes compensates
for this.  Other restrictions include whether it works on directories,
open files, or pre-existing files.  Check perlport NAME and either the
rename(2) manpage or equivalent system documentation for details.



For a platform independent move function look at the (File-Copy)
module.



Portability issues: perlport rename.





require VERSION







require EXPR






require




Demands a version of Perl specified by VERSION, or demands some semantics
specified by EXPR or by $_ if EXPR is not supplied.



VERSION may be either a numeric argument such as 5.006, which will be
compared to $], or a literal of the form v5.6.1, which will be compared
to $^V (aka $PERL_VERSION).  An exception is raised if
VERSION is greater than the version of the current Perl interpreter.
Compare with ‘use’, which can do a similar check at compile time.



Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
versions of Perl that do not support this syntax.  The equivalent numeric
version should be used instead.



    require v5.6.1;     # run time version check
    require 5.6.1;      # ditto
    require 5.006_001;  # ditto; preferred for backwards compatibility



Otherwise, require demands that a library file be included if it
hasn’t already been included.  The file is included via the do-FILE
mechanism, which is essentially just a variety of eval with the
caveat that lexical variables in the invoking script will be invisible
to the included code.  Has semantics similar to the following subroutine:



    sub require {
       my ($filename) = @_;
       if (exists $INC{$filename}) {
           return 1 if $INC{$filename};
           die "Compilation failed in require";
       }
       my ($realfilename,$result);
       ITER: {
           foreach $prefix (@INC) {
               $realfilename = "$prefix/$filename";
               if (-f $realfilename) {
                   $INC{$filename} = $realfilename;
                   $result = do $realfilename;
                   last ITER;
               }
           }
           die "Can't find $filename in \@INC";
       }
       if ($@) {
           $INC{$filename} = undef;
           die $@;
       } elsif (!$result) {
           delete $INC{$filename};
           die "$filename did not return true value";
       } else {
           return $result;
       }
    }



Note that the file will not be included twice under the same specified
name.



The file must return true as the last statement to indicate
successful execution of any initialization code, so it’s customary to
end such a file with 1; unless you’re sure it’ll return true
otherwise.  But it’s better just to put the 1;, in case you add more
statements.



If EXPR is a bareword, the require assumes a ".pm" extension and
replaces "::" with "/" in the filename for you,
to make it easy to load standard modules.  This form of loading of
modules does not risk altering your namespace.



In other words, if you try this:



        require Foo::Bar;     # a splendid bareword



The require function will actually look for the "Foo/Bar.pm" file in the
directories specified in the @INC array.



But if you try this:



        $class = 'Foo::Bar';
        require $class;       # $class is not a bareword
    #or
        require "Foo::Bar";   # not a bareword because of the ""



The require function will look for the "Foo::Bar" file in the @INC array and
will complain about not finding "Foo::Bar" there.  In this case you can do:



        eval "require $class";



Now that you understand how require looks for files with a
bareword argument, there is a little extra functionality going on behind
the scenes.  Before require looks for a ".pm" extension, it will
first look for a similar filename with a ".pmc" extension.  If this file
is found, it will be loaded in place of any file ending in a ".pm"
extension.



You can also insert hooks into the import facility by putting Perl code
directly into the @INC array.  There are three forms of hooks: subroutine
references, array references, and blessed objects.



Subroutine references are the simplest case.  When the inclusion system
walks through @INC and encounters a subroutine, this subroutine gets
called with two parameters, the first a reference to itself, and the
second the name of the file to be included (e.g., "Foo/Bar.pm").  The
subroutine should return either nothing or else a list of up to three 
values in the following order:



    

  1.  A filehandle, from which the file will be read.  


  2.  A reference to a subroutine.  If there is no filehandle (previous item),
then this subroutine is expected to generate one line of source code per
call, writing the line into $_ and returning 1, then finally at end of
file returning 0.  If there is a filehandle, then the subroutine will be
called to act as a simple source filter, with the line as read in $_.
Again, return 1 for each valid line, and 0 after all lines have been
returned.


  3.  Optional state for the subroutine.  The state is passed in as $_[1].  A
reference to the subroutine itself is passed in as $_[0].





If an empty list, undef, or nothing that matches the first 3 values above
is returned, then require looks at the remaining elements of @INC.
Note that this filehandle must be a real filehandle (strictly a typeglob
or reference to a typeglob, whether blessed or unblessed); tied filehandles 
will be ignored and processing will stop there.



If the hook is an array reference, its first element must be a subroutine
reference.  This subroutine is called as above, but the first parameter is
the array reference.  This lets you indirectly pass arguments to
the subroutine.



In other words, you can write:



    push @INC, \&my_sub;
    sub my_sub {
        my ($coderef, $filename) = @_;  # $coderef is \&my_sub
        ...
    }



or:



    push @INC, [ \&my_sub, $x, $y, ... ];
    sub my_sub {
        my ($arrayref, $filename) = @_;
        # Retrieve $x, $y, ...
        my @parameters = @$arrayref[1..$#$arrayref];
        ...
    }



If the hook is an object, it must provide an INC method that will be
called as above, the first parameter being the object itself.  (Note that
you must fully qualify the sub’s name, as unqualified INC is always forced
into package main.)  Here is a typical code layout:



    # In Foo.pm
    package Foo;
    sub new { ... }
    sub Foo::INC {
        my ($self, $filename) = @_;
        ...
    }

    # In the main program
    push @INC, Foo->new(...);



These hooks are also permitted to set the %INC entry
corresponding to the files they have loaded.  See perlvar %INC.



For a yet-more-powerful import facility, see ‘use’ and perlmod NAME.





reset EXPR







reset




Generally used in a continue block at the end of a loop to clear
variables and reset ?? searches so that they work again.  The
expression is interpreted as a list of single characters (hyphens
allowed for ranges).  All variables and arrays beginning with one of
those letters are reset to their pristine state.  If the expression is
omitted, one-match searches (?pattern?) are reset to match again.  
Only resets variables or searches in the current package.  Always returns
1.  Examples:



    reset 'X';      # reset all X variables
    reset 'a-z';    # reset lower case variables
    reset;          # just reset ?one-time? searches



Resetting "A-Z" is not recommended because you’ll wipe out your
@ARGV and @INC arrays and your %ENV hash.  Resets only package
variables; lexical variables are unaffected, but they clean themselves
up on scope exit anyway, so you’ll probably want to use them instead.
See ‘my’.





return EXPR







return




Returns from a subroutine, eval, or do FILE with the value
given in EXPR.  Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see wantarray).  If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in void context.



(In the absence of an explicit return, a subroutine, eval,
or do FILE automatically returns the value of the last expression
evaluated.)





reverse LIST







In list context, returns a list value consisting of the elements
of LIST in the opposite order.  In scalar context, concatenates the
elements of LIST and returns a string value with all characters
in the opposite order.



    print join(", ", reverse "world", "Hello"); # Hello, world

    print scalar reverse "dlrow ,", "olleH";    # Hello, world



Used without arguments in scalar context, reverse() reverses $_.



    $_ = "dlrow ,olleH";
    print reverse;                              # No output, list context
    print scalar reverse;                       # Hello, world



Note that reversing an array to itself (as in @a = reverse @a) will
preserve non-existent elements whenever possible, i.e., for non magical
arrays or tied arrays with EXISTS and DELETE methods.



This operator is also handy for inverting a hash, although there are some
caveats.  If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash.  Also, this has to
unwind one hash and build a whole new one, which may take some time
on a large hash, such as from a DBM file.



    %by_name = reverse %by_address;  # Invert the hash





rewinddir DIRHANDLE





Sets the current position to the beginning of the directory for the
readdir routine on DIRHANDLE.



Portability issues: perlport rewinddir.





rindex STR,SUBSTR,POSITION







rindex STR,SUBSTR




Works just like index() except that it returns the position of the last
occurrence of SUBSTR in STR.  If POSITION is specified, returns the
last occurrence beginning at or before that position.





rmdir FILENAME









rmdir




Deletes the directory specified by FILENAME if that directory is
empty.  If it succeeds it returns true; otherwise it returns false and
sets $! (errno).  If FILENAME is omitted, uses $_.



To remove a directory tree recursively (rm -rf on Unix) look at
the rmtree function of the (File-Path) module.





s///




The substitution operator.  See (perlop)Regexp Quote-Like Operators.





say FILEHANDLE LIST







say FILEHANDLE






say LIST






say




Just like print, but implicitly appends a newline.  say LIST is
simply an abbreviation for { local $\ = "\n"; print LIST }.  To use
FILEHANDLE without a LIST to print the contents of $_ to it, you must
use a real filehandle like FH, not an indirect one like $fh.



This keyword is available only when the "say" feature
is enabled, or when prefixed with CORE::; see
(feature).  Alternately, include a use v5.10 or later to the current
scope.





scalar EXPR






Forces EXPR to be interpreted in scalar context and returns the value
of EXPR.



    @counts = ( scalar @a, scalar @b, scalar @c );



There is no equivalent operator to force an expression to
be interpolated in list context because in practice, this is never
needed.  If you really wanted to do so, however, you could use
the construction @{[ (some expression) ]}, but usually a simple
(some expression) suffices.



Because scalar is a unary operator, if you accidentally use a
parenthesized list for the EXPR, this behaves as a scalar comma expression,
evaluating all but the last element in void context and returning the final
element evaluated in scalar context.  This is seldom what you want.



The following single statement:



    print uc(scalar(&foo,$bar)),$baz;



is the moral equivalent of these two:



    &foo;
    print(uc($bar),$baz);



See (perlop) for more details on unary operators and the comma operator.





seek FILEHANDLE,POSITION,WHENCE







Sets FILEHANDLE’s position, just like the fseek call of stdio.
FILEHANDLE may be an expression whose value gives the name of the
filehandle.  The values for WHENCE are 0 to set the new position
in bytes to POSITION; 1 to set it to the current position plus
POSITION; and 2 to set it to EOF plus POSITION, typically
negative.  For WHENCE you may use the constants SEEK_SET,
SEEK_CUR, and SEEK_END (start of the file, current position, end
of the file) from the (Fcntl) module.  Returns 1 on success, false
otherwise.



Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the :encoding(utf8) open
layer), tell() will return byte offsets, not character offsets
(because implementing that would render seek() and tell() rather slow).



If you want to position the file for sysread or syswrite, don’t use
seek, because buffering makes its effect on the file’s read-write position
unpredictable and non-portable.  Use sysseek instead.



Due to the rules and rigors of ANSI C, on some systems you have to do a
seek whenever you switch between reading and writing.  Amongst other
things, this may have the effect of calling stdio’s clearerr(3).
A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:



    seek(TEST,0,1);



This is also useful for applications emulating tail -f.  Once you hit
EOF on your read and then sleep for a while, you (probably) have to stick in a
dummy seek() to reset things.  The seek doesn’t change the position,
but it does clear the end-of-file condition on the handle, so that the
next <FILE> makes Perl try again to read something.  (We hope.)



If that doesn’t work (some I/O implementations are particularly
cantankerous), you might need something like this:



    for (;;) {
        for ($curpos = tell(FILE); $_ = <FILE>;
             $curpos = tell(FILE)) {
            # search for some stuff and put it into files
        }
        sleep($for_a_while);
        seek(FILE, $curpos, 0);
    }





seekdir DIRHANDLE,POS





Sets the current position for the readdir routine on DIRHANDLE.  POS
must be a value returned by telldir.  seekdir also has the same caveats
about possible directory compaction as the corresponding system library
routine.





select FILEHANDLE








select




Returns the currently selected filehandle.  If FILEHANDLE is supplied,
sets the new current default filehandle for output.  This has two
effects: first, a write or a print without a filehandle 
default to this FILEHANDLE.  Second, references to variables related to
output will refer to this output channel.  



For example, to set the top-of-form format for more than one
output channel, you might do the following:



    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';



FILEHANDLE may be an expression whose value gives the name of the
actual filehandle.  Thus:



    $oldfh = select(STDERR); $| = 1; select($oldfh);



Some programmers may prefer to think of filehandles as objects with
methods, preferring to write the last example as:



    use IO::Handle;
    STDERR->autoflush(1);



Portability issues: perlport select.





select RBITS,WBITS,EBITS,TIMEOUT





This calls the select(2) syscall with the bit masks specified, which
can be constructed using fileno and vec, along these lines:



    $rin = $win = $ein = '';
    vec($rin, fileno(STDIN),  1) = 1;
    vec($win, fileno(STDOUT), 1) = 1;
    $ein = $rin | $win;



If you want to select on many filehandles, you may wish to write a
subroutine like this:



    sub fhbits {
        my @fhlist = @_;
        my $bits = "";
        for my $fh (@fhlist) {
            vec($bits, fileno($fh), 1) = 1;
        }
        return $bits;
    }
    $rin = fhbits(*STDIN, *TTY, *MYSOCK);



The usual idiom is:



    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);



or to block until something becomes ready just do this



    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);



Most systems do not bother to return anything useful in $timeleft, so
calling select() in scalar context just returns $nfound.



Any of the bit masks can also be undef.  The timeout, if specified, is
in seconds, which may be fractional.  Note: not all implementations are
capable of returning the $timeleft.  If not, they always return
$timeleft equal to the supplied $timeout.



You can effect a sleep of 250 milliseconds this way:



    select(undef, undef, undef, 0.25);



Note that whether select gets restarted after signals (say, SIGALRM)
is implementation-dependent.  See also perlport NAME for notes on the
portability of select.



On error, select behaves just like select(2): it returns
-1 and sets $!.



On some Unixes, select(2) may report a socket file descriptor as "ready for
reading" even when no data is available, and thus any subsequent read
would block.  This can be avoided if you always use O_NONBLOCK on the
socket.  See select(2) and fcntl(2) for further details.



The standard IO::Select module provides a user-friendlier interface
to select, mostly because it does all the bit-mask work for you.



WARNING: One should not attempt to mix buffered I/O (like read
or <FH>) with select, except as permitted by POSIX, and even
then only on POSIX systems.  You have to use sysread instead.



Portability issues: perlport select.





semctl ID,SEMNUM,CMD,ARG





Calls the System V IPC function semctl(2).  You’ll probably have to say



    use IPC::SysV;



first to get the correct constant definitions.  If CMD is IPC_STAT or
GETALL, then ARG must be a variable that will hold the returned
semid_ds structure or semaphore value array.  Returns like ioctl:
the undefined value for error, "0 but true" for zero, or the actual
return value otherwise.  The ARG must consist of a vector of native
short integers, which may be created with pack("s!",(0)x$nsem).
See also perlipc SysV IPC, IPC::SysV, IPC::Semaphore
documentation.



Portability issues: perlport semctl.





semget KEY,NSEMS,FLAGS





Calls the System V IPC function semget(2).  Returns the semaphore id, or
the undefined value on error.  See also
perlipc SysV IPC, IPC::SysV, IPC::SysV::Semaphore
documentation.



Portability issues: perlport semget.





semop KEY,OPSTRING





Calls the System V IPC function semop(2) for semaphore operations
such as signalling and waiting.  OPSTRING must be a packed array of
semop structures.  Each semop structure can be generated with
pack("s!3", $semnum, $semop, $semflag).  The length of OPSTRING 
implies the number of semaphore operations.  Returns true if
successful, false on error.  As an example, the
following code waits on semaphore $semnum of semaphore id $semid:



    $semop = pack("s!3", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);



To signal the semaphore, replace -1 with 1.  See also
perlipc SysV IPC, IPC::SysV, and IPC::SysV::Semaphore
documentation.



Portability issues: perlport semop.





send SOCKET,MSG,FLAGS,TO







send SOCKET,MSG,FLAGS




Sends a message on a socket.  Attempts to send the scalar MSG to the SOCKET
filehandle.  Takes the same flags as the system call of the same name.  On
unconnected sockets, you must specify a destination to send to, in which
case it does a sendto(2) syscall.  Returns the number of characters sent,
or the undefined value on error.  The sendmsg(2) syscall is currently
unimplemented.  See perlipc UDP: Message Passing for examples.



Note the characters: depending on the status of the socket, either
(8-bit) bytes or characters are sent.  By default all sockets operate
on bytes, but for example if the socket has been changed using
binmode() to operate with the :encoding(utf8) I/O layer (see
‘open’, or the open pragma, (open)), the I/O will operate on UTF-8
encoded Unicode characters, not bytes.  Similarly for the :encoding
pragma: in that case pretty much any characters can be sent.





setpgrp PID,PGRP






Sets the current process group for the specified PID, 0 for the current
process.  Raises an exception when used on a machine that doesn’t
implement POSIX setpgid(2) or BSD setpgrp(2).  If the arguments are omitted,
it defaults to 0,0.  Note that the BSD 4.2 version of setpgrp does not
accept any arguments, so only setpgrp(0,0) is portable.  See also
POSIX::setsid().



Portability issues: perlport setpgrp.





setpriority WHICH,WHO,PRIORITY








Sets the current priority for a process, a process group, or a user.
(See setpriority(2).)  Raises an exception when used on a machine
that doesn’t implement setpriority(2).



Portability issues: perlport setpriority.





setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL





Sets the socket option requested.  Returns undef on error.
Use integer constants provided by the Socket module for
LEVEL and OPNAME.  Values for LEVEL can also be obtained from
getprotobyname.  OPTVAL might either be a packed string or an integer.
An integer OPTVAL is shorthand for pack("i", OPTVAL).



An example disabling Nagle’s algorithm on a socket:



    use Socket qw(IPPROTO_TCP TCP_NODELAY);
    setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);



Portability issues: perlport setsockopt.





shift ARRAY







shift EXPR






shift




Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down.  If there are no elements in the
array, returns the undefined value.  If ARRAY is omitted, shifts the
@_ array within the lexical scope of subroutines and formats, and the
@ARGV array outside a subroutine and also within the lexical scopes
established by the eval STRING, BEGIN {}, INIT {}, CHECK {},
UNITCHECK {}, and END {} constructs.



Starting with Perl 5.14, shift can take a scalar EXPR, which must hold a
reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of shift is considered highly experimental.
The exact behaviour may change in a future version of Perl.



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.014;  # so push/pop/etc work on scalars (experimental)



See also unshift, push, and pop.  shift and unshift do the
same thing to the left end of an array that pop and push do to the
right end.





shmctl ID,CMD,ARG





Calls the System V IPC function shmctl.  You’ll probably have to say



    use IPC::SysV;



first to get the correct constant definitions.  If CMD is IPC_STAT,
then ARG must be a variable that will hold the returned shmid_ds
structure.  Returns like ioctl: undef for error; "0 but
true" for zero; and the actual return value otherwise.
See also perlipc SysV IPC and IPC::SysV documentation.



Portability issues: perlport shmctl.





shmget KEY,SIZE,FLAGS





Calls the System V IPC function shmget.  Returns the shared memory
segment id, or undef on error.
See also perlipc SysV IPC and IPC::SysV documentation.



Portability issues: perlport shmget.





shmread ID,VAR,POS,SIZE








shmwrite ID,STRING,POS,SIZE




Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it.  When reading, VAR must be a variable that will
hold the data read.  When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes.  Return true if successful, false on error.
shmread() taints the variable.  See also perlipc SysV IPC,
IPC::SysV, and the IPC::Shareable module from CPAN.



Portability issues: perlport shmread and perlport shmwrite.





shutdown SOCKET,HOW





Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the syscall of the same name.



    shutdown(SOCKET, 0);    # I/we have stopped reading data
    shutdown(SOCKET, 1);    # I/we have stopped writing data
    shutdown(SOCKET, 2);    # I/we have stopped using this socket



This is useful with sockets when you want to tell the other
side you’re done writing but not done reading, or vice versa.
It’s also a more insistent form of close because it also
disables the file descriptor in any forked copies in other
processes.



Returns 1 for success; on error, returns undef if
the first argument is not a valid filehandle, or returns 0 and sets
$! for any other failure.





sin EXPR










sin




Returns the sine of EXPR (expressed in radians).  If EXPR is omitted,
returns sine of $_.



For the inverse sine operation, you may use the Math::Trig::asin
function, or use this relation:



    sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }





sleep EXPR








sleep




Causes the script to sleep for (integer) EXPR seconds, or forever if no 
argument is given.  Returns the integer number of seconds actually slept.  



May be interrupted if the process receives a signal such as SIGALRM.



    eval {
        local $SIG{ALARM} = sub { die "Alarm!\n" };
        sleep;
    };
    die $@ unless $@ eq "Alarm!\n";



You probably cannot mix alarm and sleep calls, because sleep
is often implemented using alarm.



On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds.  Most modern systems
always sleep the full amount.  They may appear to sleep longer than that,
however, because your process might not be scheduled right away in a
busy multitasking system.



For delays of finer granularity than one second, the Time::HiRes module
(from CPAN, and starting from Perl 5.8 part of the standard
distribution) provides usleep().  You may also use Perl’s four-argument
version of select() leaving the first three arguments undefined, or you
might be able to use the syscall interface to access setitimer(2) if
your system supports it.  See (perlfaq8) for details.



See also the POSIX module’s pause function.





socket SOCKET,DOMAIN,TYPE,PROTOCOL





Opens a socket of the specified kind and attaches it to filehandle
SOCKET.  DOMAIN, TYPE, and PROTOCOL are specified the same as for
the syscall of the same name.  You should use Socket first
to get the proper definitions imported.  See the examples in
perlipc Sockets: Client/Server Communication.



On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor, as determined by the
value of $^F.  See perlvar $^F.





socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL





Creates an unnamed pair of sockets in the specified domain, of the
specified type.  DOMAIN, TYPE, and PROTOCOL are specified the same as
for the syscall of the same name.  If unimplemented, raises an exception.
Returns true if successful.



On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptors, as determined by the value
of $^F.  See perlvar $^F.



Some systems defined pipe in terms of socketpair, in which a call
to pipe(Rdr, Wtr) is essentially:



    use Socket;
    socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
    shutdown(Rdr, 1);        # no more writing for reader
    shutdown(Wtr, 0);        # no more reading for writer



See perlipc NAME for an example of socketpair use.  Perl 5.8 and later will
emulate socketpair using IP sockets to localhost if your system implements
sockets but not socketpair.



Portability issues: perlport socketpair.





sort SUBNAME LIST










sort BLOCK LIST






sort LIST




In list context, this sorts the LIST and returns the sorted list value.
In scalar context, the behaviour of sort() is undefined.



If SUBNAME or BLOCK is omitted, sorts in standard string comparison
order.  If SUBNAME is specified, it gives the name of a subroutine
that returns an integer less than, equal to, or greater than 0,
depending on how the elements of the list are to be ordered.  (The 
<=> and cmp operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to) the actual
subroutine to use.  In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.



If the subroutine’s prototype is ($$), the elements to be compared are
passed by reference in @_, as for a normal subroutine.  This is slower
than unprototyped subroutines, where the elements to be compared are passed
into the subroutine as the package global variables $a and $b (see example
below).  Note that in the latter case, it is usually highly counter-productive
to declare $a and $b as lexicals.



If the subroutine is an XSUB, the elements to be compared are pushed on to
the stack, the way arguments are usually passed to XSUBs.  $a and $b are
not set.



The values to be compared are always passed by reference and should not
be modified.



You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in (perlsyn) or with goto.



When use locale (but not use locale 'not_characters') is in
effect, sort LIST sorts LIST according to the
current collation locale.  See perllocale NAME.



sort() returns aliases into the original list, much as a for loop’s index
variable aliases the list elements.  That is, modifying an element of a
list returned by sort() (for example, in a foreach, map or grep)
actually modifies the element in the original list.  This is usually
something to be avoided when writing clear code.



Perl 5.6 and earlier used a quicksort algorithm to implement sort.
That algorithm was not stable, so could go quadratic.  (A stable sort
preserves the input order of elements that compare equal.  Although
quicksort’s run time is O(NlogN) when averaged over all arrays of
length N, the time can be O(N**2), quadratic behavior, for some
inputs.)  In 5.7, the quicksort implementation was replaced with
a stable mergesort algorithm whose worst-case behavior is O(NlogN).
But benchmarks indicated that for some inputs, on some platforms,
the original quicksort was faster.  5.8 has a sort pragma for
limited control of the sort.  Its rather blunt control of the
underlying algorithm may not persist into future Perls, but the
ability to characterize the input or output in implementation
independent ways quite probably will.  See (sort)the sort pragma.



Examples:



    # sort lexically
    @articles = sort @files;
    
    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;
    
    # now case-insensitively
    @articles = sort {fc($a) cmp fc($b)} @files;
    
    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;
    
    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;
    
    # sort numerically descending
    @articles = sort {$b <=> $a} @files;
    
    # this sorts the %age hash by value instead of key
    # using an in-line function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
    
    # sort using explicit subroutine name
    sub byage {
        $age{$a} <=> $age{$b};  # presuming numeric
    }
    @sortedclass = sort byage @class;
    
    sub backwards { $b cmp $a }
    @harry  = qw(dog cat x Cain Abel);
    @george = qw(gone chased yz Punished Axed);
    print sort @harry;
        # prints AbelCaincatdogx
    print sort backwards @harry;
        # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
        # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using
    # the first integer after the first = sign, or the
    # whole record case-insensitively otherwise

    my @new = sort {
        ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                            ||
                    fc($a)  cmp  fc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    my @nums = @caps = ();
    for (@old) {
        push @nums, ( /=(\d+)/ ? $1 : undef );
        push @caps, fc($_);
    }

    my @new = @old[ sort {
                           $nums[$b] <=> $nums[$a]
                                    ||
                           $caps[$a] cmp $caps[$b]
                         } 0..$#old
                  ];

    # same thing, but without any temps
    @new = map { $_->[0] }
           sort { $b->[1] <=> $a->[1]
                           ||
                  $a->[2] cmp $b->[2]
           } map { [$_, /=(\d+)/, fc($_)] } @old;

    # using a prototype allows you to use any comparison subroutine
    # as a sort subroutine (including other package's subroutines)
    package other;
    sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are not set here
    
    package main;
    @new = sort other::backwards @old;
    
    # guarantee stability, regardless of algorithm
    use sort 'stable';
    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
    
    # force use of mergesort (not portable outside Perl 5.8)
    use sort '_mergesort';  # note discouraging _
    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;



Warning: syntactical care is required when sorting the list returned from
a function.  If you want to sort the list returned by the function call
find_records(@key), you can use:



    @contact = sort { $a cmp $b } find_records @key;
    @contact = sort +find_records(@key);
    @contact = sort &find_records(@key);
    @contact = sort(find_records(@key));



If instead you want to sort the array @key with the comparison routine
find_records() then you can use:



    @contact = sort { find_records() } @key;
    @contact = sort find_records(@key);
    @contact = sort(find_records @key);
    @contact = sort(find_records (@key));



If you’re using strict, you must not declare $a
and $b as lexicals.  They are package globals.  That means
that if you’re in the main package and type



    @articles = sort {$b <=> $a} @files;



then $a and $b are $main::a and $main::b (or $::a and $::b),
but if you’re in the FooPack package, it’s the same as typing



    @articles = sort {$FooPack::b <=> $FooPack::a} @files;



The comparison function is required to behave.  If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2] and
sometimes saying the opposite, for example) the results are not
well-defined.



Because <=> returns undef when either operand is NaN
(not-a-number), be careful when sorting with a
comparison function like $a <=> $b any lists that might contain a
NaN.  The following example takes advantage that NaN != NaN to
eliminate any NaNs from the input list.



    @result = sort { $a <=> $b } grep { $_ == $_ } @input;





splice ARRAY or EXPR,OFFSET,LENGTH,LIST







splice ARRAY or EXPR,OFFSET,LENGTH






splice ARRAY or EXPR,OFFSET






splice ARRAY or EXPR




Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any.  In list context,
returns the elements removed from the array.  In scalar context,
returns the last element removed, or undef if no elements are
removed.  The array grows or shrinks as necessary.
If OFFSET is negative then it starts that far from the end of the array.
If LENGTH is omitted, removes everything from OFFSET onward.
If LENGTH is negative, removes the elements from OFFSET onward
except for -LENGTH elements at the end of the array.
If both OFFSET and LENGTH are omitted, removes everything.  If OFFSET is
past the end of the array, Perl issues a warning, and splices at the
end of the array.



The following equivalences hold (assuming $#a >= $i )



    push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
    pop(@a)             splice(@a,-1)
    shift(@a)           splice(@a,0,1)
    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
    $a[$i] = $y         splice(@a,$i,1,$y)



Example, assuming array lengths are passed before arrays:



    sub aeq {  # compare two list values
        my(@a) = splice(@_,0,shift);
        my(@b) = splice(@_,0,shift);
        return 0 unless @a == @b;  # same len?
        while (@a) {
            return 0 if pop(@a) ne pop(@b);
        }
        return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }



Starting with Perl 5.14, splice can take scalar EXPR, which must hold a
reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of splice is considered highly experimental.
The exact behaviour may change in a future version of Perl.



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.014;  # so push/pop/etc work on scalars (experimental)





split /PATTERN/,EXPR,LIMIT







split /PATTERN/,EXPR






split /PATTERN/






split




Splits the string EXPR into a list of strings and returns the
list in list context, or the size of the list in scalar context.



If only PATTERN is given, EXPR defaults to $_.



Anything in EXPR that matches PATTERN is taken to be a separator
that separates the EXPR into substrings (called "fields") that
do not include the separator.  Note that a separator may be
longer than one character or even have no characters at all (the
empty string, which is a zero-width match).



The PATTERN need not be constant; an expression may be used
to specify a pattern that varies at runtime.



If PATTERN matches the empty string, the EXPR is split at the match
position (between characters).  As an example, the following:



    print join(':', split('b', 'abc')), "\n";



uses the ’b’ in ’abc’ as a separator to produce the output ’a:c’.
However, this:



    print join(':', split('', 'abc')), "\n";



uses empty string matches as separators to produce the output
’a:b:c’; thus, the empty string may be used to split EXPR into a
list of its component characters.



As a special case for split, the empty pattern given in
(perlop)match operator syntax (//) specifically matches the empty string, which is contrary to its usual
interpretation as the last successful match.



If PATTERN is /^/, then it is treated as if it used the
multiline modifier (/^/m), since it
isn’t much use otherwise.



As another special case, split emulates the default behavior of the
command line tool awk when the PATTERN is either omitted or a literal
string composed of a single space character (such as ' ' or
"\x20", but not e.g. / /).  In this case, any leading
whitespace in EXPR is removed before splitting occurs, and the PATTERN is
instead treated as if it were /\s+/; in particular, this means that
any contiguous whitespace (not just a single space character) is used as
a separator.  However, this special treatment can be avoided by specifying
the pattern / / instead of the string " ", thereby allowing
only a single space character to be a separator.



If omitted, PATTERN defaults to a single space, " ", triggering
the previously described awk emulation.



If LIMIT is specified and positive, it represents the maximum number
of fields into which the EXPR may be split; in other words, LIMIT is
one greater than the maximum number of times EXPR may be split.  Thus,
the LIMIT value 1 means that EXPR may be split a maximum of zero
times, producing a maximum of one field (namely, the entire value of
EXPR).  For instance:



    print join(':', split(//, 'abc', 1)), "\n";



produces the output ’abc’, and this:



    print join(':', split(//, 'abc', 2)), "\n";



produces the output ’a:bc’, and each of these:



    print join(':', split(//, 'abc', 3)), "\n";
    print join(':', split(//, 'abc', 4)), "\n";



produces the output ’a:b:c’.



If LIMIT is negative, it is treated as if it were instead arbitrarily
large; as many fields as possible are produced.



If LIMIT is omitted (or, equivalently, zero), then it is usually
treated as if it were instead negative but with the exception that
trailing empty fields are stripped (empty leading fields are always
preserved); if all fields are empty, then all fields are considered to
be trailing (and are thus stripped in this case).  Thus, the following:



    print join(':', split(',', 'a,b,c,,,')), "\n";



produces the output ’a:b:c’, but the following:



    print join(':', split(',', 'a,b,c,,,', -1)), "\n";



produces the output ’a:b:c:::’.



In time-critical applications, it is worthwhile to avoid splitting
into more fields than necessary.  Thus, when assigning to a list,
if LIMIT is omitted (or zero), then LIMIT is treated as though it
were one larger than the number of variables in the list; for the
following, LIMIT is implicitly 4:



    ($login, $passwd, $remainder) = split(/:/);



Note that splitting an EXPR that evaluates to the empty string always
produces zero fields, regardless of the LIMIT specified.



An empty leading field is produced when there is a positive-width
match at the beginning of EXPR.  For instance:



    print join(':', split(/ /, ' abc')), "\n";



produces the output ’:abc’.  However, a zero-width match at the
beginning of EXPR never produces an empty field, so that:



    print join(':', split(//, ' abc'));



produces the output ’ :a:b:c’ (rather than ’: :a:b:c’).



An empty trailing field, on the other hand, is produced when there is a
match at the end of EXPR, regardless of the length of the match
(of course, unless a non-zero LIMIT is given explicitly, such fields are
removed, as in the last example).  Thus:



    print join(':', split(//, ' abc', -1)), "\n";



produces the output ’ :a:b:c:’.



If the PATTERN contains
capturing groups,
then for each separator, an additional field is produced for each substring
captured by a group (in the order in which the groups are specified,
as per backreferences); if any group does not
match, then it captures the undef value instead of a substring.  Also,
note that any such additional field is produced whenever there is a
separator (that is, whenever a split occurs), and such an additional field
does not count towards the LIMIT.  Consider the following expressions
evaluated in list context (each returned list is provided in the associated
comment):



    split(/-|,/, "1-10,20", 3)
    # ('1', '10', '20')

    split(/(-|,)/, "1-10,20", 3)
    # ('1', '-', '10', ',', '20')

    split(/-|(,)/, "1-10,20", 3)
    # ('1', undef, '10', ',', '20')

    split(/(-)|,/, "1-10,20", 3)
    # ('1', '-', '10', undef, '20')

    split(/(-)|(,)/, "1-10,20", 3)
    # ('1', '-', undef, '10', undef, ',', '20')





sprintf FORMAT, LIST





Returns a string formatted by the usual printf conventions of the C
library function sprintf.  See below for more details
and see sprintf(3) or printf(3) on your system for an explanation of
the general principles.



For example:



        # Format number with up to 8 leading zeroes
        $result = sprintf("%08d", $number);

        # Round number to 3 digits after decimal point
        $rounded = sprintf("%.3f", $number);



Perl does its own sprintf formatting: it emulates the C
function sprintf(3), but doesn’t use it except for floating-point
numbers, and even then only standard modifiers are allowed.  
Non-standard extensions in your local sprintf(3) are 
therefore unavailable from Perl.



Unlike printf, sprintf does not do what you probably mean when you
pass it an array as your first argument. 
The array is given scalar context,
and instead of using the 0th element of the array as the format, Perl will
use the count of elements in the array as the format, which is almost never
useful.



Perl’s sprintf permits the following universally-known conversions:



   %%    a percent sign
   %c    a character with the given number
   %s    a string
   %d    a signed integer, in decimal
   %u    an unsigned integer, in decimal
   %o    an unsigned integer, in octal
   %x    an unsigned integer, in hexadecimal
   %e    a floating-point number, in scientific notation
   %f    a floating-point number, in fixed decimal notation
   %g    a floating-point number, in %e or %f notation



In addition, Perl permits the following widely-supported conversions:



   %X    like %x, but using upper-case letters
   %E    like %e, but using an upper-case "E"
   %G    like %g, but with an upper-case "E" (if applicable)
   %b    an unsigned integer, in binary
   %B    like %b, but using an upper-case "B" with the # flag
   %p    a pointer (outputs the Perl value's address in hexadecimal)
   %n    special: *stores* the number of characters output so far
         into the next argument in the parameter list



Finally, for backward (and we do mean "backward") compatibility, Perl
permits these unnecessary but widely-supported conversions:



   %i    a synonym for %d
   %D    a synonym for %ld
   %U    a synonym for %lu
   %O    a synonym for %lo
   %F    a synonym for %f



Note that the number of exponent digits in the scientific notation produced
by %e, %E, %g and %G for numbers with the modulus of the
exponent less than 100 is system-dependent: it may be three or less
(zero-padded as necessary).  In other words, 1.23 times ten to the
99th may be either "1.23e99" or "1.23e099".



Between the % and the format letter, you may specify several
additional attributes controlling the interpretation of the format.
In order, these are:





format parameter index




An explicit format parameter index, such as 2$.  By default sprintf
will format the next unused argument in the list, but this allows you
to take the arguments out of order:



  printf '%2$d %1$d', 12, 34;      # prints "34 12"
  printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"





flags




one or more of:



   space   prefix non-negative number with a space
   +       prefix non-negative number with a plus sign
   -       left-justify within the field
   0       use zeros, not spaces, to right-justify
   #       ensure the leading "0" for any octal,
           prefix non-zero hexadecimal with "0x" or "0X",
           prefix non-zero binary with "0b" or "0B"



For example:



  printf '<% d>',  12;   # prints "< 12>"
  printf '<%+d>',  12;   # prints "<+12>"
  printf '<%6s>',  12;   # prints "<    12>"
  printf '<%-6s>', 12;   # prints "<12    >"
  printf '<%06s>', 12;   # prints "<000012>"
  printf '<%#o>',  12;   # prints "<014>"
  printf '<%#x>',  12;   # prints "<0xc>"
  printf '<%#X>',  12;   # prints "<0XC>"
  printf '<%#b>',  12;   # prints "<0b1100>"
  printf '<%#B>',  12;   # prints "<0B1100>"



When a space and a plus sign are given as the flags at once,
a plus sign is used to prefix a positive number.



  printf '<%+ d>', 12;   # prints "<+12>"
  printf '<% +d>', 12;   # prints "<+12>"



When the # flag and a precision are given in the %o conversion,
the precision is incremented if it’s necessary for the leading "0".



  printf '<%#.5o>', 012;      # prints "<00012>"
  printf '<%#.5o>', 012345;   # prints "<012345>"
  printf '<%#.0o>', 0;        # prints "<0>"





vector flag




This flag tells Perl to interpret the supplied string as a vector of
integers, one for each character in the string.  Perl applies the format to
each integer in turn, then joins the resulting strings with a separator (a
dot . by default).  This can be useful for displaying ordinal values of
characters in arbitrary strings:



  printf "%vd", "AB\x{100}";           # prints "65.66.256"
  printf "version is v%vd\n", $^V;     # Perl's version



Put an asterisk * before the v to override the string to
use to separate the numbers:



  printf "address is %*vX\n", ":", $addr;   # IPv6 address
  printf "bits are %0*v8b\n", " ", $bits;   # random bitstring



You can also explicitly specify the argument number to use for
the join string using something like *2$v; for example:



  printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":";   # 3 IPv6 addresses





(minimum) width




Arguments are usually formatted to be only as wide as required to
display the given value.  You can override the width by putting
a number here, or get the width from the next argument (with *)
or from a specified argument (e.g., with *2$):



  printf "<%s>", "a";       # prints "<a>"
  printf "<%6s>", "a";      # prints "<     a>"
  printf "<%*s>", 6, "a";   # prints "<     a>"
  printf "<%*2$s>", "a", 6; # prints "<     a>"
  printf "<%2s>", "long";   # prints "<long>" (does not truncate)



If a field width obtained through * is negative, it has the same
effect as the - flag: left-justification.





precision, or maximum width





You can specify a precision (for numeric conversions) or a maximum
width (for string conversions) by specifying a . followed by a number.
For floating-point formats except g and G, this specifies
how many places right of the decimal point to show (the default being 6).
For example:



  # these examples are subject to system-specific variation
  printf '<%f>', 1;    # prints "<1.000000>"
  printf '<%.1f>', 1;  # prints "<1.0>"
  printf '<%.0f>', 1;  # prints "<1>"
  printf '<%e>', 10;   # prints "<1.000000e+01>"
  printf '<%.1e>', 10; # prints "<1.0e+01>"



For "g" and "G", this specifies the maximum number of digits to show,
including those prior to the decimal point and those after it; for 
example:



  # These examples are subject to system-specific variation.
  printf '<%g>', 1;        # prints "<1>"
  printf '<%.10g>', 1;     # prints "<1>"
  printf '<%g>', 100;      # prints "<100>"
  printf '<%.1g>', 100;    # prints "<1e+02>"
  printf '<%.2g>', 100.01; # prints "<1e+02>"
  printf '<%.5g>', 100.01; # prints "<100.01>"
  printf '<%.4g>', 100.01; # prints "<100>"



For integer conversions, specifying a precision implies that the
output of the number itself should be zero-padded to this width,
where the 0 flag is ignored:



  printf '<%.6d>', 1;      # prints "<000001>"
  printf '<%+.6d>', 1;     # prints "<+000001>"
  printf '<%-10.6d>', 1;   # prints "<000001    >"
  printf '<%10.6d>', 1;    # prints "<    000001>"
  printf '<%010.6d>', 1;   # prints "<    000001>"
  printf '<%+10.6d>', 1;   # prints "<   +000001>"

  printf '<%.6x>', 1;      # prints "<000001>"
  printf '<%#.6x>', 1;     # prints "<0x000001>"
  printf '<%-10.6x>', 1;   # prints "<000001    >"
  printf '<%10.6x>', 1;    # prints "<    000001>"
  printf '<%010.6x>', 1;   # prints "<    000001>"
  printf '<%#10.6x>', 1;   # prints "<  0x000001>"



For string conversions, specifying a precision truncates the string
to fit the specified width:



  printf '<%.5s>', "truncated";   # prints "<trunc>"
  printf '<%10.5s>', "truncated"; # prints "<     trunc>"



You can also get the precision from the next argument using .*:



  printf '<%.6x>', 1;       # prints "<000001>"
  printf '<%.*x>', 6, 1;    # prints "<000001>"



If a precision obtained through * is negative, it counts
as having no precision at all.



  printf '<%.*s>',  7, "string";   # prints "<string>"
  printf '<%.*s>',  3, "string";   # prints "<str>"
  printf '<%.*s>',  0, "string";   # prints "<>"
  printf '<%.*s>', -1, "string";   # prints "<string>"

  printf '<%.*d>',  1, 0;   # prints "<0>"
  printf '<%.*d>',  0, 0;   # prints "<>"
  printf '<%.*d>', -1, 0;   # prints "<0>"



You cannot currently get the precision from a specified number,
but it is intended that this will be possible in the future, for
example using .*2$:



  printf "<%.*2$x>", 1, 6;   # INVALID, but in future will print "<000001>"





size




For numeric conversions, you can specify the size to interpret the
number as using l, h, V, q, L, or ll.  For integer
conversions (d u o x X b i D U O), numbers are usually assumed to be
whatever the default integer size is on your platform (usually 32 or 64
bits), but you can override this to use instead one of the standard C types,
as supported by the compiler used to build Perl:



   hh          interpret integer as C type "char" or "unsigned char"
               on Perl 5.14 or later
   h           interpret integer as C type "short" or "unsigned short"
   j           interpret integer as C type "intmax_t" on Perl 5.14 
               or later, and only with a C99 compiler (unportable)
   l           interpret integer as C type "long" or "unsigned long"
   q, L, or ll interpret integer as C type "long long", "unsigned long long",
               or "quad" (typically 64-bit integers)
   t           interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
   z           interpret integer as C type "size_t" on Perl 5.14 or later



As of 5.14, none of these raises an exception if they are not supported on
your platform.  However, if warnings are enabled, a warning of the
printf warning class is issued on an unsupported conversion flag.  
Should you instead prefer an exception, do this:



    use warnings FATAL => "printf";



If you would like to know about a version dependency before you
start running the program, put something like this at its top:



    use 5.014;  # for hh/j/t/z/ printf modifiers



You can find out whether your Perl supports quads via (Config):



    use Config;
    if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
        print "Nice quads!\n";
    }



For floating-point conversions (e f g E F G), numbers are usually assumed
to be the default floating-point size on your platform (double or long double),
but you can force "long double" with q, L, or ll if your
platform supports them.  You can find out whether your Perl supports long
doubles via (Config):



    use Config;
    print "long doubles\n" if $Config{d_longdbl} eq "define";



You can find out whether Perl considers "long double" to be the default
floating-point size to use on your platform via (Config):



    use Config;
    if ($Config{uselongdouble} eq "define") {
        print "long doubles by default\n";
    }



It can also be that long doubles and doubles are the same thing:



        use Config;
        ($Config{doublesize} == $Config{longdblsize}) &&
                print "doubles are long doubles\n";



The size specifier V has no effect for Perl code, but is supported for
compatibility with XS code.  It means "use the standard size for a Perl
integer or floating-point number", which is the default.





order of arguments




Normally, sprintf() takes the next unused argument as the value to
format for each format specification.  If the format specification
uses * to require additional arguments, these are consumed from
the argument list in the order they appear in the format
specification before the value to format.  Where an argument is
specified by an explicit index, this does not affect the normal
order for the arguments, even when the explicitly specified index
would have been the next argument.



So:



    printf "<%*.*s>", $a, $b, $c;



uses $a for the width, $b for the precision, and $c
as the value to format; while:



  printf "<%*1$.*s>", $a, $b;



would use $a for the width and precision, and $b as the
value to format.



Here are some more examples; be aware that when using an explicit
index, the $ may need escaping:



  printf "%2\$d %d\n",    12, 34;        # will print "34 12\n"
  printf "%2\$d %d %d\n", 12, 34;        # will print "34 12 34\n"
  printf "%3\$d %d %d\n", 12, 34, 56;    # will print "56 12 34\n"
  printf "%2\$*3\$d %d\n", 12, 34, 3;    # will print " 34 12\n"








If use locale (including use locale 'not_characters') is in effect
and POSIX::setlocale() has been called,
the character used for the decimal separator in formatted floating-point
numbers is affected by the LC_NUMERIC locale.  See perllocale NAME
and (POSIX).





sqrt EXPR









sqrt




Return the positive square root of EXPR.  If EXPR is omitted, uses
$_.  Works only for non-negative operands unless you’ve
loaded the Math::Complex module.



    use Math::Complex;
    print sqrt(-4);    # prints 2i





srand EXPR









srand




Sets and returns the random number seed for the rand operator.



The point of the function is to "seed" the rand function so that rand
can produce a different sequence each time you run your program.  When
called with a parameter, srand uses that for the seed; otherwise it
(semi-)randomly chooses a seed.  In either case, starting with Perl 5.14,
it returns the seed.  To signal that your code will work only on Perls
of a recent vintage:



    use 5.014;  # so srand returns the seed



If srand() is not called explicitly, it is called implicitly without a
parameter at the first use of the rand operator.  However, this was not true
of versions of Perl before 5.004, so if your script will run under older
Perl versions, it should call srand; otherwise most programs won’t call
srand() at all.



But there are a few situations in recent Perls where programs are likely to
want to call srand.  One is for generating predictable results generally for
testing or debugging.  There, you use srand($seed), with the same $seed
each time.  Another case is that you may want to call srand()
after a fork() to avoid child processes sharing the same seed value as the
parent (and consequently each other).



Do not call srand() (i.e., without an argument) more than once per
process.  The internal state of the random number generator should
contain more entropy than can be provided by any seed, so calling
srand() again actually loses randomness.



Most implementations of srand take an integer and will silently
truncate decimal numbers.  This means srand(42) will usually
produce the same results as srand(42.1).  To be safe, always pass
srand an integer.



In versions of Perl prior to 5.004 the default seed was just the
current time.  This isn’t a particularly good seed, so many old
programs supply their own seed value (often time ^ $$ or time ^
($$ + ($$ << 15))), but that isn’t necessary any more.



Frequently called programs (like CGI scripts) that simply use



    time ^ $$



for a seed can fall prey to the mathematical property that



    a^b == (a+1)^(b+1)



one-third of the time.  So don’t do that.



A typical use of the returned seed is for a test program which has too many
combinations to test comprehensively in the time available to it each run.  It
can test a random subset each time, and should there be a failure, log the seed
used for that run so that it can later be used to reproduce the same results.



rand() is not cryptographically secure.  You should not rely
on it in security-sensitive situations.  As of this writing, a
number of third-party CPAN modules offer random number generators
intended by their authors to be cryptographically secure,
including: (Data-Entropy), (Crypt-Random), (Math-Random-Secure),
and (Math-TrulyRandom).





stat FILEHANDLE









stat EXPR






stat DIRHANDLE






stat




Returns a 13-element list giving the status info for a file, either
the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR.  If EXPR is 
omitted, it stats $_ (not _!).  Returns the empty list if stat fails.  Typically
used as follows:



    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);



Not all fields are supported on all filesystem types.  Here are the
meanings of the fields:



  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions)
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time in seconds since the epoch
  9 mtime    last modify time in seconds since the epoch
 10 ctime    inode change time in seconds since the epoch (*)
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated



(The epoch was at 00:00 January 1, 1970 GMT.)



(*) Not all fields are supported on all filesystem types.  Notably, the
ctime field is non-portable.  In particular, you cannot expect it to be a
"creation time"; see perlport Files and Filesystems for details.



If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat, lstat, or filetest are returned.  Example:



    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }



(This works on machines only for which the device number is negative
under NFS.)



Because the mode contains both the file type and its permissions, you
should mask off the file type portion and (s)printf using a "%o"
if you want to see the real permissions.



    $mode = (stat($filename))[2];
    printf "Permissions are %04o\n", $mode & 07777;



In scalar context, stat returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle _.



The (File-stat) module provides a convenient, by-name access mechanism:



    use File::stat;
    $sb = stat($filename);
    printf "File is %s, size is %s, perm %04o, mtime %s\n",
           $filename, $sb->size, $sb->mode & 07777,
           scalar localtime $sb->mtime;



You can import symbolic mode constants (S_IF*) and functions
(S_IS*) from the Fcntl module:



    use Fcntl ':mode';

    $mode = (stat($filename))[2];

    $user_rwx      = ($mode & S_IRWXU) >> 6;
    $group_read    = ($mode & S_IRGRP) >> 3;
    $other_execute =  $mode & S_IXOTH;

    printf "Permissions are %04o\n", S_IMODE($mode), "\n";

    $is_setuid     =  $mode & S_ISUID;
    $is_directory  =  S_ISDIR($mode);



You could write the last two using the -u and -d operators.
Commonly available S_IF* constants are:



    # Permissions: read, write, execute, for user, group, others.

    S_IRWXU S_IRUSR S_IWUSR S_IXUSR
    S_IRWXG S_IRGRP S_IWGRP S_IXGRP
    S_IRWXO S_IROTH S_IWOTH S_IXOTH

    # Setuid/Setgid/Stickiness/SaveText.
    # Note that the exact meaning of these is system-dependent.

    S_ISUID S_ISGID S_ISVTX S_ISTXT

    # File types.  Not all are necessarily available on
    # your system.

    S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
    S_IFIFO S_IFSOCK S_IFWHT S_ENFMT

    # The following are compatibility aliases for S_IRUSR,
    # S_IWUSR, and S_IXUSR.

    S_IREAD S_IWRITE S_IEXEC



and the S_IF* functions are



    S_IMODE($mode)    the part of $mode containing the permission
                      bits and the setuid/setgid/sticky bits

    S_IFMT($mode)     the part of $mode containing the file type
                      which can be bit-anded with (for example)
                      S_IFREG or with the following functions

    # The operators -f, -d, -l, -b, -c, -p, and -S.

    S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
    S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

    # No direct -X operator counterpart, but for the first one
    # the -g operator is often equivalent.  The ENFMT stands for
    # record flocking enforcement, a platform-dependent feature.

    S_ISENFMT($mode) S_ISWHT($mode)



See your native chmod(2) and stat(2) documentation for more details
about the S_* constants.  To get status info for a symbolic link
instead of the target file behind the link, use the lstat function.



Portability issues: perlport stat.





state EXPR







state TYPE EXPR






state EXPR : ATTRS






state TYPE EXPR : ATTRS




state declares a lexically scoped variable, just like my.
However, those variables will never be reinitialized, contrary to
lexical variables that are reinitialized each time their enclosing block
is entered.
See (perlsub)Persistent Private Variables for details.



state variables are enabled only when the use feature "state" pragma 
is in effect, unless the keyword is written as CORE::state.
See also (feature).





study SCALAR







study




Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching and the distribution of character
frequencies in the string to be searched; you probably want to compare
run times with and without it to see which is faster.  Those loops
that scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.
(The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are.  From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text.  Only those places
that contain this "rarest" character are examined.)



For example, here is a loop that inserts index producing entries
before any line containing a certain pattern:



    while (<>) {
        study;
        print ".IX foo\n"    if /\bfoo\b/;
        print ".IX bar\n"    if /\bbar\b/;
        print ".IX blurfl\n" if /\bblurfl\b/;
        # ...
        print;
    }



In searching for /\bfoo\b/, only locations in $_ that contain f
will be looked at, because f is rarer than o.  In general, this is
a big win except in pathological cases.  The only question is whether
it saves you more time than it took to build the linked list in the
first place.



Note that if you have to look for strings that you don’t know till
runtime, you can build an entire loop as a string and eval that to
avoid recompiling all your patterns all the time.  Together with
undefining $/ to input entire files as one record, this can be quite
fast, often faster than specialized programs like fgrep(1).  The following
scans a list of files (@files) for a list of words (@words), and prints
out the names of those files that contain a match:



    $search = 'while (<>) { study;';
    foreach $word (@words) {
        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;        # this screams
    $/ = "\n";        # put back to normal input delimiter
    foreach $file (sort keys(%seen)) {
        print $file, "\n";
    }





sub NAME BLOCK







sub NAME (PROTO) BLOCK






sub NAME : ATTRS BLOCK






sub NAME (PROTO) : ATTRS BLOCK




This is subroutine definition, not a real function per se.  Without a
BLOCK it’s just a forward declaration.  Without a NAME, it’s an anonymous
function declaration, so does return a value: the CODE ref of the closure
just created.



See (perlsub) and (perlref) for details about subroutines and
references; see (attributes) and (Attribute-Handlers) for more
information about attributes.





__SUB__





A special token that returns the a reference to the current subroutine, or
undef outside of a subroutine.



This token is only available under use v5.16 or the "current_sub"
feature.  See (feature).





substr EXPR,OFFSET,LENGTH,REPLACEMENT











substr EXPR,OFFSET,LENGTH






substr EXPR,OFFSET




Extracts a substring out of EXPR and returns it.  First character is at
offset zero.  If OFFSET is negative, starts
that far back from the end of the string.  If LENGTH is omitted, returns
everything through the end of the string.  If LENGTH is negative, leaves that
many characters off the end of the string.



    my $s = "The black cat climbed the green tree";
    my $color  = substr $s, 4, 5;      # black
    my $middle = substr $s, 4, -11;    # black cat climbed the
    my $end    = substr $s, 14;        # climbed the green tree
    my $tail   = substr $s, -4;        # tree
    my $z      = substr $s, -4, 2;     # tr



You can use the substr() function as an lvalue, in which case EXPR
must itself be an lvalue.  If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it.  To keep the string the same
length, you may need to pad or chop your value using sprintf.



If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned.  If the substring
is beyond either end of the string, substr() returns the undefined
value and produces a warning.  When used as an lvalue, specifying a
substring that is entirely outside the string raises an exception.
Here’s an example showing the behavior for boundary cases:



    my $name = 'fred';
    substr($name, 4) = 'dy';         # $name is now 'freddy'
    my $null = substr $name, 6, 2;   # returns "" (no warning)
    my $oops = substr $name, 7;      # returns undef, with warning
    substr($name, 7) = 'gap';        # raises an exception



An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument.  This allows you to replace
parts of the EXPR and return what was there before in one operation,
just as you can with splice().



    my $s = "The black cat climbed the green tree";
    my $z = substr $s, 14, 7, "jumped from";    # climbed
    # $s is now "The black cat jumped from the green tree"



Note that the lvalue returned by the three-argument version of substr() acts as
a ’magic bullet’; each time it is assigned to, it remembers which part
of the original string is being modified; for example:



    $x = '1234';
    for (substr($x,1,2)) {
        $_ = 'a';   print $x,"\n";    # prints 1a4
        $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
        $x = '56789';
        $_ = 'pq';  print $x,"\n";    # prints 5pq9
    }



With negative offsets, it remembers its position from the end of the string
when the target string is modified:



    $x = '1234';
    for (substr($x, -3, 2)) {
        $_ = 'a';   print $x,"\n";    # prints 1a4, as above
        $x = 'abcdefg';
        print $_,"\n";                # prints f
    }



Prior to Perl version 5.10, the result of using an lvalue multiple times was
unspecified.  Prior to 5.16, the result with negative offsets was
unspecified.





symlink OLDFILE,NEWFILE








Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise.  On systems that don’t support
symbolic links, raises an exception.  To check for that,
use eval:



    $symlink_exists = eval { symlink("",""); 1 };



Portability issues: perlport symlink.





syscall NUMBER, LIST






Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.  If
unimplemented, raises an exception.  The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int.  If not, the pointer to the string value is passed.  You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string.  You can’t use a
string literal (or other read-only string) as an argument to syscall
because Perl has to assume that any string pointer might be written
through.  If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
like numbers.  This emulates the syswrite function (or vice versa):



    require 'syscall.ph';        # may need to run h2ph
    $s = "hi there\n";
    syscall(&SYS_write, fileno(STDOUT), $s, length $s);



Note that Perl supports passing of up to only 14 arguments to your syscall,
which in practice should (usually) suffice.



Syscall returns whatever value returned by the system call it calls.
If the system call fails, syscall returns -1 and sets $! (errno).
Note that some system calls can legitimately return -1.  The proper
way to handle such calls is to assign $!=0 before the call, then
check the value of $! if syscall returns -1.



There’s a problem with syscall(&SYS_pipe): it returns the file
number of the read end of the pipe it creates, but there is no way
to retrieve the file number of the other end.  You can avoid this
problem by using pipe instead.



Portability issues: perlport syscall.





sysopen FILEHANDLE,FILENAME,MODE







sysopen FILEHANDLE,FILENAME,MODE,PERMS




Opens the file whose filename is given by FILENAME, and associates it with
FILEHANDLE.  If FILEHANDLE is an expression, its value is used as the real
filehandle wanted; an undefined scalar will be suitably autovivified.  This
function calls the underlying operating system’s open(2) function with the
parameters FILENAME, MODE, and PERMS.



The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module Fcntl.  See
the documentation of your operating system’s open(2) syscall to see
which values and flag bits are available.  You may combine several flags
using the |-operator.






Some of the most common values are O_RDONLY for opening the file in
read-only mode, O_WRONLY for opening the file in write-only mode,
and O_RDWR for opening the file in read-write mode.




For historical reasons, some values work on almost every system
supported by Perl: 0 means read-only, 1 means write-only, and 2
means read/write.  We know that these values do not work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don’t want to
use them in new code.




If the file named by FILENAME does not exist and the open call creates
it (typically because MODE includes the O_CREAT flag), then the value of
PERMS specifies the permissions of the newly created file.  If you omit
the PERMS argument to sysopen, Perl uses the octal value 0666.
These permission values need to be in octal, and are modified by your
process’s current umask.





In many systems the O_EXCL flag is available for opening files in
exclusive mode.  This is not locking: exclusiveness means here that
if the file already exists, sysopen() fails.  O_EXCL may not work
on network filesystems, and has no effect unless the O_CREAT flag
is set as well.  Setting O_CREAT|O_EXCL prevents the file from
being opened if it is a symbolic link.  It does not protect against
symbolic links in the file’s path.





Sometimes you may want to truncate an already-existing file.  This
can be done using the O_TRUNC flag.  The behavior of
O_TRUNC with O_RDONLY is undefined.




You should seldom if ever use 0644 as argument to sysopen, because
that takes away the user’s option to have a more permissive umask.
Better to omit it.  See the perlfunc(1) entry on umask for more
on this.



Note that sysopen depends on the fdopen() C library function.
On many Unix systems, fdopen() is known to fail when file descriptors
exceed a certain value, typically 255.  If you need more file
descriptors than that, consider rebuilding Perl to use the sfio
library, or perhaps using the POSIX::open() function.



See perlopentut NAME for a kinder, gentler explanation of opening files.



Portability issues: perlport sysopen.





sysread FILEHANDLE,SCALAR,LENGTH,OFFSET







sysread FILEHANDLE,SCALAR,LENGTH




Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the read(2).  It bypasses
buffered IO, so mixing this with other kinds of reads, print,
write, seek, tell, or eof can cause confusion because the
perlio or stdio layers usually buffers data.  Returns the number of
bytes actually read, 0 at end of file, or undef if there was an
error (in the latter case $! is also set).  SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the
scalar after the read.



An OFFSET may be specified to place the read data at some place in the
string other than the beginning.  A negative OFFSET specifies
placement at that many characters counting backwards from the end of
the string.  A positive OFFSET greater than the length of SCALAR
results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.



There is no syseof() function, which is ok, since eof() doesn’t work
well on device files (like ttys) anyway.  Use sysread() and check
for a return value for 0 to decide whether you’re done.



Note that if the filehandle has been marked as :utf8 Unicode
characters are read instead of bytes (the LENGTH, OFFSET, and the
return value of sysread() are in Unicode characters).
The :encoding(...) layer implicitly introduces the :utf8 layer.
See ‘binmode’, ‘open’, and the open pragma, (open).





sysseek FILEHANDLE,POSITION,WHENCE






Sets FILEHANDLE’s system position in bytes using lseek(2).  FILEHANDLE may
be an expression whose value gives the name of the filehandle.  The values
for WHENCE are 0 to set the new position to POSITION; 1 to set the it
to the current position plus POSITION; and 2 to set it to EOF plus
POSITION, typically negative.



Note the in bytes: even if the filehandle has been set to operate
on characters (for example by using the :encoding(utf8) I/O layer),
tell() will return byte offsets, not character offsets (because
implementing that would render sysseek() unacceptably slow).



sysseek() bypasses normal buffered IO, so mixing it with reads other
than sysread (for example <> or read()) print, write,
seek, tell, or eof may cause confusion.



For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR,
and SEEK_END (start of the file, current position, end of the file)
from the Fcntl module.  Use of the constants is also more portable
than relying on 0, 1, and 2.  For example to define a "systell" function:



    use Fcntl 'SEEK_CUR';
    sub systell { sysseek($_[0], 0, SEEK_CUR) }



Returns the new position, or the undefined value on failure.  A position
of zero is returned as the string "0 but true"; thus sysseek returns
true on success and false on failure, yet you can still easily determine
the new position.





system LIST








system PROGRAM LIST




Does exactly the same thing as exec LIST, except that a fork is
done first and the parent process waits for the child process to
exit.  Note that argument processing varies depending on the
number of arguments.  If there is more than one argument in LIST,
or if LIST is an array with more than one value, starts the program
given by the first element of the list with arguments given by the
rest of the list.  If there is only one scalar argument, the argument
is checked for shell metacharacters, and if there are any, the
entire argument is passed to the system’s command shell for parsing
(this is /bin/sh -c on Unix platforms, but varies on other
platforms).  If there are no shell metacharacters in the argument,
it is split into words and passed directly to execvp, which is
more efficient.



Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before any operation that may do a fork, but this may not be
supported on some platforms (see perlport NAME).  To be safe, you may need
to set $| ($AUTOFLUSH in English) or call the autoflush() method
of IO::Handle on any open handles.



The return value is the exit status of the program as returned by the
wait call.  To get the actual exit value, shift right by eight (see
below).  See also ‘exec’.  This is not what you want to use to capture
the output from a command; for that you should use merely backticks or
qx//, as described in (perlop)`STRING`.  Return value of -1
indicates a failure to start the program or an error of the wait(2) system
call (inspect $! for the reason).



If you’d like to make system (and many other bits of Perl) die on error,
have a look at the (autodie) pragma.



Like exec, system allows you to lie to a program about its name if
you use the system PROGRAM LIST syntax.  Again, see ‘exec’.



Since SIGINT and SIGQUIT are ignored during the execution of
system, if you expect your program to terminate on receipt of these
signals you will need to arrange to do so yourself based on the return
value.



    @args = ("command", "arg1", "arg2");
    system(@args) == 0
        or die "system @args failed: $?"



If you’d like to manually inspect system’s failure, you can check all
possible failure modes by inspecting $? like this:



    if ($? == -1) {
        print "failed to execute: $!\n";
    }
    elsif ($? & 127) {
        printf "child died with signal %d, %s coredump\n",
            ($? & 127),  ($? & 128) ? 'with' : 'without';
    }
    else {
        printf "child exited with value %d\n", $? >> 8;
    }



Alternatively, you may inspect the value of ${^CHILD_ERROR_NATIVE}
with the W*() calls from the POSIX module.



When system’s arguments are executed indirectly by the shell, 
results and return codes are subject to its quirks.
See (perlop)`STRING` and ‘exec’ for details.



Since system does a fork and wait it may affect a SIGCHLD
handler.  See perlipc NAME for details.



Portability issues: perlport system.





syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET







syswrite FILEHANDLE,SCALAR,LENGTH






syswrite FILEHANDLE,SCALAR




Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using write(2).  If LENGTH is
not specified, writes whole SCALAR.  It bypasses buffered IO, so
mixing this with reads (other than sysread()), print, write,
seek, tell, or eof may cause confusion because the perlio and
stdio layers usually buffer data.  Returns the number of bytes
actually written, or undef if there was an error (in this case the
errno variable $! is also set).  If the LENGTH is greater than the
data available in the SCALAR after the OFFSET, only as much data as is
available will be written.



An OFFSET may be specified to write the data from some part of the
string other than the beginning.  A negative OFFSET specifies writing
that many characters counting backwards from the end of the string.
If SCALAR is of length zero, you can only use an OFFSET of 0.



WARNING: If the filehandle is marked :utf8, Unicode characters
encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
return value of syswrite() are in (UTF8-encoded Unicode) characters.
The :encoding(...) layer implicitly introduces the :utf8 layer.
Alternately, if the handle is not marked with an encoding but you
attempt to write characters with code points over 255, raises an exception.
See ‘binmode’, ‘open’, and the open pragma, (open).





tell FILEHANDLE







tell




Returns the current position in bytes for FILEHANDLE, or -1 on
error.  FILEHANDLE may be an expression whose value gives the name of
the actual filehandle.  If FILEHANDLE is omitted, assumes the file
last read.



Note the in bytes: even if the filehandle has been set to
operate on characters (for example by using the :encoding(utf8) open
layer), tell() will return byte offsets, not character offsets (because
that would render seek() and tell() rather slow).



The return value of tell() for the standard streams like the STDIN
depends on the operating system: it may return -1 or something else.
tell() on pipes, fifos, and sockets usually returns -1.



There is no systell function.  Use sysseek(FH, 0, 1) for that.



Do not use tell() (or other buffered I/O operations) on a filehandle
that has been manipulated by sysread(), syswrite(), or sysseek().
Those functions ignore the buffering, while tell() does not.





telldir DIRHANDLE





Returns the current position of the readdir routines on DIRHANDLE.
Value may be given to seekdir to access a particular location in a
directory.  telldir has the same caveats about possible directory
compaction as the corresponding system library routine.





tie VARIABLE,CLASSNAME,LIST





This function binds a variable to a package class that will provide the
implementation for the variable.  VARIABLE is the name of the variable
to be enchanted.  CLASSNAME is the name of a class implementing objects
of correct type.  Any additional arguments are passed to the new
method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY,
or TIEHASH).  Typically these are arguments such as might be passed
to the dbm_open() function of C.  The object returned by the new
method is also returned by the tie function, which would be useful
if you want to access other methods in CLASSNAME.



Note that functions such as keys and values may return huge lists
when used on large objects, like DBM files.  You may prefer to use the
each function to iterate over such.  Example:



    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);



A class implementing a hash should have the following methods:



    TIEHASH classname, LIST
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    CLEAR this
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey
    SCALAR this
    DESTROY this
    UNTIE this



A class implementing an ordinary array should have the following methods:



    TIEARRAY classname, LIST
    FETCH this, key
    STORE this, key, value
    FETCHSIZE this
    STORESIZE this, count
    CLEAR this
    PUSH this, LIST
    POP this
    SHIFT this
    UNSHIFT this, LIST
    SPLICE this, offset, length, LIST
    EXTEND this, count
    DESTROY this
    UNTIE this



A class implementing a filehandle should have the following methods:



    TIEHANDLE classname, LIST
    READ this, scalar, length, offset
    READLINE this
    GETC this
    WRITE this, scalar, length, offset
    PRINT this, LIST
    PRINTF this, format, LIST
    BINMODE this
    EOF this
    FILENO this
    SEEK this, position, whence
    TELL this
    OPEN this, mode, LIST
    CLOSE this
    DESTROY this
    UNTIE this



A class implementing a scalar should have the following methods:



    TIESCALAR classname, LIST
    FETCH this,
    STORE this, value
    DESTROY this
    UNTIE this



Not all methods indicated above need be implemented.  See (perltie),
(Tie-Hash), (Tie-Array), (Tie-Scalar), and (Tie-Handle).



Unlike dbmopen, the tie function will not use or require a module
for you; you need to do that explicitly yourself.  See (DB_File)
or the Config module for interesting tie implementations.



For further details see (perltie), tied VARIABLE.





tied VARIABLE





Returns a reference to the object underlying VARIABLE (the same value
that was originally returned by the tie call that bound the variable
to a package.)  Returns the undefined value if VARIABLE isn’t tied to a
package.





time






Returns the number of non-leap seconds since whatever time the system
considers to be the epoch, suitable for feeding to gmtime and
localtime.  On most systems the epoch is 00:00:00 UTC, January 1, 1970;
a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
1904 in the current local time zone for its epoch.



For measuring time in better granularity than one second, use the
(Time-HiRes) module from Perl 5.8 onwards (or from CPAN before then), or,
if you have gettimeofday(2), you may be able to use the syscall
interface of Perl.  See (perlfaq8) for details.



For date and time processing look at the many related modules on CPAN.
For a comprehensive date and time representation look at the
(DateTime) module.





times





Returns a four-element list giving the user and system times in
seconds for this process and any exited children of this process.



    ($user,$system,$cuser,$csystem) = times;



In scalar context, times returns $user.



Children’s times are only included for terminated children.



Portability issues: perlport times.





tr///




The transliteration operator.  Same as y///.  See
(perlop)Quote and Quote-like Operators.





truncate FILEHANDLE,LENGTH







truncate EXPR,LENGTH




Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length.  Raises an exception if truncate isn’t implemented
on your system.  Returns true if successful, undef on error.



The behavior is undefined if LENGTH is greater than the length of the
file.



The position in the file of FILEHANDLE is left unchanged.  You may want to
call seek before writing to the file.



Portability issues: perlport truncate.





uc EXPR









uc




Returns an uppercased version of EXPR.  This is the internal function
implementing the \U escape in double-quoted strings.
It does not attempt to do titlecase mapping on initial letters.  See
ucfirst for that.



If EXPR is omitted, uses $_.



This function behaves the same way under various pragma, such as in a locale,
as lc does.





ucfirst EXPR








ucfirst




Returns the value of EXPR with the first character in uppercase
(titlecase in Unicode).  This is the internal function implementing
the \u escape in double-quoted strings.



If EXPR is omitted, uses $_.



This function behaves the same way under various pragma, such as in a locale,
as lc does.





umask EXPR







umask




Sets the umask for the process to EXPR and returns the previous value.
If EXPR is omitted, merely returns the current umask.



The Unix permission rwxr-x--- is represented as three sets of three
bits, or three octal digits: 0750 (the leading 0 indicates octal
and isn’t one of the digits).  The umask value is such a number
representing disabled permissions bits.  The permission (or "mode")
values you pass mkdir or sysopen are modified by your umask, so
even if you tell sysopen to create a file with permissions 0777,
if your umask is 0022, then the file will actually be created with
permissions 0755.  If your umask were 0027 (group can’t
write; others can’t read, write, or execute), then passing
sysopen 0666 would create a file with mode 0640 (because 
0666 &~ 027 is 0640).



Here’s some advice: supply a creation mode of 0666 for regular
files (in sysopen) and one of 0777 for directories (in
mkdir) and executable files.  This gives users the freedom of
choice: if they want protected files, they might choose process umasks
of 022, 027, or even the particularly antisocial mask of 077.
Programs should rarely if ever make policy decisions better left to
the user.  The exception to this is when writing files that should be
kept private: mail files, web browser cookies, .rhosts files, and
so on.



If umask(2) is not implemented on your system and you are trying to
restrict access for yourself (i.e., (EXPR & 0700) > 0), 
raises an exception.  If umask(2) is not implemented and you are
not trying to restrict access for yourself, returns undef.



Remember that a umask is a number, usually given in octal; it is not a
string of octal digits.  See also oct, if all you have is a string.



Portability issues: perlport umask.





undef EXPR








undef




Undefines the value of EXPR, which must be an lvalue.  Use only on a
scalar value, an array (using @), a hash (using %), a subroutine
(using &), or a typeglob (using *).  Saying undef $hash{$key}
will probably not do what you expect on most predefined variables or
DBM list values, so don’t do that; see ‘delete’.  Always returns the
undefined value.  You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for
instance, return from a subroutine, assign to a variable, or pass as a
parameter.  Examples:



    undef $foo;
    undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
    undef @ary;
    undef %hash;
    undef &mysub;
    undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
    select undef, undef, undef, 0.25;
    ($a, $b, undef, $c) = &foo;       # Ignore third value returned



Note that this is a unary operator, not a list operator.





unlink LIST











unlink




Deletes a list of files.  On success, it returns the number of files
it successfully deleted.  On failure, it returns false and sets $!
(errno):



    my $unlinked = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink glob "*.bak";



On error, unlink will not tell you which files it could not remove.
If you want to know which files you could not remove, try them one
at a time:



     foreach my $file ( @goners ) {
         unlink $file or warn "Could not unlink $file: $!";
     }



Note: unlink will not attempt to delete directories unless you are
superuser and the -U flag is supplied to Perl.  Even if these
conditions are met, be warned that unlinking a directory can inflict
damage on your filesystem.  Finally, using unlink on directories is
not supported on many operating systems.  Use rmdir instead.



If LIST is omitted, unlink uses $_.





unpack TEMPLATE,EXPR







unpack TEMPLATE




unpack does the reverse of pack: it takes a string
and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)



If EXPR is omitted, unpacks the $_ string.
See perlpacktut NAME for an introduction to this function.



The string is broken into chunks described by the TEMPLATE.  Each chunk
is converted separately to a value.  Typically, either the string is a result
of pack, or the characters of the string represent a C structure of some
kind.



The TEMPLATE has the same format as in the pack function.
Here’s a subroutine that does substring:



    sub substr {
        my($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }



and then there’s



    sub ordinal { unpack("W",$_[0]); } # same as ord()



In addition to fields allowed in pack(), you may prefix a field with
a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  Checksum is calculated by
summing numeric values of expanded values (for string fields the sum of
ord($char) is taken; for bit fields the sum of zeroes and ones).



For example, the following
computes the same number as the System V sum program:



    $checksum = do {
        local $/;  # slurp!
        unpack("%32W*",<>) % 65535;
    };



The following efficiently counts the number of set bits in a bit vector:



    $setbits = unpack("%32b*", $selectmask);



The p and P formats should be used with care.  Since Perl
has no way of checking whether the value passed to unpack()
corresponds to a valid memory location, passing a pointer value that’s
not known to be valid is likely to have disastrous consequences.



If there are more pack codes or if the repeat count of a field or a group
is larger than what the remainder of the input string allows, the result
is not well defined: the repeat count may be decreased, or
unpack() may produce empty strings or zeros, or it may raise an exception.
If the input string is longer than one described by the TEMPLATE,
the remainder of that input string is ignored.



See ‘pack’ for more examples and notes.





unshift ARRAY,LIST







unshift EXPR,LIST




Does the opposite of a shift.  Or the opposite of a push,
depending on how you look at it.  Prepends list to the front of the
array and returns the new number of elements in the array.



    unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;



Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order.  Use reverse to do the
reverse.



Starting with Perl 5.14, unshift can take a scalar EXPR, which must hold
a reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of unshift is considered highly
experimental.  The exact behaviour may change in a future version of Perl.



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.014;  # so push/pop/etc work on scalars (experimental)





untie VARIABLE





Breaks the binding between a variable and a package.
(See tie.)
Has no effect if the variable is not tied.





use Module VERSION LIST









use Module VERSION






use Module LIST






use Module






use VERSION




Imports some semantics into the current package from the named module,
generally by aliasing certain subroutine or variable names into your
package.  It is exactly equivalent to



    BEGIN { require Module; Module->import( LIST ); }



except that Module must be a bareword.
The importation can be made conditional; see (if).



In the peculiar use VERSION form, VERSION may be either a positive
decimal fraction such as 5.006, which will be compared to $], or a v-string
of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION).  An
exception is raised if VERSION is greater than the version of the
current Perl interpreter; Perl will not attempt to parse the rest of the
file.  Compare with require, which can do a similar check at run time.
Symmetrically, no VERSION allows you to specify that you want a version
of Perl older than the specified one.



Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
versions of Perl (that is, prior to 5.6.0) that do not support this
syntax.  The equivalent numeric version should be used instead.



    use v5.6.1;     # compile time version check
    use 5.6.1;      # ditto
    use 5.006_001;  # ditto; preferred for backwards compatibility



This is often useful if you need to check the current Perl version before
useing library modules that won’t work with older versions of Perl.
(We try not to do this more than we have to.)



use VERSION also enables all features available in the requested
version as defined by the feature pragma, disabling any features
not in the requested version’s feature bundle.  See (feature).
Similarly, if the specified Perl version is greater than or equal to
5.11.0, strictures are enabled lexically as
with use strict.  Any explicit use of
use strict or no strict overrides use VERSION, even if it comes
before it.  In both cases, the feature.pm and strict.pm files are
not actually loaded.



The BEGIN forces the require and import to happen at compile time.  The
require makes sure the module is loaded into memory if it hasn’t been
yet.  The import is not a builtin; it’s just an ordinary static method
call into the Module package to tell the module to import the list of
features back into the current package.  The module can implement its
import method any way it likes, though most modules just choose to
derive their import method via inheritance from the Exporter class that
is defined in the Exporter module.  See (Exporter).  If no import
method can be found then the call is skipped, even if there is an AUTOLOAD
method.



If you do not want to call the package’s import method (for instance,
to stop your namespace from being altered), explicitly supply the empty list:



    use Module ();



That is exactly equivalent to



    BEGIN { require Module }



If the VERSION argument is present between Module and LIST, then the
use will call the VERSION method in class Module with the given
version as an argument.  The default VERSION method, inherited from
the UNIVERSAL class, croaks if the given version is larger than the
value of the variable $Module::VERSION.



Again, there is a distinction between omitting LIST (import called
with no arguments) and an explicit empty LIST () (import not
called).  Note that there is no comma after VERSION!



Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way.  Currently implemented pragmas are:



    use constant;
    use diagnostics;
    use integer;
    use sigtrap  qw(SEGV BUS);
    use strict   qw(subs vars refs);
    use subs     qw(afunc blurfl);
    use warnings qw(all);
    use sort     qw(stable _quicksort _mergesort);



Some of these pseudo-modules import semantics into the current
block scope (like strict or integer, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).



Because use takes effect at compile time, it doesn’t respect the
ordinary flow control of the code being compiled.  In particular, putting
a use inside the false branch of a conditional doesn’t prevent it
from being processed.  If a module or pragma only needs to be loaded 
conditionally, this can be done using the (if) pragma:



    use if $] < 5.008, "utf8";
    use if WANT_WARNINGS, warnings => qw(all);



There’s a corresponding no declaration that unimports meanings imported
by use, i.e., it calls unimport Module LIST instead of import.
It behaves just as import does with VERSION, an omitted or empty LIST, 
or no unimport method being found.



    no integer;
    no strict 'refs';
    no warnings;



Care should be taken when using the no VERSION form of no.  It is
only meant to be used to assert that the running Perl is of a earlier
version than its argument and not to undo the feature-enabling side effects
of use VERSION.



See (perlmodlib) for a list of standard modules and pragmas.  See perlrun NAME
for the -M and -m command-line options to Perl that give use
functionality from the command-line.





utime LIST





Changes the access and modification times on each file of a list of
files.  The first two elements of the list must be the NUMERIC access
and modification times, in that order.  Returns the number of files
successfully changed.  The inode change time of each file is set
to the current time.  For example, this code has the same effect as the
Unix touch(1) command when the files already exist and belong to
the user running the program:



    #!/usr/bin/perl
    $atime = $mtime = time;
    utime $atime, $mtime, @ARGV;



Since Perl 5.7.2, if the first two elements of the list are undef, 
the utime(2) syscall from your C library is called with a null second
argument.  On most systems, this will set the file’s access and
modification times to the current time (i.e., equivalent to the example
above) and will work even on files you don’t own provided you have write
permission:



    for $file (@ARGV) {
        utime(undef, undef, $file) 
            || warn "couldn't touch $file: $!";
    } 



Under NFS this will use the time of the NFS server, not the time of
the local machine.  If there is a time synchronization problem, the
NFS server and local machine will have different times.  The Unix
touch(1) command will in fact normally use this form instead of the
one shown in the first example.



Passing only one of the first two elements as undef is
equivalent to passing a 0 and will not have the effect 
described when both are undef.  This also triggers an
uninitialized warning.



On systems that support futimes(2), you may pass filehandles among the
files.  On systems that don’t support futimes(2), passing filehandles raises
an exception.  Filehandles must be passed as globs or glob references to be
recognized; barewords are considered filenames.



Portability issues: perlport utime.





values HASH







values ARRAY






values EXPR




In list context, returns a list consisting of all the values of the named
hash.  In Perl 5.12 or later only, will also return a list of the values of
an array; prior to that release, attempting to use an array argument will
produce a syntax error.  In scalar context, returns the number of values.



When called on a hash, the values are returned in an apparently random
order.  The actual random order is subject to change in future versions of
Perl, but it is guaranteed to be the same order as either the keys or
each function would produce on the same (unmodified) hash.  Since Perl
5.8.1 the ordering is different even between different runs of Perl for
security reasons (see perlsec Algorithmic Complexity Attacks).



As a side effect, calling values() resets the HASH or ARRAY’s internal
iterator, see ‘each’.  (In particular, calling values() in void context
resets the iterator with no other overhead.  Apart from resetting the
iterator, values @array in list context is the same as plain @array.
(We recommend that you use void context keys @array for this, but
reasoned that taking values @array out would require more
documentation than leaving it in.)



Note that the values are not copied, which means modifying them will
modify the contents of the hash:



    for (values %hash)      { s/foo/bar/g }   # modifies %hash values
    for (@hash{keys %hash}) { s/foo/bar/g }   # same



Starting with Perl 5.14, values can take a scalar EXPR, which must hold
a reference to an unblessed hash or array.  The argument will be
dereferenced automatically.  This aspect of values is considered highly
experimental.  The exact behaviour may change in a future version of Perl.



    for (values $hashref) { ... }
    for (values $obj->get_arrayref) { ... }



To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage:



    use 5.012;  # so keys/values/each work on arrays
    use 5.014;  # so keys/values/each work on scalars (experimental)



See also keys, each, and sort.





vec EXPR,OFFSET,BITS







Treats the string in EXPR as a bit vector made up of elements of
width BITS and returns the value of the element specified by OFFSET
as an unsigned integer.  BITS therefore specifies the number of bits
that are reserved for each element in the bit vector.  This must
be a power of two from 1 to 32 (or 64, if your platform supports
that).



If BITS is 8, "elements" coincide with bytes of the input string.



If BITS is 16 or more, bytes of the input string are grouped into chunks
of size BITS/8, and each group is converted to a number as with
pack()/unpack() with big-endian formats n/N (and analogously
for BITS==64).  See ‘pack’ for details.



If bits is 4 or less, the string is broken into bytes, then the bits
of each byte are broken into 8/BITS groups.  Bits of a byte are
numbered in a little-endian-ish way, as in 0x01, 0x02,
0x04, 0x08, 0x10, 0x20, 0x40, 0x80.  For example,
breaking the single input byte chr(0x36) into two groups gives a list
(0x6, 0x3); breaking it into 4 groups gives (0x2, 0x1, 0x3, 0x0).



vec may also be assigned to, in which case parentheses are needed
to give the expression the correct precedence as in



    vec($image, $max_x * $x + $y, 8) = 3;



If the selected element is outside the string, the value 0 is returned.
If an element off the end of the string is written to, Perl will first
extend the string with sufficiently many zero bytes.   It is an error
to try to write off the beginning of the string (i.e., negative OFFSET).



If the string happens to be encoded as UTF-8 internally (and thus has
the UTF8 flag set), this is ignored by vec, and it operates on the
internal byte string, not the conceptual character string, even if you
only have characters with values less than 256. 



Strings created with vec can also be manipulated with the logical
operators |, &, ^, and ~.  These operators will assume a bit
vector operation is desired when both operands are strings.
See (perlop)Bitwise String Operators.



The following code will build up an ASCII string saying 'PerlPerlPerl'.
The comments show the string after each step.  Note that this code works
in the same way on big-endian or little-endian machines.



    my $foo = '';
    vec($foo,  0, 32) = 0x5065726C; # 'Perl'

    # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
    print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')

    vec($foo,  2, 16) = 0x5065; # 'PerlPe'
    vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
    vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
    vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
    vec($foo, 20,  4) = 2;      # 'PerlPerlPe'   . "\x02"
    vec($foo, 21,  4) = 7;      # 'PerlPerlPer'
                                   # 'r' is "\x72"
    vec($foo, 45,  2) = 3;      # 'PerlPerlPer'  . "\x0c"
    vec($foo, 93,  1) = 1;      # 'PerlPerlPer'  . "\x2c"
    vec($foo, 94,  1) = 1;      # 'PerlPerlPerl'
                                   # 'l' is "\x6c"



To transform a bit vector into a string or list of 0’s and 1’s, use these:



    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));



If you know the exact length in bits, it can be used in place of the *.



Here is an example to illustrate how the bits actually fall in place:



    #!/usr/bin/perl -wl

    print <<'EOT';
                                      0         1         2         3
                       unpack("V",$_) 01234567890123456789012345678901
    ------------------------------------------------------------------
    EOT

    for $w (0..3) {
        $width = 2**$w;
        for ($shift=0; $shift < $width; ++$shift) {
            for ($off=0; $off < 32/$width; ++$off) {
                $str = pack("B*", "0"x32);
                $bits = (1<<$shift);
                vec($str, $off, $width) = $bits;
                $res = unpack("b*",$str);
                $val = unpack("V", $str);
                write;
            }
        }
    }

    format STDOUT =
    vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
    $off, $width, $bits, $val, $res
    .
    __END__



Regardless of the machine architecture on which it runs, the 
example above should print the following table:



                                      0         1         2         3
                       unpack("V",$_) 01234567890123456789012345678901
    ------------------------------------------------------------------
    vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
    vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
    vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
    vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
    vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
    vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
    vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
    vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
    vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
    vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
    vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
    vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
    vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
    vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
    vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
    vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
    vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
    vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
    vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
    vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
    vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
    vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
    vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
    vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
    vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
    vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
    vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
    vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
    vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
    vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
    vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
    vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
    vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
    vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
    vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
    vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
    vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
    vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
    vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
    vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
    vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
    vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
    vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
    vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
    vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
    vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
    vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
    vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
    vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
    vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
    vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
    vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
    vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
    vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
    vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
    vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
    vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
    vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
    vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
    vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
    vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
    vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
    vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
    vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
    vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
    vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
    vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
    vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
    vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
    vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
    vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
    vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
    vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
    vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
    vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
    vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
    vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
    vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
    vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
    vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
    vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
    vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
    vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
    vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
    vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
    vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
    vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
    vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
    vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
    vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
    vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
    vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
    vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
    vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
    vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
    vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
    vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
    vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
    vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
    vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
    vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
    vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
    vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
    vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
    vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
    vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
    vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
    vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
    vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
    vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
    vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
    vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001





wait





Behaves like wait(2) on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
-1 if there are no child processes.  The status is returned in $?
and ${^CHILD_ERROR_NATIVE}.
Note that a return value of -1 could mean that child processes are
being automatically reaped, as described in perlipc NAME.



If you use wait in your handler for $SIG{CHLD} it may accidentally for the
child created by qx() or system().  See perlipc NAME for details.



Portability issues: perlport wait.





waitpid PID,FLAGS





Waits for a particular child process to terminate and returns the pid of
the deceased process, or -1 if there is no such child process.  On some
systems, a value of 0 indicates that there are processes still running.
The status is returned in $? and ${^CHILD_ERROR_NATIVE}.  If you say



    use POSIX ":sys_wait_h";
    #...
    do {
        $kid = waitpid(-1, WNOHANG);
    } while $kid > 0;



then you can do a non-blocking wait for all pending zombie processes.
Non-blocking wait is available on machines supporting either the
waitpid(2) or wait4(2) syscalls.  However, waiting for a particular
pid with FLAGS of 0 is implemented everywhere.  (Perl emulates the
system call by remembering the status values of processes that have
exited but have not been harvested by the Perl script yet.)



Note that on some systems, a return value of -1 could mean that child
processes are being automatically reaped.  See perlipc NAME for details,
and for other examples.



Portability issues: perlport waitpid.





wantarray






Returns true if the context of the currently executing subroutine or
eval is looking for a list value.  Returns false if the context is
looking for a scalar.  Returns the undefined value if the context is
looking for no value (void context).



    return unless defined wantarray; # don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";



wantarray()’s result is unspecified in the top level of a file,
in a BEGIN, UNITCHECK, CHECK, INIT or END block, or
in a DESTROY method.



This function should have been named wantlist() instead.





warn LIST







Prints the value of LIST to STDERR.  If the last element of LIST does
not end in a newline, it appends the same file/line number text as die
does.



If the output is empty and $@ already contains a value (typically from a
previous eval) that value is used after appending "\t...caught"
to $@.  This is useful for staying almost, but not entirely similar to
die.



If $@ is empty then the string "Warning: Something's wrong" is used.



No message is printed if there is a $SIG{__WARN__} handler
installed.  It is the handler’s responsibility to deal with the message
as it sees fit (like, for instance, converting it into a die).  Most
handlers must therefore arrange to actually display the
warnings that they are not prepared to deal with, by calling warn
again in the handler.  Note that this is quite safe and will not
produce an endless loop, since __WARN__ hooks are not called from
inside one.



You will find this behavior is slightly different from that of
$SIG{__DIE__} handlers (which don’t suppress the error text, but can
instead call die again to change it).



Using a __WARN__ handler provides a powerful way to silence all
warnings (even the so-called mandatory ones).  An example:



    # wipe out *all* compile-time warnings
    BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
    my $foo = 10;
    my $foo = 20;          # no warning about duplicate my $foo,
                           # but hey, you asked for it!
    # no compile-time or run-time warnings before here
    $DOWARN = 1;

    # run-time warnings enabled after here
    warn "\$foo is alive and $foo!";     # does show up



See perlvar NAME for details on setting %SIG entries and for more
examples.  See the Carp module for other kinds of warnings using its
carp() and cluck() functions.





write FILEHANDLE







write EXPR






write




Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file.  By default the format for
a file is the one having the same name as the filehandle, but the
format for the current output channel (see the select function) may be set
explicitly by assigning the name of the format to the $~ variable.



Top of form processing is handled automatically:  if there is insufficient
room on the current page for the formatted record, the page is advanced by
writing a form feed, a special top-of-page format is used to format the new
page header before the record is written.  By default, the top-of-page
format is the name of the filehandle with "_TOP" appended.  This would be a
problem with autovivified filehandles, but it may be dynamically set to the
format of your choice by assigning the name to the $^ variable while
that filehandle is selected.  The number of lines remaining on the current
page is in variable $-, which can be set to 0 to force a new page.



If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
select operator.  If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time.  For more on formats, see (perlform).



Note that write is not the opposite of read.  Unfortunately.





y///




The transliteration operator.  Same as tr///.  See
(perlop)Quote and Quote-like Operators.














Previous: NAME function Alphabetical Listing of Perl Functions, Up: NAME function DESCRIPTION   [Contents]





25.2.4 Non-function Keywords by Cross-reference





• NAME function perldata:  



• NAME function perlmod:  



• NAME function perlobj:  



• NAME function perlop:  



• NAME function perlsub:  



• NAME function perlsyn:  












Next: , Up: NAME function Non-function Keywords by Cross-reference   [Contents]





25.2.4.1 perldata





__DATA__






__END__




These keywords are documented in perldata Special Literals.














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25.2.4.2 perlmod





BEGIN






CHECK






END






INIT






UNITCHECK




These compile phase keywords are documented in perlmod BEGIN, UNITCHECK, CHECK, INIT and END.














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25.2.4.3 perlobj





DESTROY




This method keyword is documented in (perlobj)Destructors.














Next: , Previous: NAME function perlobj, Up: NAME function Non-function Keywords by Cross-reference   [Contents]





25.2.4.4 perlop





and






cmp






eq






ge






gt






if






le






lt






ne






not






or






x






xor




These operators are documented in (perlop).














Next: , Previous: NAME function perlop, Up: NAME function Non-function Keywords by Cross-reference   [Contents]





25.2.4.5 perlsub





AUTOLOAD




This keyword is documented in (perlsub)Autoloading.














Previous: NAME function perlsub, Up: NAME function Non-function Keywords by Cross-reference   [Contents]





25.2.4.6 perlsyn





else






elseif






elsif






for






foreach






unless






until






while




These flow-control keywords are documented in (perlsyn)Compound Statements.










default






given






when




These flow-control keywords related to the experimental switch feature are
documented in (perlsyn)Switch Statements .














Next: , Previous: NAME function, Up: Top   [Contents]





26 perlgit





• perlgit NAME:  



• perlgit DESCRIPTION:  



• perlgit CLONING THE REPOSITORY:  



• perlgit WORKING WITH THE REPOSITORY:  



• perlgit Topic branches and rewriting history:  



• perlgit WRITE ACCESS TO THE GIT REPOSITORY:  



• perlgit Accepting a patch:  












Next: , Up: perlgit   [Contents]





26.1 NAME



perlgit - Detailed information about git and the Perl repository









Next: , Previous: perlgit NAME, Up: perlgit   [Contents]





26.2 DESCRIPTION



This document provides details on using git to develop Perl. If you are
just interested in working on a quick patch, see perlhack NAME first.
This document is intended for people who are regular contributors to
Perl, including those with write access to the git repository.









Next: , Previous: perlgit DESCRIPTION, Up: perlgit   [Contents]





26.3 CLONING THE REPOSITORY



All of Perl’s source code is kept centrally in a Git repository at
perl5.git.perl.org.



You can make a read-only clone of the repository by running:



  % git clone git://perl5.git.perl.org/perl.git perl



This uses the git protocol (port 9418).



If you cannot use the git protocol for firewall reasons, you can also
clone via http, though this is much slower:



  % git clone http://perl5.git.perl.org/perl.git perl









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26.4 WORKING WITH THE REPOSITORY



Once you have changed into the repository directory, you can inspect
it. After a clone the repository will contain a single local branch,
which will be the current branch as well, as indicated by the asterisk.



  % git branch
  * blead



Using the -a switch to branch will also show the remote tracking
branches in the repository:



  % git branch -a
  * blead
    origin/HEAD
    origin/blead
  ...



The branches that begin with "origin" correspond to the "git remote"
that you cloned from (which is named "origin"). Each branch on the
remote will be exactly tracked by these branches. You should NEVER do
work on these remote tracking branches. You only ever do work in a
local branch. Local branches can be configured to automerge (on pull)
from a designated remote tracking branch. This is the case with the
default branch blead which will be configured to merge from the
remote tracking branch origin/blead.



You can see recent commits:



  % git log



And pull new changes from the repository, and update your local
repository (must be clean first)



  % git pull



Assuming we are on the branch blead immediately after a pull, this
command would be more or less equivalent to:



  % git fetch
  % git merge origin/blead



In fact if you want to update your local repository without touching
your working directory you do:



  % git fetch



And if you want to update your remote-tracking branches for all defined
remotes simultaneously you can do



  % git remote update



Neither of these last two commands will update your working directory,
however both will update the remote-tracking branches in your
repository.



To make a local branch of a remote branch:



  % git checkout -b maint-5.10 origin/maint-5.10



To switch back to blead:



  % git checkout blead





• perlgit Finding out your status:  



• perlgit Patch workflow:  



• perlgit Committing your changes:  



• perlgit Using git to send patch emails:  



• perlgit A note on derived files:  



• perlgit Cleaning a working directory:  



• perlgit Bisecting:  












Next: , Up: perlgit WORKING WITH THE REPOSITORY   [Contents]





26.4.1 Finding out your status



The most common git command you will use will probably be



  % git status



This command will produce as output a description of the current state
of the repository, including modified files and unignored untracked
files, and in addition it will show things like what files have been
staged for the next commit, and usually some useful information about
how to change things. For instance the following:



  $ git status
  # On branch blead
  # Your branch is ahead of 'origin/blead' by 1 commit.
  #
  # Changes to be committed:
  #   (use "git reset HEAD <file>..." to unstage)
  #
  #       modified:   pod/perlgit.pod
  #
  # Changed but not updated:
  #   (use "git add <file>..." to update what will be committed)
  #
  #       modified:   pod/perlgit.pod
  #
  # Untracked files:
  #   (use "git add <file>..." to include in what will be committed)
  #
  #       deliberate.untracked



This shows that there were changes to this document staged for commit,
and that there were further changes in the working directory not yet
staged. It also shows that there was an untracked file in the working
directory, and as you can see shows how to change all of this. It also
shows that there is one commit on the working branch blead which has
not been pushed to the origin remote yet. NOTE: that this output
is also what you see as a template if you do not provide a message to
git commit.









Next: , Previous: perlgit Finding out your status, Up: perlgit WORKING WITH THE REPOSITORY   [Contents]





26.4.2 Patch workflow



First, please read perlhack NAME for details on hacking the Perl core.
That document covers many details on how to create a good patch.



If you already have a Perl repository, you should ensure that you’re on
the blead branch, and your repository is up to date:



  % git checkout blead
  % git pull



It’s preferable to patch against the latest blead version, since this
is where new development occurs for all changes other than critical bug
fixes. Critical bug fix patches should be made against the relevant
maint branches, or should be submitted with a note indicating all the
branches where the fix should be applied.



Now that we have everything up to date, we need to create a temporary
new branch for these changes and switch into it:



  % git checkout -b orange



which is the short form of



  % git branch orange
  % git checkout orange



Creating a topic branch makes it easier for the maintainers to rebase
or merge back into the master blead for a more linear history. If you
don’t work on a topic branch the maintainer has to manually cherry pick
your changes onto blead before they can be applied.



That’ll get you scolded on perl5-porters, so don’t do that. Be Awesome.



Then make your changes. For example, if Leon Brocard changes his name
to Orange Brocard, we should change his name in the AUTHORS file:



  % perl -pi -e 's{Leon Brocard}{Orange Brocard}' AUTHORS



You can see what files are changed:



  % git status
  # On branch orange
  # Changes to be committed:
  #   (use "git reset HEAD <file>..." to unstage)
  #
  #    modified:   AUTHORS
  #



And you can see the changes:



  % git diff
  diff --git a/AUTHORS b/AUTHORS
  index 293dd70..722c93e 100644
  --- a/AUTHORS
  +++ b/AUTHORS
  @@ -541,7 +541,7 @@    Lars Hecking                   <lhecking@nmrc.ucc.ie>
   Laszlo Molnar                  <laszlo.molnar@eth.ericsson.se>
   Leif Huhn                      <leif@hale.dkstat.com>
   Len Johnson                    <lenjay@ibm.net>
  -Leon Brocard                   <acme@astray.com>
  +Orange Brocard                 <acme@astray.com>
   Les Peters                     <lpeters@aol.net>
   Lesley Binks                   <lesley.binks@gmail.com>
   Lincoln D. Stein               <lstein@cshl.org>



Now commit your change locally:



  % git commit -a -m 'Rename Leon Brocard to Orange Brocard'
  Created commit 6196c1d: Rename Leon Brocard to Orange Brocard
   1 files changed, 1 insertions(+), 1 deletions(-)



The -a option is used to include all files that git tracks that you
have changed. If at this time, you only want to commit some of the
files you have worked on, you can omit the -a and use the command
git add FILE ... before doing the commit. git add --interactive allows you to even just commit portions of files
instead of all the changes in them.



The -m option is used to specify the commit message. If you omit it,
git will open a text editor for you to compose the message
interactively. This is useful when the changes are more complex than
the sample given here, and, depending on the editor, to know that the
first line of the commit message doesn’t exceed the 50 character legal
maximum.



Once you’ve finished writing your commit message and exited your
editor, git will write your change to disk and tell you something like
this:



  Created commit daf8e63: explain git status and stuff about remotes
   1 files changed, 83 insertions(+), 3 deletions(-)



If you re-run git status, you should see something like this:



  % git status
  # On branch blead
  # Your branch is ahead of 'origin/blead' by 2 commits.
  #
  # Untracked files:
  #   (use "git add <file>..." to include in what will be committed)
  #
  #       deliberate.untracked
  nothing added to commit but untracked files present (use "git add" to track)



When in doubt, before you do anything else, check your status and read
it carefully, many questions are answered directly by the git status
output.



You can examine your last commit with:



  % git show HEAD



and if you are not happy with either the description or the patch
itself you can fix it up by editing the files once more and then issue:



  % git commit -a --amend



Now you should create a patch file for all your local changes:



  % git format-patch -M origin..
  0001-Rename-Leon-Brocard-to-Orange-Brocard.patch



You should now send an email to
perlbug@perl.org with a description of your
changes, and include this patch file as an attachment. In addition to
being tracked by RT, mail to perlbug will automatically be forwarded to
perl5-porters (with manual moderation, so please be patient). You
should only send patches to
perl5-porters@perl.org directly if the
patch is not ready to be applied, but intended for discussion.



See the next section for how to configure and use git to send these
emails for you.



If you want to delete your temporary branch, you may do so with:



  % git checkout blead
  % git branch -d orange
  error: The branch 'orange' is not an ancestor of your current HEAD.
  If you are sure you want to delete it, run 'git branch -D orange'.
  % git branch -D orange
  Deleted branch orange.









Next: , Previous: perlgit Patch workflow, Up: perlgit WORKING WITH THE REPOSITORY   [Contents]





26.4.3 Committing your changes



Assuming that you’d like to commit all the changes you’ve made as a
single atomic unit, run this command:



   % git commit -a



(That -a tells git to add every file you’ve changed to this commit.
New files aren’t automatically added to your commit when you use
commit -a If you want to add files or to commit some, but not all of
your changes, have a look at the documentation for git add.)



Git will start up your favorite text editor, so that you can craft a
commit message for your change. See perlhack Commit message for more
information about what makes a good commit message.



Once you’ve finished writing your commit message and exited your
editor, git will write your change to disk and tell you something like
this:



  Created commit daf8e63: explain git status and stuff about remotes
   1 files changed, 83 insertions(+), 3 deletions(-)



If you re-run git status, you should see something like this:



  % git status
  # On branch blead
  # Your branch is ahead of 'origin/blead' by 2 commits.
  #
  # Untracked files:
  #   (use "git add <file>..." to include in what will be committed)
  #
  #       deliberate.untracked
  nothing added to commit but untracked files present (use "git add" to track)



When in doubt, before you do anything else, check your status and read
it carefully, many questions are answered directly by the git status
output.









Next: , Previous: perlgit Committing your changes, Up: perlgit WORKING WITH THE REPOSITORY   [Contents]





26.4.4 Using git to send patch emails



Please read perlhack NAME first in order to figure out where your patches
should be sent.



In your ~/git/perl repository, set the destination email to perl’s bug
tracker:



  $ git config sendemail.to perlbug@perl.org



Or maybe perl5-porters:



  $ git config sendemail.to perl5-porters@perl.org



Then you can use git directly to send your patch emails:



  $ git send-email 0001-Rename-Leon-Brocard-to-Orange-Brocard.patch



You may need to set some configuration variables for your particular
email service provider. For example, to set your global git config to
send email via a gmail account:



  $ git config --global sendemail.smtpserver smtp.gmail.com
  $ git config --global sendemail.smtpssl 1
  $ git config --global sendemail.smtpuser YOURUSERNAME@gmail.com



With this configuration, you will be prompted for your gmail password
when you run ’git send-email’. You can also configure
sendemail.smtppass with your password if you don’t care about having
your password in the .gitconfig file.









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26.4.5 A note on derived files



Be aware that many files in the distribution are derivative–avoid
patching them, because git won’t see the changes to them, and the build
process will overwrite them. Patch the originals instead. Most
utilities (like perldoc) are in this category, i.e. patch
utils/perldoc.PL rather than utils/perldoc. Similarly, don’t
create patches for files under $src_root/ext from their copies found in
$install_root/lib. If you are unsure about the proper location of a
file that may have gotten copied while building the source
distribution, consult the MANIFEST.









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26.4.6 Cleaning a working directory



The command git clean can with varying arguments be used as a
replacement for make clean.



To reset your working directory to a pristine condition you can do:



  % git clean -dxf



However, be aware this will delete ALL untracked content. You can use



  % git clean -Xf



to remove all ignored untracked files, such as build and test
byproduct, but leave any  manually created files alone.



If you only want to cancel some uncommitted edits, you can use git
checkout and give it a list of files to be reverted, or git checkout
-f to revert them all.



If you want to cancel one or several commits, you can use git reset.









Previous: perlgit Cleaning a working directory, Up: perlgit WORKING WITH THE REPOSITORY   [Contents]





26.4.7 Bisecting



git provides a built-in way to determine which commit should be blamed
for introducing a given bug. git bisect performs a binary search of
history to locate the first failing commit. It is fast, powerful and
flexible, but requires some setup and to automate the process an auxiliary
shell script is needed.



The core provides a wrapper program, Porting/bisect.pl, which attempts to
simplify as much as possible, making bisecting as simple as running a Perl
one-liner. For example, if you want to know when this became an error:



    perl -e 'my $a := 2'



you simply run this:



    .../Porting/bisect.pl -e 'my $a := 2;'



Using bisect.pl, with one command (and no other files) it’s easy to find
out





usually without needing to know which versions of perl to use as start and
end revisions, as bisect.pl automatically searches to find the earliest
stable version for which the test case passes. Run
Porting/bisect.pl --help for the full documentation, including how to
set the Configure and build time options.



If you require more flexibility than Porting/bisect.pl has to offer, you’ll
need to run git bisect yourself. It’s most useful to use git bisect run
to automate the building and testing of perl revisions. For this you’ll need
a shell script for git to call to test a particular revision. An example
script is Porting/bisect-example.sh, which you should copy outside of
the repository, as the bisect process will reset the state to a clean checkout
as it runs. The instructions below assume that you copied it as ~/run and
then edited it as appropriate.



You first enter in bisect mode with:



  % git bisect start



For example, if the bug is present on HEAD but wasn’t in 5.10.0,
git will learn about this when you enter:



  % git bisect bad
  % git bisect good perl-5.10.0
  Bisecting: 853 revisions left to test after this



This results in checking out the median commit between HEAD and
perl-5.10.0. You can then run the bisecting process with:



  % git bisect run ~/run



When the first bad commit is isolated, git bisect will tell you so:



  ca4cfd28534303b82a216cfe83a1c80cbc3b9dc5 is first bad commit
  commit ca4cfd28534303b82a216cfe83a1c80cbc3b9dc5
  Author: Dave Mitchell <davem@fdisolutions.com>
  Date:   Sat Feb 9 14:56:23 2008 +0000

      [perl #49472] Attributes + Unknown Error
      ...

  bisect run success



You can peek into the bisecting process with git bisect log and
git bisect visualize. git bisect reset will get you out of bisect
mode.



Please note that the first good state must be an ancestor of the
first bad state. If you want to search for the commit that solved
some bug, you have to negate your test case (i.e. exit with 1 if OK
and 0 if not) and still mark the lower bound as good and the
upper as bad. The "first bad commit" has then to be understood as
the "first commit where the bug is solved".



git help bisect has much more information on how you can tweak your
binary searches.









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26.5 Topic branches and rewriting history



Individual committers should create topic branches under
yourname/some_descriptive_name. Other committers should check
with a topic branch’s creator before making any change to it.



The simplest way to create a remote topic branch that works on all
versions of git is to push the current head as a new branch on the
remote, then check it out locally:



  $ branch="$yourname/$some_descriptive_name"
  $ git push origin HEAD:$branch
  $ git checkout -b $branch origin/$branch



Users of git 1.7 or newer can do it in a more obvious manner:



  $ branch="$yourname/$some_descriptive_name"
  $ git checkout -b $branch
  $ git push origin -u $branch



If you are not the creator of yourname/some_descriptive_name, you
might sometimes find that the original author has edited the branch’s
history. There are lots of good reasons for this. Sometimes, an author
might simply be rebasing the branch onto a newer source point.
Sometimes, an author might have found an error in an early commit which
they wanted to fix before merging the branch to blead.



Currently the master repository is configured to forbid
non-fast-forward merges. This means that the branches within can not be
rebased and pushed as a single step.



The only way you will ever be allowed to rebase or modify the history
of a pushed branch is to delete it and push it as a new branch under
the same name. Please think carefully about doing this. It may be
better to sequentially rename your branches so that it is easier for
others working with you to cherry-pick their local changes onto the new
version. (XXX: needs explanation).



If you want to rebase a personal topic branch, you will have to delete
your existing topic branch and push as a new version of it. You can do
this via the following formula (see the explanation about refspec’s
in the git push documentation for details) after you have rebased your
branch:



   # first rebase
   $ git checkout $user/$topic
   $ git fetch
   $ git rebase origin/blead

   # then "delete-and-push"
   $ git push origin :$user/$topic
   $ git push origin $user/$topic



NOTE: it is forbidden at the repository level to delete any of the
"primary" branches. That is any branch matching
m!^(blead|maint|perl)!. Any attempt to do so will result in git
producing an error like this:



    $ git push origin :blead
    *** It is forbidden to delete blead/maint branches in this repository
    error: hooks/update exited with error code 1
    error: hook declined to update refs/heads/blead
    To ssh://perl5.git.perl.org/perl
     ! [remote rejected] blead (hook declined)
     error: failed to push some refs to 'ssh://perl5.git.perl.org/perl'



As a matter of policy we do not edit the history of the blead and
maint-* branches. If a typo (or worse) sneaks into a commit to blead or
maint-*, we’ll fix it in another commit. The only types of updates
allowed on these branches are "fast-forward’s", where all history is
preserved.



Annotated tags in the canonical perl.git repository will never be
deleted or modified. Think long and hard about whether you want to push
a local tag to perl.git before doing so. (Pushing unannotated tags is
not allowed.)





• perlgit Grafts:  












Up: perlgit Topic branches and rewriting history   [Contents]





26.5.1 Grafts



The perl history contains one mistake which was not caught in the
conversion: a merge was recorded in the history between blead and
maint-5.10 where no merge actually occurred. Due to the nature of git,
this is now impossible to fix in the public repository. You can remove
this mis-merge locally by adding the following line to your
.git/info/grafts file:



  296f12bbbbaa06de9be9d09d3dcf8f4528898a49 434946e0cb7a32589ed92d18008aaa1d88515930



It is particularly important to have this graft line if any bisecting
is done in the area of the "merge" in question.









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26.6 WRITE ACCESS TO THE GIT REPOSITORY



Once you have write access, you will need to modify the URL for the
origin remote to enable pushing. Edit .git/config with the
git-config(1) command:



  % git config remote.origin.url ssh://perl5.git.perl.org/perl.git



You can also set up your user name and e-mail address. Most people do
this once globally in their ~/.gitconfig by doing something like:



  % git config --global user.name "Ævar Arnfjörð Bjarmason"
  % git config --global user.email avarab@gmail.com



However if you’d like to override that just for perl then execute then
execute something like the following in perl:



  % git config user.email avar@cpan.org



It is also possible to keep origin as a git remote, and add a new
remote for ssh access:



  % git remote add camel perl5.git.perl.org:/perl.git



This allows you to update your local repository by pulling from
origin, which is faster and doesn’t require you to authenticate, and
to push your changes back with the camel remote:



  % git fetch camel
  % git push camel



The fetch command just updates the camel refs, as the objects
themselves should have been fetched when pulling from origin.









Previous: perlgit WRITE ACCESS TO THE GIT REPOSITORY, Up: perlgit   [Contents]





26.7 Accepting a patch



If you have received a patch file generated using the above section,
you should try out the patch.



First we need to create a temporary new branch for these changes and
switch into it:



  % git checkout -b experimental



Patches that were formatted by git format-patch are applied with
git am:



  % git am 0001-Rename-Leon-Brocard-to-Orange-Brocard.patch
  Applying Rename Leon Brocard to Orange Brocard



If just a raw diff is provided, it is also possible use this two-step
process:



  % git apply bugfix.diff
  % git commit -a -m "Some fixing" --author="That Guy <that.guy@internets.com>"



Now we can inspect the change:



  % git show HEAD
  commit b1b3dab48344cff6de4087efca3dbd63548ab5e2
  Author: Leon Brocard <acme@astray.com>
  Date:   Fri Dec 19 17:02:59 2008 +0000

    Rename Leon Brocard to Orange Brocard

  diff --git a/AUTHORS b/AUTHORS
  index 293dd70..722c93e 100644
  --- a/AUTHORS
  +++ b/AUTHORS
  @@ -541,7 +541,7 @@ Lars Hecking                        <lhecking@nmrc.ucc.ie>
   Laszlo Molnar                  <laszlo.molnar@eth.ericsson.se>
   Leif Huhn                      <leif@hale.dkstat.com>
   Len Johnson                    <lenjay@ibm.net>
  -Leon Brocard                   <acme@astray.com>
  +Orange Brocard                 <acme@astray.com>
   Les Peters                     <lpeters@aol.net>
   Lesley Binks                   <lesley.binks@gmail.com>
   Lincoln D. Stein               <lstein@cshl.org>



If you are a committer to Perl and you think the patch is good, you can
then merge it into blead then push it out to the main repository:



  % git checkout blead
  % git merge experimental
  % git push



If you want to delete your temporary branch, you may do so with:



  % git checkout blead
  % git branch -d experimental
  error: The branch 'experimental' is not an ancestor of your current HEAD.
  If you are sure you want to delete it, run 'git branch -D experimental'.
  % git branch -D experimental
  Deleted branch experimental.





• perlgit Committing to blead:  



• perlgit Committing to maintenance versions:  



• perlgit Merging from a branch via GitHub:  



• perlgit A note on camel and dromedary:  












Next: , Up: perlgit Accepting a patch   [Contents]





26.7.1 Committing to blead



The ’blead’ branch will become the next production release of Perl.



Before pushing any local change to blead, it’s incredibly important
that you do a few things, lest other committers come after you with
pitchforks and torches:







• perlgit On merging and rebasing:  












Up: perlgit Committing to blead   [Contents]





26.7.1.1 On merging and rebasing



Simple, one-off commits pushed to the ’blead’ branch should be simple
commits that apply cleanly.  In other words, you should make sure your
work is committed against the current position of blead, so that you can
push back to the master repository without merging.



Sometimes, blead will move while you’re building or testing your
changes.  When this happens, your push will be rejected with a message
like this:



  To ssh://perl5.git.perl.org/perl.git
   ! [rejected]        blead -> blead (non-fast-forward)
  error: failed to push some refs to 'ssh://perl5.git.perl.org/perl.git'
  To prevent you from losing history, non-fast-forward updates were rejected
  Merge the remote changes (e.g. 'git pull') before pushing again.  See the
  'Note about fast-forwards' section of 'git push --help' for details.



When this happens, you can just rebase your work against the new
position of blead, like this (assuming your remote for the master
repository is "p5p"):



  $ git fetch p5p
  $ git rebase p5p/blead



You will see your commits being re-applied, and you will then be able to
push safely.  More information about rebasing can be found in the
documentation for the git-rebase(1) command.



For larger sets of commits that only make sense together, or that would
benefit from a summary of the set’s purpose, you should use a merge
commit.  You should perform your work on a topic branch, which you should regularly rebase
against blead to ensure that your code is not broken by blead moving.
When you have finished your work, please perform a final rebase and
test.  Linear history is something that gets lost with every
commit on blead, but a final rebase makes the history linear
again, making it easier for future maintainers to see what has
happened.  Rebase as follows (assuming your work was on the
branch committer/somework):



  $ git checkout committer/somework
  $ git rebase blead



Then you can merge it into master like this:



  $ git checkout blead
  $ git merge --no-ff --no-commit committer/somework
  $ git commit -a



The switches above deserve explanation.  --no-ff indicates that even
if all your work can be applied linearly against blead, a merge commit
should still be prepared.  This ensures that all your work will be shown
as a side branch, with all its commits merged into the mainstream blead
by the merge commit.



--no-commit means that the merge commit will be prepared but not
committed.  The commit is then actually performed when you run the
next command, which will bring up your editor to describe the commit.
Without --no-commit, the commit would be made with nearly no useful
message, which would greatly diminish the value of the merge commit as a
placeholder for the work’s description.



When describing the merge commit, explain the purpose of the branch, and
keep in mind that this description will probably be used by the
eventual release engineer when reviewing the next perldelta document.









Next: , Previous: perlgit Committing to blead, Up: perlgit Accepting a patch   [Contents]





26.7.2 Committing to maintenance versions



Maintenance versions should only be altered to add critical bug fixes,
see perlpolicy NAME.



To commit to a maintenance version of perl, you need to create a local
tracking branch:



  % git checkout --track -b maint-5.005 origin/maint-5.005



This creates a local branch named maint-5.005, which tracks the
remote branch origin/maint-5.005. Then you can pull, commit, merge
and push as before.



You can also cherry-pick commits from blead and another branch, by
using the git cherry-pick command. It is recommended to use the
-x option to git cherry-pick in order to record the SHA1 of the
original commit in the new commit message.



Before pushing any change to a maint version, make sure you’ve
satisfied the steps in Committing to blead above.









Next: , Previous: perlgit Committing to maintenance versions, Up: perlgit Accepting a patch   [Contents]





26.7.3 Merging from a branch via GitHub



While we don’t encourage the submission of patches via GitHub, that
will still happen. Here is a guide to merging patches from a GitHub
repository.



  % git remote add avar git://github.com/avar/perl.git
  % git fetch avar



Now you can see the differences between the branch and blead:



  % git diff avar/orange



And you can see the commits:



  % git log avar/orange



If you approve of a specific commit, you can cherry pick it:



  % git cherry-pick 0c24b290ae02b2ab3304f51d5e11e85eb3659eae



Or you could just merge the whole branch if you like it all:



  % git merge avar/orange



And then push back to the repository:



  % git push









Previous: perlgit Merging from a branch via GitHub, Up: perlgit Accepting a patch   [Contents]





26.7.4 A note on camel and dromedary



The committers have SSH access to the two servers that serve
perl5.git.perl.org. One is perl5.git.perl.org itself (camel),
which is the ’master’ repository. The second one is
users.perl5.git.perl.org (dromedary), which can be used for
general testing and development. Dromedary syncs the git tree from
camel every few minutes, you should not push there. Both machines also
have a full CPAN mirror in /srv/CPAN, please use this. To share files
with the general public, dromedary serves your ~/public_html/ as
http://users.perl5.git.perl.org/~yourlogin/



These hosts have fairly strict firewalls to the outside. Outgoing, only
rsync, ssh and git are allowed. For http and ftp, you can use
http://webproxy:3128 as proxy. Incoming, the firewall tries to detect
attacks and blocks IP addresses with suspicious activity. This
sometimes (but very rarely) has false positives and you might get
blocked. The quickest way to get unblocked is to notify the admins.



These two boxes are owned, hosted, and operated by booking.com. You can
reach the sysadmins in #p5p on irc.perl.org or via mail to
perl5-porters@perl.org.









Next: , Previous: perlgit, Up: Top   [Contents]





27 perlgpl





• perlgpl NAME:  



• perlgpl SYNOPSIS:  



• perlgpl DESCRIPTION:  



• perlgpl GNU GENERAL PUBLIC LICENSE:  












Next: , Up: perlgpl   [Contents]





27.1 NAME



perlgpl - the GNU General Public License, version 1









Next: , Previous: perlgpl NAME, Up: perlgpl   [Contents]





27.2 SYNOPSIS



 You can refer to this document in Pod via "L<perlgpl>"
 Or you can see this document by entering "perldoc perlgpl"









Next: , Previous: perlgpl SYNOPSIS, Up: perlgpl   [Contents]





27.3 DESCRIPTION



Perl is free software; you can redistribute it and/or modify
it under the terms of either:



        a) the GNU General Public License as published by the Free
        Software Foundation; either version 1, or (at your option) any
        later version, or

        b) the "Artistic License" which comes with this Kit.



This is the "GNU General Public License, version 1".
It’s here so that modules, programs, etc., that want to declare
this as their distribution license can link to it.



For the Perl Artistic License, see perlartistic NAME.









Previous: perlgpl DESCRIPTION, Up: perlgpl   [Contents]





27.4 GNU GENERAL PUBLIC LICENSE



                    GNU GENERAL PUBLIC LICENSE
                     Version 1, February 1989
 
  Copyright (C) 1989 Free Software Foundation, Inc.
                     51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 
  Everyone is permitted to copy and distribute verbatim copies
  of this license document, but changing it is not allowed.
 
                            Preamble
 
   The license agreements of most software companies try to keep users
 at the mercy of those companies.  By contrast, our General Public
 License is intended to guarantee your freedom to share and change free
 software--to make sure the software is free for all its users.  The
 General Public License applies to the Free Software Foundation's
 software and to any other program whose authors commit to using it.
 You can use it for your programs, too.
 
   When we speak of free software, we are referring to freedom, not
 price.  Specifically, the General Public License is designed to make
 sure that you have the freedom to give away or sell copies of free
 software, that you receive source code or can get it if you want it,
 that you can change the software or use pieces of it in new free
 programs; and that you know you can do these things.
 
   To protect your rights, we need to make restrictions that forbid
 anyone to deny you these rights or to ask you to surrender the rights.
 These restrictions translate to certain responsibilities for you if you
 distribute copies of the software, or if you modify it.
 
   For example, if you distribute copies of a such a program, whether
 gratis or for a fee, you must give the recipients all the rights that
 you have.  You must make sure that they, too, receive or can get the
 source code.  And you must tell them their rights.
 
   We protect your rights with two steps: (1) copyright the software, and
 (2) offer you this license which gives you legal permission to copy,
 distribute and/or modify the software.
 
   Also, for each author's protection and ours, we want to make certain
 that everyone understands that there is no warranty for this free
 software.  If the software is modified by someone else and passed on, we
 want its recipients to know that what they have is not the original, so
 that any problems introduced by others will not reflect on the original
 authors' reputations.
 
   The precise terms and conditions for copying, distribution and
 modification follow.
  
                    GNU GENERAL PUBLIC LICENSE
    TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
 
   0. This License Agreement applies to any program or other work which
 contains a notice placed by the copyright holder saying it may be
 distributed under the terms of this General Public License.  The
 "Program", below, refers to any such program or work, and a "work based
 on the Program" means either the Program or any work containing the
 Program or a portion of it, either verbatim or with modifications.  Each
 licensee is addressed as "you".
 
   1. You may copy and distribute verbatim copies of the Program's source
 code as you receive it, in any medium, provided that you conspicuously and
 appropriately publish on each copy an appropriate copyright notice and
 disclaimer of warranty; keep intact all the notices that refer to this
 General Public License and to the absence of any warranty; and give any
 other recipients of the Program a copy of this General Public License
 along with the Program.  You may charge a fee for the physical act of
 transferring a copy.
 
   2. You may modify your copy or copies of the Program or any portion of
 it, and copy and distribute such modifications under the terms of Paragraph
 1 above, provided that you also do the following:
 
     a) cause the modified files to carry prominent notices stating that
     you changed the files and the date of any change; and
 
     b) cause the whole of any work that you distribute or publish, that
     in whole or in part contains the Program or any part thereof, either
     with or without modifications, to be licensed at no charge to all
     third parties under the terms of this General Public License (except
     that you may choose to grant warranty protection to some or all
     third parties, at your option).
 
     c) If the modified program normally reads commands interactively when
     run, you must cause it, when started running for such interactive use
     in the simplest and most usual way, to print or display an
     announcement including an appropriate copyright notice and a notice
     that there is no warranty (or else, saying that you provide a
     warranty) and that users may redistribute the program under these
     conditions, and telling the user how to view a copy of this General
     Public License.
 
     d) You may charge a fee for the physical act of transferring a
     copy, and you may at your option offer warranty protection in
     exchange for a fee.
 
 Mere aggregation of another independent work with the Program (or its
 derivative) on a volume of a storage or distribution medium does not bring
 the other work under the scope of these terms.
  
   3. You may copy and distribute the Program (or a portion or derivative of
 it, under Paragraph 2) in object code or executable form under the terms of
 Paragraphs 1 and 2 above provided that you also do one of the following:
 
     a) accompany it with the complete corresponding machine-readable
     source code, which must be distributed under the terms of
     Paragraphs 1 and 2 above; or,
 
     b) accompany it with a written offer, valid for at least three
     years, to give any third party free (except for a nominal charge
     for the cost of distribution) a complete machine-readable copy of the
     corresponding source code, to be distributed under the terms of
     Paragraphs 1 and 2 above; or,
 
     c) accompany it with the information you received as to where the
     corresponding source code may be obtained.  (This alternative is
     allowed only for noncommercial distribution and only if you
     received the program in object code or executable form alone.)
 
 Source code for a work means the preferred form of the work for making
 modifications to it.  For an executable file, complete source code means
 all the source code for all modules it contains; but, as a special
 exception, it need not include source code for modules which are standard
 libraries that accompany the operating system on which the executable
 file runs, or for standard header files or definitions files that
 accompany that operating system.
 
   4. You may not copy, modify, sublicense, distribute or transfer the
 Program except as expressly provided under this General Public License.
 Any attempt otherwise to copy, modify, sublicense, distribute or transfer
 the Program is void, and will automatically terminate your rights to use
 the Program under this License.  However, parties who have received
 copies, or rights to use copies, from you under this General Public
 License will not have their licenses terminated so long as such parties
 remain in full compliance.
 
   5. By copying, distributing or modifying the Program (or any work based
 on the Program) you indicate your acceptance of this license to do so,
 and all its terms and conditions.
 
   6. Each time you redistribute the Program (or any work based on the
 Program), the recipient automatically receives a license from the original
 licensor to copy, distribute or modify the Program subject to these
 terms and conditions.  You may not impose any further restrictions on the
 recipients' exercise of the rights granted herein.
  
   7. The Free Software Foundation may publish revised and/or new versions
 of the General Public License from time to time.  Such new versions will
 be similar in spirit to the present version, but may differ in detail to
 address new problems or concerns.
 
 Each version is given a distinguishing version number.  If the Program
 specifies a version number of the license which applies to it and "any
 later version", you have the option of following the terms and conditions
 either of that version or of any later version published by the Free
 Software Foundation.  If the Program does not specify a version number of
 the license, you may choose any version ever published by the Free Software
 Foundation.
 
   8. If you wish to incorporate parts of the Program into other free
 programs whose distribution conditions are different, write to the author
 to ask for permission.  For software which is copyrighted by the Free
 Software Foundation, write to the Free Software Foundation; we sometimes
 make exceptions for this.  Our decision will be guided by the two goals
 of preserving the free status of all derivatives of our free software and
 of promoting the sharing and reuse of software generally.
 
                            NO WARRANTY
 
   9. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
 FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
 OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
 PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
 OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
 TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
 PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
 REPAIR OR CORRECTION.
 
   10. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
 WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
 REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
 INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
 OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
 TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
 YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
 PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGES.
 
                     END OF TERMS AND CONDITIONS
 
        Appendix: How to Apply These Terms to Your New Programs
 
   If you develop a new program, and you want it to be of the greatest
 possible use to humanity, the best way to achieve this is to make it
 free software which everyone can redistribute and change under these
 terms.
 
   To do so, attach the following notices to the program.  It is safest to
 attach them to the start of each source file to most effectively convey
 the exclusion of warranty; and each file should have at least the
 "copyright" line and a pointer to where the full notice is found.
 
     <one line to give the program's name and a brief idea of what it does.>
     Copyright (C) 19yy  <name of author>
 
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation; either version 1, or (at your option)
     any later version.
 
     This program is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU General Public License for more details.
 
     You should have received a copy of the GNU General Public License
     along with this program; if not, write to the Free Software
     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA
     02110-1301 USA
 
 
 Also add information on how to contact you by electronic and paper mail.
 
 If the program is interactive, make it output a short notice like this
 when it starts in an interactive mode:
 
     Gnomovision version 69, Copyright (C) 19xx name of author
     Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type 'show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type 'show c' for details.
 
 The hypothetical commands 'show w' and 'show c' should show the
 appropriate parts of the General Public License.  Of course, the
 commands you use may be called something other than 'show w' and 'show
 c'; they could even be mouse-clicks or menu items--whatever suits your
 program.
 
 You should also get your employer (if you work as a programmer) or your
 school, if any, to sign a "copyright disclaimer" for the program, if
 necessary.  Here a sample; alter the names:
 
   Yoyodyne, Inc., hereby disclaims all copyright interest in the
   program 'Gnomovision' (a program to direct compilers to make passes
   at assemblers) written by James Hacker.
 
   <signature of Ty Coon>, 1 April 1989
   Ty Coon, President of Vice
 
 That's all there is to it!









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28 perlguts





• perlguts NAME:  



• perlguts DESCRIPTION:  



• perlguts Variables:  



• perlguts Subroutines:  



• perlguts Compiled code:  



• perlguts Examining internal data structures with the dump functions:  



• perlguts How multiple interpreters and concurrency are supported:  



• perlguts Internal Functions:  



• perlguts Unicode Support:  



• perlguts Custom Operators:  



• perlguts AUTHORS:  



• perlguts SEE ALSO:  












Next: , Up: perlguts   [Contents]





28.1 NAME



perlguts - Introduction to the Perl API









Next: , Previous: perlguts NAME, Up: perlguts   [Contents]





28.2 DESCRIPTION



This document attempts to describe how to use the Perl API, as well as
to provide some info on the basic workings of the Perl core. It is far
from complete and probably contains many errors. Please refer any
questions or comments to the author below.









Next: , Previous: perlguts DESCRIPTION, Up: perlguts   [Contents]





28.3 Variables





• perlguts Datatypes:  



• perlguts What is an "IV"?:  



• perlguts Working with SVs:  



• perlguts Offsets:  



• perlguts What's Really Stored in an SV?:  



• perlguts Working with AVs:  



• perlguts Working with HVs:  



• perlguts Hash API Extensions:  



• perlguts AVs, HVs and undefined values:  



• perlguts References:  



• perlguts Blessed References and Class Objects:  



• perlguts Creating New Variables:  



• perlguts Reference Counts and Mortality:  



• perlguts Stashes and Globs:  



• perlguts Double-Typed SVs:  



• perlguts Magic Variables:  



• perlguts Assigning Magic:  



• perlguts Magic Virtual Tables:  



• perlguts Finding Magic:  



• perlguts Understanding the Magic of Tied Hashes and Arrays:  



• perlguts Localizing changes:  












Next: , Up: perlguts Variables   [Contents]





28.3.1 Datatypes



Perl has three typedefs that handle Perl’s three main data types:



    SV  Scalar Value
    AV  Array Value
    HV  Hash Value



Each typedef has specific routines that manipulate the various data types.









Next: , Previous: perlguts Datatypes, Up: perlguts Variables   [Contents]





28.3.2 What is an "IV"?



Perl uses a special typedef IV which is a simple signed integer type that is
guaranteed to be large enough to hold a pointer (as well as an integer).
Additionally, there is the UV, which is simply an unsigned IV.



Perl also uses two special typedefs, I32 and I16, which will always be at
least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
as well.)  They will usually be exactly 32 and 16 bits long, but on Crays
they will both be 64 bits.









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28.3.3 Working with SVs



An SV can be created and loaded with one command.  There are five types of
values that can be loaded: an integer value (IV), an unsigned integer
value (UV), a double (NV), a string (PV), and another scalar (SV).



The seven routines are:



    SV*  newSViv(IV);
    SV*  newSVuv(UV);
    SV*  newSVnv(double);
    SV*  newSVpv(const char*, STRLEN);
    SV*  newSVpvn(const char*, STRLEN);
    SV*  newSVpvf(const char*, ...);
    SV*  newSVsv(SV*);



STRLEN is an integer type (Size_t, usually defined as size_t in
config.h) guaranteed to be large enough to represent the size of
any string that perl can handle.



In the unlikely case of a SV requiring more complex initialisation, you
can create an empty SV with newSV(len).  If len is 0 an empty SV of
type NULL is returned, else an SV of type PV is returned with len + 1 (for
the NUL) bytes of storage allocated, accessible via SvPVX.  In both cases
the SV has the undef value.



    SV *sv = newSV(0);   /* no storage allocated  */
    SV *sv = newSV(10);  /* 10 (+1) bytes of uninitialised storage
                          * allocated */



To change the value of an already-existing SV, there are eight routines:



    void  sv_setiv(SV*, IV);
    void  sv_setuv(SV*, UV);
    void  sv_setnv(SV*, double);
    void  sv_setpv(SV*, const char*);
    void  sv_setpvn(SV*, const char*, STRLEN)
    void  sv_setpvf(SV*, const char*, ...);
    void  sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
                                                    SV **, I32, bool *);
    void  sv_setsv(SV*, SV*);



Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn, newSVpvn, or newSVpv, or you may
allow Perl to calculate the length by using sv_setpv or by specifying
0 as the second argument to newSVpv.  Be warned, though, that Perl will
determine the string’s length by using strlen, which depends on the
string terminating with a NUL character, and not otherwise containing
NULs.



The arguments of sv_setpvf are processed like sprintf, and the
formatted output becomes the value.



sv_vsetpvfn is an analogue of vsprintf, but it allows you to specify
either a pointer to a variable argument list or the address and length of
an array of SVs.  The last argument points to a boolean; on return, if that
boolean is true, then locale-specific information has been used to format
the string, and the string’s contents are therefore untrustworthy (see
perlsec NAME).  This pointer may be NULL if that information is not
important.  Note that this function requires you to specify the length of
the format.



The sv_set*() functions are not generic enough to operate on values
that have "magic".  See Magic Virtual Tables later in this document.



All SVs that contain strings should be terminated with a NUL character.
If it is not NUL-terminated there is a risk of
core dumps and corruptions from code which passes the string to C
functions or system calls which expect a NUL-terminated string.
Perl’s own functions typically add a trailing NUL for this reason.
Nevertheless, you should be very careful when you pass a string stored
in an SV to a C function or system call.



To access the actual value that an SV points to, you can use the macros:



    SvIV(SV*)
    SvUV(SV*)
    SvNV(SV*)
    SvPV(SV*, STRLEN len)
    SvPV_nolen(SV*)



which will automatically coerce the actual scalar type into an IV, UV, double,
or string.



In the SvPV macro, the length of the string returned is placed into the
variable len (this is a macro, so you do not use &len).  If you do
not care what the length of the data is, use the SvPV_nolen macro.
Historically the SvPV macro with the global variable PL_na has been
used in this case.  But that can be quite inefficient because PL_na must
be accessed in thread-local storage in threaded Perl.  In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL.



Also remember that C doesn’t allow you to safely say foo(SvPV(s, len),
len);. It might work with your compiler, but it won’t work for everyone.
Break this sort of statement up into separate assignments:



    SV *s;
    STRLEN len;
    char *ptr;
    ptr = SvPV(s, len);
    foo(ptr, len);



If you want to know if the scalar value is TRUE, you can use:



    SvTRUE(SV*)



Although Perl will automatically grow strings for you, if you need to force
Perl to allocate more memory for your SV, you can use the macro



    SvGROW(SV*, STRLEN newlen)



which will determine if more memory needs to be allocated.  If so, it will
call the function sv_grow.  Note that SvGROW can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
add space for the trailing NUL byte (perl’s own string functions typically do
SvGROW(sv, len + 1)).



If you have an SV and want to know what kind of data Perl thinks is stored
in it, you can use the following macros to check the type of SV you have.



    SvIOK(SV*)
    SvNOK(SV*)
    SvPOK(SV*)



You can get and set the current length of the string stored in an SV with
the following macros:



    SvCUR(SV*)
    SvCUR_set(SV*, I32 val)



You can also get a pointer to the end of the string stored in the SV
with the macro:



    SvEND(SV*)



But note that these last three macros are valid only if SvPOK() is true.



If you want to append something to the end of string stored in an SV*,
you can use the following functions:



    void  sv_catpv(SV*, const char*);
    void  sv_catpvn(SV*, const char*, STRLEN);
    void  sv_catpvf(SV*, const char*, ...);
    void  sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
                                                             I32, bool);
    void  sv_catsv(SV*, SV*);



The first function calculates the length of the string to be appended by
using strlen.  In the second, you specify the length of the string
yourself.  The third function processes its arguments like sprintf and
appends the formatted output.  The fourth function works like vsprintf.
You can specify the address and length of an array of SVs instead of the
va_list argument. The fifth function extends the string stored in the first
SV with the string stored in the second SV.  It also forces the second SV
to be interpreted as a string.



The sv_cat*() functions are not generic enough to operate on values that
have "magic".  See Magic Virtual Tables later in this document.



If you know the name of a scalar variable, you can get a pointer to its SV
by using the following:



    SV*  get_sv("package::varname", 0);



This returns NULL if the variable does not exist.



If you want to know if this variable (or any other SV) is actually defined,
you can call:



    SvOK(SV*)



The scalar undef value is stored in an SV instance called PL_sv_undef.



Its address can be used whenever an SV* is needed. Make sure that
you don’t try to compare a random sv with &PL_sv_undef. For example
when interfacing Perl code, it’ll work correctly for:



  foo(undef);



But won’t work when called as:



  $x = undef;
  foo($x);



So to repeat always use SvOK() to check whether an sv is defined.



Also you have to be careful when using &PL_sv_undef as a value in
AVs or HVs (see AVs, HVs and undefined values).



There are also the two values PL_sv_yes and PL_sv_no, which contain
boolean TRUE and FALSE values, respectively.  Like PL_sv_undef, their
addresses can be used whenever an SV* is needed.



Do not be fooled into thinking that (SV *) 0 is the same as &PL_sv_undef.
Take this code:



    SV* sv = (SV*) 0;
    if (I-am-to-return-a-real-value) {
            sv = sv_2mortal(newSViv(42));
    }
    sv_setsv(ST(0), sv);



This code tries to return a new SV (which contains the value 42) if it should
return a real value, or undef otherwise.  Instead it has returned a NULL
pointer which, somewhere down the line, will cause a segmentation violation,
bus error, or just weird results.  Change the zero to &PL_sv_undef in the
first line and all will be well.



To free an SV that you’ve created, call SvREFCNT_dec(SV*).  Normally this
call is not necessary (see Reference Counts and Mortality).









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28.3.4 Offsets



Perl provides the function sv_chop to efficiently remove characters
from the beginning of a string; you give it an SV and a pointer to
somewhere inside the PV, and it discards everything before the
pointer. The efficiency comes by means of a little hack: instead of
actually removing the characters, sv_chop sets the flag OOK
(offset OK) to signal to other functions that the offset hack is in
effect, and it puts the number of bytes chopped off into the IV field
of the SV. It then moves the PV pointer (called SvPVX) forward that
many bytes, and adjusts SvCUR and SvLEN.



Hence, at this point, the start of the buffer that we allocated lives
at SvPVX(sv) - SvIV(sv) in memory and the PV pointer is pointing
into the middle of this allocated storage.



This is best demonstrated by example:



  % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
  SV = PVIV(0x8128450) at 0x81340f0
    REFCNT = 1
    FLAGS = (POK,OOK,pPOK)
    IV = 1  (OFFSET)
    PV = 0x8135781 ( "1" . ) "2345"\0
    CUR = 4
    LEN = 5



Here the number of bytes chopped off (1) is put into IV, and
Devel::Peek::Dump helpfully reminds us that this is an offset. The
portion of the string between the "real" and the "fake" beginnings is
shown in parentheses, and the values of SvCUR and SvLEN reflect
the fake beginning, not the real one.



Something similar to the offset hack is performed on AVs to enable
efficient shifting and splicing off the beginning of the array; while
AvARRAY points to the first element in the array that is visible from
Perl, AvALLOC points to the real start of the C array. These are
usually the same, but a shift operation can be carried out by
increasing AvARRAY by one and decreasing AvFILL and AvMAX.
Again, the location of the real start of the C array only comes into
play when freeing the array. See av_shift in av.c.









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28.3.5 What’s Really Stored in an SV?



Recall that the usual method of determining the type of scalar you have is
to use Sv*OK macros.  Because a scalar can be both a number and a string,
usually these macros will always return TRUE and calling the Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.



If you really need to know if you have an integer, double, or string
pointer in an SV, you can use the following three macros instead:



    SvIOKp(SV*)
    SvNOKp(SV*)
    SvPOKp(SV*)



These will tell you if you truly have an integer, double, or string pointer
stored in your SV.  The "p" stands for private.



There are various ways in which the private and public flags may differ.
For example, a tied SV may have a valid underlying value in the IV slot
(so SvIOKp is true), but the data should be accessed via the FETCH
routine rather than directly, so SvIOK is false. Another is when
numeric conversion has occurred and precision has been lost: only the
private flag is set on ’lossy’ values. So when an NV is converted to an
IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.



In general, though, it’s best to use the Sv*V macros.









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28.3.6 Working with AVs



There are two ways to create and load an AV.  The first method creates an
empty AV:



    AV*  newAV();



The second method both creates the AV and initially populates it with SVs:



    AV*  av_make(I32 num, SV **ptr);



The second argument points to an array containing num SV*’s.  Once the
AV has been created, the SVs can be destroyed, if so desired.



Once the AV has been created, the following operations are possible on it:



    void  av_push(AV*, SV*);
    SV*   av_pop(AV*);
    SV*   av_shift(AV*);
    void  av_unshift(AV*, I32 num);



These should be familiar operations, with the exception of av_unshift.
This routine adds num elements at the front of the array with the undef
value.  You must then use av_store (described below) to assign values
to these new elements.



Here are some other functions:



    I32   av_len(AV*);
    SV**  av_fetch(AV*, I32 key, I32 lval);
    SV**  av_store(AV*, I32 key, SV* val);



The av_len function returns the highest index value in an array (just
like $#array in Perl).  If the array is empty, -1 is returned.  The
av_fetch function returns the value at index key, but if lval
is non-zero, then av_fetch will store an undef value at that index.
The av_store function stores the value val at index key, and does
not increment the reference count of val.  Thus the caller is responsible
for taking care of that, and if av_store returns NULL, the caller will
have to decrement the reference count to avoid a memory leak.  Note that
av_fetch and av_store both return SV**’s, not SV*’s as their
return value.



A few more:



    void  av_clear(AV*);
    void  av_undef(AV*);
    void  av_extend(AV*, I32 key);



The av_clear function deletes all the elements in the AV* array, but
does not actually delete the array itself.  The av_undef function will
delete all the elements in the array plus the array itself.  The
av_extend function extends the array so that it contains at least key+1
elements.  If key+1 is less than the currently allocated length of the array,
then nothing is done.



If you know the name of an array variable, you can get a pointer to its AV
by using the following:



    AV*  get_av("package::varname", 0);



This returns NULL if the variable does not exist.



See Understanding the Magic of Tied Hashes and Arrays for more
information on how to use the array access functions on tied arrays.









Next: , Previous: perlguts Working with AVs, Up: perlguts Variables   [Contents]





28.3.7 Working with HVs



To create an HV, you use the following routine:



    HV*  newHV();



Once the HV has been created, the following operations are possible on it:



    SV**  hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
    SV**  hv_fetch(HV*, const char* key, U32 klen, I32 lval);



The klen parameter is the length of the key being passed in (Note that
you cannot pass 0 in as a value of klen to tell Perl to measure the
length of the key).  The val argument contains the SV pointer to the
scalar being stored, and hash is the precomputed hash value (zero if
you want hv_store to calculate it for you).  The lval parameter
indicates whether this fetch is actually a part of a store operation, in
which case a new undefined value will be added to the HV with the supplied
key and hv_fetch will return as if the value had already existed.



Remember that hv_store and hv_fetch return SV**’s and not just
SV*.  To access the scalar value, you must first dereference the return
value.  However, you should check to make sure that the return value is
not NULL before dereferencing it.



The first of these two functions checks if a hash table entry exists, and the 
second deletes it.



    bool  hv_exists(HV*, const char* key, U32 klen);
    SV*   hv_delete(HV*, const char* key, U32 klen, I32 flags);



If flags does not include the G_DISCARD flag then hv_delete will
create and return a mortal copy of the deleted value.



And more miscellaneous functions:



    void   hv_clear(HV*);
    void   hv_undef(HV*);



Like their AV counterparts, hv_clear deletes all the entries in the hash
table but does not actually delete the hash table.  The hv_undef deletes
both the entries and the hash table itself.



Perl keeps the actual data in a linked list of structures with a typedef of HE.
These contain the actual key and value pointers (plus extra administrative
overhead).  The key is a string pointer; the value is an SV*.  However,
once you have an HE*, to get the actual key and value, use the routines
specified below.



    I32    hv_iterinit(HV*);
            /* Prepares starting point to traverse hash table */
    HE*    hv_iternext(HV*);
            /* Get the next entry, and return a pointer to a
               structure that has both the key and value */
    char*  hv_iterkey(HE* entry, I32* retlen);
            /* Get the key from an HE structure and also return
               the length of the key string */
    SV*    hv_iterval(HV*, HE* entry);
            /* Return an SV pointer to the value of the HE
               structure */
    SV*    hv_iternextsv(HV*, char** key, I32* retlen);
            /* This convenience routine combines hv_iternext,
               hv_iterkey, and hv_iterval.  The key and retlen
               arguments are return values for the key and its
               length.  The value is returned in the SV* argument */



If you know the name of a hash variable, you can get a pointer to its HV
by using the following:



    HV*  get_hv("package::varname", 0);



This returns NULL if the variable does not exist.



The hash algorithm is defined in the PERL_HASH(hash, key, klen) macro:



    hash = 0;
    while (klen--)
        hash = (hash * 33) + *key++;
    hash = hash + (hash >> 5);                  /* after 5.6 */



The last step was added in version 5.6 to improve distribution of
lower bits in the resulting hash value.



See Understanding the Magic of Tied Hashes and Arrays for more
information on how to use the hash access functions on tied hashes.









Next: , Previous: perlguts Working with HVs, Up: perlguts Variables   [Contents]





28.3.8 Hash API Extensions



Beginning with version 5.004, the following functions are also supported:



    HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
    HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);

    bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
    SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);

    SV*     hv_iterkeysv  (HE* entry);



Note that these functions take SV* keys, which simplifies writing
of extension code that deals with hash structures.  These functions
also allow passing of SV* keys to tie functions without forcing
you to stringify the keys (unlike the previous set of functions).



They also return and accept whole hash entries (HE*), making their
use more efficient (since the hash number for a particular string
doesn’t have to be recomputed every time).  See (perlapi) for detailed
descriptions.



The following macros must always be used to access the contents of hash
entries.  Note that the arguments to these macros must be simple
variables, since they may get evaluated more than once.  See
(perlapi) for detailed descriptions of these macros.



    HePV(HE* he, STRLEN len)
    HeVAL(HE* he)
    HeHASH(HE* he)
    HeSVKEY(HE* he)
    HeSVKEY_force(HE* he)
    HeSVKEY_set(HE* he, SV* sv)



These two lower level macros are defined, but must only be used when
dealing with keys that are not SV*s:



    HeKEY(HE* he)
    HeKLEN(HE* he)



Note that both hv_store and hv_store_ent do not increment the
reference count of the stored val, which is the caller’s responsibility.
If these functions return a NULL value, the caller will usually have to
decrement the reference count of val to avoid a memory leak.









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28.3.9 AVs, HVs and undefined values



Sometimes you have to store undefined values in AVs or HVs. Although
this may be a rare case, it can be tricky. That’s because you’re
used to using &PL_sv_undef if you need an undefined SV.



For example, intuition tells you that this XS code:



    AV *av = newAV();
    av_store( av, 0, &PL_sv_undef );



is equivalent to this Perl code:



    my @av;
    $av[0] = undef;



Unfortunately, this isn’t true. AVs use &PL_sv_undef as a marker
for indicating that an array element has not yet been initialized.
Thus, exists $av[0] would be true for the above Perl code, but
false for the array generated by the XS code.



Other problems can occur when storing &PL_sv_undef in HVs:



    hv_store( hv, "key", 3, &PL_sv_undef, 0 );



This will indeed make the value undef, but if you try to modify
the value of key, you’ll get the following error:



    Modification of non-creatable hash value attempted



In perl 5.8.0, &PL_sv_undef was also used to mark placeholders
in restricted hashes. This caused such hash entries not to appear
when iterating over the hash or when checking for the keys
with the hv_exists function.



You can run into similar problems when you store &PL_sv_yes or
&PL_sv_no into AVs or HVs. Trying to modify such elements
will give you the following error:



    Modification of a read-only value attempted



To make a long story short, you can use the special variables
&PL_sv_undef, &PL_sv_yes and &PL_sv_no with AVs and
HVs, but you have to make sure you know what you’re doing.



Generally, if you want to store an undefined value in an AV
or HV, you should not use &PL_sv_undef, but rather create a
new undefined value using the newSV function, for example:



    av_store( av, 42, newSV(0) );
    hv_store( hv, "foo", 3, newSV(0), 0 );









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28.3.10 References



References are a special type of scalar that point to other data types
(including other references).



To create a reference, use either of the following functions:



    SV* newRV_inc((SV*) thing);
    SV* newRV_noinc((SV*) thing);



The thing argument can be any of an SV*, AV*, or HV*.  The
functions are identical except that newRV_inc increments the reference
count of the thing, while newRV_noinc does not.  For historical
reasons, newRV is a synonym for newRV_inc.



Once you have a reference, you can use the following macro to dereference
the reference:



    SvRV(SV*)



then call the appropriate routines, casting the returned SV* to either an
AV* or HV*, if required.



To determine if an SV is a reference, you can use the following macro:



    SvROK(SV*)



To discover what type of value the reference refers to, use the following
macro and then check the return value.



    SvTYPE(SvRV(SV*))



The most useful types that will be returned are:



    SVt_IV    Scalar
    SVt_NV    Scalar
    SVt_PV    Scalar
    SVt_RV    Scalar
    SVt_PVAV  Array
    SVt_PVHV  Hash
    SVt_PVCV  Code
    SVt_PVGV  Glob (possibly a file handle)
    SVt_PVMG  Blessed or Magical Scalar



See the sv.h header file for more details.









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28.3.11 Blessed References and Class Objects



References are also used to support object-oriented programming.  In perl’s
OO lexicon, an object is simply a reference that has been blessed into a
package (or class).  Once blessed, the programmer may now use the reference
to access the various methods in the class.



A reference can be blessed into a package with the following function:



    SV* sv_bless(SV* sv, HV* stash);



The sv argument must be a reference value.  The stash argument
specifies which class the reference will belong to.  See
Stashes and Globs for information on converting class names into stashes.



/* Still under construction */



The following function upgrades rv to reference if not already one.
Creates a new SV for rv to point to.  If classname is non-null, the SV
is blessed into the specified class.  SV is returned.



        SV* newSVrv(SV* rv, const char* classname);



The following three functions copy integer, unsigned integer or double
into an SV whose reference is rv.  SV is blessed if classname is
non-null.



        SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
        SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
        SV* sv_setref_nv(SV* rv, const char* classname, NV iv);



The following function copies the pointer value (the address, not the
string!) into an SV whose reference is rv.  SV is blessed if classname
is non-null.



        SV* sv_setref_pv(SV* rv, const char* classname, void* pv);



The following function copies a string into an SV whose reference is rv.
Set length to 0 to let Perl calculate the string length.  SV is blessed if
classname is non-null.



    SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
                                                         STRLEN length);



The following function tests whether the SV is blessed into the specified
class.  It does not check inheritance relationships.



        int  sv_isa(SV* sv, const char* name);



The following function tests whether the SV is a reference to a blessed object.



        int  sv_isobject(SV* sv);



The following function tests whether the SV is derived from the specified
class. SV can be either a reference to a blessed object or a string
containing a class name. This is the function implementing the
UNIVERSAL::isa functionality.



        bool sv_derived_from(SV* sv, const char* name);



To check if you’ve got an object derived from a specific class you have
to write:



        if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }









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28.3.12 Creating New Variables



To create a new Perl variable with an undef value which can be accessed from
your Perl script, use the following routines, depending on the variable type.



    SV*  get_sv("package::varname", GV_ADD);
    AV*  get_av("package::varname", GV_ADD);
    HV*  get_hv("package::varname", GV_ADD);



Notice the use of GV_ADD as the second parameter.  The new variable can now
be set, using the routines appropriate to the data type.



There are additional macros whose values may be bitwise OR’ed with the
GV_ADD argument to enable certain extra features.  Those bits are:





GV_ADDMULTI




Marks the variable as multiply defined, thus preventing the:



  Name <varname> used only once: possible typo



warning.





GV_ADDWARN




Issues the warning:



  Had to create <varname> unexpectedly



if the variable did not exist before the function was called.








If you do not specify a package name, the variable is created in the current
package.









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28.3.13 Reference Counts and Mortality



Perl uses a reference count-driven garbage collection mechanism. SVs,
AVs, or HVs (xV for short in the following) start their life with a
reference count of 1.  If the reference count of an xV ever drops to 0,
then it will be destroyed and its memory made available for reuse.



This normally doesn’t happen at the Perl level unless a variable is
undef’ed or the last variable holding a reference to it is changed or
overwritten.  At the internal level, however, reference counts can be
manipulated with the following macros:



    int SvREFCNT(SV* sv);
    SV* SvREFCNT_inc(SV* sv);
    void SvREFCNT_dec(SV* sv);



However, there is one other function which manipulates the reference
count of its argument.  The newRV_inc function, you will recall,
creates a reference to the specified argument.  As a side effect,
it increments the argument’s reference count.  If this is not what
you want, use newRV_noinc instead.



For example, imagine you want to return a reference from an XSUB function.
Inside the XSUB routine, you create an SV which initially has a reference
count of one.  Then you call newRV_inc, passing it the just-created SV.
This returns the reference as a new SV, but the reference count of the
SV you passed to newRV_inc has been incremented to two.  Now you
return the reference from the XSUB routine and forget about the SV.
But Perl hasn’t!  Whenever the returned reference is destroyed, the
reference count of the original SV is decreased to one and nothing happens.
The SV will hang around without any way to access it until Perl itself
terminates.  This is a memory leak.



The correct procedure, then, is to use newRV_noinc instead of
newRV_inc.  Then, if and when the last reference is destroyed,
the reference count of the SV will go to zero and it will be destroyed,
stopping any memory leak.



There are some convenience functions available that can help with the
destruction of xVs.  These functions introduce the concept of "mortality".
An xV that is mortal has had its reference count marked to be decremented,
but not actually decremented, until "a short time later".  Generally the
term "short time later" means a single Perl statement, such as a call to
an XSUB function.  The actual determinant for when mortal xVs have their
reference count decremented depends on two macros, SAVETMPS and FREETMPS.
See perlcall NAME and (perlxs) for more details on these macros.



"Mortalization" then is at its simplest a deferred SvREFCNT_dec.
However, if you mortalize a variable twice, the reference count will
later be decremented twice.



"Mortal" SVs are mainly used for SVs that are placed on perl’s stack.
For example an SV which is created just to pass a number to a called sub
is made mortal to have it cleaned up automatically when it’s popped off
the stack. Similarly, results returned by XSUBs (which are pushed on the
stack) are often made mortal.



To create a mortal variable, use the functions:



    SV*  sv_newmortal()
    SV*  sv_2mortal(SV*)
    SV*  sv_mortalcopy(SV*)



The first call creates a mortal SV (with no value), the second converts an existing
SV to a mortal SV (and thus defers a call to SvREFCNT_dec), and the
third creates a mortal copy of an existing SV.
Because sv_newmortal gives the new SV no value, it must normally be given one
via sv_setpv, sv_setiv, etc. :



    SV *tmp = sv_newmortal();
    sv_setiv(tmp, an_integer);



As that is multiple C statements it is quite common so see this idiom instead:



    SV *tmp = sv_2mortal(newSViv(an_integer));



You should be careful about creating mortal variables.  Strange things
can happen if you make the same value mortal within multiple contexts,
or if you make a variable mortal multiple times. Thinking of "Mortalization"
as deferred SvREFCNT_dec should help to minimize such problems.
For example if you are passing an SV which you know has a high enough REFCNT
to survive its use on the stack you need not do any mortalization.
If you are not sure then doing an SvREFCNT_inc and sv_2mortal, or
making a sv_mortalcopy is safer.



The mortal routines are not just for SVs; AVs and HVs can be
made mortal by passing their address (type-casted to SV*) to the
sv_2mortal or sv_mortalcopy routines.









Next: , Previous: perlguts Reference Counts and Mortality, Up: perlguts Variables   [Contents]





28.3.14 Stashes and Globs



A stash is a hash that contains all variables that are defined
within a package.  Each key of the stash is a symbol
name (shared by all the different types of objects that have the same
name), and each value in the hash table is a GV (Glob Value).  This GV
in turn contains references to the various objects of that name,
including (but not limited to) the following:



    Scalar Value
    Array Value
    Hash Value
    I/O Handle
    Format
    Subroutine



There is a single stash called PL_defstash that holds the items that exist
in the main package.  To get at the items in other packages, append the
string "::" to the package name.  The items in the Foo package are in
the stash Foo:: in PL_defstash.  The items in the Bar::Baz package are
in the stash Baz:: in Bar::’s stash.



To get the stash pointer for a particular package, use the function:



    HV*  gv_stashpv(const char* name, I32 flags)
    HV*  gv_stashsv(SV*, I32 flags)



The first function takes a literal string, the second uses the string stored
in the SV.  Remember that a stash is just a hash table, so you get back an
HV*.  The flags flag will create a new package if it is set to GV_ADD.



The name that gv_stash*v wants is the name of the package whose symbol table
you want.  The default package is called main.  If you have multiply nested
packages, pass their names to gv_stash*v, separated by :: as in the Perl
language itself.



Alternately, if you have an SV that is a blessed reference, you can find
out the stash pointer by using:



    HV*  SvSTASH(SvRV(SV*));



then use the following to get the package name itself:



    char*  HvNAME(HV* stash);



If you need to bless or re-bless an object you can use the following
function:



    SV*  sv_bless(SV*, HV* stash)



where the first argument, an SV*, must be a reference, and the second
argument is a stash.  The returned SV* can now be used in the same way
as any other SV.



For more information on references and blessings, consult (perlref).









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28.3.15 Double-Typed SVs



Scalar variables normally contain only one type of value, an integer,
double, pointer, or reference.  Perl will automatically convert the
actual scalar data from the stored type into the requested type.



Some scalar variables contain more than one type of scalar data.  For
example, the variable $! contains either the numeric value of errno
or its string equivalent from either strerror or sys_errlist[].



To force multiple data values into an SV, you must do two things: use the
sv_set*v routines to add the additional scalar type, then set a flag
so that Perl will believe it contains more than one type of data.  The
four macros to set the flags are:



        SvIOK_on
        SvNOK_on
        SvPOK_on
        SvROK_on



The particular macro you must use depends on which sv_set*v routine
you called first.  This is because every sv_set*v routine turns on
only the bit for the particular type of data being set, and turns off
all the rest.



For example, to create a new Perl variable called "dberror" that contains
both the numeric and descriptive string error values, you could use the
following code:



    extern int  dberror;
    extern char *dberror_list;

    SV* sv = get_sv("dberror", GV_ADD);
    sv_setiv(sv, (IV) dberror);
    sv_setpv(sv, dberror_list[dberror]);
    SvIOK_on(sv);



If the order of sv_setiv and sv_setpv had been reversed, then the
macro SvPOK_on would need to be called instead of SvIOK_on.









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28.3.16 Magic Variables



[This section still under construction.  Ignore everything here.  Post no
bills.  Everything not permitted is forbidden.]



Any SV may be magical, that is, it has special features that a normal
SV does not have.  These features are stored in the SV structure in a
linked list of struct magic’s, typedef’ed to MAGIC.



    struct magic {
        MAGIC*      mg_moremagic;
        MGVTBL*     mg_virtual;
        U16         mg_private;
        char        mg_type;
        U8          mg_flags;
        I32         mg_len;
        SV*         mg_obj;
        char*       mg_ptr;
    };



Note this is current as of patchlevel 0, and could change at any time.









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28.3.17 Assigning Magic



Perl adds magic to an SV using the sv_magic function:



  void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);



The sv argument is a pointer to the SV that is to acquire a new magical
feature.



If sv is not already magical, Perl uses the SvUPGRADE macro to
convert sv to type SVt_PVMG. Perl then continues by adding new magic
to the beginning of the linked list of magical features.  Any prior entry
of the same type of magic is deleted.  Note that this can be overridden,
and multiple instances of the same type of magic can be associated with an
SV.



The name and namlen arguments are used to associate a string with
the magic, typically the name of a variable. namlen is stored in the
mg_len field and if name is non-null then either a savepvn copy of
name or name itself is stored in the mg_ptr field, depending on
whether namlen is greater than zero or equal to zero respectively.  As a
special case, if (name && namlen == HEf_SVKEY) then name is assumed
to contain an SV* and is stored as-is with its REFCNT incremented.



The sv_magic function uses how to determine which, if any, predefined
"Magic Virtual Table" should be assigned to the mg_virtual field.
See the Magic Virtual Tables section below.  The how argument is also
stored in the mg_type field. The value of how should be chosen
from the set of macros PERL_MAGIC_foo found in perl.h. Note that before
these macros were added, Perl internals used to directly use character
literals, so you may occasionally come across old code or documentation
referring to ’U’ magic rather than PERL_MAGIC_uvar for example.



The obj argument is stored in the mg_obj field of the MAGIC
structure.  If it is not the same as the sv argument, the reference
count of the obj object is incremented.  If it is the same, or if
the how argument is PERL_MAGIC_arylen, or if it is a NULL pointer,
then obj is merely stored, without the reference count being incremented.



See also sv_magicext in (perlapi) for a more flexible way to add magic
to an SV.



There is also a function to add magic to an HV:



    void hv_magic(HV *hv, GV *gv, int how);



This simply calls sv_magic and coerces the gv argument into an SV.



To remove the magic from an SV, call the function sv_unmagic:



    int sv_unmagic(SV *sv, int type);



The type argument should be equal to the how value when the SV
was initially made magical.



However, note that sv_unmagic removes all magic of a certain type from the
SV. If you want to remove only certain magic of a type based on the magic
virtual table, use sv_unmagicext instead:



    int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);









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28.3.18 Magic Virtual Tables



The mg_virtual field in the MAGIC structure is a pointer to an
MGVTBL, which is a structure of function pointers and stands for
"Magic Virtual Table" to handle the various operations that might be
applied to that variable.



The MGVTBL has five (or sometimes eight) pointers to the following
routine types:



    int  (*svt_get)(SV* sv, MAGIC* mg);
    int  (*svt_set)(SV* sv, MAGIC* mg);
    U32  (*svt_len)(SV* sv, MAGIC* mg);
    int  (*svt_clear)(SV* sv, MAGIC* mg);
    int  (*svt_free)(SV* sv, MAGIC* mg);

    int  (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
                                          const char *name, I32 namlen);
    int  (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
    int  (*svt_local)(SV *nsv, MAGIC *mg);



This MGVTBL structure is set at compile-time in perl.h and there are
currently 32 types.  These different structures contain pointers to various
routines that perform additional actions depending on which function is
being called.



   Function pointer    Action taken
   ----------------    ------------
   svt_get             Do something before the value of the SV is
                       retrieved.
   svt_set             Do something after the SV is assigned a value.
   svt_len             Report on the SV's length.
   svt_clear           Clear something the SV represents.
   svt_free            Free any extra storage associated with the SV.

   svt_copy            copy tied variable magic to a tied element
   svt_dup             duplicate a magic structure during thread cloning
   svt_local           copy magic to local value during 'local'



For instance, the MGVTBL structure called vtbl_sv (which corresponds
to an mg_type of PERL_MAGIC_sv) contains:



    { magic_get, magic_set, magic_len, 0, 0 }



Thus, when an SV is determined to be magical and of type PERL_MAGIC_sv,
if a get operation is being performed, the routine magic_get is
called.  All the various routines for the various magical types begin
with magic_.  NOTE: the magic routines are not considered part of
the Perl API, and may not be exported by the Perl library.



The last three slots are a recent addition, and for source code
compatibility they are only checked for if one of the three flags
MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
code can continue declaring a vtable as a 5-element value. These three are
currently used exclusively by the threading code, and are highly subject
to change.



The current kinds of Magic Virtual Tables are:



 mg_type
 (old-style char and macro)   MGVTBL          Type of magic
 --------------------------   ------          -------------
 \0 PERL_MAGIC_sv             vtbl_sv         Special scalar variable
 #  PERL_MAGIC_arylen         vtbl_arylen     Array length ($#ary)
 %  PERL_MAGIC_rhash          (none)          extra data for restricted
                                              hashes
 .  PERL_MAGIC_pos            vtbl_pos        pos() lvalue
 :  PERL_MAGIC_symtab         (none)          extra data for symbol
                                              tables
 <  PERL_MAGIC_backref        vtbl_backref    for weak ref data
 @  PERL_MAGIC_arylen_p       (none)          to move arylen out of
                                              XPVAV
 A  PERL_MAGIC_overload       vtbl_amagic     %OVERLOAD hash
 a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
 B  PERL_MAGIC_bm             vtbl_regexp     Boyer-Moore 
                                              (fast string search)
 c  PERL_MAGIC_overload_table vtbl_ovrld      Holds overload table 
                                              (AMT) on stash
 D  PERL_MAGIC_regdata        vtbl_regdata    Regex match position data 
                                              (@+ and @- vars)
 d  PERL_MAGIC_regdatum       vtbl_regdatum   Regex match position data
                                              element
 E  PERL_MAGIC_env            vtbl_env        %ENV hash
 e  PERL_MAGIC_envelem        vtbl_envelem    %ENV hash element
 f  PERL_MAGIC_fm             vtbl_regdata    Formline 
                                              ('compiled' format)
 G  PERL_MAGIC_study          vtbl_regexp     study()ed string
 g  PERL_MAGIC_regex_global   vtbl_mglob      m//g target
 H  PERL_MAGIC_hints          vtbl_hints      %^H hash
 h  PERL_MAGIC_hintselem      vtbl_hintselem  %^H hash element
 I  PERL_MAGIC_isa            vtbl_isa        @ISA array
 i  PERL_MAGIC_isaelem        vtbl_isaelem    @ISA array element
 k  PERL_MAGIC_nkeys          vtbl_nkeys      scalar(keys()) lvalue
 L  PERL_MAGIC_dbfile         (none)          Debugger %_<filename
 l  PERL_MAGIC_dbline         vtbl_dbline     Debugger %_<filename
                                              element
 N  PERL_MAGIC_shared         (none)          Shared between threads
 n  PERL_MAGIC_shared_scalar  (none)          Shared between threads
 o  PERL_MAGIC_collxfrm       vtbl_collxfrm   Locale transformation
 P  PERL_MAGIC_tied           vtbl_pack       Tied array or hash
 p  PERL_MAGIC_tiedelem       vtbl_packelem   Tied array or hash element
 q  PERL_MAGIC_tiedscalar     vtbl_packelem   Tied scalar or handle
 r  PERL_MAGIC_qr             vtbl_regexp     precompiled qr// regex
 S  PERL_MAGIC_sig            (none)          %SIG hash
 s  PERL_MAGIC_sigelem        vtbl_sigelem    %SIG hash element
 t  PERL_MAGIC_taint          vtbl_taint      Taintedness
 U  PERL_MAGIC_uvar           vtbl_uvar       Available for use by
                                              extensions
 u  PERL_MAGIC_uvar_elem      (none)          Reserved for use by
                                              extensions
 V  PERL_MAGIC_vstring        vtbl_vstring    SV was vstring literal
 v  PERL_MAGIC_vec            vtbl_vec        vec() lvalue
 w  PERL_MAGIC_utf8           vtbl_utf8       Cached UTF-8 information
 x  PERL_MAGIC_substr         vtbl_substr     substr() lvalue
 y  PERL_MAGIC_defelem        vtbl_defelem    Shadow "foreach" iterator
                                              variable / smart parameter
                                              vivification
 ]  PERL_MAGIC_checkcall      (none)          inlining/mutation of call
                                              to this CV
 ~  PERL_MAGIC_ext            (none)          Available for use by
                                              extensions



When an uppercase and lowercase letter both exist in the table, then the
uppercase letter is typically used to represent some kind of composite type
(a list or a hash), and the lowercase letter is used to represent an element
of that composite type. Some internals code makes use of this case
relationship.  However, ’v’ and ’V’ (vec and v-string) are in no way related.



The PERL_MAGIC_ext and PERL_MAGIC_uvar magic types are defined
specifically for use by extensions and will not be used by perl itself.
Extensions can use PERL_MAGIC_ext magic to ’attach’ private information
to variables (typically objects).  This is especially useful because
there is no way for normal perl code to corrupt this private information
(unlike using extra elements of a hash object).



Similarly, PERL_MAGIC_uvar magic can be used much like tie() to call a
C function any time a scalar’s value is used or changed.  The MAGIC’s
mg_ptr field points to a ufuncs structure:



    struct ufuncs {
        I32 (*uf_val)(pTHX_ IV, SV*);
        I32 (*uf_set)(pTHX_ IV, SV*);
        IV uf_index;
    };



When the SV is read from or written to, the uf_val or uf_set
function will be called with uf_index as the first arg and a pointer to
the SV as the second.  A simple example of how to add PERL_MAGIC_uvar
magic is shown below.  Note that the ufuncs structure is copied by
sv_magic, so you can safely allocate it on the stack.



    void
    Umagic(sv)
        SV *sv;
    PREINIT:
        struct ufuncs uf;
    CODE:
        uf.uf_val   = &my_get_fn;
        uf.uf_set   = &my_set_fn;
        uf.uf_index = 0;
        sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));



Attaching PERL_MAGIC_uvar to arrays is permissible but has no effect.



For hashes there is a specialized hook that gives control over hash
keys (but not values).  This hook calls PERL_MAGIC_uvar ’get’ magic
if the "set" function in the ufuncs structure is NULL.  The hook
is activated whenever the hash is accessed with a key specified as
an SV through the functions hv_store_ent, hv_fetch_ent,
hv_delete_ent, and hv_exists_ent.  Accessing the key as a string
through the functions without the ..._ent suffix circumvents the
hook.  See (Hash-Util-FieldHash)GUTS for a detailed description.



Note that because multiple extensions may be using PERL_MAGIC_ext
or PERL_MAGIC_uvar magic, it is important for extensions to take
extra care to avoid conflict.  Typically only using the magic on
objects blessed into the same class as the extension is sufficient.
For PERL_MAGIC_ext magic, it is usually a good idea to define an
MGVTBL, even if all its fields will be 0, so that individual
MAGIC pointers can be identified as a particular kind of magic
using their magic virtual table. mg_findext provides an easy way
to do that:



    STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };

    MAGIC *mg;
    if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
        /* this is really ours, not another module's PERL_MAGIC_ext */
        my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
        ...
    }



Also note that the sv_set*() and sv_cat*() functions described
earlier do not invoke ’set’ magic on their targets.  This must
be done by the user either by calling the SvSETMAGIC() macro after
calling these functions, or by using one of the sv_set*_mg() or
sv_cat*_mg() functions.  Similarly, generic C code must call the
SvGETMAGIC() macro to invoke any ’get’ magic if they use an SV
obtained from external sources in functions that don’t handle magic.
See (perlapi) for a description of these functions.
For example, calls to the sv_cat*() functions typically need to be
followed by SvSETMAGIC(), but they don’t need a prior SvGETMAGIC()
since their implementation handles ’get’ magic.









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28.3.19 Finding Magic



    MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
                                       * type */



This routine returns a pointer to a MAGIC structure stored in the SV.
If the SV does not have that magical feature, NULL is returned. If the
SV has multiple instances of that magical feature, the first one will be
returned. mg_findext can be used to find a MAGIC structure of an SV
based on both its magic type and its magic virtual table:



    MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);



Also, if the SV passed to mg_find or mg_findext is not of type
SVt_PVMG, Perl may core dump.



    int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);



This routine checks to see what types of magic sv has.  If the mg_type
field is an uppercase letter, then the mg_obj is copied to nsv, but
the mg_type field is changed to be the lowercase letter.









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28.3.20 Understanding the Magic of Tied Hashes and Arrays



Tied hashes and arrays are magical beasts of the PERL_MAGIC_tied
magic type.



WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats.  Some
of these caveats are actually considered bugs in the API, to be fixed
in later releases, and are bracketed with [MAYCHANGE] below. If
you find yourself actually applying such information in this section, be
aware that the behavior may change in the future, umm, without warning.



The perl tie function associates a variable with an object that implements
the various GET, SET, etc methods.  To perform the equivalent of the perl
tie function from an XSUB, you must mimic this behaviour.  The code below
carries out the necessary steps - firstly it creates a new hash, and then
creates a second hash which it blesses into the class which will implement
the tie methods. Lastly it ties the two hashes together, and returns a
reference to the new tied hash.  Note that the code below does NOT call the
TIEHASH method in the MyTie class -
see Calling Perl Routines from within C Programs for details on how
to do this.



    SV*
    mytie()
    PREINIT:
        HV *hash;
        HV *stash;
        SV *tie;
    CODE:
        hash = newHV();
        tie = newRV_noinc((SV*)newHV());
        stash = gv_stashpv("MyTie", GV_ADD);
        sv_bless(tie, stash);
        hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
        RETVAL = newRV_noinc(hash);
    OUTPUT:
        RETVAL



The av_store function, when given a tied array argument, merely
copies the magic of the array onto the value to be "stored", using
mg_copy.  It may also return NULL, indicating that the value did not
actually need to be stored in the array.  [MAYCHANGE] After a call to
av_store on a tied array, the caller will usually need to call
mg_set(val) to actually invoke the perl level "STORE" method on the
TIEARRAY object.  If av_store did return NULL, a call to
SvREFCNT_dec(val) will also be usually necessary to avoid a memory
leak. [/MAYCHANGE]



The previous paragraph is applicable verbatim to tied hash access using the
hv_store and hv_store_ent functions as well.



av_fetch and the corresponding hash functions hv_fetch and
hv_fetch_ent actually return an undefined mortal value whose magic
has been initialized using mg_copy.  Note the value so returned does not
need to be deallocated, as it is already mortal.  [MAYCHANGE] But you will
need to call mg_get() on the returned value in order to actually invoke
the perl level "FETCH" method on the underlying TIE object.  Similarly,
you may also call mg_set() on the return value after possibly assigning
a suitable value to it using sv_setsv,  which will invoke the "STORE"
method on the TIE object. [/MAYCHANGE]



[MAYCHANGE]
In other words, the array or hash fetch/store functions don’t really
fetch and store actual values in the case of tied arrays and hashes.  They
merely call mg_copy to attach magic to the values that were meant to be
"stored" or "fetched".  Later calls to mg_get and mg_set actually
do the job of invoking the TIE methods on the underlying objects.  Thus
the magic mechanism currently implements a kind of lazy access to arrays
and hashes.



Currently (as of perl version 5.004), use of the hash and array access
functions requires the user to be aware of whether they are operating on
"normal" hashes and arrays, or on their tied variants.  The API may be
changed to provide more transparent access to both tied and normal data
types in future versions.
[/MAYCHANGE]



You would do well to understand that the TIEARRAY and TIEHASH interfaces
are mere sugar to invoke some perl method calls while using the uniform hash
and array syntax.  The use of this sugar imposes some overhead (typically
about two to four extra opcodes per FETCH/STORE operation, in addition to
the creation of all the mortal variables required to invoke the methods).
This overhead will be comparatively small if the TIE methods are themselves
substantial, but if they are only a few statements long, the overhead
will not be insignificant.









Previous: perlguts Understanding the Magic of Tied Hashes and Arrays, Up: perlguts Variables   [Contents]





28.3.21 Localizing changes



Perl has a very handy construction



  {
    local $var = 2;
    ...
  }



This construction is approximately equivalent to



  {
    my $oldvar = $var;
    $var = 2;
    ...
    $var = $oldvar;
  }



The biggest difference is that the first construction would
reinstate the initial value of $var, irrespective of how control exits
the block: goto, return, die/eval, etc. It is a little bit
more efficient as well.



There is a way to achieve a similar task from C via Perl API: create a
pseudo-block, and arrange for some changes to be automatically
undone at the end of it, either explicit, or via a non-local exit (via
die()). A block-like construct is created by a pair of
ENTER/LEAVE macros (see perlcall Returning a Scalar).
Such a construct may be created specially for some important localized
task, or an existing one (like boundaries of enclosing Perl
subroutine/block, or an existing pair for freeing TMPs) may be
used. (In the second case the overhead of additional localization must
be almost negligible.) Note that any XSUB is automatically enclosed in
an ENTER/LEAVE pair.



Inside such a pseudo-block the following service is available:





SAVEINT(int i)






SAVEIV(IV i)






SAVEI32(I32 i)






SAVELONG(long i)




These macros arrange things to restore the value of integer variable
i at the end of enclosing pseudo-block.





SAVESPTR(s)






SAVEPPTR(p)




These macros arrange things to restore the value of pointers s and
p. s must be a pointer of a type which survives conversion to
SV* and back, p should be able to survive conversion to char*
and back.





SAVEFREESV(SV *sv)




The refcount of sv would be decremented at the end of
pseudo-block.  This is similar to sv_2mortal in that it is also a
mechanism for doing a delayed SvREFCNT_dec.  However, while sv_2mortal
extends the lifetime of sv until the beginning of the next statement,
SAVEFREESV extends it until the end of the enclosing scope.  These
lifetimes can be wildly different.



Also compare SAVEMORTALIZESV.





SAVEMORTALIZESV(SV *sv)




Just like SAVEFREESV, but mortalizes sv at the end of the current
scope instead of decrementing its reference count.  This usually has the
effect of keeping sv alive until the statement that called the currently
live scope has finished executing.





SAVEFREEOP(OP *op)




The OP * is op_free()ed at the end of pseudo-block.





SAVEFREEPV(p)




The chunk of memory which is pointed to by p is Safefree()ed at the
end of pseudo-block.





SAVECLEARSV(SV *sv)




Clears a slot in the current scratchpad which corresponds to sv at
the end of pseudo-block.





SAVEDELETE(HV *hv, char *key, I32 length)




The key key of hv is deleted at the end of pseudo-block. The
string pointed to by key is Safefree()ed.  If one has a key in
short-lived storage, the corresponding string may be reallocated like
this:



  SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));





SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)




At the end of pseudo-block the function f is called with the
only argument p.





SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)




At the end of pseudo-block the function f is called with the
implicit context argument (if any), and p.





SAVESTACK_POS()




The current offset on the Perl internal stack (cf. SP) is restored
at the end of pseudo-block.








The following API list contains functions, thus one needs to
provide pointers to the modifiable data explicitly (either C pointers,
or Perlish GV *s).  Where the above macros take int, a similar
function takes int *.





SV* save_scalar(GV *gv)




Equivalent to Perl code local $gv.





AV* save_ary(GV *gv)






HV* save_hash(GV *gv)




Similar to save_scalar, but localize @gv and %gv.





void save_item(SV *item)




Duplicates the current value of SV, on the exit from the current
ENTER/LEAVE pseudo-block will restore the value of SV
using the stored value. It doesn’t handle magic. Use save_scalar if
magic is affected.





void save_list(SV **sarg, I32 maxsarg)




A variant of save_item which takes multiple arguments via an array
sarg of SV* of length maxsarg.





SV* save_svref(SV **sptr)




Similar to save_scalar, but will reinstate an SV *.





void save_aptr(AV **aptr)






void save_hptr(HV **hptr)




Similar to save_svref, but localize AV * and HV *.








The Alias module implements localization of the basic types within the
caller’s scope.  People who are interested in how to localize things in
the containing scope should take a look there too.









Next: , Previous: perlguts Variables, Up: perlguts   [Contents]





28.4 Subroutines





• perlguts XSUBs and the Argument Stack:  



• perlguts Autoloading with XSUBs:  



• perlguts Calling Perl Routines from within C Programs:  



• perlguts Memory Allocation:  



• perlguts PerlIO:  



• perlguts Putting a C value on Perl stack:  



• perlguts Scratchpads:  



• perlguts Scratchpads and recursion:  












Next: , Up: perlguts Subroutines   [Contents]





28.4.1 XSUBs and the Argument Stack



The XSUB mechanism is a simple way for Perl programs to access C subroutines.
An XSUB routine will have a stack that contains the arguments from the Perl
program, and a way to map from the Perl data structures to a C equivalent.



The stack arguments are accessible through the ST(n) macro, which returns
the n’th stack argument.  Argument 0 is the first argument passed in the
Perl subroutine call.  These arguments are SV*, and can be used anywhere
an SV* is used.



Most of the time, output from the C routine can be handled through use of
the RETVAL and OUTPUT directives.  However, there are some cases where the
argument stack is not already long enough to handle all the return values.
An example is the POSIX tzname() call, which takes no arguments, but returns
two, the local time zone’s standard and summer time abbreviations.



To handle this situation, the PPCODE directive is used and the stack is
extended using the macro:



    EXTEND(SP, num);



where SP is the macro that represents the local copy of the stack pointer,
and num is the number of elements the stack should be extended by.



Now that there is room on the stack, values can be pushed on it using PUSHs
macro. The pushed values will often need to be "mortal" (See
Reference Counts and Mortality):



    PUSHs(sv_2mortal(newSViv(an_integer)))
    PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
    PUSHs(sv_2mortal(newSVnv(a_double)))
    PUSHs(sv_2mortal(newSVpv("Some String",0)))
    /* Although the last example is better written as the more
     * efficient: */
    PUSHs(newSVpvs_flags("Some String", SVs_TEMP))



And now the Perl program calling tzname, the two values will be assigned
as in:



    ($standard_abbrev, $summer_abbrev) = POSIX::tzname;



An alternate (and possibly simpler) method to pushing values on the stack is
to use the macro:



    XPUSHs(SV*)



This macro automatically adjusts the stack for you, if needed.  Thus, you
do not need to call EXTEND to extend the stack.



Despite their suggestions in earlier versions of this document the macros
(X)PUSH[iunp] are not suited to XSUBs which return multiple results.
For that, either stick to the (X)PUSHs macros shown above, or use the new
m(X)PUSH[iunp] macros instead; see Putting a C value on Perl stack.



For more information, consult (perlxs) and (perlxstut).









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28.4.2 Autoloading with XSUBs



If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
of the XSUB’s package.



But it also puts the same information in certain fields of the XSUB itself:



    HV *stash           = CvSTASH(cv);
    const char *subname = SvPVX(cv);
    STRLEN name_length  = SvCUR(cv); /* in bytes */
    U32 is_utf8         = SvUTF8(cv);
    



SvPVX(cv) contains just the sub name itself, not including the package.
For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
CvSTASH(cv) returns NULL during a method call on a nonexistent package.



Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
XS AUTOLOAD subs at all.  Perl 5.8.0 introduced the use of fields in the
XSUB itself.  Perl 5.16.0 restored the setting of $AUTOLOAD.  If you need
to support 5.8-5.14, use the XSUB’s fields.









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28.4.3 Calling Perl Routines from within C Programs



There are four routines that can be used to call a Perl subroutine from
within a C program.  These four are:



    I32  call_sv(SV*, I32);
    I32  call_pv(const char*, I32);
    I32  call_method(const char*, I32);
    I32  call_argv(const char*, I32, register char**);



The routine most often used is call_sv.  The SV* argument
contains either the name of the Perl subroutine to be called, or a
reference to the subroutine.  The second argument consists of flags
that control the context in which the subroutine is called, whether
or not the subroutine is being passed arguments, how errors should be
trapped, and how to treat return values.



All four routines return the number of arguments that the subroutine returned
on the Perl stack.



These routines used to be called perl_call_sv, etc., before Perl v5.6.0,
but those names are now deprecated; macros of the same name are provided for
compatibility.



When using any of these routines (except call_argv), the programmer
must manipulate the Perl stack.  These include the following macros and
functions:



    dSP
    SP
    PUSHMARK()
    PUTBACK
    SPAGAIN
    ENTER
    SAVETMPS
    FREETMPS
    LEAVE
    XPUSH*()
    POP*()



For a detailed description of calling conventions from C to Perl,
consult perlcall NAME.









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28.4.4 Memory Allocation





• perlguts Allocation:  



• perlguts Reallocation:  



• perlguts Moving:  












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28.4.4.1 Allocation



All memory meant to be used with the Perl API functions should be manipulated
using the macros described in this section.  The macros provide the necessary
transparency between differences in the actual malloc implementation that is
used within perl.



It is suggested that you enable the version of malloc that is distributed
with Perl.  It keeps pools of various sizes of unallocated memory in
order to satisfy allocation requests more quickly.  However, on some
platforms, it may cause spurious malloc or free errors.



The following three macros are used to initially allocate memory :



    Newx(pointer, number, type);
    Newxc(pointer, number, type, cast);
    Newxz(pointer, number, type);



The first argument pointer should be the name of a variable that will
point to the newly allocated memory.



The second and third arguments number and type specify how many of
the specified type of data structure should be allocated.  The argument
type is passed to sizeof.  The final argument to Newxc, cast,
should be used if the pointer argument is different from the type
argument.



Unlike the Newx and Newxc macros, the Newxz macro calls memzero
to zero out all the newly allocated memory.









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28.4.4.2 Reallocation



    Renew(pointer, number, type);
    Renewc(pointer, number, type, cast);
    Safefree(pointer)



These three macros are used to change a memory buffer size or to free a
piece of memory no longer needed.  The arguments to Renew and Renewc
match those of New and Newc with the exception of not needing the
"magic cookie" argument.









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28.4.4.3 Moving



    Move(source, dest, number, type);
    Copy(source, dest, number, type);
    Zero(dest, number, type);



These three macros are used to move, copy, or zero out previously allocated
memory.  The source and dest arguments point to the source and
destination starting points.  Perl will move, copy, or zero out number
instances of the size of the type data structure (using the sizeof
function).









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28.4.5 PerlIO



The most recent development releases of Perl have been experimenting with
removing Perl’s dependency on the "normal" standard I/O suite and allowing
other stdio implementations to be used.  This involves creating a new
abstraction layer that then calls whichever implementation of stdio Perl
was compiled with.  All XSUBs should now use the functions in the PerlIO
abstraction layer and not make any assumptions about what kind of stdio
is being used.



For a complete description of the PerlIO abstraction, consult perlapio NAME.









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28.4.6 Putting a C value on Perl stack



A lot of opcodes (this is an elementary operation in the internal perl
stack machine) put an SV* on the stack. However, as an optimization
the corresponding SV is (usually) not recreated each time. The opcodes
reuse specially assigned SVs (targets) which are (as a corollary)
not constantly freed/created.



Each of the targets is created only once (but see
Scratchpads and recursion below), and when an opcode needs to put
an integer, a double, or a string on stack, it just sets the
corresponding parts of its target and puts the target on stack.



The macro to put this target on stack is PUSHTARG, and it is
directly used in some opcodes, as well as indirectly in zillions of
others, which use it via (X)PUSH[iunp].



Because the target is reused, you must be careful when pushing multiple
values on the stack. The following code will not do what you think:



    XPUSHi(10);
    XPUSHi(20);



This translates as "set TARG to 10, push a pointer to TARG onto
the stack; set TARG to 20, push a pointer to TARG onto the stack".
At the end of the operation, the stack does not contain the values 10
and 20, but actually contains two pointers to TARG, which we have set
to 20.



If you need to push multiple different values then you should either use
the (X)PUSHs macros, or else use the new m(X)PUSH[iunp] macros,
none of which make use of TARG.  The (X)PUSHs macros simply push an
SV* on the stack, which, as noted under XSUBs and the Argument Stack,
will often need to be "mortal".  The new m(X)PUSH[iunp] macros make
this a little easier to achieve by creating a new mortal for you (via
(X)PUSHmortal), pushing that onto the stack (extending it if necessary
in the case of the mXPUSH[iunp] macros), and then setting its value.
Thus, instead of writing this to "fix" the example above:



    XPUSHs(sv_2mortal(newSViv(10)))
    XPUSHs(sv_2mortal(newSViv(20)))



you can simply write:



    mXPUSHi(10)
    mXPUSHi(20)



On a related note, if you do use (X)PUSH[iunp], then you’re going to
need a dTARG in your variable declarations so that the *PUSH*
macros can make use of the local variable TARG.  See also dTARGET
and dXSTARG.









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28.4.7 Scratchpads



The question remains on when the SVs which are targets for opcodes
are created. The answer is that they are created when the current
unit–a subroutine or a file (for opcodes for statements outside of
subroutines)–is compiled. During this time a special anonymous Perl
array is created, which is called a scratchpad for the current unit.



A scratchpad keeps SVs which are lexicals for the current unit and are
targets for opcodes. One can deduce that an SV lives on a scratchpad
by looking on its flags: lexicals have SVs_PADMY set, and
targets have SVs_PADTMP set.



The correspondence between OPs and targets is not 1-to-1. Different
OPs in the compile tree of the unit can use the same target, if this
would not conflict with the expected life of the temporary.









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28.4.8 Scratchpads and recursion



In fact it is not 100% true that a compiled unit contains a pointer to
the scratchpad AV. In fact it contains a pointer to an AV of
(initially) one element, and this element is the scratchpad AV. Why do
we need an extra level of indirection?



The answer is recursion, and maybe threads. Both
these can create several execution pointers going into the same
subroutine. For the subroutine-child not write over the temporaries
for the subroutine-parent (lifespan of which covers the call to the
child), the parent and the child should have different
scratchpads. (And the lexicals should be separate anyway!)



So each subroutine is born with an array of scratchpads (of length 1).
On each entry to the subroutine it is checked that the current
depth of the recursion is not more than the length of this array, and
if it is, new scratchpad is created and pushed into the array.



The targets on this scratchpad are undefs, but they are already
marked with correct flags.









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28.5 Compiled code





• perlguts Code tree:  



• perlguts Examining the tree:  



• perlguts Compile pass 1: check routines:  



• perlguts Compile pass 1a: constant folding:  



• perlguts Compile pass 2: context propagation:  



• perlguts Compile pass 3: peephole optimization:  



• perlguts Pluggable runops:  



• perlguts Compile-time scope hooks:  












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28.5.1 Code tree



Here we describe the internal form your code is converted to by
Perl. Start with a simple example:



  $a = $b + $c;



This is converted to a tree similar to this one:



             assign-to
           /           \
          +             $a
        /   \
      $b     $c



(but slightly more complicated).  This tree reflects the way Perl
parsed your code, but has nothing to do with the execution order.
There is an additional "thread" going through the nodes of the tree
which shows the order of execution of the nodes.  In our simplified
example above it looks like:



     $b ---> $c ---> + ---> $a ---> assign-to



But with the actual compile tree for $a = $b + $c it is different:
some nodes optimized away.  As a corollary, though the actual tree
contains more nodes than our simplified example, the execution order
is the same as in our example.









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28.5.2 Examining the tree



If you have your perl compiled for debugging (usually done with
-DDEBUGGING on the Configure command line), you may examine the
compiled tree by specifying -Dx on the Perl command line.  The
output takes several lines per node, and for $b+$c it looks like
this:



    5           TYPE = add  ===> 6
                TARG = 1
                FLAGS = (SCALAR,KIDS)
                {
                    TYPE = null  ===> (4)
                      (was rv2sv)
                    FLAGS = (SCALAR,KIDS)
                    {
    3                   TYPE = gvsv  ===> 4
                        FLAGS = (SCALAR)
                        GV = main::b
                    }
                }
                {
                    TYPE = null  ===> (5)
                      (was rv2sv)
                    FLAGS = (SCALAR,KIDS)
                    {
    4                   TYPE = gvsv  ===> 5
                        FLAGS = (SCALAR)
                        GV = main::c
                    }
                }



This tree has 5 nodes (one per TYPE specifier), only 3 of them are
not optimized away (one per number in the left column).  The immediate
children of the given node correspond to {} pairs on the same level
of indentation, thus this listing corresponds to the tree:



                   add
                 /     \
               null    null
                |       |
               gvsv    gvsv



The execution order is indicated by ===> marks, thus it is 3
4 5 6 (node 6 is not included into above listing), i.e.,
gvsv gvsv add whatever.



Each of these nodes represents an op, a fundamental operation inside the
Perl core. The code which implements each operation can be found in the
pp*.c files; the function which implements the op with type gvsv
is pp_gvsv, and so on. As the tree above shows, different ops have
different numbers of children: add is a binary operator, as one would
expect, and so has two children. To accommodate the various different
numbers of children, there are various types of op data structure, and
they link together in different ways.



The simplest type of op structure is OP: this has no children. Unary
operators, UNOPs, have one child, and this is pointed to by the
op_first field. Binary operators (BINOPs) have not only an
op_first field but also an op_last field. The most complex type of
op is a LISTOP, which has any number of children. In this case, the
first child is pointed to by op_first and the last child by
op_last. The children in between can be found by iteratively
following the op_sibling pointer from the first child to the last.



There are also two other op types: a PMOP holds a regular expression,
and has no children, and a LOOP may or may not have children. If the
op_children field is non-zero, it behaves like a LISTOP. To
complicate matters, if a UNOP is actually a null op after
optimization (see Compile pass 2: context propagation) it will still
have children in accordance with its former type.



Another way to examine the tree is to use a compiler back-end module, such
as (B-Concise).









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28.5.3 Compile pass 1: check routines



The tree is created by the compiler while yacc code feeds it
the constructions it recognizes. Since yacc works bottom-up, so does
the first pass of perl compilation.



What makes this pass interesting for perl developers is that some
optimization may be performed on this pass.  This is optimization by
so-called "check routines".  The correspondence between node names
and corresponding check routines is described in opcode.pl (do not
forget to run make regen_headers if you modify this file).



A check routine is called when the node is fully constructed except
for the execution-order thread.  Since at this time there are no
back-links to the currently constructed node, one can do most any
operation to the top-level node, including freeing it and/or creating
new nodes above/below it.



The check routine returns the node which should be inserted into the
tree (if the top-level node was not modified, check routine returns
its argument).



By convention, check routines have names ck_*. They are usually
called from new*OP subroutines (or convert) (which in turn are
called from perly.y).









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28.5.4 Compile pass 1a: constant folding



Immediately after the check routine is called the returned node is
checked for being compile-time executable.  If it is (the value is
judged to be constant) it is immediately executed, and a constant
node with the "return value" of the corresponding subtree is
substituted instead.  The subtree is deleted.



If constant folding was not performed, the execution-order thread is
created.









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28.5.5 Compile pass 2: context propagation



When a context for a part of compile tree is known, it is propagated
down through the tree.  At this time the context can have 5 values
(instead of 2 for runtime context): void, boolean, scalar, list, and
lvalue.  In contrast with the pass 1 this pass is processed from top
to bottom: a node’s context determines the context for its children.



Additional context-dependent optimizations are performed at this time.
Since at this moment the compile tree contains back-references (via
"thread" pointers), nodes cannot be free()d now.  To allow
optimized-away nodes at this stage, such nodes are null()ified instead
of free()ing (i.e. their type is changed to OP_NULL).









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28.5.6 Compile pass 3: peephole optimization



After the compile tree for a subroutine (or for an eval or a file)
is created, an additional pass over the code is performed. This pass
is neither top-down or bottom-up, but in the execution order (with
additional complications for conditionals).  Optimizations performed
at this stage are subject to the same restrictions as in the pass 2.



Peephole optimizations are done by calling the function pointed to
by the global variable PL_peepp.  By default, PL_peepp just
calls the function pointed to by the global variable PL_rpeepp.
By default, that performs some basic op fixups and optimisations along
the execution-order op chain, and recursively calls PL_rpeepp for
each side chain of ops (resulting from conditionals).  Extensions may
provide additional optimisations or fixups, hooking into either the
per-subroutine or recursive stage, like this:



    static peep_t prev_peepp;
    static void my_peep(pTHX_ OP *o)
    {
        /* custom per-subroutine optimisation goes here */
        prev_peepp(o);
        /* custom per-subroutine optimisation may also go here */
    }
    BOOT:
        prev_peepp = PL_peepp;
        PL_peepp = my_peep;

    static peep_t prev_rpeepp;
    static void my_rpeep(pTHX_ OP *o)
    {
        OP *orig_o = o;
        for(; o; o = o->op_next) {
            /* custom per-op optimisation goes here */
        }
        prev_rpeepp(orig_o);
    }
    BOOT:
        prev_rpeepp = PL_rpeepp;
        PL_rpeepp = my_rpeep;









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28.5.7 Pluggable runops



The compile tree is executed in a runops function.  There are two runops
functions, in run.c and in dump.c.  Perl_runops_debug is used
with DEBUGGING and Perl_runops_standard is used otherwise.  For fine
control over the execution of the compile tree it is possible to provide
your own runops function.



It’s probably best to copy one of the existing runops functions and
change it to suit your needs.  Then, in the BOOT section of your XS
file, add the line:



  PL_runops = my_runops;



This function should be as efficient as possible to keep your programs
running as fast as possible.









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28.5.8 Compile-time scope hooks



As of perl 5.14 it is possible to hook into the compile-time lexical
scope mechanism using Perl_blockhook_register. This is used like
this:



    STATIC void my_start_hook(pTHX_ int full);
    STATIC BHK my_hooks;

    BOOT:
        BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
        Perl_blockhook_register(aTHX_ &my_hooks);



This will arrange to have my_start_hook called at the start of
compiling every lexical scope. The available hooks are:





void bhk_start(pTHX_ int full)




This is called just after starting a new lexical scope. Note that Perl
code like



    if ($x) { ... }



creates two scopes: the first starts at the ( and has full == 1,
the second starts at the { and has full == 0. Both end at the
}, so calls to start and pre/post_end will match. Anything
pushed onto the save stack by this hook will be popped just before the
scope ends (between the pre_ and post_end hooks, in fact).





void bhk_pre_end(pTHX_ OP **o)




This is called at the end of a lexical scope, just before unwinding the
stack. o is the root of the optree representing the scope; it is a
double pointer so you can replace the OP if you need to.





void bhk_post_end(pTHX_ OP **o)




This is called at the end of a lexical scope, just after unwinding the
stack. o is as above. Note that it is possible for calls to pre_
and post_end to nest, if there is something on the save stack that
calls string eval.





void bhk_eval(pTHX_ OP *const o)




This is called just before starting to compile an eval STRING, do
FILE, require or use, after the eval has been set up. o is the
OP that requested the eval, and will normally be an OP_ENTEREVAL,
OP_DOFILE or OP_REQUIRE.








Once you have your hook functions, you need a BHK structure to put
them in. It’s best to allocate it statically, since there is no way to
free it once it’s registered. The function pointers should be inserted
into this structure using the BhkENTRY_set macro, which will also set
flags indicating which entries are valid. If you do need to allocate
your BHK dynamically for some reason, be sure to zero it before you
start.



Once registered, there is no mechanism to switch these hooks off, so if
that is necessary you will need to do this yourself. An entry in %^H
is probably the best way, so the effect is lexically scoped; however it
is also possible to use the BhkDISABLE and BhkENABLE macros to
temporarily switch entries on and off. You should also be aware that
generally speaking at least one scope will have opened before your
extension is loaded, so you will see some pre/post_end pairs that
didn’t have a matching start.









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28.6 Examining internal data structures with the dump functions



To aid debugging, the source file dump.c contains a number of
functions which produce formatted output of internal data structures.



The most commonly used of these functions is Perl_sv_dump; it’s used
for dumping SVs, AVs, HVs, and CVs. The Devel::Peek module calls
sv_dump to produce debugging output from Perl-space, so users of that
module should already be familiar with its format.



Perl_op_dump can be used to dump an OP structure or any of its
derivatives, and produces output similar to perl -Dx; in fact,
Perl_dump_eval will dump the main root of the code being evaluated,
exactly like -Dx.



Other useful functions are Perl_dump_sub, which turns a GV into an
op tree, Perl_dump_packsubs which calls Perl_dump_sub on all the
subroutines in a package like so: (Thankfully, these are all xsubs, so
there is no op tree)



    (gdb) print Perl_dump_packsubs(PL_defstash)

    SUB attributes::bootstrap = (xsub 0x811fedc 0)

    SUB UNIVERSAL::can = (xsub 0x811f50c 0)

    SUB UNIVERSAL::isa = (xsub 0x811f304 0)

    SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)

    SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)



and Perl_dump_all, which dumps all the subroutines in the stash and
the op tree of the main root.









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28.7 How multiple interpreters and concurrency are supported





• perlguts Background and PERL_IMPLICIT_CONTEXT:  



• perlguts So what happened to dTHR?:  



• perlguts How do I use all this in extensions?:  



• perlguts Should I do anything special if I call perl from multiple threads?:  



• perlguts Future Plans and PERL_IMPLICIT_SYS:  












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28.7.1 Background and PERL_IMPLICIT_CONTEXT



The Perl interpreter can be regarded as a closed box: it has an API
for feeding it code or otherwise making it do things, but it also has
functions for its own use.  This smells a lot like an object, and
there are ways for you to build Perl so that you can have multiple
interpreters, with one interpreter represented either as a C structure,
or inside a thread-specific structure.  These structures contain all
the context, the state of that interpreter.



One macro controls the major Perl build flavor: MULTIPLICITY. The
MULTIPLICITY build has a C structure that packages all the interpreter
state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
normally defined, and enables the support for passing in a "hidden" first
argument that represents all three data structures. MULTIPLICITY makes
multi-threaded perls possible (with the ithreads threading model, related
to the macro USE_ITHREADS.)



Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
former turns on MULTIPLICITY.)  The PERL_GLOBAL_STRUCT causes all the
internal variables of Perl to be wrapped inside a single global struct,
struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
the function  Perl_GetVars().  The PERL_GLOBAL_STRUCT_PRIVATE goes
one step further, there is still a single struct (allocated in main()
either from heap or from stack) but there are no global data symbols
pointing to it.  In either case the global struct should be initialised
as the very first thing in main() using Perl_init_global_struct() and
correspondingly tear it down after perl_free() using Perl_free_global_struct(),
please see miniperlmain.c for usage details.  You may also need
to use dVAR in your coding to "declare the global variables"
when you are using them.  dTHX does this for you automatically.



To see whether you have non-const data you can use a BSD-compatible nm:



  nm libperl.a | grep -v ' [TURtr] '



If this displays any D or d symbols, you have non-const data.



For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
doesn’t actually hide all symbols inside a big global struct: some
PerlIO_xxx vtables are left visible.  The PERL_GLOBAL_STRUCT_PRIVATE
then hides everything (see how the PERLIO_FUNCS_DECL is used).



All this obviously requires a way for the Perl internal functions to be
either subroutines taking some kind of structure as the first
argument, or subroutines taking nothing as the first argument.  To
enable these two very different ways of building the interpreter,
the Perl source (as it does in so many other situations) makes heavy
use of macros and subroutine naming conventions.



First problem: deciding which functions will be public API functions and
which will be private.  All functions whose names begin S_ are private
(think "S" for "secret" or "static").  All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it is
part of the API. (See Internal Functions.) The easiest way to be sure a
function is part of the API is to find its entry in (perlapi).
If it exists in (perlapi), it’s part of the API.  If it doesn’t, and you
think it should be (i.e., you need it for your extension), send mail via
(perlbug) explaining why you think it should be.



Second problem: there must be a syntax so that the same subroutine
declarations and calls can pass a structure as their first argument,
or pass nothing.  To solve this, the subroutines are named and
declared in a particular way.  Here’s a typical start of a static
function used within the Perl guts:



  STATIC void
  S_incline(pTHX_ char *s)



STATIC becomes "static" in C, and may be #define’d to nothing in some
configurations in the future.



A public function (i.e. part of the internal API, but not necessarily
sanctioned for use in extensions) begins like this:



  void
  Perl_sv_setiv(pTHX_ SV* dsv, IV num)



pTHX_ is one of a number of macros (in perl.h) that hide the
details of the interpreter’s context.  THX stands for "thread", "this",
or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)
The first character could be ’p’ for a prototype, ’a’ for argument,
or ’d’ for declaration, so we have pTHX, aTHX and dTHX, and
their variants.



When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
first argument containing the interpreter’s context.  The trailing underscore
in the pTHX_ macro indicates that the macro expansion needs a comma
after the context argument because other arguments follow it.  If
PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
subroutine is not prototyped to take the extra argument.  The form of the
macro without the trailing underscore is used when there are no additional
explicit arguments.



When a core function calls another, it must pass the context.  This
is normally hidden via macros.  Consider sv_setiv.  It expands into
something like this:



    #ifdef PERL_IMPLICIT_CONTEXT
      #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
      /* can't do this for vararg functions, see below */
    #else
      #define sv_setiv           Perl_sv_setiv
    #endif



This works well, and means that XS authors can gleefully write:



    sv_setiv(foo, bar);



and still have it work under all the modes Perl could have been
compiled with.



This doesn’t work so cleanly for varargs functions, though, as macros
imply that the number of arguments is known in advance.  Instead we
either need to spell them out fully, passing aTHX_ as the first
argument (the Perl core tends to do this with functions like
Perl_warner), or use a context-free version.



The context-free version of Perl_warner is called
Perl_warner_nocontext, and does not take the extra argument.  Instead
it does dTHX; to get the context from thread-local storage.  We
#define warner Perl_warner_nocontext so that extensions get source
compatibility at the expense of performance.  (Passing an arg is
cheaper than grabbing it from thread-local storage.)



You can ignore [pad]THXx when browsing the Perl headers/sources.
Those are strictly for use within the core.  Extensions and embedders
need only be aware of [pad]THX.









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28.7.2 So what happened to dTHR?



dTHR was introduced in perl 5.005 to support the older thread model.
The older thread model now uses the THX mechanism to pass context
pointers around, so dTHR is not useful any more.  Perl 5.6.0 and
later still have it for backward source compatibility, but it is defined
to be a no-op.









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28.7.3 How do I use all this in extensions?



When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
any functions in the Perl API will need to pass the initial context
argument somehow.  The kicker is that you will need to write it in
such a way that the extension still compiles when Perl hasn’t been
built with PERL_IMPLICIT_CONTEXT enabled.



There are three ways to do this.  First, the easy but inefficient way,
which is also the default, in order to maintain source compatibility
with extensions: whenever XSUB.h is #included, it redefines the aTHX
and aTHX_ macros to call a function that will return the context.
Thus, something like:



        sv_setiv(sv, num);



in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
in effect:



        Perl_sv_setiv(Perl_get_context(), sv, num);



or to this otherwise:



        Perl_sv_setiv(sv, num);



You don’t have to do anything new in your extension to get this; since
the Perl library provides Perl_get_context(), it will all just
work.



The second, more efficient way is to use the following template for
your Foo.xs:



        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        STATIC void my_private_function(int arg1, int arg2);

        STATIC void
        my_private_function(int arg1, int arg2)
        {
            dTHX;       /* fetch context */
            ... call many Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(arg, 10);



Note that the only two changes from the normal way of writing an
extension is the addition of a #define PERL_NO_GET_CONTEXT before
including the Perl headers, followed by a dTHX; declaration at
the start of every function that will call the Perl API.  (You’ll
know which functions need this, because the C compiler will complain
that there’s an undeclared identifier in those functions.)  No changes
are needed for the XSUBs themselves, because the XS() macro is
correctly defined to pass in the implicit context if needed.



The third, even more efficient way is to ape how it is done within
the Perl guts:



        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        /* pTHX_ only needed for functions that call Perl API */
        STATIC void my_private_function(pTHX_ int arg1, int arg2);

        STATIC void
        my_private_function(pTHX_ int arg1, int arg2)
        {
            /* dTHX; not needed here, because THX is an argument */
            ... call Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(aTHX_ arg, 10);



This implementation never has to fetch the context using a function
call, since it is always passed as an extra argument.  Depending on
your needs for simplicity or efficiency, you may mix the previous
two approaches freely.



Never add a comma after pTHX yourself–always use the form of the
macro with the underscore for functions that take explicit arguments,
or the form without the argument for functions with no explicit arguments.



If one is compiling Perl with the -DPERL_GLOBAL_STRUCT the dVAR
definition is needed if the Perl global variables (see perlvars.h
or globvar.sym) are accessed in the function and dTHX is not
used (the dTHX includes the dVAR if necessary).  One notices
the need for dVAR only with the said compile-time define, because
otherwise the Perl global variables are visible as-is.









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28.7.4 Should I do anything special if I call perl from multiple threads?



If you create interpreters in one thread and then proceed to call them in
another, you need to make sure perl’s own Thread Local Storage (TLS) slot is
initialized correctly in each of those threads.



The perl_alloc and perl_clone API functions will automatically set
the TLS slot to the interpreter they created, so that there is no need to do
anything special if the interpreter is always accessed in the same thread that
created it, and that thread did not create or call any other interpreters
afterwards.  If that is not the case, you have to set the TLS slot of the
thread before calling any functions in the Perl API on that particular
interpreter.  This is done by calling the PERL_SET_CONTEXT macro in that
thread as the first thing you do:



        /* do this before doing anything else with some_perl */
        PERL_SET_CONTEXT(some_perl);

        ... other Perl API calls on some_perl go here ...









Previous: perlguts Should I do anything special if I call perl from multiple threads?, Up: perlguts How multiple interpreters and concurrency are supported   [Contents]





28.7.5 Future Plans and PERL_IMPLICIT_SYS



Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
that the interpreter knows about itself and pass it around, so too are
there plans to allow the interpreter to bundle up everything it knows
about the environment it’s running on.  This is enabled with the
PERL_IMPLICIT_SYS macro.  Currently it only works with USE_ITHREADS on
Windows.



This allows the ability to provide an extra pointer (called the "host"
environment) for all the system calls.  This makes it possible for
all the system stuff to maintain their own state, broken down into
seven C structures.  These are thin wrappers around the usual system
calls (see win32/perllib.c) for the default perl executable, but for a
more ambitious host (like the one that would do fork() emulation) all
the extra work needed to pretend that different interpreters are
actually different "processes", would be done here.



The Perl engine/interpreter and the host are orthogonal entities.
There could be one or more interpreters in a process, and one or
more "hosts", with free association between them.









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28.8 Internal Functions



All of Perl’s internal functions which will be exposed to the outside
world are prefixed by Perl_ so that they will not conflict with XS
functions or functions used in a program in which Perl is embedded.
Similarly, all global variables begin with PL_. (By convention,
static functions start with S_.)



Inside the Perl core (PERL_CORE defined), you can get at the functions
either with or without the Perl_ prefix, thanks to a bunch of defines
that live in embed.h. Note that extension code should not set
PERL_CORE; this exposes the full perl internals, and is likely to cause
breakage of the XS in each new perl release.



The file embed.h is generated automatically from
embed.pl and embed.fnc. embed.pl also creates the prototyping
header files for the internal functions, generates the documentation
and a lot of other bits and pieces. It’s important that when you add
a new function to the core or change an existing one, you change the
data in the table in embed.fnc as well. Here’s a sample entry from
that table:



    Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 lval



The second column is the return type, the third column the name. Columns
after that are the arguments. The first column is a set of flags:





A




This function is a part of the public API. All such functions should also
have ’d’, very few do not.





p




This function has a Perl_ prefix; i.e. it is defined as
Perl_av_fetch.





d




This function has documentation using the apidoc feature which we’ll
look at in a second.  Some functions have ’d’ but not ’A’; docs are good.








Other available flags are:





s




This is a static function and is defined as STATIC S_whatever, and
usually called within the sources as whatever(...).





n




This does not need an interpreter context, so the definition has no
pTHX, and it follows that callers don’t use aTHX.  (See
Background and PERL_IMPLICIT_CONTEXT.)





r




This function never returns; croak, exit and friends.





f




This function takes a variable number of arguments, printf style.
The argument list should end with ..., like this:



    Afprd   |void   |croak          |const char* pat|...





M




This function is part of the experimental development API, and may change
or disappear without notice.





o




This function should not have a compatibility macro to define, say,
Perl_parse to parse. It must be called as Perl_parse.





x




This function isn’t exported out of the Perl core.





m




This is implemented as a macro.





X




This function is explicitly exported.





E




This function is visible to extensions included in the Perl core.





b




Binary backward compatibility; this function is a macro but also has
a Perl_ implementation (which is exported).





others




See the comments at the top of embed.fnc for others.








If you edit embed.pl or embed.fnc, you will need to run
make regen_headers to force a rebuild of embed.h and other
auto-generated files.





• perlguts Formatted Printing of IVs, UVs, and NVs:  



• perlguts Pointer-To-Integer and Integer-To-Pointer:  



• perlguts Exception Handling:  



• perlguts Source Documentation:  



• perlguts Backwards compatibility:  












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28.8.1 Formatted Printing of IVs, UVs, and NVs



If you are printing IVs, UVs, or NVS instead of the stdio(3) style
formatting codes like %d, %ld, %f, you should use the
following macros for portability



        IVdf            IV in decimal
        UVuf            UV in decimal
        UVof            UV in octal
        UVxf            UV in hexadecimal
        NVef            NV %e-like
        NVff            NV %f-like
        NVgf            NV %g-like



These will take care of 64-bit integers and long doubles.
For example:



        printf("IV is %"IVdf"\n", iv);



The IVdf will expand to whatever is the correct format for the IVs.



If you are printing addresses of pointers, use UVxf combined
with PTR2UV(), do not use %lx or %p.









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28.8.2 Pointer-To-Integer and Integer-To-Pointer



Because pointer size does not necessarily equal integer size,
use the follow macros to do it right.



        PTR2UV(pointer)
        PTR2IV(pointer)
        PTR2NV(pointer)
        INT2PTR(pointertotype, integer)



For example:



        IV  iv = ...;
        SV *sv = INT2PTR(SV*, iv);



and



        AV *av = ...;
        UV  uv = PTR2UV(av);









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28.8.3 Exception Handling



There are a couple of macros to do very basic exception handling in XS
modules. You have to define NO_XSLOCKS before including XSUB.h to
be able to use these macros:



        #define NO_XSLOCKS
        #include "XSUB.h"



You can use these macros if you call code that may croak, but you need
to do some cleanup before giving control back to Perl. For example:



        dXCPT;    /* set up necessary variables */

        XCPT_TRY_START {
          code_that_may_croak();
        } XCPT_TRY_END

        XCPT_CATCH
        {
          /* do cleanup here */
          XCPT_RETHROW;
        }



Note that you always have to rethrow an exception that has been
caught. Using these macros, it is not possible to just catch the
exception and ignore it. If you have to ignore the exception, you
have to use the call_* function.



The advantage of using the above macros is that you don’t have
to setup an extra function for call_*, and that using these
macros is faster than using call_*.









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28.8.4 Source Documentation



There’s an effort going on to document the internal functions and
automatically produce reference manuals from them - (perlapi) is one
such manual which details all the functions which are available to XS
writers. (perlintern) is the autogenerated manual for the functions
which are not part of the API and are supposedly for internal use only.



Source documentation is created by putting POD comments into the C
source, like this:



 /*
 =for apidoc sv_setiv

 Copies an integer into the given SV.  Does not handle 'set' magic.  See
 C<sv_setiv_mg>.

 =cut
 */



Please try and supply some documentation if you add functions to the
Perl core.









Previous: perlguts Source Documentation, Up: perlguts Internal Functions   [Contents]





28.8.5 Backwards compatibility



The Perl API changes over time. New functions are added or the interfaces
of existing functions are changed. The Devel::PPPort module tries to
provide compatibility code for some of these changes, so XS writers don’t
have to code it themselves when supporting multiple versions of Perl.



Devel::PPPort generates a C header file ppport.h that can also
be run as a Perl script. To generate ppport.h, run:



    perl -MDevel::PPPort -eDevel::PPPort::WriteFile



Besides checking existing XS code, the script can also be used to retrieve
compatibility information for various API calls using the --api-info
command line switch. For example:



  % perl ppport.h --api-info=sv_magicext



For details, see perldoc ppport.h.









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28.9 Unicode Support



Perl 5.6.0 introduced Unicode support. It’s important for porters and XS
writers to understand this support and make sure that the code they
write does not corrupt Unicode data.





• perlguts What is Unicode, anyway?:  



• perlguts How can I recognise a UTF-8 string?:  



• perlguts How does UTF-8 represent Unicode characters?:  



• perlguts How does Perl store UTF-8 strings?:  



• perlguts How do I convert a string to UTF-8?:  



• perlguts Is there anything else I need to know?:  












Next: , Up: perlguts Unicode Support   [Contents]





28.9.1 What is Unicode, anyway?



In the olden, less enlightened times, we all used to use ASCII. Most of
us did, anyway. The big problem with ASCII is that it’s American. Well,
no, that’s not actually the problem; the problem is that it’s not
particularly useful for people who don’t use the Roman alphabet. What
used to happen was that particular languages would stick their own
alphabet in the upper range of the sequence, between 128 and 255. Of
course, we then ended up with plenty of variants that weren’t quite
ASCII, and the whole point of it being a standard was lost.



Worse still, if you’ve got a language like Chinese or
Japanese that has hundreds or thousands of characters, then you really
can’t fit them into a mere 256, so they had to forget about ASCII
altogether, and build their own systems using pairs of numbers to refer
to one character.



To fix this, some people formed Unicode, Inc. and
produced a new character set containing all the characters you can
possibly think of and more. There are several ways of representing these
characters, and the one Perl uses is called UTF-8. UTF-8 uses
a variable number of bytes to represent a character. You can learn more
about Unicode and Perl’s Unicode model in perlunicode NAME.









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28.9.2 How can I recognise a UTF-8 string?



You can’t. This is because UTF-8 data is stored in bytes just like
non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
capital E with a grave accent, is represented by the two bytes
v196.172. Unfortunately, the non-Unicode string chr(196).chr(172)
has that byte sequence as well. So you can’t tell just by looking - this
is what makes Unicode input an interesting problem.



In general, you either have to know what you’re dealing with, or you
have to guess.  The API function is_utf8_string can help; it’ll tell
you if a string contains only valid UTF-8 characters. However, it can’t
do the work for you. On a character-by-character basis, is_utf8_char
will tell you whether the current character in a string is valid UTF-8. 









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28.9.3 How does UTF-8 represent Unicode characters?



As mentioned above, UTF-8 uses a variable number of bytes to store a
character. Characters with values 0...127 are stored in one byte, just
like good ol’ ASCII. Character 128 is stored as v194.128; this
continues up to character 191, which is v194.191. Now we’ve run out of
bits (191 is binary 10111111) so we move on; 192 is v195.128. And
so it goes on, moving to three bytes at character 2048.



Assuming you know you’re dealing with a UTF-8 string, you can find out
how long the first character in it is with the UTF8SKIP macro:



    char *utf = "\305\233\340\240\201";
    I32 len;

    len = UTF8SKIP(utf); /* len is 2 here */
    utf += len;
    len = UTF8SKIP(utf); /* len is 3 here */



Another way to skip over characters in a UTF-8 string is to use
utf8_hop, which takes a string and a number of characters to skip
over. You’re on your own about bounds checking, though, so don’t use it
lightly.



All bytes in a multi-byte UTF-8 character will have the high bit set,
so you can test if you need to do something special with this
character like this (the UTF8_IS_INVARIANT() is a macro that tests
whether the byte can be encoded as a single byte even in UTF-8):



    U8 *utf;
    U8 *utf_end; /* 1 beyond buffer pointed to by utf */
    UV uv;      /* Note: a UV, not a U8, not a char */
    STRLEN len; /* length of character in bytes */

    if (!UTF8_IS_INVARIANT(*utf))
        /* Must treat this as UTF-8 */
        uv = utf8_to_uvchr_buf(utf, utf_end, &len);
    else
        /* OK to treat this character as a byte */
        uv = *utf;



You can also see in that example that we use utf8_to_uvchr_buf to get the
value of the character; the inverse function uvchr_to_utf8 is available
for putting a UV into UTF-8:



    if (!UTF8_IS_INVARIANT(uv))
        /* Must treat this as UTF8 */
        utf8 = uvchr_to_utf8(utf8, uv);
    else
        /* OK to treat this character as a byte */
        *utf8++ = uv;



You must convert characters to UVs using the above functions if
you’re ever in a situation where you have to match UTF-8 and non-UTF-8
characters. You may not skip over UTF-8 characters in this case. If you
do this, you’ll lose the ability to match hi-bit non-UTF-8 characters;
for instance, if your UTF-8 string contains v196.172, and you skip
that character, you can never match a chr(200) in a non-UTF-8 string.
So don’t do that!









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28.9.4 How does Perl store UTF-8 strings?



Currently, Perl deals with Unicode strings and non-Unicode strings
slightly differently. A flag in the SV, SVf_UTF8, indicates that the
string is internally encoded as UTF-8. Without it, the byte value is the
codepoint number and vice versa (in other words, the string is encoded
as iso-8859-1, but use feature 'unicode_strings' is needed to get iso-8859-1
semantics). You can check and manipulate this flag with the
following macros:



    SvUTF8(sv)
    SvUTF8_on(sv)
    SvUTF8_off(sv)



This flag has an important effect on Perl’s treatment of the string: if
Unicode data is not properly distinguished, regular expressions,
length, substr and other string handling operations will have
undesirable results.



The problem comes when you have, for instance, a string that isn’t
flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
especially when combining non-UTF-8 and UTF-8 strings.



Never forget that the SVf_UTF8 flag is separate to the PV value; you
need be sure you don’t accidentally knock it off while you’re
manipulating SVs. More specifically, you cannot expect to do this:



    SV *sv;
    SV *nsv;
    STRLEN len;
    char *p;

    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);



The char* string does not tell you the whole story, and you can’t
copy or reconstruct an SV just by copying the string value. Check if the
old SV has the UTF8 flag set, and act accordingly:



    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);
    if (SvUTF8(sv))
        SvUTF8_on(nsv);



In fact, your frobnicate function should be made aware of whether or
not it’s dealing with UTF-8 data, so that it can handle the string
appropriately.



Since just passing an SV to an XS function and copying the data of
the SV is not enough to copy the UTF8 flags, even less right is just
passing a char * to an XS function.









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28.9.5 How do I convert a string to UTF-8?



If you’re mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
one of the strings to UTF-8. If you’ve got an SV, the easiest way to do
this is:



    sv_utf8_upgrade(sv);



However, you must not do this, for example:



    if (!SvUTF8(left))
        sv_utf8_upgrade(left);



If you do this in a binary operator, you will actually change one of the
strings that came into the operator, and, while it shouldn’t be noticeable
by the end user, it can cause problems in deficient code.



Instead, bytes_to_utf8 will give you a UTF-8-encoded copy of its
string argument. This is useful for having the data available for
comparisons and so on, without harming the original SV. There’s also
utf8_to_bytes to go the other way, but naturally, this will fail if
the string contains any characters above 255 that can’t be represented
in a single byte.









Previous: perlguts How do I convert a string to UTF-8?, Up: perlguts Unicode Support   [Contents]





28.9.6 Is there anything else I need to know?



Not really. Just remember these things:











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28.10 Custom Operators



Custom operator support is a new experimental feature that allows you to
define your own ops. This is primarily to allow the building of
interpreters for other languages in the Perl core, but it also allows
optimizations through the creation of "macro-ops" (ops which perform the
functions of multiple ops which are usually executed together, such as
gvsv, gvsv, add.)



This feature is implemented as a new op type, OP_CUSTOM. The Perl
core does not "know" anything special about this op type, and so it will
not be involved in any optimizations. This also means that you can
define your custom ops to be any op structure - unary, binary, list and
so on - you like.



It’s important to know what custom operators won’t do for you. They
won’t let you add new syntax to Perl, directly. They won’t even let you
add new keywords, directly. In fact, they won’t change the way Perl
compiles a program at all. You have to do those changes yourself, after
Perl has compiled the program. You do this either by manipulating the op
tree using a CHECK block and the B::Generate module, or by adding
a custom peephole optimizer with the optimize module.



When you do this, you replace ordinary Perl ops with custom ops by
creating ops with the type OP_CUSTOM and the pp_addr of your own
PP function. This should be defined in XS code, and should look like
the PP ops in pp_*.c. You are responsible for ensuring that your op
takes the appropriate number of values from the stack, and you are
responsible for adding stack marks if necessary.



You should also "register" your op with the Perl interpreter so that it
can produce sensible error and warning messages. Since it is possible to
have multiple custom ops within the one "logical" op type OP_CUSTOM,
Perl uses the value of o->op_ppaddr to determine which custom op
it is dealing with. You should create an XOP structure for each
ppaddr you use, set the properties of the custom op with
XopENTRY_set, and register the structure against the ppaddr using
Perl_custom_op_register. A trivial example might look like:



    static XOP my_xop;
    static OP *my_pp(pTHX);

    BOOT:
        XopENTRY_set(&my_xop, xop_name, "myxop");
        XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
        Perl_custom_op_register(aTHX_ my_pp, &my_xop);



The available fields in the structure are:





xop_name




A short name for your op. This will be included in some error messages,
and will also be returned as $op->name by the (B)B module, so
it will appear in the output of module like (B-Concise)B::Concise.





xop_desc




A short description of the function of the op.





xop_class




Which of the various *OP structures this op uses. This should be one of
the OA_* constants from op.h, namely





OA_BASEOP






OA_UNOP






OA_BINOP






OA_LOGOP






OA_LISTOP






OA_PMOP






OA_SVOP






OA_PADOP






OA_PVOP_OR_SVOP




This should be interpreted as ’PVOP’ only. The _OR_SVOP is because
the only core PVOP, OP_TRANS, can sometimes be a SVOP instead.





OA_LOOP






OA_COP









The other OA_* constants should not be used.





xop_peep




This member is of type Perl_cpeep_t, which expands to void
(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop). If it is set, this function
will be called from Perl_rpeep when ops of this type are encountered
by the peephole optimizer. o is the OP that needs optimizing;
oldop is the previous OP optimized, whose op_next points to o.








B::Generate directly supports the creation of custom ops by name.









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28.11 AUTHORS



Until May 1997, this document was maintained by Jeff Okamoto
<okamoto@corp.hp.com>.  It is now maintained as part of Perl
itself by the Perl 5 Porters <perl5-porters@perl.org>.



With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
Stephen McCamant, and Gurusamy Sarathy.









Previous: perlguts AUTHORS, Up: perlguts   [Contents]





28.12 SEE ALSO



(perlapi), (perlintern), (perlxs), perlembed NAME









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29 perlhack





• perlhack NAME:  



• perlhack DESCRIPTION:  



• perlhack SUPER QUICK PATCH GUIDE:  



• perlhack BUG REPORTING:  



• perlhack PERL 5 PORTERS:  



• perlhack GETTING THE PERL SOURCE:  



• perlhack PATCHING PERL:  



• perlhack TESTING:  



• perlhack MORE READING FOR GUTS HACKERS:  



• perlhack CPAN TESTERS AND PERL SMOKERS:  



• perlhack WHAT NEXT?:  



• perlhack AUTHOR:  












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29.1 NAME



perlhack - How to hack on Perl









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29.2 DESCRIPTION



This document explains how Perl development works. It includes details
about the Perl 5 Porters email list, the Perl repository, the Perlbug
bug tracker, patch guidelines, and commentary on Perl development
philosophy.









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29.3 SUPER QUICK PATCH GUIDE



If you just want to submit a single small patch like a pod fix, a test
for a bug, comment fixes, etc., it’s easy! Here’s how:











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29.4 BUG REPORTING



If you want to report a bug in Perl, you must use the perlbug
command line tool. This tool will ensure that your bug report includes
all the relevant system and configuration information.



To browse existing Perl bugs and patches, you can use the web interface
at http://rt.perl.org/.



Please check the archive of the perl5-porters list (see below) and/or
the bug tracking system before submitting a bug report. Often, you’ll
find that the bug has been reported already.



You can log in to the bug tracking system and comment on existing bug
reports. If you have additional information regarding an existing bug,
please add it. This will help the porters fix the bug.









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29.5 PERL 5 PORTERS



The perl5-porters (p5p) mailing list is where the Perl standard
distribution is maintained and developed. The people who maintain Perl
are also referred to as the "Perl 5 Porters", "p5p" or just the
"porters".



A searchable archive of the list is available at
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/. There is
also another archive at
http://archive.develooper.com/perl5-porters@perl.org/.





• perlhack perl-changes mailing list:  



• perlhack #p5p on IRC:  












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29.5.1 perl-changes mailing list



The perl5-changes mailing list receives a copy of each patch that gets
submitted to the maintenance and development branches of the perl
repository. See http://lists.perl.org/list/perl5-changes.html for
subscription and archive information.









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29.5.2 #p5p on IRC



Many porters are also active on the irc://irc.perl.org/#p5p channel.
Feel free to join the channel and ask questions about hacking on the
Perl core.









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29.6 GETTING THE PERL SOURCE



All of Perl’s source code is kept centrally in a Git repository at
perl5.git.perl.org. The repository contains many Perl revisions from
Perl 1 onwards and all the revisions from Perforce, the previous
version control system.



For much more detail on using git with the Perl repository, please see
perlgit NAME.





• perlhack Read access via Git:  



• perlhack Read access via the web:  



• perlhack Read access via rsync:  



• perlhack Write access via git:  












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29.6.1 Read access via Git



You will need a copy of Git for your computer. You can fetch a copy of
the repository using the git protocol:



  % git clone git://perl5.git.perl.org/perl.git perl



This clones the repository and makes a local copy in the perl
directory.



If you cannot use the git protocol for firewall reasons, you can also
clone via http, though this is much slower:



  % git clone http://perl5.git.perl.org/perl.git perl









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29.6.2 Read access via the web



You may access the repository over the web. This allows you to browse
the tree, see recent commits, subscribe to RSS feeds for the changes,
search for particular commits and more. You may access it at
http://perl5.git.perl.org/perl.git. A mirror of the repository is
found at http://github.com/mirrors/perl.









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29.6.3 Read access via rsync



You can also choose to use rsync to get a copy of the current source
tree for the bleadperl branch and all maintenance branches:



    % rsync -avz rsync://perl5.git.perl.org/perl-current .
    % rsync -avz rsync://perl5.git.perl.org/perl-5.12.x .
    % rsync -avz rsync://perl5.git.perl.org/perl-5.10.x .
    % rsync -avz rsync://perl5.git.perl.org/perl-5.8.x .
    % rsync -avz rsync://perl5.git.perl.org/perl-5.6.x .
    % rsync -avz rsync://perl5.git.perl.org/perl-5.005xx .



(Add the --delete option to remove leftover files.)



To get a full list of the available sync points:



    % rsync perl5.git.perl.org::









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29.6.4 Write access via git



If you have a commit bit, please see perlgit NAME for more details on
using git.









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29.7 PATCHING PERL



If you’re planning to do more extensive work than a single small fix,
we encourage you to read the documentation below. This will help you
focus your work and make your patches easier to incorporate into the
Perl source.





• perlhack Submitting patches:  



• perlhack Getting your patch accepted:  



• perlhack Patching a core module:  



• perlhack Updating perldelta:  



• perlhack What makes for a good patch?:  












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29.7.1 Submitting patches



If you have a small patch to submit, please submit it via perlbug. You
can also send email directly to perlbug@perl.org. Please note that
messages sent to perlbug may be held in a moderation queue, so you
won’t receive a response immediately.



You’ll know your submission has been processed when you receive an
email from our ticket tracking system. This email will give you a
ticket number. Once your patch has made it to the ticket tracking
system, it will also be sent to the perl5-porters@perl.org list.



Patches are reviewed and discussed on the p5p list. Simple,
uncontroversial patches will usually be applied without any discussion.
When the patch is applied, the ticket will be updated and you will
receive email. In addition, an email will be sent to the p5p list.



In other cases, the patch will need more work or discussion. That will
happen on the p5p list.



You are encouraged to participate in the discussion and advocate for
your patch. Sometimes your patch may get lost in the shuffle. It’s
appropriate to send a reminder email to p5p if no action has been taken
in a month. Please remember that the Perl 5 developers are all
volunteers, and be polite.



Changes are always applied directly to the main development branch,
called "blead". Some patches may be backported to a maintenance branch.
If you think your patch is appropriate for the maintenance branch,
please explain why when you submit it.









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29.7.2 Getting your patch accepted



If you are submitting a code patch there are several things that you
can do to help the Perl 5 Porters accept your patch.





• perlhack Patch style:  



• perlhack Commit message:  



• perlhack Comments, Comments, Comments:  



• perlhack Style:  



• perlhack Test suite:  












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29.7.2.1 Patch style



If you used git to check out the Perl source, then using git
format-patch will produce a patch in a style suitable for Perl. The
format-patch command produces one patch file for each commit you
made. If you prefer to send a single patch for all commits, you can use
git diff.



  % git checkout blead
  % git pull
  % git diff blead my-branch-name



This produces a patch based on the difference between blead and your
current branch. It’s important to make sure that blead is up to date
before producing the diff, that’s why we call git pull first.



We strongly recommend that you use git if possible. It will make your
life easier, and ours as well.



However, if you’re not using git, you can still produce a suitable
patch. You’ll need a pristine copy of the Perl source to diff against.
The porters prefer unified diffs. Using GNU diff, you can produce a
diff like this:



  % diff -Npurd perl.pristine perl.mine



Make sure that you make realclean in your copy of Perl to remove any
build artifacts, or you may get a confusing result.









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29.7.2.2 Commit message



As you craft each patch you intend to submit to the Perl core, it’s
important to write a good commit message. This is especially important
if your submission will consist of a series of commits.



The first line of the commit message should be a short description
without a period. It should be no longer than the subject line of an
email, 50 characters being a good rule of thumb.



A lot of Git tools (Gitweb, GitHub, git log –pretty=oneline, ...) will
only display the first line (cut off at 50 characters) when presenting
commit summaries.



The commit message should include a description of the problem that the
patch corrects or new functionality that the patch adds.



As a general rule of thumb, your commit message should help a
programmer who knows the Perl core quickly understand what you were
trying to do, how you were trying to do it, and why the change matters
to Perl.





A commit message isn’t intended to take the place of comments in your
code. Commit messages should describe the change you made, while code
comments should describe the current state of the code.



If you’ve just implemented a new feature, complete with doc, tests and
well-commented code, a brief commit message will often suffice. If,
however, you’ve just changed a single character deep in the parser or
lexer, you might need to write a small novel to ensure that future
readers understand what you did and why you did it.









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29.7.2.3 Comments, Comments, Comments



Be sure to adequately comment your code. While commenting every line is
unnecessary, anything that takes advantage of side effects of
operators, that creates changes that will be felt outside of the
function being patched, or that others may find confusing should be
documented. If you are going to err, it is better to err on the side of
adding too many comments than too few.



The best comments explain why the code does what it does, not what
it does.









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29.7.2.4 Style



In general, please follow the particular style of the code you are
patching.



In particular, follow these general guidelines for patching Perl
sources:











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29.7.2.5 Test suite



If your patch changes code (rather than just changing documentation),
you should also include one or more test cases which illustrate the bug
you’re fixing or validate the new functionality you’re adding. In
general, you should update an existing test file rather than create a
new one.



Your test suite additions should generally follow these guidelines
(courtesy of Gurusamy Sarathy <gsar@activestate.com>):











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29.7.3 Patching a core module



This works just like patching anything else, with one extra
consideration.



Modules in the cpan/ directory of the source tree are maintained
outside of the Perl core. When the author updates the module, the
updates are simply copied into the core.  See that module’s
documentation or its listing on http://search.cpan.org/ for more
information on reporting bugs and submitting patches.



In most cases, patches to modules in cpan/ should be sent upstream
and should not be applied to the Perl core individually.  If a patch to
a file in cpan/ absolutely cannot wait for the fix to be made
upstream, released to CPAN and copied to blead, you must add (or
update) a CUSTOMIZED entry in the "Porting/Maintainers.pl" file
to flag that a local modification has been made.  See
"Porting/Maintainers.pl" for more details.



In contrast, modules in the dist/ directory are maintained in the
core.









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29.7.4 Updating perldelta



For changes significant enough to warrant a pod/perldelta.pod entry,
the porters will greatly appreciate it if you submit a delta entry
along with your actual change. Significant changes include, but are not
limited to:





Please make sure you add the perldelta entry to the right section
within pod/perldelta.pod. More information on how to write good
perldelta entries is available in the Style section of
Porting/how_to_write_a_perldelta.pod.









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29.7.5 What makes for a good patch?



New features and extensions to the language can be contentious. There
is no specific set of criteria which determine what features get added,
but here are some questions to consider when developing a patch:





• perlhack Does the concept match the general goals of Perl?:  



• perlhack Where is the implementation?:  



• perlhack Backwards compatibility:  



• perlhack Could it be a module instead?:  



• perlhack Is the feature generic enough?:  



• perlhack Does it potentially introduce new bugs?:  



• perlhack How big is it?:  



• perlhack Does it preclude other desirable features?:  



• perlhack Is the implementation robust?:  



• perlhack Is the implementation generic enough to be portable?:  



• perlhack Is the implementation tested?:  



• perlhack Is there enough documentation?:  



• perlhack Is there another way to do it?:  



• perlhack Does it create too much work?:  



• perlhack Patches speak louder than words:  












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29.7.5.1 Does the concept match the general goals of Perl?



Our goals include, but are not limited to:



    

  1.  Keep it fast, simple, and useful.


  2.  Keep features/concepts as orthogonal as possible.


  3.  No arbitrary limits (platforms, data sizes, cultures).


  4.  Keep it open and exciting to use/patch/advocate Perl everywhere.


  5.  Either assimilate new technologies, or build bridges to them.











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29.7.5.2 Where is the implementation?



All the talk in the world is useless without an implementation. In
almost every case, the person or people who argue for a new feature
will be expected to be the ones who implement it. Porters capable of
coding new features have their own agendas, and are not available to
implement your (possibly good) idea.









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29.7.5.3 Backwards compatibility



It’s a cardinal sin to break existing Perl programs. New warnings can
be contentious–some say that a program that emits warnings is not
broken, while others say it is. Adding keywords has the potential to
break programs, changing the meaning of existing token sequences or
functions might break programs.



The Perl 5 core includes mechanisms to help porters make backwards
incompatible changes more compatible such as the (feature) and
(deprecate) modules. Please use them when appropriate.









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29.7.5.4 Could it be a module instead?



Perl 5 has extension mechanisms, modules and XS, specifically to avoid
the need to keep changing the Perl interpreter. You can write modules
that export functions, you can give those functions prototypes so they
can be called like built-in functions, you can even write XS code to
mess with the runtime data structures of the Perl interpreter if you
want to implement really complicated things.



Whenever possible, new features should be prototyped in a CPAN module
before they will be considered for the core.









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29.7.5.5 Is the feature generic enough?



Is this something that only the submitter wants added to the language,
or is it broadly useful?  Sometimes, instead of adding a feature with a
tight focus, the porters might decide to wait until someone implements
the more generalized feature.









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29.7.5.6 Does it potentially introduce new bugs?



Radical rewrites of large chunks of the Perl interpreter have the
potential to introduce new bugs.









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29.7.5.7 How big is it?



The smaller and more localized the change, the better. Similarly, a
series of small patches is greatly preferred over a single large patch.









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29.7.5.8 Does it preclude other desirable features?



A patch is likely to be rejected if it closes off future avenues of
development. For instance, a patch that placed a true and final
interpretation on prototypes is likely to be rejected because there are
still options for the future of prototypes that haven’t been addressed.









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29.7.5.9 Is the implementation robust?



Good patches (tight code, complete, correct) stand more chance of going
in. Sloppy or incorrect patches might be placed on the back burner
until the pumpking has time to fix, or might be discarded altogether
without further notice.









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29.7.5.10 Is the implementation generic enough to be portable?



The worst patches make use of system-specific features. It’s highly
unlikely that non-portable additions to the Perl language will be
accepted.









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29.7.5.11 Is the implementation tested?



Patches which change behaviour (fixing bugs or introducing new
features) must include regression tests to verify that everything works
as expected.



Without tests provided by the original author, how can anyone else
changing perl in the future be sure that they haven’t unwittingly
broken the behaviour the patch implements? And without tests, how can
the patch’s author be confident that his/her hard work put into the
patch won’t be accidentally thrown away by someone in the future?









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29.7.5.12 Is there enough documentation?



Patches without documentation are probably ill-thought out or
incomplete. No features can be added or changed without documentation,
so submitting a patch for the appropriate pod docs as well as the
source code is important.









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29.7.5.13 Is there another way to do it?



Larry said "Although the Perl Slogan is There’s More Than One Way to
Do It, I hesitate to make 10 ways to do something". This is a tricky
heuristic to navigate, though–one man’s essential addition is another
man’s pointless cruft.









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29.7.5.14 Does it create too much work?



Work for the pumpking, work for Perl programmers, work for module
authors, ... Perl is supposed to be easy.









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29.7.5.15 Patches speak louder than words



Working code is always preferred to pie-in-the-sky ideas. A patch to
add a feature stands a much higher chance of making it to the language
than does a random feature request, no matter how fervently argued the
request might be. This ties into "Will it be useful?", as the fact that
someone took the time to make the patch demonstrates a strong desire
for the feature.









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29.8 TESTING



The core uses the same testing style as the rest of Perl, a simple
"ok/not ok" run through Test::Harness, but there are a few special
considerations.



There are three ways to write a test in the core. (Test-More),
t/test.pl and ad hoc print $test ? "ok 42\n" : "not ok 42\n". The
decision of which to use depends on what part of the test suite you’re
working on. This is a measure to prevent a high-level failure (such as
Config.pm breaking) from causing basic functionality tests to fail.



The t/test.pl library provides some of the features of
(Test-More), but avoids loading most modules and uses as few core
features as possible.



If you write your own test, use the Test Anything
Protocol.





When you say "make test", Perl uses the t/TEST program to run the
test suite (except under Win32 where it uses t/harness instead). All
tests are run from the t/ directory, not the directory which
contains the test. This causes some problems with the tests in lib/,
so here’s some opportunity for some patching.



You must be triply conscious of cross-platform concerns. This usually
boils down to using (File-Spec) and avoiding things like fork()
and system() unless absolutely necessary.





• perlhack Special make test targets:  



• perlhack Parallel tests:  



• perlhack Running tests by hand:  



• perlhack Using t/harness for testing:  












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29.8.1 Special make test targets



There are various special make targets that can be used to test Perl
slightly differently than the standard "test" target. Not all them are
expected to give a 100% success rate. Many of them have several
aliases, and many of them are not available on certain operating
systems.











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29.8.2 Parallel tests



The core distribution can now run its regression tests in parallel on
Unix-like platforms. Instead of running make test, set TEST_JOBS
in your environment to the number of tests to run in parallel, and run
make test_harness. On a Bourne-like shell, this can be done as



    TEST_JOBS=3 make test_harness  # Run 3 tests in parallel



An environment variable is used, rather than parallel make itself,
because (TAP-Harness) needs to be able to schedule individual
non-conflicting test scripts itself, and there is no standard interface
to make utilities to interact with their job schedulers.



Note that currently some test scripts may fail when run in parallel
(most notably ext/IO/t/io_dir.t). If necessary, run just the failing
scripts again sequentially and see if the failures go away.









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29.8.3 Running tests by hand



You can run part of the test suite by hand by using one of the
following commands from the t/ directory:



    ./perl -I../lib TEST list-of-.t-files



or



    ./perl -I../lib harness list-of-.t-files



(If you don’t specify test scripts, the whole test suite will be run.)









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29.8.4 Using t/harness for testing



If you use harness for testing, you have several command line
options available to you. The arguments are as follows, and are in the
order that they must appear if used together.



    harness -v -torture -re=pattern LIST OF FILES TO TEST
    harness -v -torture -re LIST OF PATTERNS TO MATCH



If LIST OF FILES TO TEST is omitted, the file list is obtained from
the manifest. The file list may include shell wildcards which will be
expanded out.





You can run an individual test by a command similar to



    ./perl -I../lib path/to/foo.t



except that the harnesses set up some environment variables that may
affect the execution of the test:







• perlhack Other environment variables that may influence tests:  












Up: perlhack Using t/harness for testing   [Contents]





29.8.4.1 Other environment variables that may influence tests





See also the documentation for the Test and Test::Harness modules, for
more environment variables that affect testing.









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29.9 MORE READING FOR GUTS HACKERS



To hack on the Perl guts, you’ll need to read the following things:











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29.10 CPAN TESTERS AND PERL SMOKERS



The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers
who test CPAN modules on a variety of platforms.



Perl Smokers ( http://www.nntp.perl.org/group/perl.daily-build/ and
http://www.nntp.perl.org/group/perl.daily-build.reports/ )
automatically test Perl source releases on platforms with various
configurations.



Both efforts welcome volunteers. In order to get involved in smoke
testing of the perl itself visit
http://search.cpan.org/dist/Test-Smoke/. In order to start smoke
testing CPAN modules visit
http://search.cpan.org/dist/CPANPLUS-YACSmoke/ or
http://search.cpan.org/dist/minismokebox/ or
http://search.cpan.org/dist/CPAN-Reporter/.









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29.11 WHAT NEXT?



If you’ve read all the documentation in the document and the ones
listed above, you’re more than ready to hack on Perl.



Here’s some more recommendations







• perlhack "The Road goes ever on and on, down from the door where it began.":  



• perlhack Metaphoric Quotations:  












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29.11.1 "The Road goes ever on and on, down from the door where it began."



If you can do these things, you’ve started on the long road to Perl
porting. Thanks for wanting to help make Perl better - and happy
hacking!









Previous: perlhack "The Road goes ever on and on, down from the door where it began.", Up: perlhack WHAT NEXT?   [Contents]





29.11.2 Metaphoric Quotations



If you recognized the quote about the Road above, you’re in luck.



Most software projects begin each file with a literal description of
each file’s purpose. Perl instead begins each with a literary allusion
to that file’s purpose.



Like chapters in many books, all top-level Perl source files (along
with a few others here and there) begin with an epigrammatic
inscription that alludes, indirectly and metaphorically, to the
material you’re about to read.



Quotations are taken from writings of J.R.R. Tolkien pertaining to his
Legendarium, almost always from The Lord of the Rings. Chapters and
page numbers are given using the following editions:





Other JRRT books fair game for quotes would thus include The
Adventures of Tom Bombadil, The Silmarillion, Unfinished Tales,
and The Tale of the Children of Hurin, all but the first
posthumously assembled by CJRT. But The Lord of the Rings itself is
perfectly fine and probably best to quote from, provided you can find a
suitable quote there.



So if you were to supply a new, complete, top-level source file to add
to Perl, you should conform to this peculiar practice by yourself
selecting an appropriate quotation from Tolkien, retaining the original
spelling and punctuation and using the same format the rest of the
quotes are in. Indirect and oblique is just fine; remember, it’s a
metaphor, so being meta is, after all, what it’s for.









Previous: perlhack WHAT NEXT?, Up: perlhack   [Contents]





29.12 AUTHOR



This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.









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30 perlhacktips





• perlhacktips NAME:  



• perlhacktips DESCRIPTION:  



• perlhacktips COMMON PROBLEMS:  



• perlhacktips DEBUGGING:  



• perlhacktips SOURCE CODE STATIC ANALYSIS:  



• perlhacktips MEMORY DEBUGGERS:  



• perlhacktips PROFILING:  



• perlhacktips MISCELLANEOUS TRICKS:  



• perlhacktips AUTHOR:  












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30.1 NAME



perlhacktips - Tips for Perl core C code hacking









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30.2 DESCRIPTION



This document will help you learn the best way to go about hacking on
the Perl core C code. It covers common problems, debugging, profiling,
and more.



If you haven’t read perlhack NAME and perlhacktut NAME yet, you might want
to do that first.









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30.3 COMMON PROBLEMS



Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
some cases we have to take pre-ANSI requirements into consideration.
You don’t care about some particular platform having broken Perl? I
hear there is still a strong demand for J2EE programmers.





• perlhacktips Perl environment problems:  



• perlhacktips Portability problems:  



• perlhacktips Problematic System Interfaces:  



• perlhacktips Security problems:  












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30.3.1 Perl environment problems











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30.3.2 Portability problems



The following are common causes of compilation and/or execution
failures, not common to Perl as such. The C FAQ is good bedtime
reading. Please test your changes with as many C compilers and
platforms as possible; we will, anyway, and it’s nice to save oneself
from public embarrassment.



If using gcc, you can add the -std=c89 option which will hopefully
catch most of these unportabilities. (However it might also catch
incompatibilities in your system’s header files.)



Use the Configure -Dgccansipedantic flag to enable the gcc -ansi
-pedantic flags which enforce stricter ANSI rules.



If using the gcc -Wall note that not all the possible warnings (like
-Wunitialized) are given unless you also compile with -O.



Note that if using gcc, starting from Perl 5.9.5 the Perl core source
code files (the ones at the top level of the source code distribution,
but not e.g. the extensions under ext/) are automatically compiled with
as many as possible of the -std=c89, -ansi, -pedantic, and a
selection of -W flags (see cflags.SH).



Also study perlport NAME carefully to avoid any bad assumptions about the
operating system, filesystems, and so forth.



You may once in a while try a "make microperl" to see whether we can
still compile Perl with just the bare minimum of interfaces. (See
README.micro.)



Do not assume an operating system indicates a certain compiler.











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30.3.3 Problematic System Interfaces











Previous: perlhacktips Problematic System Interfaces, Up: perlhacktips COMMON PROBLEMS   [Contents]





30.3.4 Security problems



Last but not least, here are various tips for safer coding.











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30.4 DEBUGGING



You can compile a special debugging version of Perl, which allows you
to use the -D option of Perl to tell more about what Perl is doing.
But sometimes there is no alternative than to dive in with a debugger,
either to see the stack trace of a core dump (very useful in a bug
report), or trying to figure out what went wrong before the core dump
happened, or how did we end up having wrong or unexpected results.





• perlhacktips Poking at Perl:  



• perlhacktips Using a source-level debugger:  



• perlhacktips gdb macro support:  



• perlhacktips Dumping Perl Data Structures:  












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30.4.1 Poking at Perl



To really poke around with Perl, you’ll probably want to build Perl for
debugging, like this:



    ./Configure -d -D optimize=-g
    make



-g is a flag to the C compiler to have it produce debugging
information which will allow us to step through a running program, and
to see in which C function we are at (without the debugging information
we might see only the numerical addresses of the functions, which is
not very helpful).



Configure will also turn on the DEBUGGING compilation symbol
which enables all the internal debugging code in Perl. There are a
whole bunch of things you can debug with this: perlrun NAME lists them
all, and the best way to find out about them is to play about with
them. The most useful options are probably



    l  Context (loop) stack processing
    t  Trace execution
    o  Method and overloading resolution
    c  String/numeric conversions



Some of the functionality of the debugging code can be achieved using
XS modules.



    -Dr => use re 'debug'
    -Dx => use O 'Debug'









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30.4.2 Using a source-level debugger



If the debugging output of -D doesn’t help you, it’s time to step
through perl’s execution with a source-level debugger.





To fire up the debugger, type



    gdb ./perl



Or if you have a core dump:



    gdb ./perl core



You’ll want to do that in your Perl source tree so the debugger can
read the source code. You should see the copyright message, followed by
the prompt.



    (gdb)



help will get you into the documentation, but here are the most
useful commands:





You may find it helpful to have a "macro dictionary", which you can
produce by saying cpp -dM perl.c | sort. Even then, cpp won’t
recursively apply those macros for you.









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30.4.3 gdb macro support



Recent versions of gdb have fairly good macro support, but in order
to use it you’ll need to compile perl with macro definitions included
in the debugging information. Using gcc version 3.1, this means
configuring with -Doptimize=-g3. Other compilers might use a
different switch (if they support debugging macros at all).









Previous: perlhacktips gdb macro support, Up: perlhacktips DEBUGGING   [Contents]





30.4.4 Dumping Perl Data Structures



One way to get around this macro hell is to use the dumping functions
in dump.c; these work a little like an internal
(Devel-Peek)Devel::Peek, but they also cover OPs and other
structures that you can’t get at from Perl. Let’s take an example.
We’ll use the $a = $b + $c we used before, but give it a bit of
context: $b = "6XXXX"; $c = 2.3;. Where’s a good place to stop and
poke around?



What about pp_add, the function we examined earlier to implement the
+ operator:



    (gdb) break Perl_pp_add
    Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.



Notice we use Perl_pp_add and not pp_add - see
perlguts Internal Functions. With the breakpoint in place, we can
run our program:



    (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'



Lots of junk will go past as gdb reads in the relevant source files and
libraries, and then:



    Breakpoint 1, Perl_pp_add () at pp_hot.c:309
    309         dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
    (gdb) step
    311           dPOPTOPnnrl_ul;
    (gdb)



We looked at this bit of code before, and we said that
dPOPTOPnnrl_ul arranges for two NVs to be placed into left and
right - let’s slightly expand it:



 #define dPOPTOPnnrl_ul  NV right = POPn; \
                         SV *leftsv = TOPs; \
                         NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0



POPn takes the SV from the top of the stack and obtains its NV
either directly (if SvNOK is set) or by calling the sv_2nv
function. TOPs takes the next SV from the top of the stack - yes,
POPn uses TOPs - but doesn’t remove it. We then use SvNV to
get the NV from leftsv in the same way as before - yes, POPn uses
SvNV.



Since we don’t have an NV for $b, we’ll have to use sv_2nv to
convert it. If we step again, we’ll find ourselves there:



    Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
    1669        if (!sv)
    (gdb)



We can now use Perl_sv_dump to investigate the SV:



    SV = PV(0xa057cc0) at 0xa0675d0
    REFCNT = 1
    FLAGS = (POK,pPOK)
    PV = 0xa06a510 "6XXXX"\0
    CUR = 5
    LEN = 6
    $1 = void



We know we’re going to get 6 from this, so let’s finish the
subroutine:



    (gdb) finish
    Run till exit from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
    0x462669 in Perl_pp_add () at pp_hot.c:311
    311           dPOPTOPnnrl_ul;



We can also dump out this op: the current op is always stored in
PL_op, and we can dump it with Perl_op_dump. This’ll give us
similar output to (B-Debug)B::Debug.



    {
    13  TYPE = add  ===> 14
        TARG = 1
        FLAGS = (SCALAR,KIDS)
        {
            TYPE = null  ===> (12)
              (was rv2sv)
            FLAGS = (SCALAR,KIDS)
            {
    11          TYPE = gvsv  ===> 12
                FLAGS = (SCALAR)
                GV = main::b
            }
        }



# finish this later #









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30.5 SOURCE CODE STATIC ANALYSIS



Various tools exist for analysing C source code statically, as
opposed to dynamically, that is, without executing the code. It is
possible to detect resource leaks, undefined behaviour, type
mismatches, portability problems, code paths that would cause illegal
memory accesses, and other similar problems by just parsing the C code
and looking at the resulting graph, what does it tell about the
execution and data flows. As a matter of fact, this is exactly how C
compilers know to give warnings about dubious code.





• perlhacktips lint, splint:  



• perlhacktips Coverity:  



• perlhacktips cpd (cut-and-paste detector):  



• perlhacktips gcc warnings:  



• perlhacktips Warnings of other C compilers:  












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30.5.1 lint, splint



The good old C code quality inspector, lint, is available in several
platforms, but please be aware that there are several different
implementations of it by different vendors, which means that the flags
are not identical across different platforms.



There is a lint variant called splint (Secure Programming Lint)
available from http://www.splint.org/ that should compile on any
Unix-like platform.



There are lint and <splint> targets in Makefile, but you may have to
diddle with the flags (see above).









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30.5.2 Coverity



Coverity (http://www.coverity.com/) is a product similar to lint and as
a testbed for their product they periodically check several open source
projects, and they give out accounts to open source developers to the
defect databases.









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30.5.3 cpd (cut-and-paste detector)



The cpd tool detects cut-and-paste coding. If one instance of the
cut-and-pasted code changes, all the other spots should probably be
changed, too. Therefore such code should probably be turned into a
subroutine or a macro.



cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
(http://pmd.sourceforge.net/). pmd was originally written for static
analysis of Java code, but later the cpd part of it was extended to
parse also C and C++.



Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
pmd-X.Y.jar from it, and then run that on source code thusly:



  java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt



You may run into memory limits, in which case you should use the -Xmx
option:



  java -Xmx512M ...









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30.5.4 gcc warnings



Though much can be written about the inconsistency and coverage
problems of gcc warnings (like -Wall not meaning "all the warnings",
or some common portability problems not being covered by -Wall, or
-ansi and -pedantic both being a poorly defined collection of
warnings, and so forth), gcc is still a useful tool in keeping our
coding nose clean.



The -Wall is by default on.



The -ansi (and its sidekick, -pedantic) would be nice to be on
always, but unfortunately they are not safe on all platforms, they can
for example cause fatal conflicts with the system headers (Solaris
being a prime example). If Configure -Dgccansipedantic is used, the
cflags frontend selects -ansi -pedantic for the platforms where
they are known to be safe.



Starting from Perl 5.9.4 the following extra flags are added:





The following flags would be nice to have but they would first need
their own Augean stablemaster:





The -Wtraditional is another example of the annoying tendency of gcc
to bundle a lot of warnings under one switch (it would be impossible to
deploy in practice because it would complain a lot) but it does contain
some warnings that would be beneficial to have available on their own,
such as the warning about string constants inside macros containing the
macro arguments: this behaved differently pre-ANSI than it does in
ANSI, and some C compilers are still in transition, AIX being an
example.









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30.5.5 Warnings of other C compilers



Other C compilers (yes, there are other C compilers than gcc) often
have their "strict ANSI" or "strict ANSI with some portability
extensions" modes on, like for example the Sun Workshop has its -Xa
mode on (though implicitly), or the DEC (these days, HP...) has its
-std1 mode on.









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30.6 MEMORY DEBUGGERS



NOTE 1: Running under memory debuggers such as Purify, valgrind, or
Third Degree greatly slows down the execution: seconds become minutes,
minutes become hours. For example as of Perl 5.8.1, the
ext/Encode/t/Unicode.t takes extraordinarily long to complete under
e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
than six hours, even on a snappy computer. The said test must be doing
something that is quite unfriendly for memory debuggers. If you don’t
feel like waiting, that you can simply kill away the perl process.



NOTE 2: To minimize the number of memory leak false alarms (see
PERL_DESTRUCT_LEVEL for more information), you have to set the
environment variable PERL_DESTRUCT_LEVEL to 2.



For csh-like shells:



    setenv PERL_DESTRUCT_LEVEL 2



For Bourne-type shells:



    PERL_DESTRUCT_LEVEL=2
    export PERL_DESTRUCT_LEVEL



In Unixy environments you can also use the env command:



    env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...



NOTE 3: There are known memory leaks when there are compile-time
errors within eval or require, seeing S_doeval in the call stack is
a good sign of these. Fixing these leaks is non-trivial, unfortunately,
but they must be fixed eventually.



NOTE 4: (DynaLoader) will not clean up after itself completely
unless Perl is built with the Configure option
-Accflags=-DDL_UNLOAD_ALL_AT_EXIT.





• perlhacktips Rational Software's Purify:  



• perlhacktips valgrind:  












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30.6.1 Rational Software’s Purify



Purify is a commercial tool that is helpful in identifying memory
overruns, wild pointers, memory leaks and other such badness. Perl must
be compiled in a specific way for optimal testing with Purify.  Purify
is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.





• perlhacktips Purify on Unix:  



• perlhacktips Purify on NT:  












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30.6.1.1 Purify on Unix



On Unix, Purify creates a new Perl binary. To get the most benefit out
of Purify, you should create the perl to Purify using:



    sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
     -Uusemymalloc -Dusemultiplicity



where these arguments mean:





Once you’ve compiled a perl suitable for Purify’ing, then you can just:



    make pureperl



which creates a binary named ’pureperl’ that has been Purify’ed. This
binary is used in place of the standard ’perl’ binary when you want to
debug Perl memory problems.



As an example, to show any memory leaks produced during the standard
Perl testset you would create and run the Purify’ed perl as:



    make pureperl
    cd t
    ../pureperl -I../lib harness



which would run Perl on test.pl and report any memory problems.



Purify outputs messages in "Viewer" windows by default. If you don’t
have a windowing environment or if you simply want the Purify output to
unobtrusively go to a log file instead of to the interactive window,
use these following options to output to the log file "perl.log":



    setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
     -log-file=perl.log -append-logfile=yes"



If you plan to use the "Viewer" windows, then you only need this
option:



    setenv PURIFYOPTIONS "-chain-length=25"



In Bourne-type shells:



    PURIFYOPTIONS="..."
    export PURIFYOPTIONS



or if you have the "env" utility:



    env PURIFYOPTIONS="..." ../pureperl ...









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30.6.1.2 Purify on NT



Purify on Windows NT instruments the Perl binary ’perl.exe’ on the fly.
 There are several options in the makefile you should change to get the
most use out of Purify:





As an example, to show any memory leaks produced during the standard
Perl testset you would create and run Purify as:



    cd win32
    make
    cd ../t
    purify ../perl -I../lib harness



which would instrument Perl in memory, run Perl on test.pl, then
finally report any memory problems.









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30.6.2 valgrind



The excellent valgrind tool can be used to find out both memory leaks
and illegal memory accesses. As of version 3.3.0, Valgrind only
supports Linux on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and
x86-64). The special "test.valgrind" target can be used to run the
tests under valgrind. Found errors and memory leaks are logged in
files named testfile.valgrind.



Valgrind also provides a cachegrind tool, invoked on perl as:



    VG_OPTS=--tool=cachegrind make test.valgrind



As system libraries (most notably glibc) are also triggering errors,
valgrind allows to suppress such errors using suppression files. The
default suppression file that comes with valgrind already catches a lot
of them. Some additional suppressions are defined in t/perl.supp.



To get valgrind and for more information see



    http://valgrind.org/









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30.7 PROFILING



Depending on your platform there are various ways of profiling Perl.



There are two commonly used techniques of profiling executables:
statistical time-sampling and basic-block counting.



The first method takes periodically samples of the CPU program counter,
and since the program counter can be correlated with the code generated
for functions, we get a statistical view of in which functions the
program is spending its time. The caveats are that very small/fast
functions have lower probability of showing up in the profile, and that
periodically interrupting the program (this is usually done rather
frequently, in the scale of milliseconds) imposes an additional
overhead that may skew the results. The first problem can be alleviated
by running the code for longer (in general this is a good idea for
profiling), the second problem is usually kept in guard by the
profiling tools themselves.



The second method divides up the generated code into basic blocks.
Basic blocks are sections of code that are entered only in the
beginning and exited only at the end. For example, a conditional jump
starts a basic block. Basic block profiling usually works by
instrumenting the code by adding enter basic block #nnnn
book-keeping code to the generated code. During the execution of the
code the basic block counters are then updated appropriately. The
caveat is that the added extra code can skew the results: again, the
profiling tools usually try to factor their own effects out of the
results.





• perlhacktips Gprof Profiling:  



• perlhacktips GCC gcov Profiling:  












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30.7.1 Gprof Profiling



gprof is a profiling tool available in many Unix platforms, it uses
statistical time-sampling.



You can build a profiled version of perl called "perl.gprof" by
invoking the make target "perl.gprof"  (What is required is that Perl
must be compiled using the -pg flag, you may need to re-Configure).
Running the profiled version of Perl will create an output file called
gmon.out is created which contains the profiling data collected
during the execution.



The gprof tool can then display the collected data in various ways.
Usually gprof understands the following options:





For more detailed explanation of the available commands and output
formats, see your own local documentation of gprof.



quick hint:



    $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
    $ ./perl.gprof someprog # creates gmon.out in current directory
    $ gprof ./perl.gprof > out
    $ view out









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30.7.2 GCC gcov Profiling



Starting from GCC 3.0 basic block profiling is officially available
for the GNU CC.



You can build a profiled version of perl called perl.gcov by
invoking the make target "perl.gcov" (what is required that Perl must
be compiled using gcc with the flags -fprofile-arcs -ftest-coverage,
you may need to re-Configure).



Running the profiled version of Perl will cause profile output to be
generated. For each source file an accompanying ".da" file will be
created.



To display the results you use the "gcov" utility (which should be
installed if you have gcc 3.0 or newer installed). gcov is run on
source code files, like this



    gcov sv.c



which will cause sv.c.gcov to be created. The .gcov files contain
the source code annotated with relative frequencies of execution
indicated by "#" markers.



Useful options of gcov include -b which will summarise the basic
block, branch, and function call coverage, and -c which instead of
relative frequencies will use the actual counts. For more information
on the use of gcov and basic block profiling with gcc, see the
latest GNU CC manual, as of GCC 3.0 see



    http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html



and its section titled "8. gcov: a Test Coverage Program"



    http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132



quick hint:



    $ sh Configure -des -Dusedevel -Doptimize='-g' \
        -Accflags='-fprofile-arcs -ftest-coverage' \
        -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
    $ rm -f regexec.c.gcov regexec.gcda
    $ ./perl.gcov
    $ gcov regexec.c
    $ view regexec.c.gcov









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30.8 MISCELLANEOUS TRICKS





• perlhacktips PERL_DESTRUCT_LEVEL:  



• perlhacktips PERL_MEM_LOG:  



• perlhacktips DDD over gdb:  



• perlhacktips Poison:  



• perlhacktips Read-only optrees:  



• perlhacktips The .i Targets:  












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30.8.1 PERL_DESTRUCT_LEVEL



If you want to run any of the tests yourself manually using e.g.
valgrind, or the pureperl or perl.third executables, please note that
by default perl does not explicitly cleanup all the memory it has
allocated (such as global memory arenas) but instead lets the exit() of
the whole program "take care" of such allocations, also known as
"global destruction of objects".



There is a way to tell perl to do complete cleanup: set the environment
variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
does set this to 2, and this is what you need to do too, if you don’t
want to see the "global leaks": For example, for "third-degreed" Perl:



        env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t



(Note: the mod_perl apache module uses also this environment variable
for its own purposes and extended its semantics. Refer to the mod_perl
documentation for more information. Also, spawned threads do the
equivalent of setting this variable to the value 1.)



If, at the end of a run you get the message N scalars leaked, you
can recompile with -DDEBUG_LEAKING_SCALARS, which will cause the
addresses of all those leaked SVs to be dumped along with details as to
where each SV was originally allocated. This information is also
displayed by Devel::Peek. Note that the extra details recorded with
each SV increases memory usage, so it shouldn’t be used in production
environments. It also converts new_SV() from a macro into a real
function, so you can use your favourite debugger to discover where
those pesky SVs were allocated.



If you see that you’re leaking memory at runtime, but neither valgrind
nor -DDEBUG_LEAKING_SCALARS will find anything, you’re probably
leaking SVs that are still reachable and will be properly cleaned up
during destruction of the interpreter. In such cases, using the -Dm
switch can point you to the source of the leak. If the executable was
built with -DDEBUG_LEAKING_SCALARS, -Dm will output SV
allocations in addition to memory allocations. Each SV allocation has a
distinct serial number that will be written on creation and destruction
of the SV. So if you’re executing the leaking code in a loop, you need
to look for SVs that are created, but never destroyed between each
cycle. If such an SV is found, set a conditional breakpoint within
new_SV() and make it break only when PL_sv_serial is equal to the
serial number of the leaking SV. Then you will catch the interpreter in
exactly the state where the leaking SV is allocated, which is
sufficient in many cases to find the source of the leak.



As -Dm is using the PerlIO layer for output, it will by itself
allocate quite a bunch of SVs, which are hidden to avoid recursion. You
can bypass the PerlIO layer if you use the SV logging provided by
-DPERL_MEM_LOG instead.









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30.8.2 PERL_MEM_LOG



If compiled with -DPERL_MEM_LOG, both memory and SV allocations go
through logging functions, which is handy for breakpoint setting.



Unless -DPERL_MEM_LOG_NOIMPL is also compiled, the logging functions
read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
so how:



    $ENV{PERL_MEM_LOG} =~ /m/           Log all memory ops
    $ENV{PERL_MEM_LOG} =~ /s/           Log all SV ops
    $ENV{PERL_MEM_LOG} =~ /t/           include timestamp in Log
    $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)



Memory logging is somewhat similar to -Dm but is independent of
-DDEBUGGING, and at a higher level; all uses of Newx(), Renew(), and
Safefree() are logged with the caller’s source code file and line
number (and C function name, if supported by the C compiler). In
contrast, -Dm is directly at the point of malloc(). SV logging is
similar.



Since the logging doesn’t use PerlIO, all SV allocations are logged and
no extra SV allocations are introduced by enabling the logging. If
compiled with -DDEBUG_LEAKING_SCALARS, the serial number for each SV
allocation is also logged.









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30.8.3 DDD over gdb



Those debugging perl with the DDD frontend over gdb may find the
following useful:



You can extend the data conversion shortcuts menu, so for example you
can display an SV’s IV value with one click, without doing any typing.
To do that simply edit ~/.ddd/init file and add after:



  ! Display shortcuts.
  Ddd*gdbDisplayShortcuts: \
  /t ()   // Convert to Bin\n\
  /d ()   // Convert to Dec\n\
  /x ()   // Convert to Hex\n\
  /o ()   // Convert to Oct(\n\



the following two lines:



  ((XPV*) (())->sv_any )->xpv_pv  // 2pvx\n\
  ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx



so now you can do ivx and pvx lookups or you can plug there the sv_peek
"conversion":



  Perl_sv_peek(my_perl, (SV*)()) // sv_peek



(The my_perl is for threaded builds.) Just remember that every line,
but the last one, should end with \n\



Alternatively edit the init file interactively via: 3rd mouse button ->
New Display -> Edit Menu



Note: you can define up to 20 conversion shortcuts in the gdb section.









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30.8.4 Poison



If you see in a debugger a memory area mysteriously full of 0xABABABAB
or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
perlclib NAME.









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30.8.5 Read-only optrees



Under ithreads the optree is read only. If you want to enforce this, to
check for write accesses from buggy code, compile with
-DPL_OP_SLAB_ALLOC to enable the OP slab allocator and
-DPERL_DEBUG_READONLY_OPS to enable code that allocates op memory
via mmap, and sets it read-only at run time. Any write access to an
op results in a SIGBUS and abort.



This code is intended for development only, and may not be portable
even to all Unix variants. Also, it is an 80% solution, in that it
isn’t able to make all ops read only. Specifically it





It’s not possible to turn the slabs to read-only after an action
requiring read-write access, as either can happen during op tree
building time, so there may still be legitimate write access.



However, as an 80% solution it is still effective, as currently it
catches a write access during the generation of Config.pm, which
means that we can’t yet build perl with this enabled.









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30.8.6 The .i Targets



You can expand the macros in a foo.c file by saying



    make foo.i



which will expand the macros using cpp.  Don’t be scared by the results.









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30.9 AUTHOR



This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.









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31 perlhacktut





• perlhacktut NAME:  



• perlhacktut DESCRIPTION:  



• perlhacktut EXAMPLE OF A SIMPLE PATCH:  



• perlhacktut AUTHOR:  












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31.1 NAME



perlhacktut - Walk through the creation of a simple C code patch









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31.2 DESCRIPTION



This document takes you through a simple patch example.



If you haven’t read perlhack NAME yet, go do that first! You might also
want to read through perlsource NAME too.



Once you’re done here, check out perlhacktips NAME next.









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31.3 EXAMPLE OF A SIMPLE PATCH



Let’s take a simple patch from start to finish.



Here’s something Larry suggested: if a U is the first active format
during a pack, (for example, pack "U3C8", @stuff) then the
resulting string should be treated as UTF-8 encoded.



If you are working with a git clone of the Perl repository, you will
want to create a branch for your changes. This will make creating a
proper patch much simpler. See the perlgit NAME for details on how to do
this.





• perlhacktut Writing the patch:  



• perlhacktut Testing the patch:  



• perlhacktut Documenting the patch:  



• perlhacktut Submit:  












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31.3.1 Writing the patch



How do we prepare to fix this up? First we locate the code in question
- the pack happens at runtime, so it’s going to be in one of the
pp files. Sure enough, pp_pack is in pp.c. Since we’re going
to be altering this file, let’s copy it to pp.c~.



[Well, it was in pp.c when this tutorial was written. It has now
been split off with pp_unpack to its own file, pp_pack.c]



Now let’s look over pp_pack: we take a pattern into pat, and then
loop over the pattern, taking each format character in turn into
datum_type. Then for each possible format character, we swallow up
the other arguments in the pattern (a field width, an asterisk, and so
on) and convert the next chunk input into the specified format, adding
it onto the output SV cat.



How do we know if the U is the first format in the pat? Well, if
we have a pointer to the start of pat then, if we see a U we can
test whether we’re still at the start of the string. So, here’s where
pat is set up:



    STRLEN fromlen;
    register char *pat = SvPVx(*++MARK, fromlen);
    register char *patend = pat + fromlen;
    register I32 len;
    I32 datumtype;
    SV *fromstr;



We’ll have another string pointer in there:



    STRLEN fromlen;
    register char *pat = SvPVx(*++MARK, fromlen);
    register char *patend = pat + fromlen;
 +  char *patcopy;
    register I32 len;
    I32 datumtype;
    SV *fromstr;



And just before we start the loop, we’ll set patcopy to be the start
of pat:



    items = SP - MARK;
    MARK++;
    sv_setpvn(cat, "", 0);
 +  patcopy = pat;
    while (pat < patend) {



Now if we see a U which was at the start of the string, we turn on
the UTF8 flag for the output SV, cat:



 +  if (datumtype == 'U' && pat==patcopy+1)
 +      SvUTF8_on(cat);
    if (datumtype == '#') {
        while (pat < patend && *pat != '\n')
            pat++;



Remember that it has to be patcopy+1 because the first character of
the string is the U which has been swallowed into datumtype!



Oops, we forgot one thing: what if there are spaces at the start of the
pattern? pack("  U*", @stuff) will have U as the first active
character, even though it’s not the first thing in the pattern. In this
case, we have to advance patcopy along with pat when we see
spaces:



    if (isSPACE(datumtype))
        continue;



needs to become



    if (isSPACE(datumtype)) {
        patcopy++;
        continue;
    }



OK. That’s the C part done. Now we must do two additional things before
this patch is ready to go: we’ve changed the behaviour of Perl, and so
we must document that change. We must also provide some more regression
tests to make sure our patch works and doesn’t create a bug somewhere
else along the line.









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31.3.2 Testing the patch



The regression tests for each operator live in t/op/, and so we make
a copy of t/op/pack.t to t/op/pack.t~. Now we can add our tests
to the end. First, we’ll test that the U does indeed create Unicode
strings.



t/op/pack.t has a sensible ok() function, but if it didn’t we could use
the one from t/test.pl.



 require './test.pl';
 plan( tests => 159 );



so instead of this:



 print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
                                               pack("U*",1,20,300,4000);
 print "ok $test\n"; $test++;



we can write the more sensible (see (Test-More) for a full
explanation of is() and other testing functions).



 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
                                       "U* produces Unicode" );



Now we’ll test that we got that space-at-the-beginning business right:



 is( "1.20.300.4000", sprintf "%vd", pack("  U*",1,20,300,4000),
                                     "  with spaces at the beginning" );



And finally we’ll test that we don’t make Unicode strings if U is
not the first active format:



 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
                                       "U* not first isn't Unicode" );



Mustn’t forget to change the number of tests which appears at the top,
or else the automated tester will get confused. This will either look
like this:



 print "1..156\n";



or this:



 plan( tests => 156 );



We now compile up Perl, and run it through the test suite. Our new
tests pass, hooray!









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31.3.3 Documenting the patch



Finally, the documentation. The job is never done until the paperwork
is over, so let’s describe the change we’ve just made. The relevant
place is pod/perlfunc.pod; again, we make a copy, and then we’ll
insert this text in the description of pack:



 =item *

 If the pattern begins with a C<U>, the resulting string will be treated
 as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
 with an initial C<U0>, and the bytes that follow will be interpreted as
 Unicode characters. If you don't want this to happen, you can begin
 your pattern with C<C0> (or anything else) to force Perl not to UTF-8
 encode your string, and then follow this with a C<U*> somewhere in your
 pattern.









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31.3.4 Submit



See perlhack NAME for details on how to submit this patch.









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31.4 AUTHOR



This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.









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32 perlhist





• perlhist NAME:  



• perlhist DESCRIPTION:  



• perlhist INTRODUCTION:  



• perlhist THE KEEPERS OF THE PUMPKIN:  



• perlhist THE RECORDS:  



• perlhist THE KEEPERS OF THE RECORDS:  












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32.1 NAME



perlhist - the Perl history records









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32.2 DESCRIPTION



This document aims to record the Perl source code releases.









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32.3 INTRODUCTION



Perl history in brief, by Larry Wall:



    Perl 0 introduced Perl to my officemates.
    Perl 1 introduced Perl to the world, and changed /\(...\|...\)/ to
        /(...|...)/.  \(Dan Faigin still hasn't forgiven me. :-\)
    Perl 2 introduced Henry Spencer's regular expression package.
    Perl 3 introduced the ability to handle binary data (embedded nulls).
    Perl 4 introduced the first Camel book.  Really.  We mostly just
        switched version numbers so the book could refer to 4.000.
    Perl 5 introduced everything else, including the ability to
        introduce everything else.









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32.4 THE KEEPERS OF THE PUMPKIN



Larry Wall, Andy Dougherty, Tom Christiansen, Charles Bailey, Nick
Ing-Simmons, Chip Salzenberg, Tim Bunce, Malcolm Beattie, Gurusamy
Sarathy, Graham Barr, Jarkko Hietaniemi, Hugo van der Sanden,
Michael Schwern, Rafael Garcia-Suarez, Nicholas Clark, Richard Clamp,
Leon Brocard, Dave Mitchell, Jesse Vincent, Ricardo Signes, Steve Hay,
Matt S Trout, David Golden, Florian Ragwitz, Tatsuhiko Miyagawa,
Chris BinGOs Williams, Zefram, Ævar Arnfjörð Bjarmason, Stevan
Little, Dave Rolsky, Max Maischein, Abigail, Jesse Luehrs and Tony
Cook.





• perlhist PUMPKIN?:  












Up: perlhist THE KEEPERS OF THE PUMPKIN   [Contents]





32.4.1 PUMPKIN?



[from Porting/pumpkin.pod in the Perl source code distribution]



Chip Salzenberg gets credit for that, with a nod to his cow orker,
David Croy.  We had passed around various names (baton, token, hot
potato) but none caught on.  Then, Chip asked:



[begin quote]



   Who has the patch pumpkin?



To explain:  David Croy once told me that at a previous job,
there was one tape drive and multiple systems that used it for backups.
But instead of some high-tech exclusion software, they used a low-tech
method to prevent multiple simultaneous backups: a stuffed pumpkin.
No one was allowed to make backups unless they had the "backup pumpkin".



[end quote]



The name has stuck.  The holder of the pumpkin is sometimes called
the pumpking (keeping the source afloat?) or the pumpkineer (pulling
the strings?).









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32.5 THE RECORDS



 Pump-  Release         Date            Notes
 king                                   (by no means
                                         comprehensive,
                                         see Changes*
                                         for details)
 ======================================================================

 Larry   0              Classified.     Don't ask.

 Larry   1.000          1987-Dec-18

          1.001..10     1988-Jan-30
          1.011..14     1988-Feb-02
 Schwern  1.0.15        2002-Dec-18     Modernization
 Richard  1.0_16        2003-Dec-18

 Larry   2.000          1988-Jun-05

          2.001         1988-Jun-28

 Larry   3.000          1989-Oct-18

          3.001         1989-Oct-26
          3.002..4      1989-Nov-11
          3.005         1989-Nov-18
          3.006..8      1989-Dec-22
          3.009..13     1990-Mar-02
          3.014         1990-Mar-13
          3.015         1990-Mar-14
          3.016..18     1990-Mar-28
          3.019..27     1990-Aug-10     User subs.
          3.028         1990-Aug-14
          3.029..36     1990-Oct-17
          3.037         1990-Oct-20
          3.040         1990-Nov-10
          3.041         1990-Nov-13
          3.042..43     1991-Jan-??
          3.044         1991-Jan-12

 Larry   4.000          1991-Mar-21

          4.001..3      1991-Apr-12
          4.004..9      1991-Jun-07
          4.010         1991-Jun-10
          4.011..18     1991-Nov-05
          4.019         1991-Nov-11     Stable.
          4.020..33     1992-Jun-08
          4.034         1992-Jun-11
          4.035         1992-Jun-23
 Larry    4.036         1993-Feb-05     Very stable.

          5.000alpha1   1993-Jul-31
          5.000alpha2   1993-Aug-16
          5.000alpha3   1993-Oct-10
          5.000alpha4   1993-???-??
          5.000alpha5   1993-???-??
          5.000alpha6   1994-Mar-18
          5.000alpha7   1994-Mar-25
 Andy     5.000alpha8   1994-Apr-04
 Larry    5.000alpha9   1994-May-05     ext appears.
          5.000alpha10  1994-Jun-11
          5.000alpha11  1994-Jul-01
 Andy     5.000a11a     1994-Jul-07     To fit 14.
          5.000a11b     1994-Jul-14
          5.000a11c     1994-Jul-19
          5.000a11d     1994-Jul-22
 Larry    5.000alpha12  1994-Aug-04
 Andy     5.000a12a     1994-Aug-08
          5.000a12b     1994-Aug-15
          5.000a12c     1994-Aug-22
          5.000a12d     1994-Aug-22
          5.000a12e     1994-Aug-22
          5.000a12f     1994-Aug-24
          5.000a12g     1994-Aug-24
          5.000a12h     1994-Aug-24
 Larry    5.000beta1    1994-Aug-30
 Andy     5.000b1a      1994-Sep-06
 Larry    5.000beta2    1994-Sep-14     Core slushified.
 Andy     5.000b2a      1994-Sep-14
          5.000b2b      1994-Sep-17
          5.000b2c      1994-Sep-17
 Larry    5.000beta3    1994-Sep-??
 Andy     5.000b3a      1994-Sep-18
          5.000b3b      1994-Sep-22
          5.000b3c      1994-Sep-23
          5.000b3d      1994-Sep-27
          5.000b3e      1994-Sep-28
          5.000b3f      1994-Sep-30
          5.000b3g      1994-Oct-04
 Andy     5.000b3h      1994-Oct-07
 Larry?   5.000gamma    1994-Oct-13?

 Larry   5.000          1994-Oct-17

 Andy     5.000a        1994-Dec-19
          5.000b        1995-Jan-18
          5.000c        1995-Jan-18
          5.000d        1995-Jan-18
          5.000e        1995-Jan-18
          5.000f        1995-Jan-18
          5.000g        1995-Jan-18
          5.000h        1995-Jan-18
          5.000i        1995-Jan-26
          5.000j        1995-Feb-07
          5.000k        1995-Feb-11
          5.000l        1995-Feb-21
          5.000m        1995-Feb-28
          5.000n        1995-Mar-07
          5.000o        1995-Mar-13?

 Larry   5.001          1995-Mar-13

 Andy     5.001a        1995-Mar-15
          5.001b        1995-Mar-31
          5.001c        1995-Apr-07
          5.001d        1995-Apr-14
          5.001e        1995-Apr-18     Stable.
          5.001f        1995-May-31
          5.001g        1995-May-25
          5.001h        1995-May-25
          5.001i        1995-May-30
          5.001j        1995-Jun-05
          5.001k        1995-Jun-06
          5.001l        1995-Jun-06     Stable.
          5.001m        1995-Jul-02     Very stable.
          5.001n        1995-Oct-31     Very unstable.
          5.002beta1    1995-Nov-21
          5.002b1a      1995-Dec-04
          5.002b1b      1995-Dec-04
          5.002b1c      1995-Dec-04
          5.002b1d      1995-Dec-04
          5.002b1e      1995-Dec-08
          5.002b1f      1995-Dec-08
 Tom      5.002b1g      1995-Dec-21     Doc release.
 Andy     5.002b1h      1996-Jan-05
          5.002b2       1996-Jan-14
 Larry    5.002b3       1996-Feb-02
 Andy     5.002gamma    1996-Feb-11
 Larry    5.002delta    1996-Feb-27

 Larry   5.002          1996-Feb-29     Prototypes.

 Charles  5.002_01      1996-Mar-25

         5.003          1996-Jun-25     Security release.

          5.003_01      1996-Jul-31
 Nick     5.003_02      1996-Aug-10
 Andy     5.003_03      1996-Aug-28
          5.003_04      1996-Sep-02
          5.003_05      1996-Sep-12
          5.003_06      1996-Oct-07
          5.003_07      1996-Oct-10
 Chip     5.003_08      1996-Nov-19
          5.003_09      1996-Nov-26
          5.003_10      1996-Nov-29
          5.003_11      1996-Dec-06
          5.003_12      1996-Dec-19
          5.003_13      1996-Dec-20
          5.003_14      1996-Dec-23
          5.003_15      1996-Dec-23
          5.003_16      1996-Dec-24
          5.003_17      1996-Dec-27
          5.003_18      1996-Dec-31
          5.003_19      1997-Jan-04
          5.003_20      1997-Jan-07
          5.003_21      1997-Jan-15
          5.003_22      1997-Jan-16
          5.003_23      1997-Jan-25
          5.003_24      1997-Jan-29
          5.003_25      1997-Feb-04
          5.003_26      1997-Feb-10
          5.003_27      1997-Feb-18
          5.003_28      1997-Feb-21
          5.003_90      1997-Feb-25     Ramping up to the 5.004 release.
          5.003_91      1997-Mar-01
          5.003_92      1997-Mar-06
          5.003_93      1997-Mar-10
          5.003_94      1997-Mar-22
          5.003_95      1997-Mar-25
          5.003_96      1997-Apr-01
          5.003_97      1997-Apr-03     Fairly widely used.
          5.003_97a     1997-Apr-05
          5.003_97b     1997-Apr-08
          5.003_97c     1997-Apr-10
          5.003_97d     1997-Apr-13
          5.003_97e     1997-Apr-15
          5.003_97f     1997-Apr-17
          5.003_97g     1997-Apr-18
          5.003_97h     1997-Apr-24
          5.003_97i     1997-Apr-25
          5.003_97j     1997-Apr-28
          5.003_98      1997-Apr-30
          5.003_99      1997-May-01
          5.003_99a     1997-May-09
          p54rc1        1997-May-12     Release Candidates.
          p54rc2        1997-May-14

 Chip    5.004          1997-May-15     A major maintenance release.

 Tim      5.004_01-t1   1997-???-??     The 5.004 maintenance track.
          5.004_01-t2   1997-Jun-11     aka perl5.004m1t2
          5.004_01      1997-Jun-13
          5.004_01_01   1997-Jul-29     aka perl5.004m2t1
          5.004_01_02   1997-Aug-01     aka perl5.004m2t2
          5.004_01_03   1997-Aug-05     aka perl5.004m2t3
          5.004_02      1997-Aug-07
          5.004_02_01   1997-Aug-12     aka perl5.004m3t1
          5.004_03-t2   1997-Aug-13     aka perl5.004m3t2
          5.004_03      1997-Sep-05
          5.004_04-t1   1997-Sep-19     aka perl5.004m4t1
          5.004_04-t2   1997-Sep-23     aka perl5.004m4t2
          5.004_04-t3   1997-Oct-10     aka perl5.004m4t3
          5.004_04-t4   1997-Oct-14     aka perl5.004m4t4
          5.004_04      1997-Oct-15
          5.004_04-m1   1998-Mar-04     (5.004m5t1) Maint. trials for 5.004_05.
          5.004_04-m2   1998-May-01
          5.004_04-m3   1998-May-15
          5.004_04-m4   1998-May-19
          5.004_05-MT5  1998-Jul-21
          5.004_05-MT6  1998-Oct-09
          5.004_05-MT7  1998-Nov-22
          5.004_05-MT8  1998-Dec-03
 Chip     5.004_05-MT9  1999-Apr-26
          5.004_05      1999-Apr-29

 Malcolm  5.004_50      1997-Sep-09     The 5.005 development track.
          5.004_51      1997-Oct-02
          5.004_52      1997-Oct-15
          5.004_53      1997-Oct-16
          5.004_54      1997-Nov-14
          5.004_55      1997-Nov-25
          5.004_56      1997-Dec-18
          5.004_57      1998-Feb-03
          5.004_58      1998-Feb-06
          5.004_59      1998-Feb-13
          5.004_60      1998-Feb-20
          5.004_61      1998-Feb-27
          5.004_62      1998-Mar-06
          5.004_63      1998-Mar-17
          5.004_64      1998-Apr-03
          5.004_65      1998-May-15
          5.004_66      1998-May-29
 Sarathy  5.004_67      1998-Jun-15
          5.004_68      1998-Jun-23
          5.004_69      1998-Jun-29
          5.004_70      1998-Jul-06
          5.004_71      1998-Jul-09
          5.004_72      1998-Jul-12
          5.004_73      1998-Jul-13
          5.004_74      1998-Jul-14     5.005 beta candidate.
          5.004_75      1998-Jul-15     5.005 beta1.
          5.004_76      1998-Jul-21     5.005 beta2.

 Sarathy  5.005         1998-Jul-22     Oneperl.

 Sarathy  5.005_01      1998-Jul-27     The 5.005 maintenance track.
          5.005_02-T1   1998-Aug-02
          5.005_02-T2   1998-Aug-05
          5.005_02      1998-Aug-08
 Graham   5.005_03-MT1  1998-Nov-30
          5.005_03-MT2  1999-Jan-04
          5.005_03-MT3  1999-Jan-17
          5.005_03-MT4  1999-Jan-26
          5.005_03-MT5  1999-Jan-28
          5.005_03-MT6  1999-Mar-05
          5.005_03      1999-Mar-28
 Leon     5.005_04-RC1  2004-Feb-05
          5.005_04-RC2  2004-Feb-18
          5.005_04      2004-Feb-23
          5.005_05-RC1  2009-Feb-16

 Sarathy  5.005_50      1998-Jul-26     The 5.6 development track.
          5.005_51      1998-Aug-10
          5.005_52      1998-Sep-25
          5.005_53      1998-Oct-31
          5.005_54      1998-Nov-30
          5.005_55      1999-Feb-16
          5.005_56      1999-Mar-01
          5.005_57      1999-May-25
          5.005_58      1999-Jul-27
          5.005_59      1999-Aug-02
          5.005_60      1999-Aug-02
          5.005_61      1999-Aug-20
          5.005_62      1999-Oct-15
          5.005_63      1999-Dec-09
          5.5.640       2000-Feb-02
          5.5.650       2000-Feb-08     beta1
          5.5.660       2000-Feb-22     beta2
          5.5.670       2000-Feb-29     beta3
          5.6.0-RC1     2000-Mar-09     Release candidate 1.
          5.6.0-RC2     2000-Mar-14     Release candidate 2.
          5.6.0-RC3     2000-Mar-21     Release candidate 3.

 Sarathy  5.6.0         2000-Mar-22

 Sarathy  5.6.1-TRIAL1  2000-Dec-18     The 5.6 maintenance track.
          5.6.1-TRIAL2  2001-Jan-31
          5.6.1-TRIAL3  2001-Mar-19
          5.6.1-foolish 2001-Apr-01     The "fools-gold" release.
          5.6.1         2001-Apr-08
 Rafael   5.6.2-RC1     2003-Nov-08
          5.6.2         2003-Nov-15     Fix new build issues

 Jarkko   5.7.0         2000-Sep-02     The 5.7 track: Development.
          5.7.1         2001-Apr-09
          5.7.2         2001-Jul-13     Virtual release candidate 0.
          5.7.3         2002-Mar-05
          5.8.0-RC1     2002-Jun-01
          5.8.0-RC2     2002-Jun-21
          5.8.0-RC3     2002-Jul-13

 Jarkko   5.8.0         2002-Jul-18

 Jarkko   5.8.1-RC1     2003-Jul-10     The 5.8 maintenance track
          5.8.1-RC2     2003-Jul-11
          5.8.1-RC3     2003-Jul-30
          5.8.1-RC4     2003-Aug-01
          5.8.1-RC5     2003-Sep-22
          5.8.1         2003-Sep-25
 Nicholas 5.8.2-RC1     2003-Oct-27
          5.8.2-RC2     2003-Nov-03
          5.8.2         2003-Nov-05
          5.8.3-RC1     2004-Jan-07
          5.8.3         2004-Jan-14
          5.8.4-RC1     2004-Apr-05
          5.8.4-RC2     2004-Apr-15
          5.8.4         2004-Apr-21
          5.8.5-RC1     2004-Jul-06
          5.8.5-RC2     2004-Jul-08
          5.8.5         2004-Jul-19
          5.8.6-RC1     2004-Nov-11
          5.8.6         2004-Nov-27
          5.8.7-RC1     2005-May-18
          5.8.7         2005-May-30
          5.8.8-RC1     2006-Jan-20
          5.8.8         2006-Jan-31
          5.8.9-RC1     2008-Nov-10
          5.8.9-RC2     2008-Dec-06
          5.8.9         2008-Dec-14

 Hugo     5.9.0         2003-Oct-27     The 5.9 development track
 Rafael   5.9.1         2004-Mar-16
          5.9.2         2005-Apr-01
          5.9.3         2006-Jan-28
          5.9.4         2006-Aug-15
          5.9.5         2007-Jul-07
          5.10.0-RC1    2007-Nov-17
          5.10.0-RC2    2007-Nov-25

 Rafael   5.10.0        2007-Dec-18

 David M  5.10.1-RC1    2009-Aug-06     The 5.10 maintenance track
          5.10.1-RC2    2009-Aug-18
          5.10.1        2009-Aug-22

 Jesse    5.11.0        2009-Oct-02     The 5.11 development track
          5.11.1        2009-Oct-20
 Leon     5.11.2        2009-Nov-20
 Jesse    5.11.3        2009-Dec-20
 Ricardo  5.11.4        2010-Jan-20
 Steve    5.11.5        2010-Feb-20
 Jesse    5.12.0-RC0    2010-Mar-21
          5.12.0-RC1    2010-Mar-29
          5.12.0-RC2    2010-Apr-01
          5.12.0-RC3    2010-Apr-02
          5.12.0-RC4    2010-Apr-06
          5.12.0-RC5    2010-Apr-09

 Jesse    5.12.0        2010-Apr-12

 Jesse    5.12.1-RC2    2010-May-13     The 5.12 maintenance track
          5.12.1-RC1    2010-May-09
          5.12.1        2010-May-16
          5.12.2-RC2    2010-Aug-31
          5.12.2        2010-Sep-06
 Ricardo  5.12.3-RC1    2011-Jan-09
 Ricardo  5.12.3-RC2    2011-Jan-14
 Ricardo  5.12.3-RC3    2011-Jan-17
 Ricardo  5.12.3        2011-Jan-21
 Leon     5.12.4-RC1    2011-Jun-08
 Leon     5.12.4        2011-Jun-20

 Leon     5.13.0        2010-Apr-20     The 5.13 development track
 Ricardo  5.13.1        2010-May-20
 Matt     5.13.2        2010-Jun-22
 David G  5.13.3        2010-Jul-20
 Florian  5.13.4        2010-Aug-20
 Steve    5.13.5        2010-Sep-19
 Miyagawa 5.13.6        2010-Oct-20
 BinGOs   5.13.7        2010-Nov-20
 Zefram   5.13.8        2010-Dec-20
 Jesse    5.13.9        2011-Jan-20
 Ævar     5.13.10       2011-Feb-20
 Florian  5.13.11       2011-Mar-20
 Jesse    5.14.0RC1     2011-Apr-20
 Jesse    5.14.0RC2     2011-May-04
 Jesse    5.14.0RC3     2011-May-11

 Jesse    5.14.0        2011-May-14     The 5.14 maintenance track
 Jesse    5.14.1        2011-Jun-16
 Florian  5.14.2-RC1    2011-Sep-19
          5.14.2        2011-Sep-26

 David G  5.15.0        2011-Jun-20     The 5.15 development track
 Zefram   5.15.1        2011-Jul-20
 Ricardo  5.15.2        2011-Aug-20
 Stevan   5.15.3        2011-Sep-20
 Florian  5.15.4        2011-Oct-20
 Steve    5.15.5        2011-Nov-20
 Dave R   5.15.6        2011-Dec-20
 BinGOs   5.15.7        2012-Jan-20
 Max M    5.15.8        2012-Feb-20
 Abigail  5.15.9        2012-Mar-20
 Ricardo  5.16.0-RC0    2012-May-10
 Ricardo  5.16.0-RC1    2012-May-14
 Ricardo  5.16.0-RC2    2012-May-15

 Ricardo  5.16.0        2012-May-20     The 5.16 maintenance track
 Ricardo  5.16.1-RC1    2012-Aug-03
 Ricardo  5.16.1        2012-Aug-08
 Ricardo  5.16.2        2012-Nov-01
 Ricardo  5.16.3-RC1    2013-Mar-06
 Ricardo  5.16.3        2013-Mar-11

 Zefram   5.17.0        2012-May-26     The 5.17 development track
 Jesse L  5.17.1        2012-Jun-20
 TonyC    5.17.2        2012-Jul-20
 Steve    5.17.3        2012-Aug-20
 Florian  5.17.4        2012-Sep-20
 Florian  5.17.5        2012-Oct-20
 Ricardo  5.17.6        2012-Nov-20
 Dave R   5.17.7        2012-Dec-18
 Aaron    5.17.8        2013-Jan-20
 BinGOs   5.17.9        2013-Feb-20





• perlhist SELECTED RELEASE SIZES:  



• perlhist SELECTED PATCH SIZES:  












Next: , Up: perlhist THE RECORDS   [Contents]





32.5.1 SELECTED RELEASE SIZES



For example the notation "core: 212  29" in the release 1.000 means that
it had in the core 212 kilobytes, in 29 files.  The "core".."doc" are
explained below.



 release        core       lib         ext        t         doc
 ======================================================================

 1.000           212  29      -   -      -    -     38   51     62   3
 1.014           219  29      -   -      -    -     39   52     68   4
 2.000           309  31      2   3      -    -     55   57     92   4
 2.001           312  31      2   3      -    -     55   57     94   4
 3.000           508  36     24  11      -    -     79   73    156   5
 3.044           645  37     61  20      -    -     90   74    190   6
 4.000           635  37     59  20      -    -     91   75    198   4
 4.019           680  37     85  29      -    -     98   76    199   4
 4.036           709  37     89  30      -    -     98   76    208   5
 5.000alpha2     785  50    114  32      -    -    112   86    209   5
 5.000alpha3     801  50    117  33      -    -    121   87    209   5
 5.000alpha9    1022  56    149  43    116   29    125   90    217   6
 5.000a12h       978  49    140  49    205   46    152   97    228   9
 5.000b3h       1035  53    232  70    216   38    162   94    218  21
 5.000          1038  53    250  76    216   38    154   92    536  62
 5.001m         1071  54    388  82    240   38    159   95    544  29
 5.002          1121  54    661 101    287   43    155   94    847  35
 5.003          1129  54    680 102    291   43    166  100    853  35
 5.003_07       1231  60    748 106    396   53    213  137    976  39
 5.004          1351  60   1230 136    408   51    355  161   1587  55
 5.004_01       1356  60   1258 138    410   51    358  161   1587  55
 5.004_04       1375  60   1294 139    413   51    394  162   1629  55
 5.004_05       1463  60   1435 150    394   50    445  175   1855  59
 5.004_51       1401  61   1260 140    413   53    358  162   1594  56
 5.004_53       1422  62   1295 141    438   70    394  162   1637  56
 5.004_56       1501  66   1301 140    447   74    408  165   1648  57
 5.004_59       1555  72   1317 142    448   74    424  171   1678  58
 5.004_62       1602  77   1327 144    629   92    428  173   1674  58
 5.004_65       1626  77   1358 146    615   92    446  179   1698  60
 5.004_68       1856  74   1382 152    619   92    463  187   1784  60
 5.004_70       1863  75   1456 154    675   92    494  194   1809  60
 5.004_73       1874  76   1467 152    762  102    506  196   1883  61
 5.004_75       1877  76   1467 152    770  103    508  196   1896  62
 5.005          1896  76   1469 152    795  103    509  197   1945  63
 5.005_03       1936  77   1541 153    813  104    551  201   2176  72
 5.005_50       1969  78   1842 301    795  103    514  198   1948  63
 5.005_53       1999  79   1885 303    806  104    602  224   2002  67
 5.005_56       2086  79   1970 307    866  113    672  238   2221  75
 5.6.0          2820  79   2626 364   1096  129    863  280   2840  93
 5.6.1          2946  78   2921 430   1171  132   1024  304   3330 102
 5.6.2          2947  78   3143 451   1247  127   1303  387   3406 102
 5.7.0          2977  80   2801 425   1250  132    975  307   3206 100
 5.7.1          3351  84   3442 455   1944  167   1334  357   3698 124
 5.7.2          3491  87   4858 618   3290  298   1598  449   3910 139
 5.7.3          3299  85   4295 537   2196  300   2176  626   4171 120
 5.8.0          3489  87   4533 585   2437  331   2588  726   4368 125
 5.8.1          3674  90   5104 623   2604  353   2983  836   4625 134
 5.8.2          3633  90   5111 623   2623  357   3019  848   4634 135
 5.8.3          3625  90   5141 624   2660  363   3083  869   4669 136
 5.8.4          3653  90   5170 634   2684  368   3148  885   4689 137
 5.8.5          3664  90   4260 303   2707  369   3208  898   4689 138
 5.8.6          3690  90   4271 303   3141  396   3411  925   4709 139
 5.8.7          3788  90   4322 307   3297  401   3485  964   4744 141
 5.8.8          3895  90   4357 314   3409  431   3622 1017   4979 144
 5.8.9          4132  93   5508 330   3826  529   4364 1234   5348 152
 5.9.0          3657  90   4951 626   2603  354   3011  841   4609 135
 5.9.1          3580  90   5196 634   2665  367   3186  889   4725 138
 5.9.2          3863  90   4654 312   3283  403   3551  973   4800 142
 5.9.3          4096  91   5318 381   4806  597   4272 1214   5139 147
 5.9.4          4393  94   5718 415   4578  642   4646 1310   5335 153
 5.9.5          4681  96   6849 479   4827  671   5155 1490   5572 159
 5.10.0         4710  97   7050 486   4899  673   5275 1503   5673 160
 5.10.1         4858  98   7440 519   6195  921   6147 1751   5151 163
 5.12.0         4999 100   1146 121  15227 2176   6400 1843   5342 168
 5.12.1         5000 100   1146 121  15283 2178   6407 1846   5354 169
 5.12.2         5003 100   1146 121  15404 2178   6413 1846   5376 170
 5.12.3         5004 100   1146 121  15529 2180   6417 1848   5391 171



The "core"..."doc" mean the following files from the Perl source code
distribution.  The glob notation ** means recursively, (.) means
regular files.



 core   *.[hcy]
 lib    lib/**/*.p[ml]
 ext    ext/**/*.{[hcyt],xs,pm} (for -5.10.1) or
        {dist,ext,cpan}/**/*.{[hcyt],xs,pm} (for 5.12.0-)
 t      t/**/*(.) (for 1-5.005_56) or **/*.t (for 5.6.0-5.7.3)
 doc    {README*,INSTALL,*[_.]man{,.?},pod/**/*.pod}



Here are some statistics for the other subdirectories and one file in
the Perl source distribution for somewhat more selected releases.



 ======================================================================
   Legend:  kB   #

                  1.014      2.001      3.044

 Configure      31    1    37    1    62    1
 eg              -    -    34   28    47   39
 h2pl            -    -     -    -    12   12
 msdos           -    -     -    -    41   13
 os2             -    -     -    -    63   22
 usub            -    -     -    -    21   16
 x2p           103   17   104   17   137   17

 ======================================================================

                  4.000      4.019      4.036

 atarist         -    -     -    -   113   31
 Configure      73    1    83    1    86    1
 eg             47   39    47   39    47   39
 emacs          67    4    67    4    67    4
 h2pl           12   12    12   12    12   12
 hints           -    -     5   42    11   56
 msdos          57   15    58   15    60   15
 os2            81   29    81   29   113   31
 usub           25    7    43    8    43    8
 x2p           147   18   152   19   154   19

 ======================================================================

                5.000a2  5.000a12h   5.000b3h      5.000     5.001m

 apollo          8    3     8    3     8    3     8    3     8    3
 atarist       113   31   113   31     -    -     -    -     -    -
 bench           -    -     0    1     -    -     -    -     -    -
 Bugs            2    5    26    1     -    -     -    -     -    -
 dlperl         40    5     -    -     -    -     -    -     -    -
 do            127   71     -    -     -    -     -    -     -    -
 Configure       -    -   153    1   159    1   160    1   180    1
 Doc             -    -    26    1    75    7    11    1    11    1
 eg             79   58    53   44    51   43    54   44    54   44
 emacs          67    4   104    6   104    6   104    1   104    6
 h2pl           12   12    12   12    12   12    12   12    12   12
 hints          11   56    12   46    18   48    18   48    44   56
 msdos          60   15    60   15     -    -     -    -     -    -
 os2           113   31   113   31     -    -     -    -     -    -
 U               -    -    62    8   112   42     -    -     -    -
 usub           43    8     -    -     -    -     -    -     -    -
 vms             -    -    80    7   123    9   184   15   304   20
 x2p           171   22   171   21   162   20   162   20   279   20

 ======================================================================

                  5.002      5.003   5.003_07

 Configure     201    1   201    1   217    1
 eg             54   44    54   44    54   44
 emacs         108    1   108    1   143    1
 h2pl           12   12    12   12    12   12
 hints          73   59    77   60    90   62
 os2            84   17    56   10   117   42
 plan9           -    -     -    -    79   15
 Porting         -    -     -    -    51    1
 utils          87    7    88    7    97    7
 vms           500   24   475   26   505   27
 x2p           280   20   280   20   280   19

 ======================================================================

                  5.004   5.004_04   5.004_62   5.004_65   5.004_68

 beos            -    -     -    -     -    -      1   1      1   1
 Configure     225    1   225    1   240    1    248   1    256   1
 cygwin32       23    5    23    5    23    5     24   5     24   5
 djgpp           -    -     -    -    14    5     14   5     14   5
 eg             81   62    81   62    81   62     81  62     81  62
 emacs         194    1   204    1   212    2    212   2    212   2
 h2pl           12   12    12   12    12   12     12  12     12  12
 hints         129   69   132   71   144   72    151  74    155  74
 os2           121   42   127   42   127   44    129  44    129  44
 plan9          82   15    82   15    82   15     82  15     82  15
 Porting        94    2   109    4   203    6    234   8    241   9
 qnx             1    2     1    2     1    2      1   2      1   2
 utils         112    8   118    8   124    8    156   9    159   9
 vms           518   34   524   34   538   34    569  34    569  34
 win32         285   33   378   36   470   39    493  39    575  41
 x2p           281   19   281   19   281   19    282  19    281  19

 ======================================================================

               5.004_70   5.004_73   5.004_75      5.005   5.005_03

 apollo          -    -     -    -     -    -     -    -      0   1
 beos            1    1     1    1     1    1     1    1      1   1
 Configure     256    1   256    1   264    1   264    1    270   1
 cygwin32       24    5    24    5    24    5    24    5     24   5
 djgpp          14    5    14    5    14    5    14    5     15   5
 eg             86   65    86   65    86   65    86   65     86  65
 emacs         262    2   262    2   262    2   262    2    274   2
 h2pl           12   12    12   12    12   12    12   12     12  12
 hints         157   74   157   74   159   74   160   74    179  77
 mint            -    -     -    -     -    -     -    -      4   7
 mpeix           -    -     -    -     5    3     5    3      5   3
 os2           129   44   139   44   142   44   143   44    148  44
 plan9          82   15    82   15    82   15    82   15     82  15
 Porting       241    9   253    9   259   10   264   12    272  13
 qnx             1    2     1    2     1    2     1    2      1   2
 utils         160    9   160    9   160    9   160    9    164   9
 vms           570   34   572   34   573   34   575   34    583  34
 vos             -    -     -    -     -    -     -   -     156  10
 win32         577   41   585   41   585   41   587   41    600  42
 x2p           281   19   281   19   281   19   281   19    281  19

 ======================================================================

                  5.6.0      5.6.1      5.6.2      5.7.3

 apollo          8    3     8    3     8    3     8    3
 beos            5    2     5    2     5    2     6    4
 Configure     346    1   361    1   363    1   394    1
 Cross           -    -     -    -     -    -     4    2
 djgpp          19    6    19    6    19    6    21    7
 eg            112   71   112   71   112   71     -    -
 emacs         303    4   319    4   319    4   319    4
 epoc           29    8    35    8    35    8    36    8
 h2pl           24   15    24   15    24   15    24   15
 hints         242   83   250   84   321   89   272   87
 mint           11    9    11    9    11    9    11    9
 mpeix           9    4     9    4     9    4     9    4
 NetWare         -    -     -    -     -    -   423   57
 os2           214   59   224   60   224   60   357   66
 plan9          92   17    92   17    92   17    85   15
 Porting       361   15   390   16   390   16   425   21
 qnx             5    3     5    3     5    3     5    3
 utils         228   12   221   11   222   11   267   13
 uts             -    -     -    -     -    -    12    3
 vmesa          25    4    25    4    25    4    25    4
 vms           686   38   627   38   627   38   649   36
 vos           227   12   249   15   248   15   281   17
 win32         755   41   782   42   801   42  1006   50
 x2p           307   20   307   20   307   20   345   20

 ======================================================================

                  5.8.0      5.8.1      5.8.2      5.8.3      5.8.4

 apollo          8    3     8    3     8    3     8    3     8    3
 beos            6    4     6    4     6    4     6    4     6    4
 Configure     472    1   493    1   493    1   493    1   494    1
 Cross           4    2    45   10    45   10    45   10    45   10
 djgpp          21    7    21    7    21    7    21    7    21    7
 emacs         319    4   329    4   329    4   329    4   329    4
 epoc           33    8    33    8    33    8    33    8    33    8
 h2pl           24   15    24   15    24   15    24   15    24   15
 hints         294   88   321   89   321   89   321   89   348   91
 mint           11    9    11    9    11    9    11    9    11    9
 mpeix          24    5    25    5    25    5    25    5    25    5
 NetWare       488   61   490   61   490   61   490   61   488   61
 os2           361   66   445   67   450   67   488   67   488   67
 plan9          85   15   325   17   325   17   325   17   321   17
 Porting       479   22   537   32   538   32   539   32   538   33
 qnx             5    3     5    3     5    3     5    3     5    3
 utils         275   15   258   16   258   16   263   19   263   19
 uts            12    3    12    3    12    3    12    3    12    3
 vmesa          25    4    25    4    25    4    25    4    25    4
 vms           648   36   654   36   654   36   656   36   656   36
 vos           330   20   335   20   335   20   335   20   335   20
 win32        1062   49  1125   49  1127   49  1126   49  1181   56
 x2p           347   20   348   20   348   20   348   20   348   20

 ======================================================================

                  5.8.5      5.8.6      5.8.7      5.8.8      5.8.9

 apollo          8    3     8    3     8    3     8    3     8    3
 beos            6    4     6    4     8    4     8    4     8    4
 Configure     494    1   494    1   495    1   506    1   520    1
 Cross          45   10    45   10    45   10    45   10    46   10
 djgpp          21    7    21    7    21    7    21    7    21    7
 emacs         329    4   329    4   329    4   329    4   406    4
 epoc           33    8    33    8    33    8    34    8    35    8
 h2pl           24   15    24   15    24   15    24   15    24   15
 hints         350   91   352   91   355   94   360   94   387   99
 mint           11    9    11    9    11    9    11    9    11    9
 mpeix          25    5    25    5    25    5    49    6    49    6
 NetWare       488   61   488   61   488   61   490   61   491   61
 os2           488   67   488   67   488   67   488   67   552   70
 plan9         321   17   321   17   321   17   322   17   324   17
 Porting       538   34   548   35   549   35   564   37   625   41
 qnx             5    3     5    3     5    3     5    3     5    3
 utils         265   19   265   19   266   19   267   19   281   21
 uts            12    3    12    3    12    3    12    3    12    3
 vmesa          25    4    25    4    25    4    25    4    25    4
 vms           657   36   658   36   662   36   664   36   716   35
 vos           335   20   335   20   335   20   336   21   345   22
 win32        1183   56  1190   56  1199   56  1219   56  1484   68
 x2p           349   20   349   20   349   20   349   19   350   19

 ======================================================================

                  5.9.0      5.9.1      5.9.2      5.9.3      5.9.4

 apollo          8    3     8    3     8    3     8    3     8    3
 beos            6    4     6    4     8    4     8    4     8    4
 Configure     493    1   493    1   495    1   508    1   512    1
 Cross          45   10    45   10    45   10    45   10    46   10
 djgpp          21    7    21    7    21    7    21    7    21    7
 emacs         329    4   329    4   329    4   329    4   329    4
 epoc           33    8    33    8    33    8    34    8    34    8
 h2pl           24   15    24   15    24   15    24   15    24   15
 hints         321   89   346   91   355   94   359   94   366   96
 mad             -    -     -    -     -    -     -    -   174    6
 mint           11    9    11    9    11    9    11    9    11    9
 mpeix          25    5    25    5    25    5    49    6    49    6
 NetWare       489   61   487   61   487   61   489   61   489   61
 os2           444   67   488   67   488   67   488   67   488   67
 plan9         325   17   321   17   321   17   322   17   323   17
 Porting       537   32   536   33   549   36   564   38   576   38
 qnx             5    3     5    3     5    3     5    3     5    3
 symbian         -    -     -    -     -    -   293   53   293   53
 utils         258   16   263   19   268   20   273   23   275   24
 uts            12    3    12    3    12    3    12    3    12    3
 vmesa          25    4    25    4    25    4    25    4    25    4
 vms           660   36   547   33   553   33   661   33   696   33
 vos            11    7    11    7    11    7    11    7    11    7
 win32        1120   49  1124   51  1191   56  1209   56  1719   90
 x2p           348   20   348   20   349   20   349   19   349   19

 ======================================================================

                  5.9.5     5.10.0     5.10.1     5.12.0     5.12.1

 apollo          8    3     8    3     0    3     0    3     0    3
 beos            8    4     8    4     4    4     4    4     4    4
 Configure     518    1   518    1   533    1   536    1   536    1
 Cross         122   15   122   15   119   15   118   15   118   15
 djgpp          21    7    21    7    17    7    17    7    17    7
 emacs         329    4   406    4   402    4   402    4   402    4
 epoc           34    8    35    8    31    8    31    8    31    8
 h2pl           24   15    24   15    12   15    12   15    12   15
 hints         377   98   381   98   385  100   368   97   368   97
 mad           182    8   182    8   174    8   174    8   174    8
 mint           11    9    11    9     3    9     -    -     -    -
 mpeix          49    6    49    6    45    6    45    6    45    6
 NetWare       489   61   489   61   465   61   466   61   466   61
 os2           552   70   552   70   507   70   507   70   507   70
 plan9         324   17   324   17   316   17   316   17   316   17
 Porting       627   40   632   40   933   53   749   54   749   54
 qnx             5    3     5    4     1    4     1    4     1    4
 symbian       300   54   300   54   290   54   288   54   288   54
 utils         260   26   264   27   268   27   269   27   269   27
 uts            12    3    12    3     8    3     8    3     8    3
 vmesa          25    4    25    4    21    4    21    4    21    4
 vms           690   32   722   32   693   30   645   18   645   18
 vos            19    8    19    8    16    8    16    8    16    8
 win32        1482   68  1485   68  1497   70  1841   73  1841   73
 x2p           349   19   349   19   345   19   345   19   345   19

 ======================================================================

                  5.12.2     5.12.3

 apollo          0    3     0    3
 beos            4    4     4    4
 Configure     536    1   536    1
 Cross         118   15   118   15
 djgpp          17    7    17    7
 emacs         402    4   402    4
 epoc           31    8    31    8
 h2pl           12   15    12   15
 hints         368   97   368   97
 mad           174    8   174    8
 mpeix          45    6    45    6
 NetWare       466   61   466   61
 os2           507   70   507   70
 plan9         316   17   316   17
 Porting       750   54   750   54
 qnx             1    4     1    4
 symbian       288   54   288   54
 utils         269   27   269   27
 uts             8    3     8    3
 vmesa          21    4    21    4
 vms           646   18   644   18
 vos            16    8    16    8
 win32        1841   73  1841   73
 x2p           345   19   345   19









Previous: perlhist SELECTED RELEASE SIZES, Up: perlhist THE RECORDS   [Contents]





32.5.2 SELECTED PATCH SIZES



The "diff lines kB" means that for example the patch 5.003_08, to be
applied on top of the 5.003_07 (or whatever was before the 5.003_08)
added lines for 110 kilobytes, it removed lines for 19 kilobytes, and
changed lines for 424 kilobytes.  Just the lines themselves are
counted, not their context.  The "+ - !" become from the diff(1)
context diff output format.



 Pump-  Release         Date              diff lines kB
 king                                     -------------
                                          +     -     !
 ======================================================================

 Chip     5.003_08      1996-Nov-19     110    19   424
          5.003_09      1996-Nov-26      38     9   248
          5.003_10      1996-Nov-29      29     2    27
          5.003_11      1996-Dec-06      73    12   165
          5.003_12      1996-Dec-19     275     6   436
          5.003_13      1996-Dec-20      95     1    56
          5.003_14      1996-Dec-23      23     7   333
          5.003_15      1996-Dec-23       0     0     1
          5.003_16      1996-Dec-24      12     3    50
          5.003_17      1996-Dec-27      19     1    14
          5.003_18      1996-Dec-31      21     1    32
          5.003_19      1997-Jan-04      80     3    85
          5.003_20      1997-Jan-07      18     1   146
          5.003_21      1997-Jan-15      38    10   221
          5.003_22      1997-Jan-16       4     0    18
          5.003_23      1997-Jan-25      71    15   119
          5.003_24      1997-Jan-29     426     1    20
          5.003_25      1997-Feb-04      21     8   169
          5.003_26      1997-Feb-10      16     1    15
          5.003_27      1997-Feb-18      32    10    38
          5.003_28      1997-Feb-21      58     4    66
          5.003_90      1997-Feb-25      22     2    34
          5.003_91      1997-Mar-01      37     1    39
          5.003_92      1997-Mar-06      16     3    69
          5.003_93      1997-Mar-10      12     3    15
          5.003_94      1997-Mar-22     407     7   200
          5.003_95      1997-Mar-25      41     1    37
          5.003_96      1997-Apr-01     283     5   261
          5.003_97      1997-Apr-03      13     2    34
          5.003_97a     1997-Apr-05      57     1    27
          5.003_97b     1997-Apr-08      14     1    20
          5.003_97c     1997-Apr-10      20     1    16
          5.003_97d     1997-Apr-13       8     0    16
          5.003_97e     1997-Apr-15      15     4    46
          5.003_97f     1997-Apr-17       7     1    33
          5.003_97g     1997-Apr-18       6     1    42
          5.003_97h     1997-Apr-24      23     3    68
          5.003_97i     1997-Apr-25      23     1    31
          5.003_97j     1997-Apr-28      36     1    49
          5.003_98      1997-Apr-30     171    12   539
          5.003_99      1997-May-01       6     0     7
          5.003_99a     1997-May-09      36     2    61
          p54rc1        1997-May-12       8     1    11
          p54rc2        1997-May-14       6     0    40

        5.004           1997-May-15       4     0     4

 Tim      5.004_01      1997-Jun-13     222    14    57
          5.004_02      1997-Aug-07     112    16   119
          5.004_03      1997-Sep-05     109     0    17
          5.004_04      1997-Oct-15      66     8   173





• perlhist The patch-free era:  












Up: perlhist SELECTED PATCH SIZES   [Contents]





32.5.2.1 The patch-free era



In more modern times, named releases don’t come as often, and as progress
can be followed (nearly) instantly (with rsync, and since late 2008, git)
patches between versions are no longer provided. However, that doesn’t
keep us from calculating how large a patch could have been. Which is
shown in the table below. Unless noted otherwise, the size mentioned is
the patch to bring version x.y.z to x.y.z+1.



 Sarathy  5.6.1         2001-Apr-08     531    44   651
 Rafael   5.6.2         2003-Nov-15      20    11  1819

 Jarkko   5.8.0         2002-Jul-18    1205    31   471   From 5.7.3

          5.8.1         2003-Sep-25     243   102  6162
 Nicholas 5.8.2         2003-Nov-05      10    50   788
          5.8.3         2004-Jan-14      31    13   360
          5.8.4         2004-Apr-21      33     8   299
          5.8.5         2004-Jul-19      11    19   255
          5.8.6         2004-Nov-27      35     3   192
          5.8.7         2005-May-30      75    34   778
          5.8.8         2006-Jan-31     131    42  1251
          5.8.9         2008-Dec-14     340   132 12988

 Hugo     5.9.0         2003-Oct-27     281   168  7132   From 5.8.0
 Rafael   5.9.1         2004-Mar-16      57   250  2107
          5.9.2         2005-Apr-01     720    57   858
          5.9.3         2006-Jan-28    1124   102  1906
          5.9.4         2006-Aug-15     896    60   862
          5.9.5         2007-Jul-07    1149   128  1062

          5.10.0        2007-Dec-18      50    31 13111   From 5.9.5









Previous: perlhist THE RECORDS, Up: perlhist   [Contents]





32.6 THE KEEPERS OF THE RECORDS



Jarkko Hietaniemi <jhi@iki.fi>.



Thanks to the collective memory of the Perlfolk.  In addition to the
Keepers of the Pumpkin also Alan Champion, Mark Dominus,
Andreas König, John Macdonald, Matthias Neeracher, Jeff Okamoto,
Michael Peppler, Randal Schwartz, and Paul D. Smith sent corrections
and additions. Abigail added file and patch size data for the 5.6.0 - 5.10
era.









Next: , Previous: perlhist, Up: Top   [Contents]





33 perlinterp





• perlinterp NAME:  



• perlinterp DESCRIPTION:  



• perlinterp ELEMENTS OF THE INTERPRETER:  



• perlinterp OP TREES:  



• perlinterp STACKS:  



• perlinterp MILLIONS OF MACROS:  



• perlinterp FURTHER READING:  












Next: , Up: perlinterp   [Contents]





33.1 NAME



perlinterp - An overview of the Perl interpreter









Next: , Previous: perlinterp NAME, Up: perlinterp   [Contents]





33.2 DESCRIPTION



This document provides an overview of how the Perl interpreter works at
the level of C code, along with pointers to the relevant C source code
files.









Next: , Previous: perlinterp DESCRIPTION, Up: perlinterp   [Contents]





33.3 ELEMENTS OF THE INTERPRETER



The work of the interpreter has two main stages: compiling the code
into the internal representation, or bytecode, and then executing it.
perlguts Compiled code explains exactly how the compilation stage
happens.



Here is a short breakdown of perl’s operation:





• perlinterp Startup:  



• perlinterp Parsing:  



• perlinterp Optimization:  



• perlinterp Running:  



• perlinterp Exception handing:  



• perlinterp INTERNAL VARIABLE TYPES:  












Next: , Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.1 Startup



The action begins in perlmain.c. (or miniperlmain.c for miniperl)
This is very high-level code, enough to fit on a single screen, and it
resembles the code found in perlembed NAME; most of the real action takes
place in perl.c



perlmain.c is generated by ExtUtils::Miniperl from
miniperlmain.c at make time, so you should make perl to follow this
along.



First, perlmain.c allocates some memory and constructs a Perl
interpreter, along these lines:



    1 PERL_SYS_INIT3(&argc,&argv,&env);
    2
    3 if (!PL_do_undump) {
    4     my_perl = perl_alloc();
    5     if (!my_perl)
    6         exit(1);
    7     perl_construct(my_perl);
    8     PL_perl_destruct_level = 0;
    9 }



Line 1 is a macro, and its definition is dependent on your operating
system. Line 3 references PL_do_undump, a global variable - all
global variables in Perl start with PL_. This tells you whether the
current running program was created with the -u flag to perl and
then undump, which means it’s going to be false in any sane context.



Line 4 calls a function in perl.c to allocate memory for a Perl
interpreter. It’s quite a simple function, and the guts of it looks
like this:



 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));



Here you see an example of Perl’s system abstraction, which we’ll see
later: PerlMem_malloc is either your system’s malloc, or Perl’s
own malloc as defined in malloc.c if you selected that option at
configure time.



Next, in line 7, we construct the interpreter using perl_construct,
also in perl.c; this sets up all the special variables that Perl
needs, the stacks, and so on.



Now we pass Perl the command line options, and tell it to go:



 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
 if (!exitstatus)
     perl_run(my_perl);

 exitstatus = perl_destruct(my_perl);

 perl_free(my_perl);



perl_parse is actually a wrapper around S_parse_body, as defined
in perl.c, which processes the command line options, sets up any
statically linked XS modules, opens the program and calls yyparse to
parse it.









Next: , Previous: perlinterp Startup, Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.2 Parsing



The aim of this stage is to take the Perl source, and turn it into an
op tree. We’ll see what one of those looks like later. Strictly
speaking, there’s three things going on here.



yyparse, the parser, lives in perly.c, although you’re better off
reading the original YACC input in perly.y. (Yes, Virginia, there
is a YACC grammar for Perl!) The job of the parser is to take your
code and "understand" it, splitting it into sentences, deciding which
operands go with which operators and so on.



The parser is nobly assisted by the lexer, which chunks up your input
into tokens, and decides what type of thing each token is: a variable
name, an operator, a bareword, a subroutine, a core function, and so
on. The main point of entry to the lexer is yylex, and that and its
associated routines can be found in toke.c. Perl isn’t much like
other computer languages; it’s highly context sensitive at times, it
can be tricky to work out what sort of token something is, or where a
token ends. As such, there’s a lot of interplay between the tokeniser
and the parser, which can get pretty frightening if you’re not used to
it.



As the parser understands a Perl program, it builds up a tree of
operations for the interpreter to perform during execution. The
routines which construct and link together the various operations are
to be found in op.c, and will be examined later.









Next: , Previous: perlinterp Parsing, Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.3 Optimization



Now the parsing stage is complete, and the finished tree represents the
operations that the Perl interpreter needs to perform to execute our
program. Next, Perl does a dry run over the tree looking for
optimisations: constant expressions such as 3 + 4 will be computed
now, and the optimizer will also see if any multiple operations can be
replaced with a single one. For instance, to fetch the variable
$foo, instead of grabbing the glob *foo and looking at the scalar
component, the optimizer fiddles the op tree to use a function which
directly looks up the scalar in question. The main optimizer is peep
in op.c, and many ops have their own optimizing functions.









Next: , Previous: perlinterp Optimization, Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.4 Running



Now we’re finally ready to go: we have compiled Perl byte code, and all
that’s left to do is run it. The actual execution is done by the
runops_standard function in run.c; more specifically, it’s done
by these three innocent looking lines:



    while ((PL_op = PL_op->op_ppaddr(aTHX))) {
        PERL_ASYNC_CHECK();
    }



You may be more comfortable with the Perl version of that:



    PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};



Well, maybe not. Anyway, each op contains a function pointer, which
stipulates the function which will actually carry out the operation.
This function will return the next op in the sequence - this allows for
things like if which choose the next op dynamically at run time. The
PERL_ASYNC_CHECK makes sure that things like signals interrupt
execution if required.



The actual functions called are known as PP code, and they’re spread
between four files: pp_hot.c contains the "hot" code, which is most
often used and highly optimized, pp_sys.c contains all the
system-specific functions, pp_ctl.c contains the functions which
implement control structures (if, while and the like) and pp.c
contains everything else. These are, if you like, the C code for Perl’s
built-in functions and operators.



Note that each pp_ function is expected to return a pointer to the
next op. Calls to perl subs (and eval blocks) are handled within the
same runops loop, and do not consume extra space on the C stack. For
example, pp_entersub and pp_entertry just push a CxSUB or
CxEVAL block struct onto the context stack which contain the address
of the op following the sub call or eval. They then return the first op
of that sub or eval block, and so execution continues of that sub or
block. Later, a pp_leavesub or pp_leavetry op pops the CxSUB
or CxEVAL, retrieves the return op from it, and returns it.









Next: , Previous: perlinterp Running, Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.5 Exception handing



Perl’s exception handing (i.e. die etc.) is built on top of the
low-level setjmp()/longjmp() C-library functions. These basically
provide a way to capture the current PC and SP registers and later
restore them; i.e. a longjmp() continues at the point in code where
a previous setjmp() was done, with anything further up on the C
stack being lost. This is why code should always save values using
SAVE_FOO rather than in auto variables.



The perl core wraps setjmp() etc in the macros JMPENV_PUSH and
JMPENV_JUMP. The basic rule of perl exceptions is that exit, and
die (in the absence of eval) perform a JMPENV_JUMP(2), while
die within eval does a JMPENV_JUMP(3).



At entry points to perl, such as perl_parse(), perl_run() and
call_sv(cv, G_EVAL) each does a JMPENV_PUSH, then enter a runops
loop or whatever, and handle possible exception returns. For a 2
return, final cleanup is performed, such as popping stacks and calling
CHECK or END blocks. Amongst other things, this is how scope
cleanup still occurs during an exit.



If a die can find a CxEVAL block on the context stack, then the
stack is popped to that level and the return op in that block is
assigned to PL_restartop; then a JMPENV_JUMP(3) is performed.
This normally passes control back to the guard. In the case of
perl_run and call_sv, a non-null PL_restartop triggers
re-entry to the runops loop. The is the normal way that die or
croak is handled within an eval.



Sometimes ops are executed within an inner runops loop, such as tie,
sort or overload code. In this case, something like



    sub FETCH { eval { die } }



would cause a longjmp right back to the guard in perl_run, popping
both runops loops, which is clearly incorrect. One way to avoid this is
for the tie code to do a JMPENV_PUSH before executing FETCH in
the inner runops loop, but for efficiency reasons, perl in fact just
sets a flag, using CATCH_SET(TRUE). The pp_require,
pp_entereval and pp_entertry ops check this flag, and if true,
they call docatch, which does a JMPENV_PUSH and starts a new
runops level to execute the code, rather than doing it on the current
loop.



As a further optimisation, on exit from the eval block in the FETCH,
execution of the code following the block is still carried on in the
inner loop. When an exception is raised, docatch compares the
JMPENV level of the CxEVAL with PL_top_env and if they differ,
just re-throws the exception. In this way any inner loops get popped.



Here’s an example.



    1: eval { tie @a, 'A' };
    2: sub A::TIEARRAY {
    3:     eval { die };
    4:     die;
    5: }



To run this code, perl_run is called, which does a JMPENV_PUSH
then enters a runops loop. This loop executes the eval and tie ops on
line 1, with the eval pushing a CxEVAL onto the context stack.



The pp_tie does a CATCH_SET(TRUE), then starts a second runops
loop to execute the body of TIEARRAY. When it executes the entertry
op on line 3, CATCH_GET is true, so pp_entertry calls docatch
which does a JMPENV_PUSH and starts a third runops loop, which then
executes the die op. At this point the C call stack looks like this:



    Perl_pp_die
    Perl_runops      # third loop
    S_docatch_body
    S_docatch
    Perl_pp_entertry
    Perl_runops      # second loop
    S_call_body
    Perl_call_sv
    Perl_pp_tie
    Perl_runops      # first loop
    S_run_body
    perl_run
    main



and the context and data stacks, as shown by -Dstv, look like:



    STACK 0: MAIN
      CX 0: BLOCK  =>
      CX 1: EVAL   => AV()  PV("A"\0)
      retop=leave
    STACK 1: MAGIC
      CX 0: SUB    =>
      retop=(null)
      CX 1: EVAL   => *
    retop=nextstate



The die pops the first CxEVAL off the context stack, sets
PL_restartop from it, does a JMPENV_JUMP(3), and control returns
to the top docatch. This then starts another third-level runops
level, which executes the nextstate, pushmark and die ops on line 4. At
the point that the second pp_die is called, the C call stack looks
exactly like that above, even though we are no longer within an inner
eval; this is because of the optimization mentioned earlier. However,
the context stack now looks like this, ie with the top CxEVAL popped:



    STACK 0: MAIN
      CX 0: BLOCK  =>
      CX 1: EVAL   => AV()  PV("A"\0)
      retop=leave
    STACK 1: MAGIC
      CX 0: SUB    =>
      retop=(null)



The die on line 4 pops the context stack back down to the CxEVAL,
leaving it as:



    STACK 0: MAIN
      CX 0: BLOCK  =>



As usual, PL_restartop is extracted from the CxEVAL, and a
JMPENV_JUMP(3) done, which pops the C stack back to the docatch:



    S_docatch
    Perl_pp_entertry
    Perl_runops      # second loop
    S_call_body
    Perl_call_sv
    Perl_pp_tie
    Perl_runops      # first loop
    S_run_body
    perl_run
    main



In  this case, because the JMPENV level recorded in the CxEVAL
differs from the current one, docatch just does a JMPENV_JUMP(3)
and the C stack unwinds to:



    perl_run
    main



Because PL_restartop is non-null, run_body starts a new runops
loop and execution continues.









Previous: perlinterp Exception handing, Up: perlinterp ELEMENTS OF THE INTERPRETER   [Contents]





33.3.6 INTERNAL VARIABLE TYPES



You should by now have had a look at perlguts NAME, which tells you about
Perl’s internal variable types: SVs, HVs, AVs and the rest. If not, do
that now.



These variables are used not only to represent Perl-space variables,
but also any constants in the code, as well as some structures
completely internal to Perl. The symbol table, for instance, is an
ordinary Perl hash. Your code is represented by an SV as it’s read into
the parser; any program files you call are opened via ordinary Perl
filehandles, and so on.



The core (Devel-Peek)Devel::Peek module lets us examine SVs from a
Perl program. Let’s see, for instance, how Perl treats the constant
"hello".



      % perl -MDevel::Peek -e 'Dump("hello")'
    1 SV = PV(0xa041450) at 0xa04ecbc
    2   REFCNT = 1
    3   FLAGS = (POK,READONLY,pPOK)
    4   PV = 0xa0484e0 "hello"\0
    5   CUR = 5
    6   LEN = 6



Reading Devel::Peek output takes a bit of practise, so let’s go
through it line by line.



Line 1 tells us we’re looking at an SV which lives at 0xa04ecbc in
memory. SVs themselves are very simple structures, but they contain a
pointer to a more complex structure. In this case, it’s a PV, a
structure which holds a string value, at location 0xa041450. Line 2
is the reference count; there are no other references to this data, so
it’s 1.



Line 3 are the flags for this SV - it’s OK to use it as a PV, it’s a
read-only SV (because it’s a constant) and the data is a PV internally.
Next we’ve got the contents of the string, starting at location
0xa0484e0.



Line 5 gives us the current length of the string - note that this does
not include the null terminator. Line 6 is not the length of the
string, but the length of the currently allocated buffer; as the string
grows, Perl automatically extends the available storage via a routine
called SvGROW.



You can get at any of these quantities from C very easily; just add
Sv to the name of the field shown in the snippet, and you’ve got a
macro which will return the value: SvCUR(sv) returns the current
length of the string, SvREFCOUNT(sv) returns the reference count,
SvPV(sv, len) returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in perlguts NAME.



Let’s take an example of manipulating a PV, from sv_catpvn, in
sv.c



     1  void
     2  Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
     3  {
     4      STRLEN tlen;
     5      char *junk;

     6      junk = SvPV_force(sv, tlen);
     7      SvGROW(sv, tlen + len + 1);
     8      if (ptr == junk)
     9          ptr = SvPVX(sv);
    10      Move(ptr,SvPVX(sv)+tlen,len,char);
    11      SvCUR(sv) += len;
    12      *SvEND(sv) = '\0';
    13      (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    14      SvTAINT(sv);
    15  }



This is a function which adds a string, ptr, of length len onto
the end of the PV stored in sv. The first thing we do in line 6 is
make sure that the SV has a valid PV, by calling the SvPV_force
macro to force a PV. As a side effect, tlen gets set to the current
value of the PV, and the PV itself is returned to junk.



In line 7, we make sure that the SV will have enough room to
accommodate the old string, the new string and the null terminator. If
LEN isn’t big enough, SvGROW will reallocate space for us.



Now, if junk is the same as the string we’re trying to add, we can
grab the string directly from the SV; SvPVX is the address of the PV
in the SV.



Line 10 does the actual catenation: the Move macro moves a chunk of
memory around: we move the string ptr to the end of the PV - that’s
the start of the PV plus its current length. We’re moving len bytes
of type char. After doing so, we need to tell Perl we’ve extended
the string, by altering CUR to reflect the new length. SvEND is a
macro which gives us the end of the string, so that needs to be a
"\0".



Line 13 manipulates the flags; since we’ve changed the PV, any IV or NV
values will no longer be valid: if we have $a=10; $a.="6"; we don’t
want to use the old IV of 10. SvPOK_only_utf8 is a special
UTF-8-aware version of SvPOK_only, a macro which turns off the IOK
and NOK flags and turns on POK. The final SvTAINT is a macro which
launders tainted data if taint mode is turned on.



AVs and HVs are more complicated, but SVs are by far the most common
variable type being thrown around. Having seen something of how we
manipulate these, let’s go on and look at how the op tree is
constructed.









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33.4 OP TREES



First, what is the op tree, anyway? The op tree is the parsed
representation of your program, as we saw in our section on parsing,
and it’s the sequence of operations that Perl goes through to execute
your program, as we saw in Running.



An op is a fundamental operation that Perl can perform: all the
built-in functions and operators are ops, and there are a series of ops
which deal with concepts the interpreter needs internally - entering
and leaving a block, ending a statement, fetching a variable, and so
on.



The op tree is connected in two ways: you can imagine that there are
two "routes" through it, two orders in which you can traverse the tree.
First, parse order reflects how the parser understood the code, and
secondly, execution order tells perl what order to perform the
operations in.



The easiest way to examine the op tree is to stop Perl after it has
finished parsing, and get it to dump out the tree. This is exactly what
the compiler backends (B-Terse)B::Terse, (B-Concise)B::Concise
and (B-Debug)B::Debug do.



Let’s have a look at how Perl sees $a = $b + $c:



     % perl -MO=Terse -e '$a=$b+$c'
     1  LISTOP (0x8179888) leave
     2      OP (0x81798b0) enter
     3      COP (0x8179850) nextstate
     4      BINOP (0x8179828) sassign
     5          BINOP (0x8179800) add [1]
     6              UNOP (0x81796e0) null [15]
     7                  SVOP (0x80fafe0) gvsv  GV (0x80fa4cc) *b
     8              UNOP (0x81797e0) null [15]
     9                  SVOP (0x8179700) gvsv  GV (0x80efeb0) *c
    10          UNOP (0x816b4f0) null [15]
    11              SVOP (0x816dcf0) gvsv  GV (0x80fa460) *a



Let’s start in the middle, at line 4. This is a BINOP, a binary
operator, which is at location 0x8179828. The specific operator in
question is sassign - scalar assignment - and you can find the code
which implements it in the function pp_sassign in pp_hot.c. As a
binary operator, it has two children: the add operator, providing the
result of $b+$c, is uppermost on line 5, and the left hand side is
on line 10.



Line 10 is the null op: this does exactly nothing. What is that doing
there? If you see the null op, it’s a sign that something has been
optimized away after parsing. As we mentioned in Optimization, the
optimization stage sometimes converts two operations into one, for
example when fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers, it’s
easier just to replace the redundant operation with the null op.
Originally, the tree would have looked like this:



    10          SVOP (0x816b4f0) rv2sv [15]
    11              SVOP (0x816dcf0) gv  GV (0x80fa460) *a



That is, fetch the a entry from the main symbol table, and then look
at the scalar component of it: gvsv (pp_gvsv into pp_hot.c)
happens to do both these things.



The right hand side, starting at line 5 is similar to what we’ve just
seen: we have the add op (pp_add also in pp_hot.c) add
together two gvsvs.



Now, what’s this about?



     1  LISTOP (0x8179888) leave
     2      OP (0x81798b0) enter
     3      COP (0x8179850) nextstate



enter and leave are scoping ops, and their job is to perform any
housekeeping every time you enter and leave a block: lexical variables
are tidied up, unreferenced variables are destroyed, and so on. Every
program will have those first three lines: leave is a list, and its
children are all the statements in the block. Statements are delimited
by nextstate, so a block is a collection of nextstate ops, with
the ops to be performed for each statement being the children of
nextstate. enter is a single op which functions as a marker.



That’s how Perl parsed the program, from top to bottom:



                        Program
                           |
                       Statement
                           |
                           =
                          / \
                         /   \
                        $a   +
                            / \
                          $b   $c



However, it’s impossible to perform the operations in this order:
you have to find the values of $b and $c before you add them
together, for instance. So, the other thread that runs through the op
tree is the execution order: each op has a field op_next which
points to the next op to be run, so following these pointers tells us
how perl executes the code. We can traverse the tree in this order
using the exec option to B::Terse:



     % perl -MO=Terse,exec -e '$a=$b+$c'
     1  OP (0x8179928) enter
     2  COP (0x81798c8) nextstate
     3  SVOP (0x81796c8) gvsv  GV (0x80fa4d4) *b
     4  SVOP (0x8179798) gvsv  GV (0x80efeb0) *c
     5  BINOP (0x8179878) add [1]
     6  SVOP (0x816dd38) gvsv  GV (0x80fa468) *a
     7  BINOP (0x81798a0) sassign
     8  LISTOP (0x8179900) leave



This probably makes more sense for a human: enter a block, start a
statement. Get the values of $b and $c, and add them together.
Find $a, and assign one to the other. Then leave.



The way Perl builds up these op trees in the parsing process can be
unravelled by examining perly.y, the YACC grammar. Let’s take the
piece we need to construct the tree for $a = $b + $c



    1 term    :   term ASSIGNOP term
    2                { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
    3         |   term ADDOP term
    4                { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }



If you’re not used to reading BNF grammars, this is how it works:
You’re fed certain things by the tokeniser, which generally end up in
upper case. Here, ADDOP, is provided when the tokeniser sees + in
your code. ASSIGNOP is provided when = is used for assigning.
These are "terminal symbols", because you can’t get any simpler than
them.



The grammar, lines one and three of the snippet above, tells you how to
build up more complex forms. These complex forms, "non-terminal
symbols" are generally placed in lower case. term here is a
non-terminal symbol, representing a single expression.



The grammar gives you the following rule: you can make the thing on the
left of the colon if you see all the things on the right in sequence.
This is called a "reduction", and the aim of parsing is to completely
reduce the input. There are several different ways you can perform a
reduction, separated by vertical bars: so, term followed by =
followed by term makes a term, and term followed by +
followed by term can also make a term.



So, if you see two terms with an = or +, between them, you can
turn them into a single expression. When you do this, you execute the
code in the block on the next line: if you see =, you’ll do the code
in line 2. If you see +, you’ll do the code in line 4. It’s this
code which contributes to the op tree.



            |   term ADDOP term
            { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }



What this does is creates a new binary op, and feeds it a number of
variables. The variables refer to the tokens: $1 is the first token
in the input, $2 the second, and so on - think regular expression
backreferences. $$ is the op returned from this reduction. So, we
call newBINOP to create a new binary operator. The first parameter
to newBINOP, a function in op.c, is the op type. It’s an addition
operator, so we want the type to be ADDOP. We could specify this
directly, but it’s right there as the second token in the input, so we
use $2. The second parameter is the op’s flags: 0 means "nothing
special". Then the things to add: the left and right hand side of our
expression, in scalar context.









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33.5 STACKS



When perl executes something like addop, how does it pass on its
results to the next op? The answer is, through the use of stacks. Perl
has a number of stacks to store things it’s currently working on, and
we’ll look at the three most important ones here.





• perlinterp Argument stack:  



• perlinterp Mark stack:  



• perlinterp Save stack:  












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33.5.1 Argument stack



Arguments are passed to PP code and returned from PP code using the
argument stack, ST. The typical way to handle arguments is to pop
them off the stack, deal with them how you wish, and then push the
result back onto the stack. This is how, for instance, the cosine
operator works:



      NV value;
      value = POPn;
      value = Perl_cos(value);
      XPUSHn(value);



We’ll see a more tricky example of this when we consider Perl’s macros
below. POPn gives you the NV (floating point value) of the top SV on
the stack: the $x in cos($x). Then we compute the cosine, and
push the result back as an NV. The X in XPUSHn means that the
stack should be extended if necessary - it can’t be necessary here,
because we know there’s room for one more item on the stack, since
we’ve just removed one! The XPUSH* macros at least guarantee safety.



Alternatively, you can fiddle with the stack directly: SP gives you
the first element in your portion of the stack, and TOP* gives you
the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
negation of an integer:



     SETi(-TOPi);



Just set the integer value of the top stack entry to its negation.



Argument stack manipulation in the core is exactly the same as it is in
XSUBs - see (perlxstut), (perlxs) and perlguts NAME for a longer
description of the macros used in stack manipulation.









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33.5.2 Mark stack



I say "your portion of the stack" above because PP code doesn’t
necessarily get the whole stack to itself: if your function calls
another function, you’ll only want to expose the arguments aimed for
the called function, and not (necessarily) let it get at your own data.
The way we do this is to have a "virtual" bottom-of-stack, exposed to
each function. The mark stack keeps bookmarks to locations in the
argument stack usable by each function. For instance, when dealing with
a tied variable, (internally, something with "P" magic) Perl has to
call methods for accesses to the tied variables. However, we need to
separate the arguments exposed to the method to the argument exposed to
the original function - the store or fetch or whatever it may be.
Here’s roughly how the tied push is implemented; see av_push in
av.c:



     1  PUSHMARK(SP);
     2  EXTEND(SP,2);
     3  PUSHs(SvTIED_obj((SV*)av, mg));
     4  PUSHs(val);
     5  PUTBACK;
     6  ENTER;
     7  call_method("PUSH", G_SCALAR|G_DISCARD);
     8  LEAVE;



Let’s examine the whole implementation, for practice:



     1  PUSHMARK(SP);



Push the current state of the stack pointer onto the mark stack. This
is so that when we’ve finished adding items to the argument stack, Perl
knows how many things we’ve added recently.



     2  EXTEND(SP,2);
     3  PUSHs(SvTIED_obj((SV*)av, mg));
     4  PUSHs(val);



We’re going to add two more items onto the argument stack: when you
have a tied array, the PUSH subroutine receives the object and the
value to be pushed, and that’s exactly what we have here - the tied
object, retrieved with SvTIED_obj, and the value, the SV val.



     5  PUTBACK;



Next we tell Perl to update the global stack pointer from our internal
variable: dSP only gave us a local copy, not a reference to the
global.



     6  ENTER;
     7  call_method("PUSH", G_SCALAR|G_DISCARD);
     8  LEAVE;



ENTER and LEAVE localise a block of code - they make sure that
all variables are tidied up, everything that has been localised gets
its previous value returned, and so on. Think of them as the { and
} of a Perl block.



To actually do the magic method call, we have to call a subroutine in
Perl space: call_method takes care of that, and it’s described in
perlcall NAME. We call the PUSH method in scalar context, and we’re
going to discard its return value. The call_method() function removes
the top element of the mark stack, so there is nothing for the caller
to clean up.









Previous: perlinterp Mark stack, Up: perlinterp STACKS   [Contents]





33.5.3 Save stack



C doesn’t have a concept of local scope, so perl provides one. We’ve
seen that ENTER and LEAVE are used as scoping braces; the save
stack implements the C equivalent of, for example:



    {
        local $foo = 42;
        ...
    }



See perlguts Localizing changes for how to use the save stack.









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33.6 MILLIONS OF MACROS



One thing you’ll notice about the Perl source is that it’s full of
macros. Some have called the pervasive use of macros the hardest thing
to understand, others find it adds to clarity. Let’s take an example,
the code which implements the addition operator:



   1  PP(pp_add)
   2  {
   3      dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
   4      {
   5        dPOPTOPnnrl_ul;
   6        SETn( left + right );
   7        RETURN;
   8      }
   9  }



Every line here (apart from the braces, of course) contains a macro.
The first line sets up the function declaration as Perl expects for PP
code; line 3 sets up variable declarations for the argument stack and
the target, the return value of the operation. Finally, it tries to see
if the addition operation is overloaded; if so, the appropriate
subroutine is called.



Line 5 is another variable declaration - all variable declarations
start with d - which pops from the top of the argument stack two NVs
(hence nn) and puts them into the variables right and left,
hence the rl. These are the two operands to the addition operator.
Next, we call SETn to set the NV of the return value to the result
of adding the two values. This done, we return - the RETURN macro
makes sure that our return value is properly handled, and we pass the
next operator to run back to the main run loop.



Most of these macros are explained in (perlapi), and some of the more
important ones are explained in (perlxs) as well. Pay special
attention to perlguts Background and PERL_IMPLICIT_CONTEXT for
information on the [pad]THX_? macros.









Previous: perlinterp MILLIONS OF MACROS, Up: perlinterp   [Contents]





33.7 FURTHER READING



For more information on the Perl internals, please see the documents
listed at perl Internals and C Language Interface.









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34 perlintro





• perlintro NAME:  



• perlintro DESCRIPTION:  



• perlintro AUTHOR:  












Next: , Up: perlintro   [Contents]





34.1 NAME



perlintro – a brief introduction and overview of Perl









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34.2 DESCRIPTION



This document is intended to give you a quick overview of the Perl
programming language, along with pointers to further documentation.  It
is intended as a "bootstrap" guide for those who are new to the
language, and provides just enough information for you to be able to
read other peoples’ Perl and understand roughly what it’s doing, or
write your own simple scripts.



This introductory document does not aim to be complete.  It does not
even aim to be entirely accurate.  In some cases perfection has been
sacrificed in the goal of getting the general idea across.  You are
strongly advised to follow this introduction with more information
from the full Perl manual, the table of contents to which can be found
in (perltoc).



Throughout this document you’ll see references to other parts of the
Perl documentation.  You can read that documentation using the perldoc
command or whatever method you’re using to read this document.



Throughout Perl’s documentation, you’ll find numerous examples intended
to help explain the discussed features.  Please keep in mind that many
of them are code fragments rather than complete programs.



These examples often reflect the style and preference of the author of
that piece of the documentation, and may be briefer than a corresponding
line of code in a real program.  Except where otherwise noted, you
should assume that use strict and use warnings statements
appear earlier in the "program", and that any variables used have
already been declared, even if those declarations have been omitted
to make the example easier to read.



Do note that the examples have been written by many different authors over
a period of several decades.  Styles and techniques will therefore differ,
although some effort has been made to not vary styles too widely in the
same sections.  Do not consider one style to be better than others - "There’s
More Than One Way To Do It" is one of Perl’s mottos.  After all, in your
journey as a programmer, you are likely to encounter different styles.





• perlintro What is Perl?:  



• perlintro Running Perl programs:  



• perlintro Safety net:  



• perlintro Basic syntax overview:  



• perlintro Perl variable types:  



• perlintro Variable scoping:  



• perlintro Conditional and looping constructs:  



• perlintro Builtin operators and functions:  



• perlintro Files and I/O:  



• perlintro Regular expressions:  



• perlintro Writing subroutines:  



• perlintro OO Perl:  



• perlintro Using Perl modules:  












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34.2.1 What is Perl?



Perl is a general-purpose programming language originally developed for
text manipulation and now used for a wide range of tasks including
system administration, web development, network programming, GUI
development, and more.



The language is intended to be practical (easy to use, efficient,
complete) rather than beautiful (tiny, elegant, minimal).  Its major
features are that it’s easy to use, supports both procedural and
object-oriented (OO) programming, has powerful built-in support for text
processing, and has one of the world’s most impressive collections of
third-party modules.



Different definitions of Perl are given in perl NAME, (perlfaq1) and
no doubt other places.  From this we can determine that Perl is different
things to different people, but that lots of people think it’s at least
worth writing about.









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34.2.2 Running Perl programs



To run a Perl program from the Unix command line:



    perl progname.pl



Alternatively, put this as the first line of your script:



    #!/usr/bin/env perl



... and run the script as /path/to/script.pl.  Of course, it’ll need
to be executable first, so chmod 755 script.pl (under Unix).



(This start line assumes you have the env program.  You can also put
directly the path to your perl executable, like in #!/usr/bin/perl).



For more information, including instructions for other platforms such as
Windows and Mac OS, read perlrun NAME.









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34.2.3 Safety net



Perl by default is very forgiving.  In order to make it more robust
it is recommended to start every program with the following lines:



    #!/usr/bin/perl
    use strict;
    use warnings;



The two additional lines request from perl to catch various common
problems in your code.  They check different things so you need both.  A
potential problem caught by use strict; will cause your code to stop
immediately when it is encountered, while use warnings; will merely
give a warning (like the command-line switch -w) and let your code run.
To read more about them check their respective manual pages at (strict)
and (warnings).









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34.2.4 Basic syntax overview



A Perl script or program consists of one or more statements.  These
statements are simply written in the script in a straightforward
fashion.  There is no need to have a main() function or anything of
that kind.



Perl statements end in a semi-colon:



    print "Hello, world";



Comments start with a hash symbol and run to the end of the line



    # This is a comment



Whitespace is irrelevant:



    print
        "Hello, world"
        ;



... except inside quoted strings:



    # this would print with a linebreak in the middle
    print "Hello
    world";



Double quotes or single quotes may be used around literal strings:



    print "Hello, world";
    print 'Hello, world';



However, only double quotes "interpolate" variables and special
characters such as newlines (\n):



    print "Hello, $name\n";     # works fine
    print 'Hello, $name\n';     # prints $name\n literally



Numbers don’t need quotes around them:



    print 42;



You can use parentheses for functions’ arguments or omit them
according to your personal taste.  They are only required
occasionally to clarify issues of precedence.



    print("Hello, world\n");
    print "Hello, world\n";



More detailed information about Perl syntax can be found in (perlsyn).









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34.2.5 Perl variable types



Perl has three main variable types: scalars, arrays, and hashes.





Scalars




A scalar represents a single value:



    my $animal = "camel";
    my $answer = 42;



Scalar values can be strings, integers or floating point numbers, and Perl
will automatically convert between them as required.  There is no need
to pre-declare your variable types, but you have to declare them using
the my keyword the first time you use them.  (This is one of the
requirements of use strict;.)



Scalar values can be used in various ways:



    print $animal;
    print "The animal is $animal\n";
    print "The square of $answer is ", $answer * $answer, "\n";



There are a number of "magic" scalars with names that look like
punctuation or line noise.  These special variables are used for all
kinds of purposes, and are documented in perlvar NAME.  The only one you
need to know about for now is $_ which is the "default variable".
It’s used as the default argument to a number of functions in Perl, and
it’s set implicitly by certain looping constructs.



    print;          # prints contents of $_ by default





Arrays




An array represents a list of values:



    my @animals = ("camel", "llama", "owl");
    my @numbers = (23, 42, 69);
    my @mixed   = ("camel", 42, 1.23);



Arrays are zero-indexed.  Here’s how you get at elements in an array:



    print $animals[0];              # prints "camel"
    print $animals[1];              # prints "llama"



The special variable $#array tells you the index of the last element
of an array:



    print $mixed[$#mixed];       # last element, prints 1.23



You might be tempted to use $#array + 1 to tell you how many items there
are in an array.  Don’t bother.  As it happens, using @array where Perl
expects to find a scalar value ("in scalar context") will give you the number
of elements in the array:



    if (@animals < 5) { ... }



The elements we’re getting from the array start with a $ because
we’re getting just a single value out of the array; you ask for a scalar,
you get a scalar.



To get multiple values from an array:



    @animals[0,1];                  # gives ("camel", "llama");
    @animals[0..2];                 # gives ("camel", "llama", "owl");
    @animals[1..$#animals];         # gives all except the first element



This is called an "array slice".



You can do various useful things to lists:



    my @sorted    = sort @animals;
    my @backwards = reverse @numbers;



There are a couple of special arrays too, such as @ARGV (the command
line arguments to your script) and @_ (the arguments passed to a
subroutine).  These are documented in perlvar NAME.





Hashes




A hash represents a set of key/value pairs:



    my %fruit_color = ("apple", "red", "banana", "yellow");



You can use whitespace and the => operator to lay them out more
nicely:



    my %fruit_color = (
        apple  => "red",
        banana => "yellow",
    );



To get at hash elements:



    $fruit_color{"apple"};           # gives "red"



You can get at lists of keys and values with keys() and
values().



    my @fruits = keys %fruit_colors;
    my @colors = values %fruit_colors;



Hashes have no particular internal order, though you can sort the keys
and loop through them.



Just like special scalars and arrays, there are also special hashes.
The most well known of these is %ENV which contains environment
variables.  Read all about it (and other special variables) in
perlvar NAME.








Scalars, arrays and hashes are documented more fully in perldata NAME.



More complex data types can be constructed using references, which allow
you to build lists and hashes within lists and hashes.



A reference is a scalar value and can refer to any other Perl data
type.  So by storing a reference as the value of an array or hash
element, you can easily create lists and hashes within lists and
hashes.  The following example shows a 2 level hash of hash
structure using anonymous hash references.



    my $variables = {
        scalar  =>  {
                     description => "single item",
                     sigil => '$',
                    },
        array   =>  {
                     description => "ordered list of items",
                     sigil => '@',
                    },
        hash    =>  {
                     description => "key/value pairs",
                     sigil => '%',
                    },
    };

    print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";



Exhaustive information on the topic of references can be found in
perlreftut NAME, perllol NAME, (perlref) and (perldsc).









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34.2.6 Variable scoping



Throughout the previous section all the examples have used the syntax:



    my $var = "value";



The my is actually not required; you could just use:



    $var = "value";



However, the above usage will create global variables throughout your
program, which is bad programming practice.  my creates lexically
scoped variables instead.  The variables are scoped to the block
(i.e. a bunch of statements surrounded by curly-braces) in which they
are defined.



    my $x = "foo";
    my $some_condition = 1;
    if ($some_condition) {
        my $y = "bar";
        print $x;           # prints "foo"
        print $y;           # prints "bar"
    }
    print $x;               # prints "foo"
    print $y;               # prints nothing; $y has fallen out of scope



Using my in combination with a use strict; at the top of
your Perl scripts means that the interpreter will pick up certain common
programming errors.  For instance, in the example above, the final
print $y would cause a compile-time error and prevent you from
running the program.  Using strict is highly recommended.









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34.2.7 Conditional and looping constructs



Perl has most of the usual conditional and looping constructs.  As of Perl
5.10, it even has a case/switch statement (spelled given/when).  See
(perlsyn)Switch Statements for more details.



The conditions can be any Perl expression.  See the list of operators in
the next section for information on comparison and boolean logic operators,
which are commonly used in conditional statements.





if




    if ( condition ) {
        ...
    } elsif ( other condition ) {
        ...
    } else {
        ...
    }



There’s also a negated version of it:



    unless ( condition ) {
        ...
    }



This is provided as a more readable version of if (!condition).



Note that the braces are required in Perl, even if you’ve only got one
line in the block.  However, there is a clever way of making your one-line
conditional blocks more English like:



    # the traditional way
    if ($zippy) {
        print "Yow!";
    }

    # the Perlish post-condition way
    print "Yow!" if $zippy;
    print "We have no bananas" unless $bananas;





while




    while ( condition ) {
        ...
    }



There’s also a negated version, for the same reason we have unless:



    until ( condition ) {
        ...
    }



You can also use while in a post-condition:



    print "LA LA LA\n" while 1;          # loops forever





for




Exactly like C:



    for ($i = 0; $i <= $max; $i++) {
        ...
    }



The C style for loop is rarely needed in Perl since Perl provides
the more friendly list scanning foreach loop.





foreach




    foreach (@array) {
        print "This element is $_\n";
    }

    print $list[$_] foreach 0 .. $max;

    # you don't have to use the default $_ either...
    foreach my $key (keys %hash) {
        print "The value of $key is $hash{$key}\n";
    }



The foreach keyword is actually a synonym for the for
keyword.  See (perlsyn)Foreach Loops.








For more detail on looping constructs (and some that weren’t mentioned in
this overview) see (perlsyn).









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34.2.8 Builtin operators and functions



Perl comes with a wide selection of builtin functions.  Some of the ones
we’ve already seen include print, sort and reverse.  A list of
them is given at the start of (perlfunc) and you can easily read
about any given function by using perldoc -f functionname.



Perl operators are documented in full in (perlop), but here are a few
of the most common ones:





Arithmetic




    +   addition
    -   subtraction
    *   multiplication
    /   division





Numeric comparison




    ==  equality
    !=  inequality
    <   less than
    >   greater than
    <=  less than or equal
    >=  greater than or equal





String comparison




    eq  equality
    ne  inequality
    lt  less than
    gt  greater than
    le  less than or equal
    ge  greater than or equal



(Why do we have separate numeric and string comparisons?  Because we don’t
have special variable types, and Perl needs to know whether to sort
numerically (where 99 is less than 100) or alphabetically (where 100 comes
before 99).





Boolean logic




    &&  and
    ||  or
    !   not



(and, or and not aren’t just in the above table as descriptions
of the operators.  They’re also supported as operators in their own
right.  They’re more readable than the C-style operators, but have
different precedence to && and friends.  Check (perlop) for more
detail.)





Miscellaneous




    =   assignment
    .   string concatenation
    x   string multiplication
    ..  range operator (creates a list of numbers)








Many operators can be combined with a = as follows:



    $a += 1;        # same as $a = $a + 1
    $a -= 1;        # same as $a = $a - 1
    $a .= "\n";     # same as $a = $a . "\n";









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34.2.9 Files and I/O



You can open a file for input or output using the open() function.
It’s documented in extravagant detail in (perlfunc) and perlopentut NAME,
but in short:



    open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
    open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
    open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";



You can read from an open filehandle using the <> operator.  In
scalar context it reads a single line from the filehandle, and in list
context it reads the whole file in, assigning each line to an element of
the list:



    my $line  = <$in>;
    my @lines = <$in>;



Reading in the whole file at one time is called slurping.  It can
be useful but it may be a memory hog.  Most text file processing
can be done a line at a time with Perl’s looping constructs.



The <> operator is most often seen in a while loop:



    while (<$in>) {     # assigns each line in turn to $_
        print "Just read in this line: $_";
    }



We’ve already seen how to print to standard output using print().
However, print() can also take an optional first argument specifying
which filehandle to print to:



    print STDERR "This is your final warning.\n";
    print $out $record;
    print $log $logmessage;



When you’re done with your filehandles, you should close() them
(though to be honest, Perl will clean up after you if you forget):



    close $in or die "$in: $!";









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34.2.10 Regular expressions



Perl’s regular expression support is both broad and deep, and is the
subject of lengthy documentation in perlrequick NAME, perlretut NAME, and
elsewhere.  However, in short:





Simple matching




    if (/foo/)       { ... }  # true if $_ contains "foo"
    if ($a =~ /foo/) { ... }  # true if $a contains "foo"



The // matching operator is documented in (perlop).  It operates on
$_ by default, or can be bound to another variable using the =~
binding operator (also documented in (perlop)).





Simple substitution




    s/foo/bar/;               # replaces foo with bar in $_
    $a =~ s/foo/bar/;         # replaces foo with bar in $a
    $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $a



The s/// substitution operator is documented in (perlop).





More complex regular expressions




You don’t just have to match on fixed strings.  In fact, you can match
on just about anything you could dream of by using more complex regular
expressions.  These are documented at great length in (perlre), but for
the meantime, here’s a quick cheat sheet:



    .                   a single character
    \s                  a whitespace character (space, tab, newline, ...)
    \S                  non-whitespace character
    \d                  a digit (0-9)
    \D                  a non-digit
    \w                  a word character (a-z, A-Z, 0-9, _)
    \W                  a non-word character
    [aeiou]             matches a single character in the given set
    [^aeiou]            matches a single character outside the given set
    (foo|bar|baz)       matches any of the alternatives specified

    ^                   start of string
    $                   end of string



Quantifiers can be used to specify how many of the previous thing you
want to match on, where "thing" means either a literal character, one
of the metacharacters listed above, or a group of characters or
metacharacters in parentheses.



    *                   zero or more of the previous thing
    +                   one or more of the previous thing
    ?                   zero or one of the previous thing
    {3}                 matches exactly 3 of the previous thing
    {3,6}               matches between 3 and 6 of the previous thing
    {3,}                matches 3 or more of the previous thing



Some brief examples:



    /^\d+/              string starts with one or more digits
    /^$/                nothing in the string (start and end are adjacent)
    /(\d\s){3}/         a three digits, each followed by a whitespace
                        character (eg "3 4 5 ")
    /(a.)+/             matches a string in which every odd-numbered letter
                        is a (eg "abacadaf")

    # This loop reads from STDIN, and prints non-blank lines:
    while (<>) {
        next if /^$/;
        print;
    }





Parentheses for capturing




As well as grouping, parentheses serve a second purpose.  They can be
used to capture the results of parts of the regexp match for later use.
The results end up in $1, $2 and so on.



    # a cheap and nasty way to break an email address up into parts

    if ($email =~ /([^@]+)@(.+)/) {
        print "Username is $1\n";
        print "Hostname is $2\n";
    }





Other regexp features




Perl regexps also support backreferences, lookaheads, and all kinds of
other complex details.  Read all about them in perlrequick NAME,
perlretut NAME, and (perlre).














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34.2.11 Writing subroutines



Writing subroutines is easy:



    sub logger {
        my $logmessage = shift;
        open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
        print $logfile $logmessage;
    }



Now we can use the subroutine just as any other built-in function:



    logger("We have a logger subroutine!");



What’s that shift?  Well, the arguments to a subroutine are available
to us as a special array called @_ (see perlvar NAME for more on that).
The default argument to the shift function just happens to be @_.
So my $logmessage = shift; shifts the first item off the list of
arguments and assigns it to $logmessage.



We can manipulate @_ in other ways too:



    my ($logmessage, $priority) = @_;       # common
    my $logmessage = $_[0];                 # uncommon, and ugly



Subroutines can also return values:



    sub square {
        my $num = shift;
        my $result = $num * $num;
        return $result;
    }



Then use it like:



    $sq = square(8);



For more information on writing subroutines, see (perlsub).









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34.2.12 OO Perl



OO Perl is relatively simple and is implemented using references which
know what sort of object they are based on Perl’s concept of packages.
However, OO Perl is largely beyond the scope of this document.
Read perlootut NAME and (perlobj).



As a beginning Perl programmer, your most common use of OO Perl will be
in using third-party modules, which are documented below.









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34.2.13 Using Perl modules



Perl modules provide a range of features to help you avoid reinventing
the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ).  A
number of popular modules are included with the Perl distribution
itself.



Categories of modules range from text manipulation to network protocols
to database integration to graphics.  A categorized list of modules is
also available from CPAN.



To learn how to install modules you download from CPAN, read
perlmodinstall NAME.



To learn how to use a particular module, use perldoc Module::Name.
Typically you will want to use Module::Name, which will then give
you access to exported functions or an OO interface to the module.



(perlfaq) contains questions and answers related to many common
tasks, and often provides suggestions for good CPAN modules to use.



perlmod NAME describes Perl modules in general.  (perlmodlib) lists the
modules which came with your Perl installation.



If you feel the urge to write Perl modules, perlnewmod NAME will give you
good advice.









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34.3 AUTHOR



Kirrily "Skud" Robert <skud@cpan.org>









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35 perliol





• perliol NAME:  



• perliol SYNOPSIS:  



• perliol DESCRIPTION:  



• perliol TODO:  












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35.1 NAME



perliol - C API for Perl’s implementation of IO in Layers.









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35.2 SYNOPSIS



    /* Defining a layer ... */
    #include <perliol.h>









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35.3 DESCRIPTION



This document describes the behavior and implementation of the PerlIO
abstraction described in perlapio NAME when USE_PERLIO is defined (and
USE_SFIO is not).





• perliol History and Background:  



• perliol Basic Structure:  



• perliol Layers vs Disciplines:  



• perliol Data Structures:  



• perliol Functions and Attributes:  



• perliol Per-instance Data:  



• perliol Layers in action.:  



• perliol Per-instance flag bits:  



• perliol Methods in Detail:  



• perliol Utilities:  



• perliol Implementing PerlIO Layers:  



• perliol Core Layers:  



• perliol Extension Layers:  












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35.3.1 History and Background



The PerlIO abstraction was introduced in perl5.003_02 but languished as
just an abstraction until perl5.7.0. However during that time a number
of perl extensions switched to using it, so the API is mostly fixed to
maintain (source) compatibility.



The aim of the implementation is to provide the PerlIO API in a flexible
and platform neutral manner. It is also a trial of an "Object Oriented
C, with vtables" approach which may be applied to Perl 6.









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35.3.2 Basic Structure



PerlIO is a stack of layers.



The low levels of the stack work with the low-level operating system
calls (file descriptors in C) getting bytes in and out, the higher
layers of the stack buffer, filter, and otherwise manipulate the I/O,
and return characters (or bytes) to Perl.  Terms above and below
are used to refer to the relative positioning of the stack layers.



A layer contains a "vtable", the table of I/O operations (at C level
a table of function pointers), and status flags.  The functions in the
vtable implement operations like "open", "read", and "write".



When I/O, for example "read", is requested, the request goes from Perl
first down the stack using "read" functions of each layer, then at the
bottom the input is requested from the operating system services, then
the result is returned up the stack, finally being interpreted as Perl
data.



The requests do not necessarily go always all the way down to the
operating system: that’s where PerlIO buffering comes into play.



When you do an open() and specify extra PerlIO layers to be deployed,
the layers you specify are "pushed" on top of the already existing
default stack.  One way to see it is that "operating system is
on the left" and "Perl is on the right".



What exact layers are in this default stack depends on a lot of
things: your operating system, Perl version, Perl compile time
configuration, and Perl runtime configuration.  See (PerlIO),
perlrun PERLIO, and (open) for more information.



binmode() operates similarly to open(): by default the specified
layers are pushed on top of the existing stack.



However, note that even as the specified layers are "pushed on top"
for open() and binmode(), this doesn’t mean that the effects are
limited to the "top": PerlIO layers can be very ’active’ and inspect
and affect layers also deeper in the stack.  As an example there
is a layer called "raw" which repeatedly "pops" layers until
it reaches the first layer that has declared itself capable of
handling binary data.  The "pushed" layers are processed in left-to-right
order.



sysopen() operates (unsurprisingly) at a lower level in the stack than
open().  For example in Unix or Unix-like systems sysopen() operates
directly at the level of file descriptors: in the terms of PerlIO
layers, it uses only the "unix" layer, which is a rather thin wrapper
on top of the Unix file descriptors.









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35.3.3 Layers vs Disciplines



Initial discussion of the ability to modify IO streams behaviour used
the term "discipline" for the entities which were added. This came (I
believe) from the use of the term in "sfio", which in turn borrowed it
from "line disciplines" on Unix terminals. However, this document (and
the C code) uses the term "layer".



This is, I hope, a natural term given the implementation, and should
avoid connotations that are inherent in earlier uses of "discipline"
for things which are rather different.









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35.3.4 Data Structures



The basic data structure is a PerlIOl:



        typedef struct _PerlIO PerlIOl;
        typedef struct _PerlIO_funcs PerlIO_funcs;
        typedef PerlIOl *PerlIO;

        struct _PerlIO
        {
         PerlIOl *      next;       /* Lower layer */
         PerlIO_funcs * tab;        /* Functions for this layer */
         IV             flags;      /* Various flags for state */
        };



A PerlIOl * is a pointer to the struct, and the application
level PerlIO * is a pointer to a PerlIOl * - i.e. a pointer
to a pointer to the struct. This allows the application level PerlIO *
to remain constant while the actual PerlIOl * underneath
changes. (Compare perl’s SV * which remains constant while its
sv_any field changes as the scalar’s type changes.) An IO stream is
then in general represented as a pointer to this linked-list of
"layers".



It should be noted that because of the double indirection in a PerlIO *,
a &(perlio->next) "is" a PerlIO *, and so to some degree
at least one layer can use the "standard" API on the next layer down.



A "layer" is composed of two parts:



    

  1.  The functions and attributes of the "layer class".


  2.  The per-instance data for a particular handle.











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35.3.5 Functions and Attributes



The functions and attributes are accessed via the "tab" (for table)
member of PerlIOl. The functions (methods of the layer "class") are
fixed, and are defined by the PerlIO_funcs type. They are broadly the
same as the public PerlIO_xxxxx functions:



  struct _PerlIO_funcs
  {
   Size_t               fsize;
   char *               name;
   Size_t               size;
   IV           kind;
   IV           (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
   IV           (*Popped)(pTHX_ PerlIO *f);
   PerlIO *     (*Open)(pTHX_ PerlIO_funcs *tab,
                        PerlIO_list_t *layers, IV n,
                        const char *mode,
                        int fd, int imode, int perm,
                        PerlIO *old,
                        int narg, SV **args);
   IV           (*Binmode)(pTHX_ PerlIO *f);
   SV *         (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
   IV           (*Fileno)(pTHX_ PerlIO *f);
   PerlIO *     (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
   /* Unix-like functions - cf sfio line disciplines */
   SSize_t      (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
   SSize_t      (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   SSize_t      (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   IV           (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
   Off_t        (*Tell)(pTHX_ PerlIO *f);
   IV           (*Close)(pTHX_ PerlIO *f);
   /* Stdio-like buffered IO functions */
   IV           (*Flush)(pTHX_ PerlIO *f);
   IV           (*Fill)(pTHX_ PerlIO *f);
   IV           (*Eof)(pTHX_ PerlIO *f);
   IV           (*Error)(pTHX_ PerlIO *f);
   void         (*Clearerr)(pTHX_ PerlIO *f);
   void         (*Setlinebuf)(pTHX_ PerlIO *f);
   /* Perl's snooping functions */
   STDCHAR *    (*Get_base)(pTHX_ PerlIO *f);
   Size_t       (*Get_bufsiz)(pTHX_ PerlIO *f);
   STDCHAR *    (*Get_ptr)(pTHX_ PerlIO *f);
   SSize_t      (*Get_cnt)(pTHX_ PerlIO *f);
   void         (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
  };



The first few members of the struct give a function table size for
compatibility check "name" for the layer, the  size to malloc for the per-instance data,
and some flags which are attributes of the class as whole (such as whether it is a buffering
layer), then follow the functions which fall into four basic groups:



    

  1.  Opening and setup functions


  2.  Basic IO operations


  3.  Stdio class buffering options.


  4.  Functions to support Perl’s traditional "fast" access to the buffer.





A layer does not have to implement all the functions, but the whole
table has to be present. Unimplemented slots can be NULL (which will
result in an error when called) or can be filled in with stubs to
"inherit" behaviour from a "base class". This "inheritance" is fixed
for all instances of the layer, but as the layer chooses which stubs
to populate the table, limited "multiple inheritance" is possible.









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35.3.6 Per-instance Data



The per-instance data are held in memory beyond the basic PerlIOl
struct, by making a PerlIOl the first member of the layer’s struct
thus:



        typedef struct
        {
         struct _PerlIO base;       /* Base "class" info */
         STDCHAR *      buf;        /* Start of buffer */
         STDCHAR *      end;        /* End of valid part of buffer */
         STDCHAR *      ptr;        /* Current position in buffer */
         Off_t          posn;       /* Offset of buf into the file */
         Size_t         bufsiz;     /* Real size of buffer */
         IV             oneword;    /* Emergency buffer */
        } PerlIOBuf;



In this way (as for perl’s scalars) a pointer to a PerlIOBuf can be
treated as a pointer to a PerlIOl.









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35.3.7 Layers in action.



                table           perlio          unix
            |           |
            +-----------+    +----------+    +--------+
   PerlIO ->|           |--->|  next    |--->|  NULL  |
            +-----------+    +----------+    +--------+
            |           |    |  buffer  |    |   fd   |
            +-----------+    |          |    +--------+
            |           |    +----------+



The above attempts to show how the layer scheme works in a simple case.
The application’s PerlIO * points to an entry in the table(s)
representing open (allocated) handles. For example the first three slots
in the table correspond to stdin,stdout and stderr. The table
in turn points to the current "top" layer for the handle - in this case
an instance of the generic buffering layer "perlio". That layer in turn
points to the next layer down - in this case the low-level "unix" layer.



The above is roughly equivalent to a "stdio" buffered stream, but with
much more flexibility:











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35.3.8 Per-instance flag bits



The generic flag bits are a hybrid of O_XXXXX style flags deduced
from the mode string passed to PerlIO_open(), and state bits for
typical buffer layers.





PERLIO_F_EOF




End of file.





PERLIO_F_CANWRITE




Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.





PERLIO_F_CANREAD




Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).





PERLIO_F_ERROR




An error has occurred (for PerlIO_error()).





PERLIO_F_TRUNCATE




Truncate file suggested by open mode.





PERLIO_F_APPEND




All writes should be appends.





PERLIO_F_CRLF




Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
mapped to "\n" for input. Normally the provided "crlf" layer is the only
layer that need bother about this. PerlIO_binmode() will mess with this
flag rather than add/remove layers if the PERLIO_K_CANCRLF bit is set
for the layers class.





PERLIO_F_UTF8




Data written to this layer should be UTF-8 encoded; data provided
by this layer should be considered UTF-8 encoded. Can be set on any layer
by ":utf8" dummy layer. Also set on ":encoding" layer.





PERLIO_F_UNBUF




Layer is unbuffered - i.e. write to next layer down should occur for
each write to this layer.





PERLIO_F_WRBUF




The buffer for this layer currently holds data written to it but not sent
to next layer.





PERLIO_F_RDBUF




The buffer for this layer currently holds unconsumed data read from
layer below.





PERLIO_F_LINEBUF




Layer is line buffered. Write data should be passed to next layer down
whenever a "\n" is seen. Any data beyond the "\n" should then be
processed.





PERLIO_F_TEMP




File has been unlink()ed, or should be deleted on close().





PERLIO_F_OPEN




Handle is open.





PERLIO_F_FASTGETS




This instance of this layer supports the "fast gets" interface.
Normally set based on PERLIO_K_FASTGETS for the class and by the
existence of the function(s) in the table. However a class that
normally provides that interface may need to avoid it on a
particular instance. The "pending" layer needs to do this when
it is pushed above a layer which does not support the interface.
(Perl’s sv_gets() does not expect the streams fast gets behaviour
to change during one "get".)














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35.3.9 Methods in Detail





fsize




        Size_t fsize;



Size of the function table. This is compared against the value PerlIO
code "knows" as a compatibility check. Future versions may be able
to tolerate layers compiled against an old version of the headers.





name




        char * name;



The name of the layer whose open() method Perl should invoke on
open().  For example if the layer is called APR, you will call:



  open $fh, ">:APR", ...



and Perl knows that it has to invoke the PerlIOAPR_open() method
implemented by the APR layer.





size




        Size_t size;



The size of the per-instance data structure, e.g.:



  sizeof(PerlIOAPR)



If this field is zero then PerlIO_pushed does not malloc anything
and assumes layer’s Pushed function will do any required layer stack
manipulation - used to avoid malloc/free overhead for dummy layers.
If the field is non-zero it must be at least the size of PerlIOl,
PerlIO_pushed will allocate memory for the layer’s data structures
and link new layer onto the stream’s stack. (If the layer’s Pushed
method returns an error indication the layer is popped again.)





kind




        IV kind;







Pushed




        IV      (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);



The only absolutely mandatory method. Called when the layer is pushed
onto the stack.  The mode argument may be NULL if this occurs
post-open. The arg will be non-NULL if an argument string was
passed. In most cases this should call PerlIOBase_pushed() to
convert mode into the appropriate PERLIO_F_XXXXX flags in
addition to any actions the layer itself takes.  If a layer is not
expecting an argument it need neither save the one passed to it, nor
provide Getarg() (it could perhaps Perl_warn that the argument
was un-expected).



Returns 0 on success. On failure returns -1 and should set errno.





Popped




        IV      (*Popped)(pTHX_ PerlIO *f);



Called when the layer is popped from the stack. A layer will normally
be popped after Close() is called. But a layer can be popped
without being closed if the program is dynamically managing layers on
the stream. In such cases Popped() should free any resources
(buffers, translation tables, ...) not held directly in the layer’s
struct.  It should also Unread() any unconsumed data that has been
read and buffered from the layer below back to that layer, so that it
can be re-provided to what ever is now above.



Returns 0 on success and failure.  If Popped() returns true then
perlio.c assumes that either the layer has popped itself, or the
layer is super special and needs to be retained for other reasons.
In most cases it should return false.





Open




        PerlIO *        (*Open)(...);



The Open() method has lots of arguments because it combines the
functions of perl’s open, PerlIO_open, perl’s sysopen,
PerlIO_fdopen and PerlIO_reopen.  The full prototype is as
follows:



 PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                        PerlIO_list_t *layers, IV n,
                        const char *mode,
                        int fd, int imode, int perm,
                        PerlIO *old,
                        int narg, SV **args);



Open should (perhaps indirectly) call PerlIO_allocate() to allocate
a slot in the table and associate it with the layers information for
the opened file, by calling PerlIO_push.  The layers is an
array of all the layers destined for the PerlIO *, and any
arguments passed to them, n is the index into that array of the
layer being called. The macro PerlIOArg will return a (possibly
NULL) SV * for the argument passed to the layer.



The mode string is an "fopen()-like" string which would match
the regular expression /^[I#]?[rwa]\+?[bt]?$/.



The 'I' prefix is used during creation of stdin..stderr via
special PerlIO_fdopen calls; the '#' prefix means that this is
sysopen and that imode and perm should be passed to
PerlLIO_open3; 'r' means read, 'w' means write and
'a' means append. The '+' suffix means that both reading and
writing/appending are permitted.  The 'b' suffix means file should
be binary, and 't' means it is text. (Almost all layers should do
the IO in binary mode, and ignore the b/t bits. The :crlf layer
should be pushed to handle the distinction.)



If old is not NULL then this is a PerlIO_reopen. Perl itself
does not use this (yet?) and semantics are a little vague.



If fd not negative then it is the numeric file descriptor fd,
which will be open in a manner compatible with the supplied mode
string, the call is thus equivalent to PerlIO_fdopen. In this case
nargs will be zero.



If nargs is greater than zero then it gives the number of arguments
passed to open, otherwise it will be 1 if for example
PerlIO_open was called.  In simple cases SvPV_nolen(*args) is the
pathname to open.



If a layer provides Open() it should normally call the Open()
method of next layer down (if any) and then push itself on top if that
succeeds.  PerlIOBase_open is provided to do exactly that, so in
most cases you don’t have to write your own Open() method.  If this
method is not defined, other layers may have difficulty pushing
themselves on top of it during open.



If PerlIO_push was performed and open has failed, it must
PerlIO_pop itself, since if it’s not, the layer won’t be removed
and may cause bad problems.



Returns NULL on failure.





Binmode




        IV        (*Binmode)(pTHX_ PerlIO *f);



Optional. Used when :raw layer is pushed (explicitly or as a result
of binmode(FH)). If not present layer will be popped. If present
should configure layer as binary (or pop itself) and return 0.
If it returns -1 for error binmode will fail with layer
still on the stack.





Getarg




        SV *      (*Getarg)(pTHX_ PerlIO *f,
                            CLONE_PARAMS *param, int flags);



Optional. If present should return an SV * representing the string
argument passed to the layer when it was
pushed. e.g. ":encoding(ascii)" would return an SvPV with value
"ascii". (param and flags arguments can be ignored in most
cases)



Dup uses Getarg to retrieve the argument originally passed to
Pushed, so you must implement this function if your layer has an
extra argument to Pushed and will ever be Duped.





Fileno




        IV        (*Fileno)(pTHX_ PerlIO *f);



Returns the Unix/Posix numeric file descriptor for the handle. Normally
PerlIOBase_fileno() (which just asks next layer down) will suffice
for this.



Returns -1 on error, which is considered to include the case where the
layer cannot provide such a file descriptor.





Dup




        PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                        CLONE_PARAMS *param, int flags);



XXX: Needs more docs.



Used as part of the "clone" process when a thread is spawned (in which
case param will be non-NULL) and when a stream is being duplicated via
’&’ in the open.



Similar to Open, returns PerlIO* on success, NULL on failure.





Read




        SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);



Basic read operation.



Typically will call Fill and manipulate pointers (possibly via the
API).  PerlIOBuf_read() may be suitable for derived classes which
provide "fast gets" methods.



Returns actual bytes read, or -1 on an error.





Unread




        SSize_t (*Unread)(pTHX_ PerlIO *f,
                          const void *vbuf, Size_t count);



A superset of stdio’s ungetc(). Should arrange for future reads to
see the bytes in vbuf. If there is no obviously better implementation
then PerlIOBase_unread() provides the function by pushing a "fake"
"pending" layer above the calling layer.



Returns the number of unread chars.





Write




        SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);



Basic write operation.



Returns bytes written or -1 on an error.





Seek




        IV      (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);



Position the file pointer. Should normally call its own Flush
method and then the Seek method of next layer down.



Returns 0 on success, -1 on failure.





Tell




        Off_t   (*Tell)(pTHX_ PerlIO *f);



Return the file pointer. May be based on layers cached concept of
position to avoid overhead.



Returns -1 on failure to get the file pointer.





Close




        IV      (*Close)(pTHX_ PerlIO *f);



Close the stream. Should normally call PerlIOBase_close() to flush
itself and close layers below, and then deallocate any data structures
(buffers, translation tables, ...) not  held directly in the data
structure.



Returns 0 on success, -1 on failure.





Flush




        IV      (*Flush)(pTHX_ PerlIO *f);



Should make stream’s state consistent with layers below. That is, any
buffered write data should be written, and file position of lower layers
adjusted for data read from below but not actually consumed.
(Should perhaps Unread() such data to the lower layer.)



Returns 0 on success, -1 on failure.





Fill




        IV      (*Fill)(pTHX_ PerlIO *f);



The buffer for this layer should be filled (for read) from layer
below.  When you "subclass" PerlIOBuf layer, you want to use its
_read method and to supply your own fill method, which fills the
PerlIOBuf’s buffer.



Returns 0 on success, -1 on failure.





Eof




        IV      (*Eof)(pTHX_ PerlIO *f);



Return end-of-file indicator. PerlIOBase_eof() is normally sufficient.



Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.





Error




        IV      (*Error)(pTHX_ PerlIO *f);



Return error indicator. PerlIOBase_error() is normally sufficient.



Returns 1 if there is an error (usually when PERLIO_F_ERROR is set,
0 otherwise.





Clearerr




        void    (*Clearerr)(pTHX_ PerlIO *f);



Clear end-of-file and error indicators. Should call PerlIOBase_clearerr()
to set the PERLIO_F_XXXXX flags, which may suffice.





Setlinebuf




        void    (*Setlinebuf)(pTHX_ PerlIO *f);



Mark the stream as line buffered. PerlIOBase_setlinebuf() sets the
PERLIO_F_LINEBUF flag and is normally sufficient.





Get_base




        STDCHAR *       (*Get_base)(pTHX_ PerlIO *f);



Allocate (if not already done so) the read buffer for this layer and
return pointer to it. Return NULL on failure.





Get_bufsiz




        Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);



Return the number of bytes that last Fill() put in the buffer.





Get_ptr




        STDCHAR *       (*Get_ptr)(pTHX_ PerlIO *f);



Return the current read pointer relative to this layer’s buffer.





Get_cnt




        SSize_t (*Get_cnt)(pTHX_ PerlIO *f);



Return the number of bytes left to be read in the current buffer.





Set_ptrcnt




        void    (*Set_ptrcnt)(pTHX_ PerlIO *f,
                              STDCHAR *ptr, SSize_t cnt);



Adjust the read pointer and count of bytes to match ptr and/or cnt.
The application (or layer above) must ensure they are consistent.
(Checking is allowed by the paranoid.)














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35.3.10 Utilities



To ask for the next layer down use PerlIONext(PerlIO *f).



To check that a PerlIO* is valid use PerlIOValid(PerlIO *f).  (All
this does is really just to check that the pointer is non-NULL and
that the pointer behind that is non-NULL.)



PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words,
the PerlIOl* pointer.



PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.



Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
calls the callback from the functions of the layer f (just by
the name of the IO function, like "Read") with the args, or if
there is no such callback, calls the base version of the callback
with the same args, or if the f is invalid, set errno to EBADF and
return failure.



Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
the callback of the functions of the layer f with the args,
or if there is no such callback, set errno to EINVAL.  Or if the f is
invalid, set errno to EBADF and return failure.



Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
the callback of the functions of the layer f with the args,
or if there is no such callback, calls the base version of the
callback with the same args, or if the f is invalid, set errno to
EBADF.



Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
callback of the functions of the layer f with the args, or if
there is no such callback, set errno to EINVAL.  Or if the f is
invalid, set errno to EBADF.









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35.3.11 Implementing PerlIO Layers



If you find the implementation document unclear or not sufficient,
look at the existing PerlIO layer implementations, which include:





If you are creating a PerlIO layer, you may want to be lazy, in other
words, implement only the methods that interest you.  The other methods
you can either replace with the "blank" methods



    PerlIOBase_noop_ok
    PerlIOBase_noop_fail



(which do nothing, and return zero and -1, respectively) or for
certain methods you may assume a default behaviour by using a NULL
method.  The Open method looks for help in the ’parent’ layer.
The following table summarizes the behaviour:



    method      behaviour with NULL

    Clearerr    PerlIOBase_clearerr
    Close       PerlIOBase_close
    Dup         PerlIOBase_dup
    Eof         PerlIOBase_eof
    Error       PerlIOBase_error
    Fileno      PerlIOBase_fileno
    Fill        FAILURE
    Flush       SUCCESS
    Getarg      SUCCESS
    Get_base    FAILURE
    Get_bufsiz  FAILURE
    Get_cnt     FAILURE
    Get_ptr     FAILURE
    Open        INHERITED
    Popped      SUCCESS
    Pushed      SUCCESS
    Read        PerlIOBase_read
    Seek        FAILURE
    Set_cnt     FAILURE
    Set_ptrcnt  FAILURE
    Setlinebuf  PerlIOBase_setlinebuf
    Tell        FAILURE
    Unread      PerlIOBase_unread
    Write       FAILURE

 FAILURE        Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS) and
                return -1 (for numeric return values) or NULL (for pointers)
 INHERITED      Inherited from the layer below
 SUCCESS        Return 0 (for numeric return values) or a pointer 









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35.3.12 Core Layers



The file perlio.c provides the following layers:





"unix"




A basic non-buffered layer which calls Unix/POSIX read(), write(),
lseek(), close(). No buffering. Even on platforms that distinguish
between O_TEXT and O_BINARY this layer is always O_BINARY.





"perlio"




A very complete generic buffering layer which provides the whole of
PerlIO API. It is also intended to be used as a "base class" for other
layers. (For example its Read() method is implemented in terms of
the Get_cnt()/Get_ptr()/Set_ptrcnt() methods).



"perlio" over "unix" provides a complete replacement for stdio as seen
via PerlIO API. This is the default for USE_PERLIO when system’s stdio
does not permit perl’s "fast gets" access, and which do not
distinguish between O_TEXT and O_BINARY.





"stdio"




A layer which provides the PerlIO API via the layer scheme, but
implements it by calling system’s stdio. This is (currently) the default
if system’s stdio provides sufficient access to allow perl’s "fast gets"
access and which do not distinguish between O_TEXT and O_BINARY.





"crlf"




A layer derived using "perlio" as a base class. It provides Win32-like
"\n" to CR,LF translation. Can either be applied above "perlio" or serve
as the buffer layer itself. "crlf" over "unix" is the default if system
distinguishes between O_TEXT and O_BINARY opens. (At some point
"unix" will be replaced by a "native" Win32 IO layer on that platform,
as Win32’s read/write layer has various drawbacks.) The "crlf" layer is
a reasonable model for a layer which transforms data in some way.





"mmap"




If Configure detects mmap() functions this layer is provided (with
"perlio" as a "base") which does "read" operations by mmap()ing the
file. Performance improvement is marginal on modern systems, so it is
mainly there as a proof of concept. It is likely to be unbundled from
the core at some point. The "mmap" layer is a reasonable model for a
minimalist "derived" layer.





"pending"




An "internal" derivative of "perlio" which can be used to provide
Unread() function for layers which have no buffer or cannot be
bothered.  (Basically this layer’s Fill() pops itself off the stack
and so resumes reading from layer below.)





"raw"




A dummy layer which never exists on the layer stack. Instead when
"pushed" it actually pops the stack removing itself, it then calls
Binmode function table entry on all the layers in the stack - normally
this (via PerlIOBase_binmode) removes any layers which do not have
PERLIO_K_RAW bit set. Layers can modify that behaviour by defining
their own Binmode entry.





"utf8"




Another dummy layer. When pushed it pops itself and sets the
PERLIO_F_UTF8 flag on the layer which was (and now is once more)
the top of the stack.








In addition perlio.c also provides a number of PerlIOBase_xxxx()
functions which are intended to be used in the table slots of classes
which do not need to do anything special for a particular method.









Previous: perliol Core Layers, Up: perliol DESCRIPTION   [Contents]





35.3.13 Extension Layers



Layers can be made available by extension modules. When an unknown layer
is encountered the PerlIO code will perform the equivalent of :



   use PerlIO 'layer';



Where layer is the unknown layer. PerlIO.pm will then attempt to:



   require PerlIO::layer;



If after that process the layer is still not defined then the open
will fail.



The following extension layers are bundled with perl:





":encoding"




   use Encoding;



makes this layer available, although PerlIO.pm "knows" where to
find it.  It is an example of a layer which takes an argument as it is
called thus:



   open( $fh, "<:encoding(iso-8859-7)", $pathname );





":scalar"




Provides support for reading data from and writing data to a scalar.



   open( $fh, "+<:scalar", \$scalar );



When a handle is so opened, then reads get bytes from the string value
of $scalar, and writes change the value. In both cases the position
in $scalar starts as zero but can be altered via seek, and
determined via tell.



Please note that this layer is implied when calling open() thus:



   open( $fh, "+<", \$scalar );





":via"




Provided to allow layers to be implemented as Perl code.  For instance:



   use PerlIO::via::StripHTML;
   open( my $fh, "<:via(StripHTML)", "index.html" );



See (PerlIO-via) for details.














Previous: perliol DESCRIPTION, Up: perliol   [Contents]





35.4 TODO



Things that need to be done to improve this document.











Next: , Previous: perliol, Up: Top   [Contents]





36 perlipc





• perlipc NAME:  



• perlipc DESCRIPTION:  



• perlipc Signals:  



• perlipc Named Pipes:  



• perlipc Using open() for IPC:  



• perlipc Sockets: Client/Server Communication:  



• perlipc TCP Clients with IO::Socket:  



• perlipc TCP Servers with IO::Socket:  



• perlipc UDP: Message Passing:  



• perlipc SysV IPC:  



• perlipc NOTES:  



• perlipc BUGS:  



• perlipc AUTHOR:  



• perlipc SEE ALSO:  












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36.1 NAME



perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)









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36.2 DESCRIPTION



The basic IPC facilities of Perl are built out of the good old Unix
signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
IPC calls.  Each is used in slightly different situations.









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36.3 Signals



Perl uses a simple signal handling model: the %SIG hash contains names
or references of user-installed signal handlers.  These handlers will
be called with an argument which is the name of the signal that
triggered it.  A signal may be generated intentionally from a
particular keyboard sequence like control-C or control-Z, sent to you
from another process, or triggered automatically by the kernel when
special events transpire, like a child process exiting, your own process
running out of stack space, or hitting a process file-size limit.



For example, to trap an interrupt signal, set up a handler like this:



    our $shucks;

    sub catch_zap {
        my $signame = shift;
        $shucks++;
        die "Somebody sent me a SIG$signame";
    }
    $SIG{INT} = __PACKAGE__ . "::catch_zap";  
    $SIG{INT} = \&catch_zap;  # best strategy



Prior to Perl 5.7.3 it was necessary to do as little as you possibly
could in your handler; notice how all we do is set a global variable
and then raise an exception.  That’s because on most systems,
libraries are not re-entrant; particularly, memory allocation and I/O
routines are not.  That meant that doing nearly anything in your
handler could in theory trigger a memory fault and subsequent core
dump - see Deferred Signals (Safe Signals) below.



The names of the signals are the ones listed out by kill -l on your
system, or you can retrieve them using the CPAN module (IPC-Signal).



You may also choose to assign the strings "IGNORE" or "DEFAULT" as
the handler, in which case Perl will try to discard the signal or do the
default thing.



On most Unix platforms, the CHLD (sometimes also known as CLD) signal
has special behavior with respect to a value of "IGNORE".
Setting $SIG{CHLD} to "IGNORE" on such a platform has the effect of
not creating zombie processes when the parent process fails to wait()
on its child processes (i.e., child processes are automatically reaped).
Calling wait() with $SIG{CHLD} set to "IGNORE" usually returns
-1 on such platforms.



Some signals can be neither trapped nor ignored, such as the KILL and STOP
(but not the TSTP) signals. Note that ignoring signals makes them disappear.
If you only want them blocked temporarily without them getting lost you’ll
have to use POSIX’ sigprocmask.



Sending a signal to a negative process ID means that you send the signal
to the entire Unix process group.  This code sends a hang-up signal to all
processes in the current process group, and also sets $SIG{HUP} to "IGNORE" 
so it doesn’t kill itself:



    # block scope for local
    {
        local $SIG{HUP} = "IGNORE";
        kill HUP => -$$;
        # snazzy writing of: kill("HUP", -$$)
    }



Another interesting signal to send is signal number zero.  This doesn’t
actually affect a child process, but instead checks whether it’s alive
or has changed its UIDs.



    unless (kill 0 => $kid_pid) {
        warn "something wicked happened to $kid_pid";
    }



Signal number zero may fail because you lack permission to send the
signal when directed at a process whose real or saved UID is not
identical to the real or effective UID of the sending process, even
though the process is alive.  You may be able to determine the cause of
failure using $! or %!.



    unless (kill(0 => $pid) || $!{EPERM}) {
        warn "$pid looks dead";
    }



You might also want to employ anonymous functions for simple signal
handlers:



    $SIG{INT} = sub { die "\nOutta here!\n" };



SIGCHLD handlers require some special care.  If a second child dies
while in the signal handler caused by the first death, we won’t get
another signal. So must loop here else we will leave the unreaped child
as a zombie. And the next time two children die we get another zombie.
And so on.



    use POSIX ":sys_wait_h";
    $SIG{CHLD} = sub {
        while ((my $child = waitpid(-1, WNOHANG)) > 0) {
            $Kid_Status{$child} = $?;
        }
    };
    # do something that forks...



Be careful: qx(), system(), and some modules for calling external commands
do a fork(), then wait() for the result. Thus, your signal handler
will be called. Because wait() was already called by system() or qx(),
the wait() in the signal handler will see no more zombies and will
therefore block.



The best way to prevent this issue is to use waitpid(), as in the following
example:



    use POSIX ":sys_wait_h"; # for nonblocking read

    my %children;

    $SIG{CHLD} = sub {
        # don't change $! and $? outside handler
        local ($!, $?);
        my $pid = waitpid(-1, WNOHANG);
        return if $pid == -1;
        return unless defined $children{$pid};
        delete $children{$pid};
        cleanup_child($pid, $?);
    };

    while (1) {
        my $pid = fork();
        die "cannot fork" unless defined $pid;
        if ($pid == 0) {
            # ...
            exit 0;
        } else {
            $children{$pid}=1;
            # ...
            system($command);
            # ...
       }
    }



Signal handling is also used for timeouts in Unix.  While safely
protected within an eval{} block, you set a signal handler to trap
alarm signals and then schedule to have one delivered to you in some
number of seconds.  Then try your blocking operation, clearing the alarm
when it’s done but not before you’ve exited your eval{} block.  If it
goes off, you’ll use die() to jump out of the block.



Here’s an example:



    my $ALARM_EXCEPTION = "alarm clock restart";
    eval {
        local $SIG{ALRM} = sub { die $ALARM_EXCEPTION };
        alarm 10;
        flock(FH, 2)    # blocking write lock
                        || die "cannot flock: $!";
        alarm 0;
    };
    if ($@ && $@ !~ quotemeta($ALARM_EXCEPTION)) { die }



If the operation being timed out is system() or qx(), this technique
is liable to generate zombies.    If this matters to you, you’ll
need to do your own fork() and exec(), and kill the errant child process.



For more complex signal handling, you might see the standard POSIX
module.  Lamentably, this is almost entirely undocumented, but
the t/lib/posix.t file from the Perl source distribution has some
examples in it.





• perlipc Handling the SIGHUP Signal in Daemons:  



• perlipc Deferred Signals (Safe Signals):  












Next: , Up: perlipc Signals   [Contents]





36.3.1 Handling the SIGHUP Signal in Daemons



A process that usually starts when the system boots and shuts down
when the system is shut down is called a daemon (Disk And Execution
MONitor). If a daemon process has a configuration file which is
modified after the process has been started, there should be a way to
tell that process to reread its configuration file without stopping
the process. Many daemons provide this mechanism using a SIGHUP
signal handler. When you want to tell the daemon to reread the file,
simply send it the SIGHUP signal.



The following example implements a simple daemon, which restarts
itself every time the SIGHUP signal is received. The actual code is
located in the subroutine code(), which just prints some debugging
info to show that it works; it should be replaced with the real code.



  #!/usr/bin/perl -w

  use POSIX ();
  use FindBin ();
  use File::Basename ();
  use File::Spec::Functions;

  $| = 1;

  # make the daemon cross-platform, so exec always calls the script
  # itself with the right path, no matter how the script was invoked.
  my $script = File::Basename::basename($0);
  my $SELF  = catfile($FindBin::Bin, $script);

  # POSIX unmasks the sigprocmask properly
  $SIG{HUP} = sub {
      print "got SIGHUP\n";
      exec($SELF, @ARGV)        || die "$0: couldn't restart: $!";
  };

  code();

  sub code {
      print "PID: $$\n";
      print "ARGV: @ARGV\n";
      my $count = 0;
      while (++$count) {
          sleep 2;
          print "$count\n";
      }
  }









Previous: perlipc Handling the SIGHUP Signal in Daemons, Up: perlipc Signals   [Contents]





36.3.2 Deferred Signals (Safe Signals)



Before Perl 5.7.3, installing Perl code to deal with signals exposed you to
danger from two things.  First, few system library functions are
re-entrant.  If the signal interrupts while Perl is executing one function
(like malloc(3) or printf(3)), and your signal handler then calls the same
function again, you could get unpredictable behavior–often, a core dump.
Second, Perl isn’t itself re-entrant at the lowest levels.  If the signal
interrupts Perl while Perl is changing its own internal data structures,
similarly unpredictable behavior may result.



There were two things you could do, knowing this: be paranoid or be
pragmatic.  The paranoid approach was to do as little as possible in your
signal handler.  Set an existing integer variable that already has a
value, and return.  This doesn’t help you if you’re in a slow system call,
which will just restart.  That means you have to die to longjmp(3) out
of the handler.  Even this is a little cavalier for the true paranoiac,
who avoids die in a handler because the system is out to get you.
The pragmatic approach was to say "I know the risks, but prefer the
convenience", and to do anything you wanted in your signal handler,
and be prepared to clean up core dumps now and again.



Perl 5.7.3 and later avoid these problems by "deferring" signals.  That is,
when the signal is delivered to the process by the system (to the C code
that implements Perl) a flag is set, and the handler returns immediately.
Then at strategic "safe" points in the Perl interpreter (e.g. when it is
about to execute a new opcode) the flags are checked and the Perl level
handler from %SIG is executed. The "deferred" scheme allows much more
flexibility in the coding of signal handlers as we know the Perl
interpreter is in a safe state, and that we are not in a system library
function when the handler is called.  However the implementation does
differ from previous Perls in the following ways:





Long-running opcodes




As the Perl interpreter looks at signal flags only when it is about
to execute a new opcode, a signal that arrives during a long-running
opcode (e.g. a regular expression operation on a very large string) will
not be seen until the current opcode completes.



If a signal of any given type fires multiple times during an opcode 
(such as from a fine-grained timer), the handler for that signal will
be called only once, after the opcode completes; all other
instances will be discarded.  Furthermore, if your system’s signal queue
gets flooded to the point that there are signals that have been raised
but not yet caught (and thus not deferred) at the time an opcode
completes, those signals may well be caught and deferred during
subsequent opcodes, with sometimes surprising results.  For example, you
may see alarms delivered even after calling alarm(0) as the latter
stops the raising of alarms but does not cancel the delivery of alarms
raised but not yet caught.  Do not depend on the behaviors described in
this paragraph as they are side effects of the current implementation and
may change in future versions of Perl.





Interrupting IO




When a signal is delivered (e.g., SIGINT from a control-C) the operating
system breaks into IO operations like read(2), which is used to
implement Perl’s readline() function, the <> operator. On older
Perls the handler was called immediately (and as read is not "unsafe",
this worked well). With the "deferred" scheme the handler is not called
immediately, and if Perl is using the system’s stdio library that
library may restart the read without returning to Perl to give it a
chance to call the %SIG handler. If this happens on your system the
solution is to use the :perlio layer to do IO–at least on those handles
that you want to be able to break into with signals. (The :perlio layer
checks the signal flags and calls %SIG handlers before resuming IO
operation.)



The default in Perl 5.7.3 and later is to automatically use
the :perlio layer.



Note that it is not advisable to access a file handle within a signal
handler where that signal has interrupted an I/O operation on that same
handle. While perl will at least try hard not to crash, there are no
guarantees of data integrity; for example, some data might get dropped or
written twice.



Some networking library functions like gethostbyname() are known to have
their own implementations of timeouts which may conflict with your
timeouts.  If you have problems with such functions, try using the POSIX
sigaction() function, which bypasses Perl safe signals.  Be warned that
this does subject you to possible memory corruption, as described above.



Instead of setting $SIG{ALRM}:



   local $SIG{ALRM} = sub { die "alarm" };



try something like the following:



  use POSIX qw(SIGALRM);
  POSIX::sigaction(SIGALRM, POSIX::SigAction->new(sub { die "alarm" }))
          || die "Error setting SIGALRM handler: $!\n";



Another way to disable the safe signal behavior locally is to use
the Perl::Unsafe::Signals module from CPAN, which affects
all signals.





Restartable system calls




On systems that supported it, older versions of Perl used the
SA_RESTART flag when installing %SIG handlers.  This meant that
restartable system calls would continue rather than returning when
a signal arrived.  In order to deliver deferred signals promptly,
Perl 5.7.3 and later do not use SA_RESTART.  Consequently, 
restartable system calls can fail (with $! set to EINTR) in places
where they previously would have succeeded.



The default :perlio layer retries read, write
and close as described above; interrupted wait and 
waitpid calls will always be retried.





Signals as "faults"




Certain signals like SEGV, ILL, and BUS are generated by virtual memory
addressing errors and similar "faults". These are normally fatal: there is
little a Perl-level handler can do with them.  So Perl delivers them
immediately rather than attempting to defer them.





Signals triggered by operating system state




On some operating systems certain signal handlers are supposed to "do
something" before returning. One example can be CHLD or CLD, which
indicates a child process has completed. On some operating systems the
signal handler is expected to wait for the completed child
process. On such systems the deferred signal scheme will not work for
those signals: it does not do the wait. Again the failure will
look like a loop as the operating system will reissue the signal because
there are completed child processes that have not yet been waited for.








If you want the old signal behavior back despite possible
memory corruption, set the environment variable PERL_SIGNALS to
"unsafe".  This feature first appeared in Perl 5.8.1.









Next: , Previous: perlipc Signals, Up: perlipc   [Contents]





36.4 Named Pipes



A named pipe (often referred to as a FIFO) is an old Unix IPC
mechanism for processes communicating on the same machine.  It works
just like regular anonymous pipes, except that the
processes rendezvous using a filename and need not be related.



To create a named pipe, use the POSIX::mkfifo() function.



    use POSIX qw(mkfifo);
    mkfifo($path, 0700)     ||  die "mkfifo $path failed: $!";



You can also use the Unix command mknod(1), or on some
systems, mkfifo(1).  These may not be in your normal path, though.



    # system return val is backwards, so && not ||
    #
    $ENV{PATH} .= ":/etc:/usr/etc";
    if  (      system("mknod",  $path, "p")
            && system("mkfifo", $path) )
    {
        die "mk{nod,fifo} $path failed";
    }



A fifo is convenient when you want to connect a process to an unrelated
one.  When you open a fifo, the program will block until there’s something
on the other end.



For example, let’s say you’d like to have your .signature file be a
named pipe that has a Perl program on the other end.  Now every time any
program (like a mailer, news reader, finger program, etc.) tries to read
from that file, the reading program will read the new signature from your
program.  We’ll use the pipe-checking file-test operator, -p, to find
out whether anyone (or anything) has accidentally removed our fifo.



    chdir();    # go home
    my $FIFO = ".signature";

    while (1) {
        unless (-p $FIFO) {
            unlink $FIFO;   # discard any failure, will catch later
            require POSIX;  # delayed loading of heavy module
            POSIX::mkfifo($FIFO, 0700)
                                || die "can't mkfifo $FIFO: $!";
        }

        # next line blocks till there's a reader
        open (FIFO, "> $FIFO")  || die "can't open $FIFO: $!";
        print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
        close(FIFO)             || die "can't close $FIFO: $!";
        sleep 2;                # to avoid dup signals
    }









Next: , Previous: perlipc Named Pipes, Up: perlipc   [Contents]





36.5 Using open() for IPC



Perl’s basic open() statement can also be used for unidirectional
interprocess communication by either appending or prepending a pipe
symbol to the second argument to open().  Here’s how to start
something up in a child process you intend to write to:



    open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
                        || die "can't fork: $!";
    local $SIG{PIPE} = sub { die "spooler pipe broke" };
    print SPOOLER "stuff\n";
    close SPOOLER       || die "bad spool: $! $?";



And here’s how to start up a child process you intend to read from:



    open(STATUS, "netstat -an 2>&1 |")
                        || die "can't fork: $!";
    while (<STATUS>) {
        next if /^(tcp|udp)/;
        print;
    }
    close STATUS        || die "bad netstat: $! $?";



If one can be sure that a particular program is a Perl script expecting
filenames in @ARGV, the clever programmer can write something like this:



    % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile



and no matter which sort of shell it’s called from, the Perl program will
read from the file f1, the process cmd1, standard input (tmpfile
in this case), the f2 file, the cmd2 command, and finally the f3
file.  Pretty nifty, eh?



You might notice that you could use backticks for much the
same effect as opening a pipe for reading:



    print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
    die "bad netstatus ($?)" if $?;



While this is true on the surface, it’s much more efficient to process the
file one line or record at a time because then you don’t have to read the
whole thing into memory at once.  It also gives you finer control of the
whole process, letting you kill off the child process early if you’d like.



Be careful to check the return values from both open() and close().  If
you’re writing to a pipe, you should also trap SIGPIPE.  Otherwise,
think of what happens when you start up a pipe to a command that doesn’t
exist: the open() will in all likelihood succeed (it only reflects the
fork()’s success), but then your output will fail–spectacularly.  Perl
can’t know whether the command worked, because your command is actually
running in a separate process whose exec() might have failed.  Therefore,
while readers of bogus commands return just a quick EOF, writers
to bogus commands will get hit with a signal, which they’d best be prepared
to handle.  Consider:



    open(FH, "|bogus")      || die "can't fork: $!";
    print FH "bang\n";      #  neither necessary nor sufficient 
                            #  to check print retval!
    close(FH)               || die "can't close: $!";



The reason for not checking the return value from print() is because of
pipe buffering; physical writes are delayed.  That won’t blow up until the
close, and it will blow up with a SIGPIPE.  To catch it, you could use
this:



    $SIG{PIPE} = "IGNORE";
    open(FH, "|bogus")  || die "can't fork: $!";
    print FH "bang\n";
    close(FH)           || die "can't close: status=$?";





• perlipc Filehandles:  



• perlipc Background Processes:  



• perlipc Complete Dissociation of Child from Parent:  



• perlipc Safe Pipe Opens:  



• perlipc Avoiding Pipe Deadlocks:  



• perlipc Bidirectional Communication with Another Process:  



• perlipc Bidirectional Communication with Yourself:  












Next: , Up: perlipc Using open() for IPC   [Contents]





36.5.1 Filehandles



Both the main process and any child processes it forks share the same
STDIN, STDOUT, and STDERR filehandles.  If both processes try to access
them at once, strange things can happen.  You may also want to close
or reopen the filehandles for the child.  You can get around this by
opening your pipe with open(), but on some systems this means that the
child process cannot outlive the parent.









Next: , Previous: perlipc Filehandles, Up: perlipc Using open() for IPC   [Contents]





36.5.2 Background Processes



You can run a command in the background with:



    system("cmd &");



The command’s STDOUT and STDERR (and possibly STDIN, depending on your
shell) will be the same as the parent’s.  You won’t need to catch
SIGCHLD because of the double-fork taking place; see below for details.









Next: , Previous: perlipc Background Processes, Up: perlipc Using open() for IPC   [Contents]





36.5.3 Complete Dissociation of Child from Parent



In some cases (starting server processes, for instance) you’ll want to
completely dissociate the child process from the parent.  This is
often called daemonization.  A well-behaved daemon will also chdir()
to the root directory so it doesn’t prevent unmounting the filesystem
containing the directory from which it was launched, and redirect its
standard file descriptors from and to /dev/null so that random
output doesn’t wind up on the user’s terminal.



    use POSIX "setsid";

    sub daemonize {
        chdir("/")                      || die "can't chdir to /: $!";
        open(STDIN,  "< /dev/null")     || die "can't read /dev/null: $!";
        open(STDOUT, "> /dev/null")     || die "can't write to /dev/null: $!";
        defined(my $pid = fork())       || die "can't fork: $!";
        exit if $pid;                   # non-zero now means I am the parent
        (setsid() != -1)                || die "Can't start a new session: $!" 
        open(STDERR, ">&STDOUT")        || die "can't dup stdout: $!";
    }



The fork() has to come before the setsid() to ensure you aren’t a
process group leader; the setsid() will fail if you are.  If your
system doesn’t have the setsid() function, open /dev/tty and use the
TIOCNOTTY ioctl() on it instead.  See tty(4) for details.



Non-Unix users should check their Your_OS::Process module for 
other possible solutions.









Next: , Previous: perlipc Complete Dissociation of Child from Parent, Up: perlipc Using open() for IPC   [Contents]





36.5.4 Safe Pipe Opens



Another interesting approach to IPC is making your single program go
multiprocess and communicate between–or even amongst–yourselves.  The
open() function will accept a file argument of either "-|" or "|-"
to do a very interesting thing: it forks a child connected to the
filehandle you’ve opened.  The child is running the same program as the
parent.  This is useful for safely opening a file when running under an
assumed UID or GID, for example.  If you open a pipe to minus, you can
write to the filehandle you opened and your kid will find it in his
STDIN.  If you open a pipe from minus, you can read from the filehandle
you opened whatever your kid writes to his STDOUT.



    use English qw[ -no_match_vars ];
    my $PRECIOUS = "/path/to/some/safe/file";
    my $sleep_count;
    my $pid;

    do {
        $pid = open(KID_TO_WRITE, "|-");
        unless (defined $pid) {
            warn "cannot fork: $!";
            die "bailing out" if $sleep_count++ > 6;
            sleep 10;
        }
    } until defined $pid;

    if ($pid) {                 # I am the parent 
        print KID_TO_WRITE @some_data;
        close(KID_TO_WRITE)     || warn "kid exited $?";
    } else {                    # I am the child
        # drop permissions in setuid and/or setgid programs:
        ($EUID, $EGID) = ($UID, $GID);  
        open (OUTFILE, "> $PRECIOUS") 
                                || die "can't open $PRECIOUS: $!";
        while (<STDIN>) {
            print OUTFILE;      # child's STDIN is parent's KID_TO_WRITE
        }
        close(OUTFILE)          || die "can't close $PRECIOUS: $!";
        exit(0);                # don't forget this!!
    }



Another common use for this construct is when you need to execute
something without the shell’s interference.  With system(), it’s
straightforward, but you can’t use a pipe open or backticks safely.
That’s because there’s no way to stop the shell from getting its hands on
your arguments.   Instead, use lower-level control to call exec() directly.



Here’s a safe backtick or pipe open for read:



    my $pid = open(KID_TO_READ, "-|");
    defined($pid)           || die "can't fork: $!";

    if ($pid) {             # parent
        while (<KID_TO_READ>) {
                            # do something interesting
        }
        close(KID_TO_READ)  || warn "kid exited $?";

    } else {                # child
        ($EUID, $EGID) = ($UID, $GID); # suid only
        exec($program, @options, @args)
                            || die "can't exec program: $!";
        # NOTREACHED
    }



And here’s a safe pipe open for writing:



    my $pid = open(KID_TO_WRITE, "|-");
    defined($pid)           || die "can't fork: $!";

    $SIG{PIPE} = sub { die "whoops, $program pipe broke" };

    if ($pid) {             # parent
        print KID_TO_WRITE @data;
        close(KID_TO_WRITE) || warn "kid exited $?";

    } else {                # child
        ($EUID, $EGID) = ($UID, $GID);
        exec($program, @options, @args)
                            || die "can't exec program: $!";
        # NOTREACHED
    }



It is very easy to dead-lock a process using this form of open(), or
indeed with any use of pipe() with multiple subprocesses.  The 
example above is "safe" because it is simple and calls exec().  See
Avoiding Pipe Deadlocks for general safety principles, but there
are extra gotchas with Safe Pipe Opens.



In particular, if you opened the pipe using open FH, "|-", then you
cannot simply use close() in the parent process to close an unwanted
writer.  Consider this code:



    my $pid = open(WRITER, "|-");        # fork open a kid
    defined($pid)               || die "first fork failed: $!";
    if ($pid) {
        if (my $sub_pid = fork()) {
            defined($sub_pid)   || die "second fork failed: $!";
            close(WRITER)       || die "couldn't close WRITER: $!";
            # now do something else...
        }
        else {
            # first write to WRITER
            # ...
            # then when finished
            close(WRITER)       || die "couldn't close WRITER: $!";
            exit(0);
        }
    }
    else {
        # first do something with STDIN, then
        exit(0);
    }



In the example above, the true parent does not want to write to the WRITER
filehandle, so it closes it.  However, because WRITER was opened using
open FH, "|-", it has a special behavior: closing it calls
waitpid() (see (perlfunc)waitpid), which waits for the subprocess
to exit.  If the child process ends up waiting for something happening
in the section marked "do something else", you have deadlock.



This can also be a problem with intermediate subprocesses in more
complicated code, which will call waitpid() on all open filehandles
during global destruction–in no predictable order.



To solve this, you must manually use pipe(), fork(), and the form of
open() which sets one file descriptor to another, as shown below:



    pipe(READER, WRITER)        || die "pipe failed: $!";
    $pid = fork();
    defined($pid)               || die "first fork failed: $!";
    if ($pid) {
        close READER;
        if (my $sub_pid = fork()) {
            defined($sub_pid)   || die "first fork failed: $!";
            close(WRITER)       || die "can't close WRITER: $!";
        }
        else {
            # write to WRITER...
            # ...
            # then  when finished
            close(WRITER)       || die "can't close WRITER: $!";
            exit(0);
        }
        # write to WRITER...
    }
    else {
        open(STDIN, "<&READER") || die "can't reopen STDIN: $!";
        close(WRITER)           || die "can't close WRITER: $!";
        # do something...
        exit(0);
    }



Since Perl 5.8.0, you can also use the list form of open for pipes.
This is preferred when you wish to avoid having the shell interpret
metacharacters that may be in your command string.



So for example, instead of using:



    open(PS_PIPE, "ps aux|")    || die "can't open ps pipe: $!";



One would use either of these:



    open(PS_PIPE, "-|", "ps", "aux") 
                                || die "can't open ps pipe: $!";

    @ps_args = qw[ ps aux ];
    open(PS_PIPE, "-|", @ps_args)
                                || die "can't open @ps_args|: $!";



Because there are more than three arguments to open(), forks the ps(1)
command without spawning a shell, and reads its standard output via the
PS_PIPE filehandle.  The corresponding syntax to write to command
pipes is to use "|-" in place of "-|".  



This was admittedly a rather silly example, because you’re using string
literals whose content is perfectly safe.  There is therefore no cause to
resort to the harder-to-read, multi-argument form of pipe open().  However,
whenever you cannot be assured that the program arguments are free of shell
metacharacters, the fancier form of open() should be used.  For example:



    @grep_args = ("egrep", "-i", $some_pattern, @many_files);
    open(GREP_PIPE, "-|", @grep_args)
                        || die "can't open @grep_args|: $!";



Here the multi-argument form of pipe open() is preferred because the
pattern and indeed even the filenames themselves might hold metacharacters.



Be aware that these operations are full Unix forks, which means they may
not be correctly implemented on all alien systems.  Additionally, these are
not true multithreading.  To learn more about threading, see the modules
file mentioned below in the SEE ALSO section.









Next: , Previous: perlipc Safe Pipe Opens, Up: perlipc Using open() for IPC   [Contents]





36.5.5 Avoiding Pipe Deadlocks



Whenever you have more than one subprocess, you must be careful that each
closes whichever half of any pipes created for interprocess communication
it is not using.  This is because any child process reading from the pipe
and expecting an EOF will never receive it, and therefore never exit. A
single process closing a pipe is not enough to close it; the last process
with the pipe open must close it for it to read EOF.



Certain built-in Unix features help prevent this most of the time.  For
instance, filehandles have a "close on exec" flag, which is set en masse
under control of the $^F variable.  This is so any filehandles you
didn’t explicitly route to the STDIN, STDOUT or STDERR of a child
program will be automatically closed.



Always explicitly and immediately call close() on the writable end of any
pipe, unless that process is actually writing to it.  Even if you don’t
explicitly call close(), Perl will still close() all filehandles during
global destruction.  As previously discussed, if those filehandles have
been opened with Safe Pipe Open, this will result in calling waitpid(),
which may again deadlock.









Next: , Previous: perlipc Avoiding Pipe Deadlocks, Up: perlipc Using open() for IPC   [Contents]





36.5.6 Bidirectional Communication with Another Process



While this works reasonably well for unidirectional communication, what
about bidirectional communication?  The most obvious approach doesn’t work:



    # THIS DOES NOT WORK!!
    open(PROG_FOR_READING_AND_WRITING, "| some program |")



If you forget to use warnings, you’ll miss out entirely on the
helpful diagnostic message:



    Can't do bidirectional pipe at -e line 1.



If you really want to, you can use the standard open2() from the
IPC::Open2 module to catch both ends.  There’s also an open3() in
IPC::Open3 for tridirectional I/O so you can also catch your child’s
STDERR, but doing so would then require an awkward select() loop and
wouldn’t allow you to use normal Perl input operations.



If you look at its source, you’ll see that open2() uses low-level
primitives like the pipe() and exec() syscalls to create all the
connections.  Although it might have been more efficient by using
socketpair(), this would have been even less portable than it already
is. The open2() and open3() functions are unlikely to work anywhere
except on a Unix system, or at least one purporting POSIX compliance.



Here’s an example of using open2():



    use FileHandle;
    use IPC::Open2;
    $pid = open2(*Reader, *Writer, "cat -un");
    print Writer "stuff\n";
    $got = <Reader>;



The problem with this is that buffering is really going to ruin your
day.  Even though your Writer filehandle is auto-flushed so the process
on the other end gets your data in a timely manner, you can’t usually do
anything to force that process to give its data to you in a similarly quick
fashion.  In this special case, we could actually so, because we gave
cat a -u flag to make it unbuffered.  But very few commands are
designed to operate over pipes, so this seldom works unless you yourself
wrote the program on the other end of the double-ended pipe.



A solution to this is to use a library which uses pseudottys to make your
program behave more reasonably.  This way you don’t have to have control
over the source code of the program you’re using.  The Expect module
from CPAN also addresses this kind of thing.  This module requires two
other modules from CPAN, IO::Pty and IO::Stty.  It sets up a pseudo
terminal to interact with programs that insist on talking to the terminal
device driver.  If your system is supported, this may be your best bet.









Previous: perlipc Bidirectional Communication with Another Process, Up: perlipc Using open() for IPC   [Contents]





36.5.7 Bidirectional Communication with Yourself



If you want, you may make low-level pipe() and fork() syscalls to stitch
this together by hand.  This example only talks to itself, but you could
reopen the appropriate handles to STDIN and STDOUT and call other processes.
(The following example lacks proper error checking.)



    #!/usr/bin/perl -w
    # pipe1 - bidirectional communication using two pipe pairs
    #         designed for the socketpair-challenged
    use IO::Handle;               # thousands of lines just for autoflush :-(
    pipe(PARENT_RDR, CHILD_WTR);  # XXX: check failure?
    pipe(CHILD_RDR,  PARENT_WTR); # XXX: check failure?
    CHILD_WTR->autoflush(1);
    PARENT_WTR->autoflush(1);

    if ($pid = fork()) {
        close PARENT_RDR; 
        close PARENT_WTR;
        print CHILD_WTR "Parent Pid $$ is sending this\n";
        chomp($line = <CHILD_RDR>);
        print "Parent Pid $$ just read this: '$line'\n";
        close CHILD_RDR; close CHILD_WTR;
        waitpid($pid, 0);
    } else {
        die "cannot fork: $!" unless defined $pid;
        close CHILD_RDR; 
        close CHILD_WTR;
        chomp($line = <PARENT_RDR>);
        print "Child Pid $$ just read this: '$line'\n";
        print PARENT_WTR "Child Pid $$ is sending this\n";
        close PARENT_RDR; 
        close PARENT_WTR;
        exit(0);
    }



But you don’t actually have to make two pipe calls.  If you
have the socketpair() system call, it will do this all for you.



    #!/usr/bin/perl -w
    # pipe2 - bidirectional communication using socketpair
    #   "the best ones always go both ways"

    use Socket;
    use IO::Handle;  # thousands of lines just for autoflush :-(

    # We say AF_UNIX because although *_LOCAL is the
    # POSIX 1003.1g form of the constant, many machines
    # still don't have it.
    socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                ||  die "socketpair: $!";

    CHILD->autoflush(1);
    PARENT->autoflush(1);

    if ($pid = fork()) {
        close PARENT;
        print CHILD "Parent Pid $$ is sending this\n";
        chomp($line = <CHILD>);
        print "Parent Pid $$ just read this: '$line'\n";
        close CHILD;
        waitpid($pid, 0);
    } else {
        die "cannot fork: $!" unless defined $pid;
        close CHILD;
        chomp($line = <PARENT>);
        print "Child Pid $$ just read this: '$line'\n";
        print PARENT "Child Pid $$ is sending this\n";
        close PARENT;
        exit(0);
    }









Next: , Previous: perlipc Using open() for IPC, Up: perlipc   [Contents]





36.6 Sockets: Client/Server Communication



While not entirely limited to Unix-derived operating systems (e.g., WinSock
on PCs provides socket support, as do some VMS libraries), you might not have
sockets on your system, in which case this section probably isn’t going to
do you much good.  With sockets, you can do both virtual circuits like TCP
streams and datagrams like UDP packets.  You may be able to do even more
depending on your system.



The Perl functions for dealing with sockets have the same names as
the corresponding system calls in C, but their arguments tend to differ
for two reasons.  First, Perl filehandles work differently than C file
descriptors.  Second, Perl already knows the length of its strings, so you
don’t need to pass that information.



One of the major problems with ancient, antemillennial socket code in Perl
was that it used hard-coded values for some of the constants, which
severely hurt portability.  If you ever see code that does anything like
explicitly setting $AF_INET = 2, you know you’re in for big trouble.  
An immeasurably superior approach is to use the Socket module, which more
reliably grants access to the various constants and functions you’ll need.



If you’re not writing a server/client for an existing protocol like
NNTP or SMTP, you should give some thought to how your server will
know when the client has finished talking, and vice-versa.  Most
protocols are based on one-line messages and responses (so one party
knows the other has finished when a "\n" is received) or multi-line
messages and responses that end with a period on an empty line
("\n.\n" terminates a message/response).





• perlipc Internet Line Terminators:  



• perlipc Internet TCP Clients and Servers:  



• perlipc Unix-Domain TCP Clients and Servers:  












Next: , Up: perlipc Sockets: Client/Server Communication   [Contents]





36.6.1 Internet Line Terminators



The Internet line terminator is "\015\012".  Under ASCII variants of
Unix, that could usually be written as "\r\n", but under other systems,
"\r\n" might at times be "\015\015\012", "\012\012\015", or something
completely different.  The standards specify writing "\015\012" to be
conformant (be strict in what you provide), but they also recommend
accepting a lone "\012" on input (be lenient in what you require).
We haven’t always been very good about that in the code in this manpage,
but unless you’re on a Mac from way back in its pre-Unix dark ages, you’ll 
probably be ok.









Next: , Previous: perlipc Internet Line Terminators, Up: perlipc Sockets: Client/Server Communication   [Contents]





36.6.2 Internet TCP Clients and Servers



Use Internet-domain sockets when you want to do client-server
communication that might extend to machines outside of your own system.



Here’s a sample TCP client using Internet-domain sockets:



    #!/usr/bin/perl -w
    use strict;
    use Socket;
    my ($remote, $port, $iaddr, $paddr, $proto, $line);

    $remote  = shift || "localhost";
    $port    = shift || 2345;  # random port
    if ($port =~ /\D/) { $port = getservbyname($port, "tcp") }
    die "No port" unless $port;
    $iaddr   = inet_aton($remote)       || die "no host: $remote";
    $paddr   = sockaddr_in($port, $iaddr);

    $proto   = getprotobyname("tcp");
    socket(SOCK, PF_INET, SOCK_STREAM, $proto)  || die "socket: $!";
    connect(SOCK, $paddr)               || die "connect: $!";
    while ($line = <SOCK>) {
        print $line;
    }

    close (SOCK)                        || die "close: $!";
    exit(0);



And here’s a corresponding server to go along with it.  We’ll
leave the address as INADDR_ANY so that the kernel can choose
the appropriate interface on multihomed hosts.  If you want sit
on a particular interface (like the external side of a gateway
or firewall machine), fill this in with your real address instead.



    #!/usr/bin/perl -Tw
    use strict;
    BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
    use Socket;
    use Carp;
    my $EOL = "\015\012";

    sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }

    my $port  = shift || 2345;
    die "invalid port" unless if $port =~ /^ \d+ $/x;

    my $proto = getprotobyname("tcp");

    socket(Server, PF_INET, SOCK_STREAM, $proto)    || die "socket: $!";
    setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))    
                                                    || die "setsockopt: $!";
    bind(Server, sockaddr_in($port, INADDR_ANY))    || die "bind: $!";
    listen(Server, SOMAXCONN)                       || die "listen: $!";

    logmsg "server started on port $port";

    my $paddr;

    $SIG{CHLD} = \&REAPER;

    for ( ; $paddr = accept(Client, Server); close Client) {
        my($port, $iaddr) = sockaddr_in($paddr);
        my $name = gethostbyaddr($iaddr, AF_INET);

        logmsg "connection from $name [",
                inet_ntoa($iaddr), "]
                at port $port";

        print Client "Hello there, $name, it's now ",
                        scalar localtime(), $EOL;
    }



And here’s a multithreaded version.  It’s multithreaded in that
like most typical servers, it spawns (fork()s) a slave server to
handle the client request so that the master server can quickly
go back to service a new client.



    #!/usr/bin/perl -Tw
    use strict;
    BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
    use Socket;
    use Carp;
    my $EOL = "\015\012";

    sub spawn;  # forward declaration
    sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }

    my $port  = shift || 2345;
    die "invalid port" unless if $port =~ /^ \d+ $/x;

    my $proto = getprotobyname("tcp");

    socket(Server, PF_INET, SOCK_STREAM, $proto)    || die "socket: $!";
    setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))         
                                                    || die "setsockopt: $!";
    bind(Server, sockaddr_in($port, INADDR_ANY))    || die "bind: $!";
    listen(Server, SOMAXCONN)                       || die "listen: $!";

    logmsg "server started on port $port";

    my $waitedpid = 0;
    my $paddr;

    use POSIX ":sys_wait_h";
    use Errno;

    sub REAPER {
        local $!;   # don't let waitpid() overwrite current error
        while ((my $pid = waitpid(-1, WNOHANG)) > 0 && WIFEXITED($?)) {
            logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
        }
        $SIG{CHLD} = \&REAPER;  # loathe SysV
    }

    $SIG{CHLD} = \&REAPER;

    while (1) {
        $paddr = accept(Client, Server) || do {
            # try again if accept() returned because got a signal
            next if $!{EINTR};
            die "accept: $!";
        };
        my ($port, $iaddr) = sockaddr_in($paddr);
        my $name = gethostbyaddr($iaddr, AF_INET);

        logmsg "connection from $name [",
               inet_ntoa($iaddr),
               "] at port $port";

        spawn sub {
            $| = 1;
            print "Hello there, $name, it's now ", scalar localtime(), $EOL;
            exec "/usr/games/fortune"       # XXX: "wrong" line terminators
                or confess "can't exec fortune: $!";
        };
        close Client;
    }

    sub spawn {
        my $coderef = shift;

        unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
            confess "usage: spawn CODEREF";
        }

        my $pid;
        unless (defined($pid = fork())) {
            logmsg "cannot fork: $!";
            return;
        } 
        elsif ($pid) {
            logmsg "begat $pid";
            return; # I'm the parent
        }
        # else I'm the child -- go spawn

        open(STDIN,  "<&Client")    || die "can't dup client to stdin";
        open(STDOUT, ">&Client")    || die "can't dup client to stdout";
        ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
        exit($coderef->());
    }



This server takes the trouble to clone off a child version via fork()
for each incoming request.  That way it can handle many requests at
once, which you might not always want.  Even if you don’t fork(), the
listen() will allow that many pending connections.  Forking servers
have to be particularly careful about cleaning up their dead children
(called "zombies" in Unix parlance), because otherwise you’ll quickly
fill up your process table.  The REAPER subroutine is used here to
call waitpid() for any child processes that have finished, thereby
ensuring that they terminate cleanly and don’t join the ranks of the
living dead.



Within the while loop we call accept() and check to see if it returns
a false value.  This would normally indicate a system error needs
to be reported.  However, the introduction of safe signals (see
Deferred Signals (Safe Signals) above) in Perl 5.7.3 means that
accept() might also be interrupted when the process receives a signal.
This typically happens when one of the forked subprocesses exits and
notifies the parent process with a CHLD signal.  



If accept() is interrupted by a signal, $! will be set to EINTR.
If this happens, we can safely continue to the next iteration of
the loop and another call to accept().  It is important that your
signal handling code not modify the value of $!, or else this test 
will likely fail.  In the REAPER subroutine we create a local version
of $! before calling waitpid().  When waitpid() sets $! to ECHILD as
it inevitably does when it has no more children waiting, it 
updates the local copy and leaves the original unchanged.



You should use the -T flag to enable taint checking (see perlsec NAME)
even if we aren’t running setuid or setgid.  This is always a good idea
for servers or any program run on behalf of someone else (like CGI
scripts), because it lessens the chances that people from the outside will
be able to compromise your system.



Let’s look at another TCP client.  This one connects to the TCP "time"
service on a number of different machines and shows how far their clocks
differ from the system on which it’s being run:



    #!/usr/bin/perl  -w
    use strict;
    use Socket;

    my $SECS_OF_70_YEARS = 2208988800;
    sub ctime { scalar localtime(shift() || time()) }

    my $iaddr = gethostbyname("localhost");
    my $proto = getprotobyname("tcp");
    my $port = getservbyname("time", "tcp");
    my $paddr = sockaddr_in(0, $iaddr);
    my($host);

    $| = 1;
    printf "%-24s %8s %s\n", "localhost", 0, ctime();

    foreach $host (@ARGV) {
        printf "%-24s ", $host;
        my $hisiaddr = inet_aton($host)     || die "unknown host";
        my $hispaddr = sockaddr_in($port, $hisiaddr);
        socket(SOCKET, PF_INET, SOCK_STREAM, $proto)   
                                            || die "socket: $!";
        connect(SOCKET, $hispaddr)          || die "connect: $!";
        my $rtime = pack("C4", ());
        read(SOCKET, $rtime, 4);
        close(SOCKET);
        my $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
        printf "%8d %s\n", $histime - time(), ctime($histime);
    }









Previous: perlipc Internet TCP Clients and Servers, Up: perlipc Sockets: Client/Server Communication   [Contents]





36.6.3 Unix-Domain TCP Clients and Servers



That’s fine for Internet-domain clients and servers, but what about local
communications?  While you can use the same setup, sometimes you don’t
want to.  Unix-domain sockets are local to the current host, and are often
used internally to implement pipes.  Unlike Internet domain sockets, Unix
domain sockets can show up in the file system with an ls(1) listing.



    % ls -l /dev/log
    srw-rw-rw-  1 root            0 Oct 31 07:23 /dev/log



You can test for these with Perl’s -S file test:



    unless (-S "/dev/log") {
        die "something's wicked with the log system";
    }



Here’s a sample Unix-domain client:



    #!/usr/bin/perl -w
    use Socket;
    use strict;
    my ($rendezvous, $line);

    $rendezvous = shift || "catsock";
    socket(SOCK, PF_UNIX, SOCK_STREAM, 0)     || die "socket: $!";
    connect(SOCK, sockaddr_un($rendezvous))   || die "connect: $!";
    while (defined($line = <SOCK>)) {
        print $line;
    }
    exit(0);



And here’s a corresponding server.  You don’t have to worry about silly
network terminators here because Unix domain sockets are guaranteed
to be on the localhost, and thus everything works right.



    #!/usr/bin/perl -Tw
    use strict;
    use Socket;
    use Carp;

    BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
    sub spawn;  # forward declaration
    sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }

    my $NAME = "catsock";
    my $uaddr = sockaddr_un($NAME);
    my $proto = getprotobyname("tcp");

    socket(Server, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
    unlink($NAME);
    bind  (Server, $uaddr)                  || die "bind: $!";
    listen(Server, SOMAXCONN)               || die "listen: $!";

    logmsg "server started on $NAME";

    my $waitedpid;

    use POSIX ":sys_wait_h";
    sub REAPER {
        my $child;
        while (($waitedpid = waitpid(-1, WNOHANG)) > 0) {
            logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
        }
        $SIG{CHLD} = \&REAPER;  # loathe SysV
    }

    $SIG{CHLD} = \&REAPER;


    for ( $waitedpid = 0;
          accept(Client, Server) || $waitedpid;
          $waitedpid = 0, close Client)
    {
        next if $waitedpid;
        logmsg "connection on $NAME";
        spawn sub {
            print "Hello there, it's now ", scalar localtime(), "\n";
            exec("/usr/games/fortune")  || die "can't exec fortune: $!";
        };
    }

    sub spawn {
        my $coderef = shift();

        unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
            confess "usage: spawn CODEREF";
        }

        my $pid;
        unless (defined($pid = fork())) {
            logmsg "cannot fork: $!";
            return;
        } 
        elsif ($pid) {
            logmsg "begat $pid";
            return; # I'm the parent
        } 
        else {
            # I'm the child -- go spawn
        }

        open(STDIN,  "<&Client")    || die "can't dup client to stdin";
        open(STDOUT, ">&Client")    || die "can't dup client to stdout";
        ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
        exit($coderef->());
    }



As you see, it’s remarkably similar to the Internet domain TCP server, so
much so, in fact, that we’ve omitted several duplicate functions–spawn(),
logmsg(), ctime(), and REAPER()–which are the same as in the other server.



So why would you ever want to use a Unix domain socket instead of a
simpler named pipe?  Because a named pipe doesn’t give you sessions.  You
can’t tell one process’s data from another’s.  With socket programming,
you get a separate session for each client; that’s why accept() takes two
arguments.



For example, let’s say that you have a long-running database server daemon
that you want folks to be able to access from the Web, but only
if they go through a CGI interface.  You’d have a small, simple CGI
program that does whatever checks and logging you feel like, and then acts
as a Unix-domain client and connects to your private server.









Next: , Previous: perlipc Sockets: Client/Server Communication, Up: perlipc   [Contents]





36.7 TCP Clients with IO::Socket



For those preferring a higher-level interface to socket programming, the
IO::Socket module provides an object-oriented approach.  IO::Socket has
been included in the standard Perl distribution ever since Perl 5.004.  If
you’re running an earlier version of Perl (in which case, how are you
reading this manpage?), just fetch IO::Socket from CPAN, where you’ll also
find modules providing easy interfaces to the following systems: DNS, FTP,
Ident (RFC 931), NIS and NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay,
Telnet, and Time–to name just a few.





• perlipc A Simple Client:  



• perlipc A Webget Client:  



• perlipc Interactive Client with IO::Socket:  












Next: , Up: perlipc TCP Clients with IO::Socket   [Contents]





36.7.1 A Simple Client



Here’s a client that creates a TCP connection to the "daytime"
service at port 13 of the host name "localhost" and prints out everything
that the server there cares to provide.



    #!/usr/bin/perl -w
    use IO::Socket;
    $remote = IO::Socket::INET->new(
                        Proto    => "tcp",
                        PeerAddr => "localhost",
                        PeerPort => "daytime(13)",
                    )
                  || die "can't connect to daytime service on localhost";
    while (<$remote>) { print }



When you run this program, you should get something back that
looks like this:



    Wed May 14 08:40:46 MDT 1997



Here are what those parameters to the new() constructor mean:





Proto




This is which protocol to use.  In this case, the socket handle returned
will be connected to a TCP socket, because we want a stream-oriented
connection, that is, one that acts pretty much like a plain old file.
Not all sockets are this of this type.  For example, the UDP protocol
can be used to make a datagram socket, used for message-passing.





PeerAddr




This is the name or Internet address of the remote host the server is
running on.  We could have specified a longer name like "www.perl.com",
or an address like "207.171.7.72".  For demonstration purposes, we’ve
used the special hostname "localhost", which should always mean the
current machine you’re running on.  The corresponding Internet address
for localhost is "127.0.0.1", if you’d rather use that.





PeerPort




This is the service name or port number we’d like to connect to.
We could have gotten away with using just "daytime" on systems with a
well-configured system services file,[FOOTNOTE: The system services file
is found in /etc/services under Unixy systems.] but here we’ve specified the
port number (13) in parentheses.  Using just the number would have also
worked, but numeric literals make careful programmers nervous.








Notice how the return value from the new constructor is used as
a filehandle in the while loop?  That’s what’s called an indirect
filehandle, a scalar variable containing a filehandle.  You can use
it the same way you would a normal filehandle.  For example, you
can read one line from it this way:



    $line = <$handle>;



all remaining lines from is this way:



    @lines = <$handle>;



and send a line of data to it this way:



    print $handle "some data\n";









Next: , Previous: perlipc A Simple Client, Up: perlipc TCP Clients with IO::Socket   [Contents]





36.7.2 A Webget Client



Here’s a simple client that takes a remote host to fetch a document
from, and then a list of files to get from that host.  This is a
more interesting client than the previous one because it first sends
something to the server before fetching the server’s response.



    #!/usr/bin/perl -w
    use IO::Socket;
    unless (@ARGV > 1) { die "usage: $0 host url ..." }
    $host = shift(@ARGV);
    $EOL = "\015\012";
    $BLANK = $EOL x 2;
    for my $document (@ARGV) {
        $remote = IO::Socket::INET->new( Proto     => "tcp",
                                         PeerAddr  => $host,
                                         PeerPort  => "http(80)",
                  )     || die "cannot connect to httpd on $host";
        $remote->autoflush(1);
        print $remote "GET $document HTTP/1.0" . $BLANK;
        while ( <$remote> ) { print }
        close $remote;
    }



The web server handling the HTTP service is assumed to be at
its standard port, number 80.  If the server you’re trying to
connect to is at a different port, like 1080 or 8080, you should specify it
as the named-parameter pair, PeerPort => 8080.  The autoflush
method is used on the socket because otherwise the system would buffer
up the output we sent it.  (If you’re on a prehistoric Mac, you’ll also
need to change every "\n" in your code that sends data over the network
to be a "\015\012" instead.)



Connecting to the server is only the first part of the process: once you
have the connection, you have to use the server’s language.  Each server
on the network has its own little command language that it expects as
input.  The string that we send to the server starting with "GET" is in
HTTP syntax.  In this case, we simply request each specified document.
Yes, we really are making a new connection for each document, even though
it’s the same host.  That’s the way you always used to have to speak HTTP.
Recent versions of web browsers may request that the remote server leave
the connection open a little while, but the server doesn’t have to honor
such a request.



Here’s an example of running that program, which we’ll call webget:



    % webget www.perl.com /guanaco.html
    HTTP/1.1 404 File Not Found
    Date: Thu, 08 May 1997 18:02:32 GMT
    Server: Apache/1.2b6
    Connection: close
    Content-type: text/html

    <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
    <BODY><H1>File Not Found</H1>
    The requested URL /guanaco.html was not found on this server.<P>
    </BODY>



Ok, so that’s not very interesting, because it didn’t find that
particular document.  But a long response wouldn’t have fit on this page.



For a more featureful version of this program, you should look to
the lwp-request program included with the LWP modules from CPAN.









Previous: perlipc A Webget Client, Up: perlipc TCP Clients with IO::Socket   [Contents]





36.7.3 Interactive Client with IO::Socket



Well, that’s all fine if you want to send one command and get one answer,
but what about setting up something fully interactive, somewhat like
the way telnet works?  That way you can type a line, get the answer,
type a line, get the answer, etc.



This client is more complicated than the two we’ve done so far, but if
you’re on a system that supports the powerful fork call, the solution
isn’t that rough.  Once you’ve made the connection to whatever service
you’d like to chat with, call fork to clone your process.  Each of
these two identical process has a very simple job to do: the parent
copies everything from the socket to standard output, while the child
simultaneously copies everything from standard input to the socket.
To accomplish the same thing using just one process would be much
harder, because it’s easier to code two processes to do one thing than it
is to code one process to do two things.  (This keep-it-simple principle
a cornerstones of the Unix philosophy, and good software engineering as
well, which is probably why it’s spread to other systems.)



Here’s the code:



    #!/usr/bin/perl -w
    use strict;
    use IO::Socket;
    my ($host, $port, $kidpid, $handle, $line);

    unless (@ARGV == 2) { die "usage: $0 host port" }
    ($host, $port) = @ARGV;

    # create a tcp connection to the specified host and port
    $handle = IO::Socket::INET->new(Proto     => "tcp",
                                    PeerAddr  => $host,
                                    PeerPort  => $port)
               || die "can't connect to port $port on $host: $!";

    $handle->autoflush(1);       # so output gets there right away
    print STDERR "[Connected to $host:$port]\n";

    # split the program into two processes, identical twins
    die "can't fork: $!" unless defined($kidpid = fork());

    # the if{} block runs only in the parent process
    if ($kidpid) {
        # copy the socket to standard output
        while (defined ($line = <$handle>)) {
            print STDOUT $line;
        }
        kill("TERM", $kidpid);   # send SIGTERM to child
    }
    # the else{} block runs only in the child process
    else {
        # copy standard input to the socket
        while (defined ($line = <STDIN>)) {
            print $handle $line;
        }
        exit(0);                # just in case
    }



The kill function in the parent’s if block is there to send a
signal to our child process, currently running in the else block,
as soon as the remote server has closed its end of the connection.



If the remote server sends data a byte at time, and you need that
data immediately without waiting for a newline (which might not happen),
you may wish to replace the while loop in the parent with the
following:



    my $byte;
    while (sysread($handle, $byte, 1) == 1) {
        print STDOUT $byte;
    }



Making a system call for each byte you want to read is not very efficient
(to put it mildly) but is the simplest to explain and works reasonably
well.









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36.8 TCP Servers with IO::Socket



As always, setting up a server is little bit more involved than running a client.
The model is that the server creates a special kind of socket that
does nothing but listen on a particular port for incoming connections.
It does this by calling the IO::Socket::INET->new() method with
slightly different arguments than the client did.





Proto




This is which protocol to use.  Like our clients, we’ll
still specify "tcp" here.





LocalPort




We specify a local
port in the LocalPort argument, which we didn’t do for the client.
This is service name or port number for which you want to be the
server. (Under Unix, ports under 1024 are restricted to the
superuser.)  In our sample, we’ll use port 9000, but you can use
any port that’s not currently in use on your system.  If you try
to use one already in used, you’ll get an "Address already in use"
message.  Under Unix, the netstat -a command will show
which services current have servers.





Listen




The Listen parameter is set to the maximum number of
pending connections we can accept until we turn away incoming clients.
Think of it as a call-waiting queue for your telephone.
The low-level Socket module has a special symbol for the system maximum, which
is SOMAXCONN.





Reuse




The Reuse parameter is needed so that we restart our server
manually without waiting a few minutes to allow system buffers to
clear out.








Once the generic server socket has been created using the parameters
listed above, the server then waits for a new client to connect
to it.  The server blocks in the accept method, which eventually accepts a
bidirectional connection from the remote client.  (Make sure to autoflush
this handle to circumvent buffering.)



To add to user-friendliness, our server prompts the user for commands.
Most servers don’t do this.  Because of the prompt without a newline,
you’ll have to use the sysread variant of the interactive client above.



This server accepts one of five different commands, sending output back to
the client.  Unlike most network servers, this one handles only one
incoming client at a time.  Multithreaded servers are covered in 
Chapter 16 of the Camel.



Here’s the code.  We’ll



 #!/usr/bin/perl -w
 use IO::Socket;
 use Net::hostent;      # for OOish version of gethostbyaddr

 $PORT = 9000;          # pick something not in use

 $server = IO::Socket::INET->new( Proto     => "tcp",
                                  LocalPort => $PORT,
                                  Listen    => SOMAXCONN,
                                  Reuse     => 1);

 die "can't setup server" unless $server;
 print "[Server $0 accepting clients]\n";

 while ($client = $server->accept()) {
   $client->autoflush(1);
   print $client "Welcome to $0; type help for command list.\n";
   $hostinfo = gethostbyaddr($client->peeraddr);
   printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost;
   print $client "Command? ";
   while ( <$client>) {
     next unless /\S/;       # blank line
     if    (/quit|exit/i)    { last                                      }
     elsif (/date|time/i)    { printf $client "%s\n", scalar localtime() }
     elsif (/who/i )         { print  $client `who 2>&1`                 }
     elsif (/cookie/i )      { print  $client `/usr/games/fortune 2>&1`  }
     elsif (/motd/i )        { print  $client `cat /etc/motd 2>&1`       }
     else {
       print $client "Commands: quit date who cookie motd\n";
     }
   } continue {
      print $client "Command? ";
   }
   close $client;
 }









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36.9 UDP: Message Passing



Another kind of client-server setup is one that uses not connections, but
messages.  UDP communications involve much lower overhead but also provide
less reliability, as there are no promises that messages will arrive at
all, let alone in order and unmangled.  Still, UDP offers some advantages
over TCP, including being able to "broadcast" or "multicast" to a whole
bunch of destination hosts at once (usually on your local subnet).  If you
find yourself overly concerned about reliability and start building checks
into your message system, then you probably should use just TCP to start
with.



UDP datagrams are not a bytestream and should not be treated as such.
This makes using I/O mechanisms with internal buffering like stdio (i.e.
print() and friends) especially cumbersome. Use syswrite(), or better
send(), like in the example below.



Here’s a UDP program similar to the sample Internet TCP client given
earlier.  However, instead of checking one host at a time, the UDP version
will check many of them asynchronously by simulating a multicast and then
using select() to do a timed-out wait for I/O.  To do something similar
with TCP, you’d have to use a different socket handle for each host.



    #!/usr/bin/perl -w
    use strict;
    use Socket;
    use Sys::Hostname;

    my ( $count, $hisiaddr, $hispaddr, $histime,
         $host, $iaddr, $paddr, $port, $proto,
         $rin, $rout, $rtime, $SECS_OF_70_YEARS);

    $SECS_OF_70_YEARS = 2_208_988_800;

    $iaddr = gethostbyname(hostname());
    $proto = getprotobyname("udp");
    $port = getservbyname("time", "udp");
    $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick

    socket(SOCKET, PF_INET, SOCK_DGRAM, $proto)   || die "socket: $!";
    bind(SOCKET, $paddr)                          || die "bind: $!";

    $| = 1;
    printf "%-12s %8s %s\n",  "localhost", 0, scalar localtime();
    $count = 0;
    for $host (@ARGV) {
        $count++;
        $hisiaddr = inet_aton($host)              || die "unknown host";
        $hispaddr = sockaddr_in($port, $hisiaddr);
        defined(send(SOCKET, 0, 0, $hispaddr))    || die "send $host: $!";
    }

    $rin = "";
    vec($rin, fileno(SOCKET), 1) = 1;

    # timeout after 10.0 seconds
    while ($count && select($rout = $rin, undef, undef, 10.0)) {
        $rtime = "";
        $hispaddr = recv(SOCKET, $rtime, 4, 0)    || die "recv: $!";
        ($port, $hisiaddr) = sockaddr_in($hispaddr);
        $host = gethostbyaddr($hisiaddr, AF_INET);
        $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
        printf "%-12s ", $host;
        printf "%8d %s\n", $histime - time(), scalar localtime($histime);
        $count--;
    }



This example does not include any retries and may consequently fail to
contact a reachable host. The most prominent reason for this is congestion
of the queues on the sending host if the number of hosts to contact is
sufficiently large.









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36.10 SysV IPC



While System V IPC isn’t so widely used as sockets, it still has some
interesting uses.  However, you cannot use SysV IPC or Berkeley mmap() to
have a variable shared amongst several processes.  That’s because Perl
would reallocate your string when you weren’t wanting it to.  You might
look into the IPC::Shareable or threads::shared modules for that.



Here’s a small example showing shared memory usage.



    use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR);

    $size = 2000;
    $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR);
    defined($id)                    || die "shmget: $!";
    print "shm key $id\n";

    $message = "Message #1";
    shmwrite($id, $message, 0, 60)  || die "shmwrite: $!";
    print "wrote: '$message'\n";
    shmread($id, $buff, 0, 60)      || die "shmread: $!";
    print "read : '$buff'\n";

    # the buffer of shmread is zero-character end-padded.
    substr($buff, index($buff, "\0")) = "";
    print "un" unless $buff eq $message;
    print "swell\n";

    print "deleting shm $id\n";
    shmctl($id, IPC_RMID, 0)        || die "shmctl: $!";



Here’s an example of a semaphore:



    use IPC::SysV qw(IPC_CREAT);

    $IPC_KEY = 1234;
    $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT);
    defined($id)                    || die "shmget: $!";
    print "shm key $id\n";



Put this code in a separate file to be run in more than one process.
Call the file take:



    # create a semaphore

    $IPC_KEY = 1234;
    $id = semget($IPC_KEY, 0, 0);
    defined($id)                    || die "shmget: $!";

    $semnum  = 0;
    $semflag = 0;

    # "take" semaphore
    # wait for semaphore to be zero
    $semop = 0;
    $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);

    # Increment the semaphore count
    $semop = 1;
    $opstring2 = pack("s!s!s!", $semnum, $semop,  $semflag);
    $opstring  = $opstring1 . $opstring2;

    semop($id, $opstring)   || die "semop: $!";



Put this code in a separate file to be run in more than one process.
Call this file give:



    # "give" the semaphore
    # run this in the original process and you will see
    # that the second process continues

    $IPC_KEY = 1234;
    $id = semget($IPC_KEY, 0, 0);
    die unless defined($id);

    $semnum  = 0;
    $semflag = 0;

    # Decrement the semaphore count
    $semop = -1;
    $opstring = pack("s!s!s!", $semnum, $semop, $semflag);

    semop($id, $opstring)   || die "semop: $!";



The SysV IPC code above was written long ago, and it’s definitely
clunky looking.  For a more modern look, see the IPC::SysV module
which is included with Perl starting from Perl 5.005.



A small example demonstrating SysV message queues:



    use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR);

    my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR);
    defined($id)                || die "msgget failed: $!";

    my $sent      = "message";
    my $type_sent = 1234;

    msgsnd($id, pack("l! a*", $type_sent, $sent), 0)
                                || die "msgsnd failed: $!";

    msgrcv($id, my $rcvd_buf, 60, 0, 0)
                                || die "msgrcv failed: $!";

    my($type_rcvd, $rcvd) = unpack("l! a*", $rcvd_buf);

    if ($rcvd eq $sent) {
        print "okay\n";
    } else {
        print "not okay\n";
    }

    msgctl($id, IPC_RMID, 0)    || die "msgctl failed: $!\n";









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36.11 NOTES



Most of these routines quietly but politely return undef when they
fail instead of causing your program to die right then and there due to
an uncaught exception.  (Actually, some of the new Socket conversion
functions do croak() on bad arguments.)  It is therefore essential to
check return values from these functions.  Always begin your socket
programs this way for optimal success, and don’t forget to add the -T
taint-checking flag to the #! line for servers:



    #!/usr/bin/perl -Tw
    use strict;
    use sigtrap;
    use Socket;









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36.12 BUGS



These routines all create system-specific portability problems.  As noted
elsewhere, Perl is at the mercy of your C libraries for much of its system
behavior.  It’s probably safest to assume broken SysV semantics for
signals and to stick with simple TCP and UDP socket operations; e.g., don’t
try to pass open file descriptors over a local UDP datagram socket if you
want your code to stand a chance of being portable.









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36.13 AUTHOR



Tom Christiansen, with occasional vestiges of Larry Wall’s original
version and suggestions from the Perl Porters.









Previous: perlipc AUTHOR, Up: perlipc   [Contents]





36.14 SEE ALSO



There’s a lot more to networking than this, but this should get you
started.



For intrepid programmers, the indispensable textbook is Unix Network
Programming, 2nd Edition, Volume 1 by W. Richard Stevens (published by
Prentice-Hall).  Most books on networking address the subject from the
perspective of a C programmer; translation to Perl is left as an exercise
for the reader.



The IO::Socket(3) manpage describes the object library, and the Socket(3)
manpage describes the low-level interface to sockets.  Besides the obvious
functions in (perlfunc), you should also check out the modules file at
your nearest CPAN site, especially
http://www.cpan.org/modules/00modlist.long.html#ID5_Networking_.  
See (perlmodlib) or best yet, the Perl FAQ for a description
of what CPAN is and where to get it if the previous link doesn’t work 
for you.



Section 5 of CPAN’s modules file is devoted to "Networking, Device
Control (modems), and Interprocess Communication", and contains numerous
unbundled modules numerous networking modules, Chat and Expect operations,
CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet,
Threads, and ToolTalk–to name just a few.









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37 NAME warning, lexical warnings warning





• NAME warning, lexical warnings warning NAME:  



• NAME warning, lexical warnings warning DESCRIPTION:  



• NAME warning, lexical warnings warning SEE ALSO:  



• NAME warning, lexical warnings warning AUTHOR:  












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37.1 NAME






perllexwarn - Perl Lexical Warnings









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37.2 DESCRIPTION



The use warnings pragma enables to control precisely what warnings are
to be enabled in which parts of a Perl program. It’s a more flexible
alternative for both the command line flag -w and the equivalent Perl
variable, $^W.



This pragma works just like the strict pragma.
This means that the scope of the warning pragma is limited to the
enclosing block. It also means that the pragma setting will not
leak across files (via use, require or do). This allows
authors to independently define the degree of warning checks that will
be applied to their module.



By default, optional warnings are disabled, so any legacy code that
doesn’t attempt to control the warnings will work unchanged.



All warnings are enabled in a block by either of these:



    use warnings;
    use warnings 'all';



Similarly all warnings are disabled in a block by either of these:



    no warnings;
    no warnings 'all';



For example, consider the code below:



    use warnings;
    my @a;
    {
        no warnings;
        my $b = @a[0];
    }
    my $c = @a[0];



The code in the enclosing block has warnings enabled, but the inner
block has them disabled. In this case that means the assignment to the
scalar $c will trip the "Scalar value @a[0] better written as $a[0]"
warning, but the assignment to the scalar $b will not.





• NAME warning, lexical warnings warning Default Warnings and Optional Warnings:  



• NAME warning, lexical warnings warning What's wrong with -w and $^W:  



• NAME warning, lexical warnings warning Controlling Warnings from the Command Line:  



• NAME warning, lexical warnings warning Backward Compatibility:  



• NAME warning, lexical warnings warning Category Hierarchy:  



• NAME warning, lexical warnings warning Fatal Warnings:  



• NAME warning, lexical warnings warning Reporting Warnings from a Module:  












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37.2.1 Default Warnings and Optional Warnings



Before the introduction of lexical warnings, Perl had two classes of
warnings: mandatory and optional. 



As its name suggests, if your code tripped a mandatory warning, you
would get a warning whether you wanted it or not.
For example, the code below would always produce an "isn't numeric"
warning about the "2:".



    my $a = "2:" + 3;



With the introduction of lexical warnings, mandatory warnings now become
default warnings. The difference is that although the previously
mandatory warnings are still enabled by default, they can then be
subsequently enabled or disabled with the lexical warning pragma. For
example, in the code below, an "isn't numeric" warning will only
be reported for the $a variable.



    my $a = "2:" + 3;
    no warnings;
    my $b = "2:" + 3;



Note that neither the -w flag or the $^W can be used to
disable/enable default warnings. They are still mandatory in this case.









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37.2.2 What’s wrong with -w and $^W



Although very useful, the big problem with using -w on the command
line to enable warnings is that it is all or nothing. Take the typical
scenario when you are writing a Perl program. Parts of the code you
will write yourself, but it’s very likely that you will make use of
pre-written Perl modules. If you use the -w flag in this case, you
end up enabling warnings in pieces of code that you haven’t written.



Similarly, using $^W to either disable or enable blocks of code is
fundamentally flawed. For a start, say you want to disable warnings in
a block of code. You might expect this to be enough to do the trick:



     {
         local ($^W) = 0;
         my $a =+ 2;
         my $b; chop $b;
     }



When this code is run with the -w flag, a warning will be produced
for the $a line:  "Reversed += operator".



The problem is that Perl has both compile-time and run-time warnings. To
disable compile-time warnings you need to rewrite the code like this:



     {
         BEGIN { $^W = 0 }
         my $a =+ 2;
         my $b; chop $b;
     }



The other big problem with $^W is the way you can inadvertently
change the warning setting in unexpected places in your code. For example,
when the code below is run (without the -w flag), the second call
to doit will trip a "Use of uninitialized value" warning, whereas
the first will not.



    sub doit
    {
        my $b; chop $b;
    }

    doit();

    {
        local ($^W) = 1;
        doit()
    }



This is a side-effect of $^W being dynamically scoped.



Lexical warnings get around these limitations by allowing finer control
over where warnings can or can’t be tripped.









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37.2.3 Controlling Warnings from the Command Line



There are three Command Line flags that can be used to control when
warnings are (or aren’t) produced:





-w





This is  the existing flag. If the lexical warnings pragma is not
used in any of you code, or any of the modules that you use, this flag
will enable warnings everywhere. See Backward Compatibility for
details of how this flag interacts with lexical warnings.





-W





If the -W flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
locally using no warnings or $^W =0. This includes all files that get
included via use, require or do.
Think of it as the Perl equivalent of the "lint" command.





-X





Does the exact opposite to the -W flag, i.e. it disables all warnings.














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37.2.4 Backward Compatibility



If you are used to working with a version of Perl prior to the
introduction of lexically scoped warnings, or have code that uses both
lexical warnings and $^W, this section will describe how they interact.



How Lexical Warnings interact with -w/$^W:



    

  1.  If none of the three command line flags (-w, -W or -X) that
control warnings is used and neither $^W nor the warnings pragma
are used, then default warnings will be enabled and optional warnings
disabled.
This means that legacy code that doesn’t attempt to control the warnings
will work unchanged.


  2.  The -w flag just sets the global $^W variable as in 5.005. This
means that any legacy code that currently relies on manipulating $^W
to control warning behavior will still work as is. 


  3.  Apart from now being a boolean, the $^W variable operates in exactly
the same horrible uncontrolled global way, except that it cannot
disable/enable default warnings.


  4.  If a piece of code is under the control of the warnings pragma,
both the $^W variable and the -w flag will be ignored for the
scope of the lexical warning.


  5.  The only way to override a lexical warnings setting is with the -W
or -X command line flags.





The combined effect of 3 & 4 is that it will allow code which uses
the warnings pragma to control the warning behavior of $^W-type
code (using a local $^W=0) if it really wants to, but not vice-versa.









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37.2.5 Category Hierarchy




A hierarchy of "categories" have been defined to allow groups of warnings
to be enabled/disabled in isolation.



The current hierarchy is:



  all -+
       |
       +- closure
       |
       +- deprecated
       |
       +- exiting
       |
       +- glob
       |
       +- io -----------+
       |                |
       |                +- closed
       |                |
       |                +- exec
       |                |
       |                +- layer
       |                |
       |                +- newline
       |                |
       |                +- pipe
       |                |
       |                +- unopened
       |
       +- imprecision
       |
       +- misc
       |
       +- numeric
       |
       +- once
       |
       +- overflow
       |
       +- pack
       |
       +- portable
       |
       +- recursion
       |
       +- redefine
       |
       +- regexp
       |
       +- severe -------+
       |                |
       |                +- debugging
       |                |
       |                +- inplace
       |                |
       |                +- internal
       |                |
       |                +- malloc
       |
       +- signal
       |
       +- substr
       |
       +- syntax -------+
       |                |
       |                +- ambiguous
       |                |
       |                +- bareword
       |                |
       |                +- digit
       |                |
       |                +- illegalproto
       |                |
       |                +- parenthesis
       |                |
       |                +- precedence
       |                |
       |                +- printf
       |                |
       |                +- prototype
       |                |
       |                +- qw
       |                |
       |                +- reserved
       |                |
       |                +- semicolon
       |
       +- taint
       |
       +- threads
       |
       +- uninitialized
       |
       +- unpack
       |
       +- untie
       |
       +- utf8----------+
       |                |
       |                +- surrogate
       |                |
       |                +- non_unicode
       |                |
       |                +- nonchar
       |
       +- void



Just like the "strict" pragma any of these categories can be combined



    use warnings qw(void redefine);
    no warnings qw(io syntax untie);



Also like the "strict" pragma, if there is more than one instance of the
warnings pragma in a given scope the cumulative effect is additive. 



    use warnings qw(void); # only "void" warnings enabled
    ...
    use warnings qw(io);   # only "void" & "io" warnings enabled
    ...
    no warnings qw(void);  # only "io" warnings enabled



To determine which category a specific warning has been assigned to see
perldiag NAME.



Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
sub-category of the "syntax" category. It is now a top-level category
in its own right.









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37.2.6 Fatal Warnings




The presence of the word "FATAL" in the category list will escalate any
warnings detected from the categories specified in the lexical scope
into fatal errors. In the code below, the use of time, length
and join can all produce a "Useless use of xxx in void context"
warning.



    use warnings;

    time;

    {
        use warnings FATAL => qw(void);
        length "abc";
    }

    join "", 1,2,3;

    print "done\n";



When run it produces this output



    Useless use of time in void context at fatal line 3.
    Useless use of length in void context at fatal line 7.  



The scope where length is used has escalated the void warnings
category into a fatal error, so the program terminates immediately it
encounters the warning.



To explicitly turn off a "FATAL" warning you just disable the warning
it is associated with.  So, for example, to disable the "void" warning
in the example above, either of these will do the trick:



    no warnings qw(void);
    no warnings FATAL => qw(void);



If you want to downgrade a warning that has been escalated into a fatal
error back to a normal warning, you can use the "NONFATAL" keyword. For
example, the code below will promote all warnings into fatal errors,
except for those in the "syntax" category.



    use warnings FATAL => 'all', NONFATAL => 'syntax';









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37.2.7 Reporting Warnings from a Module





The warnings pragma provides a number of functions that are useful for
module authors. These are used when you want to report a module-specific
warning to a calling module has enabled warnings via the warnings
pragma.



Consider the module MyMod::Abc below.



    package MyMod::Abc;

    use warnings::register;

    sub open {
        my $path = shift;
        if ($path !~ m#^/#) {
            warnings::warn("changing relative path to /var/abc")
                if warnings::enabled();
            $path = "/var/abc/$path";
        }
    }

    1;



The call to warnings::register will create a new warnings category
called "MyMod::Abc", i.e. the new category name matches the current
package name. The open function in the module will display a warning
message if it gets given a relative path as a parameter. This warnings
will only be displayed if the code that uses MyMod::Abc has actually
enabled them with the warnings pragma like below.



    use MyMod::Abc;
    use warnings 'MyMod::Abc';
    ...
    abc::open("../fred.txt");



It is also possible to test whether the pre-defined warnings categories are
set in the calling module with the warnings::enabled function. Consider
this snippet of code:



    package MyMod::Abc;

    sub open {
        warnings::warnif("deprecated", 
                         "open is deprecated, use new instead");
        new(@_);
    }

    sub new
    ...
    1;



The function open has been deprecated, so code has been included to
display a warning message whenever the calling module has (at least) the
"deprecated" warnings category enabled. Something like this, say.



    use warnings 'deprecated';
    use MyMod::Abc;
    ...
    MyMod::Abc::open($filename);



Either the warnings::warn or warnings::warnif function should be
used to actually display the warnings message. This is because they can
make use of the feature that allows warnings to be escalated into fatal
errors. So in this case



    use MyMod::Abc;
    use warnings FATAL => 'MyMod::Abc';
    ...
    MyMod::Abc::open('../fred.txt');



the warnings::warnif function will detect this and die after
displaying the warning message.



The three warnings functions, warnings::warn, warnings::warnif
and warnings::enabled can optionally take an object reference in place
of a category name. In this case the functions will use the class name
of the object as the warnings category.



Consider this example:



    package Original;

    no warnings;
    use warnings::register;

    sub new
    {
        my $class = shift;
        bless [], $class;
    }

    sub check
    {
        my $self = shift;
        my $value = shift;

        if ($value % 2 && warnings::enabled($self))
          { warnings::warn($self, "Odd numbers are unsafe") }
    }

    sub doit
    {
        my $self = shift;
        my $value = shift;
        $self->check($value);
        # ...
    }

    1;

    package Derived;

    use warnings::register;
    use Original;
    our @ISA = qw( Original );
    sub new
    {
        my $class = shift;
        bless [], $class;
    }


    1;



The code below makes use of both modules, but it only enables warnings from 
Derived.



    use Original;
    use Derived;
    use warnings 'Derived';
    my $a = Original->new();
    $a->doit(1);
    my $b = Derived->new();
    $a->doit(1);



When this code is run only the Derived object, $b, will generate
a warning. 



    Odd numbers are unsafe at main.pl line 7



Notice also that the warning is reported at the line where the object is first
used.



When registering new categories of warning, you can supply more names to
warnings::register like this:



    package MyModule;
    use warnings::register qw(format precision);

    ...

    warnings::warnif('MyModule::format', '...');









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37.3 SEE ALSO



(warnings), perldiag NAME.









Previous: NAME warning, lexical warnings warning SEE ALSO, Up: NAME warning, lexical warnings warning   [Contents]





37.4 AUTHOR



Paul Marquess









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38 perllocale





• perllocale NAME:  



• perllocale DESCRIPTION:  



• perllocale WHAT IS A LOCALE:  



• perllocale PREPARING TO USE LOCALES:  



• perllocale USING LOCALES:  



• perllocale LOCALE CATEGORIES:  



• perllocale SECURITY:  



• perllocale ENVIRONMENT:  



• perllocale NOTES:  



• perllocale Unicode and UTF-8:  



• perllocale BUGS:  



• perllocale SEE ALSO:  



• perllocale HISTORY:  












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38.1 NAME



perllocale - Perl locale handling (internationalization and localization)









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38.2 DESCRIPTION



In the beginning there was ASCII, the "American Standard Code for
Information Interchange", which works quite well for Americans with
their English alphabet and dollar-denominated currency.  But it doesn’t
work so well even for other English speakers, who may use different
currencies, such as the pound sterling (as the symbol for that currency
is not in ASCII); and it’s hopelessly inadequate for many of the
thousands of the world’s other languages.



To address these deficiencies, the concept of locales was invented
(formally the ISO C, XPG4, POSIX 1.c "locale system").  And applications
were and are being written that use the locale mechanism.  The process of
making such an application take account of its users’ preferences in
these kinds of matters is called internationalization (often
abbreviated as i18n); telling such an application about a particular
set of preferences is known as localization (l10n).



Perl was extended, starting in 5.004, to support the locale system.  This
is controlled per application by using one pragma, one function call,
and several environment variables.



Unfortunately, there are quite a few deficiencies with the design (and
often, the implementations) of locales, and their use for character sets
has mostly been supplanted by Unicode (see perlunitut NAME for an
introduction to that, and keep on reading here for how Unicode interacts
with locales in Perl).



Perl continues to support the old locale system, and starting in v5.16,
provides a hybrid way to use the Unicode character set, along with the
other portions of locales that may not be so problematic.
(Unicode is also creating CLDR, the "Common Locale Data Repository",
http://cldr.unicode.org/ which includes more types of information than
are available in the POSIX locale system.  At the time of this writing,
there was no CPAN module that provides access to this XML-encoded data.
However, many of its locales have the POSIX-only data extracted, and are
available at http://unicode.org/Public/cldr/latest/.)









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38.3 WHAT IS A LOCALE



A locale is a set of data that describes various aspects of how various
communities in the world categorize their world.  These categories are
broken down into the following types (some of which include a brief
note here):





Category LC_NUMERIC: Numeric formatting




This indicates how numbers should be formatted for human readability,
for example the character used as the decimal point.





Category LC_MONETARY: Formatting of monetary amounts




 





Category LC_TIME: Date/Time formatting




 





Category LC_MESSAGES: Error and other messages




This for the most part is beyond the scope of Perl





Category LC_COLLATE: Collation




This indicates the ordering of letters for comparision and sorting.
In Latin alphabets, for example, "b", generally follows "a".





Category LC_CTYPE: Character Types




This indicates, for example if a character is an uppercase letter.








More details on the categories are given below in LOCALE CATEGORIES.



Together, these categories go a long way towards being able to customize
a single program to run in many different locations.  But there are
deficiencies, so keep reading.









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38.4 PREPARING TO USE LOCALES



Perl will not use locales unless specifically requested to (see NOTES below
for the partial exception of write()).  But even if there is such a
request, all of the following must be true for it to work properly:





If you want a Perl application to process and present your data
according to a particular locale, the application code should include
the use locale pragma (see The use locale pragma) where
appropriate, and at least one of the following must be true:



    

  1.  The locale-determining environment variables (see ENVIRONMENT)
must be correctly set up at the time the application is started, either
by yourself or by whomever set up your system account; or


  2.  The application must set its own locale using the method described in
The setlocale function.











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38.5 USING LOCALES





• perllocale The use locale pragma:  



• perllocale The setlocale function:  



• perllocale Finding locales:  



• perllocale LOCALE PROBLEMS:  



• perllocale Temporarily fixing locale problems:  



• perllocale Permanently fixing locale problems:  



• perllocale Permanently fixing your system's locale configuration:  



• perllocale Fixing system locale configuration:  



• perllocale The localeconv function:  



• perllocale I18N::Langinfo:  












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38.5.1 The use locale pragma



By default, Perl ignores the current locale.  The use locale
pragma tells Perl to use the current locale for some operations.
Starting in v5.16, there is an optional parameter to this pragma:



    use locale ':not_characters';



This parameter allows better mixing of locales and Unicode, and is
described fully in Unicode and UTF-8, but briefly, it tells Perl to
not use the character portions of the locale definition, that is
the LC_CTYPE and LC_COLLATE categories.  Instead it will use the
native (extended by Unicode) character set.  When using this parameter,
you are responsible for getting the external character set translated
into the native/Unicode one (which it already will be if it is one of
the increasingly popular UTF-8 locales).  There are convenient ways of
doing this, as described in Unicode and UTF-8.



The current locale is set at execution time by
setlocale() described below.  If that function
hasn’t yet been called in the course of the program’s execution, the
current locale is that which was determined by the ENVIRONMENT in
effect at the start of the program, except that
LC_NUMERIC is always
initialized to the C locale (mentioned under Finding locales).
If there is no valid environment, the current locale is undefined.  It
is likely, but not necessarily, the "C" locale.



The operations that are affected by locale are:





Under use locale ':not_characters';






 





Under just plain use locale;




The above operations are affected, as well as the following:










The default behavior is restored with the no locale pragma, or
upon reaching the end of the block enclosing use locale.
Note that use locale and use locale ':not_characters' may be
nested, and that what is in effect within an inner scope will revert to
the outer scope’s rules at the end of the inner scope.



The string result of any operation that uses locale
information is tainted, as it is possible for a locale to be
untrustworthy.  See SECURITY.









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38.5.2 The setlocale function



You can switch locales as often as you wish at run time with the
POSIX::setlocale() function:



        # This functionality not usable prior to Perl 5.004
        require 5.004;

        # Import locale-handling tool set from POSIX module.
        # This example uses: setlocale -- the function call
        #                    LC_CTYPE -- explained below
        use POSIX qw(locale_h);

        # query and save the old locale
        $old_locale = setlocale(LC_CTYPE);

        setlocale(LC_CTYPE, "fr_CA.ISO8859-1");
        # LC_CTYPE now in locale "French, Canada, codeset ISO 8859-1"

        setlocale(LC_CTYPE, "");
        # LC_CTYPE now reset to default defined by LC_ALL/LC_CTYPE/LANG
        # environment variables.  See below for documentation.

        # restore the old locale
        setlocale(LC_CTYPE, $old_locale);



The first argument of setlocale() gives the category, the second the
locale.  The category tells in what aspect of data processing you
want to apply locale-specific rules.  Category names are discussed in
LOCALE CATEGORIES and ENVIRONMENT.  The locale is the name of a
collection of customization information corresponding to a particular
combination of language, country or territory, and codeset.  Read on for
hints on the naming of locales: not all systems name locales as in the
example.



If no second argument is provided and the category is something else
than LC_ALL, the function returns a string naming the current locale
for the category.  You can use this value as the second argument in a
subsequent call to setlocale().



If no second argument is provided and the category is LC_ALL, the
result is implementation-dependent.  It may be a string of
concatenated locale names (separator also implementation-dependent)
or a single locale name.  Please consult your setlocale(3) man page for
details.



If a second argument is given and it corresponds to a valid locale,
the locale for the category is set to that value, and the function
returns the now-current locale value.  You can then use this in yet
another call to setlocale().  (In some implementations, the return
value may sometimes differ from the value you gave as the second
argument–think of it as an alias for the value you gave.)



As the example shows, if the second argument is an empty string, the
category’s locale is returned to the default specified by the
corresponding environment variables.  Generally, this results in a
return to the default that was in force when Perl started up: changes
to the environment made by the application after startup may or may not
be noticed, depending on your system’s C library.



If the second argument does not correspond to a valid locale, the locale
for the category is not changed, and the function returns undef.



Note that Perl ignores the current LC_CTYPE and LC_COLLATE locales
within the scope of a use locale ':not_characters'.



For further information about the categories, consult setlocale(3).









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38.5.3 Finding locales



For locales available in your system, consult also setlocale(3) to
see whether it leads to the list of available locales (search for the
SEE ALSO section).  If that fails, try the following command lines:



        locale -a

        nlsinfo

        ls /usr/lib/nls/loc

        ls /usr/lib/locale

        ls /usr/lib/nls

        ls /usr/share/locale



and see whether they list something resembling these



        en_US.ISO8859-1     de_DE.ISO8859-1     ru_RU.ISO8859-5
        en_US.iso88591      de_DE.iso88591      ru_RU.iso88595
        en_US               de_DE               ru_RU
        en                  de                  ru
        english             german              russian
        english.iso88591    german.iso88591     russian.iso88595
        english.roman8                          russian.koi8r



Sadly, even though the calling interface for setlocale() has been
standardized, names of locales and the directories where the
configuration resides have not been.  The basic form of the name is
language_territory.codeset, but the latter parts after
language are not always present.  The language and country
are usually from the standards ISO 3166 and ISO 639, the
two-letter abbreviations for the countries and the languages of the
world, respectively.  The codeset part often mentions some ISO
8859 character set, the Latin codesets.  For example, ISO 8859-1
is the so-called "Western European codeset" that can be used to encode
most Western European languages adequately.  Again, there are several
ways to write even the name of that one standard.  Lamentably.



Two special locales are worth particular mention: "C" and "POSIX".
Currently these are effectively the same locale: the difference is
mainly that the first one is defined by the C standard, the second by
the POSIX standard.  They define the default locale in which
every program starts in the absence of locale information in its
environment.  (The default default locale, if you will.)  Its language
is (American) English and its character codeset ASCII.
Warning. The C locale delivered by some vendors may not
actually exactly match what the C standard calls for.  So beware.



NOTE: Not all systems have the "POSIX" locale (not all systems are
POSIX-conformant), so use "C" when you need explicitly to specify this
default locale.









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38.5.4 LOCALE PROBLEMS



You may encounter the following warning message at Perl startup:



        perl: warning: Setting locale failed.
        perl: warning: Please check that your locale settings:
                LC_ALL = "En_US",
                LANG = (unset)
            are supported and installed on your system.
        perl: warning: Falling back to the standard locale ("C").



This means that your locale settings had LC_ALL set to "En_US" and
LANG exists but has no value.  Perl tried to believe you but could not.
Instead, Perl gave up and fell back to the "C" locale, the default locale
that is supposed to work no matter what.  This usually means your locale
settings were wrong, they mention locales your system has never heard
of, or the locale installation in your system has problems (for example,
some system files are broken or missing).  There are quick and temporary
fixes to these problems, as well as more thorough and lasting fixes.









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38.5.5 Temporarily fixing locale problems



The two quickest fixes are either to render Perl silent about any
locale inconsistencies or to run Perl under the default locale "C".



Perl’s moaning about locale problems can be silenced by setting the
environment variable PERL_BADLANG to a zero value, for example "0".
This method really just sweeps the problem under the carpet: you tell
Perl to shut up even when Perl sees that something is wrong.  Do not
be surprised if later something locale-dependent misbehaves.



Perl can be run under the "C" locale by setting the environment
variable LC_ALL to "C".  This method is perhaps a bit more civilized
than the PERL_BADLANG approach, but setting LC_ALL (or
other locale variables) may affect other programs as well, not just
Perl.  In particular, external programs run from within Perl will see
these changes.  If you make the new settings permanent (read on), all
programs you run see the changes.  See ENVIRONMENT for
the full list of relevant environment variables and USING LOCALES
for their effects in Perl.  Effects in other programs are
easily deducible.  For example, the variable LC_COLLATE may well affect
your sort program (or whatever the program that arranges "records"
alphabetically in your system is called).



You can test out changing these variables temporarily, and if the
new settings seem to help, put those settings into your shell startup
files.  Consult your local documentation for the exact details.  For in
Bourne-like shells (sh, ksh, bash, zsh):



        LC_ALL=en_US.ISO8859-1
        export LC_ALL



This assumes that we saw the locale "en_US.ISO8859-1" using the commands
discussed above.  We decided to try that instead of the above faulty
locale "En_US"–and in Cshish shells (csh, tcsh)



        setenv LC_ALL en_US.ISO8859-1



or if you have the "env" application you can do in any shell



        env LC_ALL=en_US.ISO8859-1 perl ...



If you do not know what shell you have, consult your local
helpdesk or the equivalent.









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38.5.6 Permanently fixing locale problems



The slower but superior fixes are when you may be able to yourself
fix the misconfiguration of your own environment variables.  The
mis(sing)configuration of the whole system’s locales usually requires
the help of your friendly system administrator.



First, see earlier in this document about Finding locales.  That tells
how to find which locales are really supported–and more importantly,
installed–on your system.  In our example error message, environment
variables affecting the locale are listed in the order of decreasing
importance (and unset variables do not matter).  Therefore, having
LC_ALL set to "En_US" must have been the bad choice, as shown by the
error message.  First try fixing locale settings listed first.



Second, if using the listed commands you see something exactly
(prefix matches do not count and case usually counts) like "En_US"
without the quotes, then you should be okay because you are using a
locale name that should be installed and available in your system.
In this case, see Permanently fixing your system’s locale configuration.









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38.5.7 Permanently fixing your system’s locale configuration



This is when you see something like:



        perl: warning: Please check that your locale settings:
                LC_ALL = "En_US",
                LANG = (unset)
            are supported and installed on your system.



but then cannot see that "En_US" listed by the above-mentioned
commands.  You may see things like "en_US.ISO8859-1", but that isn’t
the same.  In this case, try running under a locale
that you can list and which somehow matches what you tried.  The
rules for matching locale names are a bit vague because
standardization is weak in this area.  See again the
Finding locales about general rules.









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38.5.8 Fixing system locale configuration



Contact a system administrator (preferably your own) and report the exact
error message you get, and ask them to read this same documentation you
are now reading.  They should be able to check whether there is something
wrong with the locale configuration of the system.  The Finding locales
section is unfortunately a bit vague about the exact commands and places
because these things are not that standardized.









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38.5.9 The localeconv function



The POSIX::localeconv() function allows you to get particulars of the
locale-dependent numeric formatting information specified by the current
LC_NUMERIC and LC_MONETARY locales.  (If you just want the name of
the current locale for a particular category, use POSIX::setlocale()
with a single parameter–see The setlocale function.)



        use POSIX qw(locale_h);

        # Get a reference to a hash of locale-dependent info
        $locale_values = localeconv();

        # Output sorted list of the values
        for (sort keys %$locale_values) {
            printf "%-20s = %s\n", $_, $locale_values->{$_}
        }



localeconv() takes no arguments, and returns a reference to a hash.
The keys of this hash are variable names for formatting, such as
decimal_point and thousands_sep.  The values are the
corresponding, er, values.  See (POSIX)localeconv for a longer
example listing the categories an implementation might be expected to
provide; some provide more and others fewer.  You don’t need an
explicit use locale, because localeconv() always observes the
current locale.



Here’s a simple-minded example program that rewrites its command-line
parameters as integers correctly formatted in the current locale:



    # See comments in previous example
    require 5.004;
    use POSIX qw(locale_h);

    # Get some of locale's numeric formatting parameters
    my ($thousands_sep, $grouping) =
            @{localeconv()}{'thousands_sep', 'grouping'};

    # Apply defaults if values are missing
    $thousands_sep = ',' unless $thousands_sep;

    # grouping and mon_grouping are packed lists
    # of small integers (characters) telling the
    # grouping (thousand_seps and mon_thousand_seps
    # being the group dividers) of numbers and
    # monetary quantities.  The integers' meanings:
    # 255 means no more grouping, 0 means repeat
    # the previous grouping, 1-254 means use that
    # as the current grouping.  Grouping goes from
    # right to left (low to high digits).  In the
    # below we cheat slightly by never using anything
    # else than the first grouping (whatever that is).
    if ($grouping) {
        @grouping = unpack("C*", $grouping);
    } else {
        @grouping = (3);
    }

    # Format command line params for current locale
    for (@ARGV) {
        $_ = int;    # Chop non-integer part
        1 while
        s/(\d)(\d{$grouping[0]}($|$thousands_sep))/$1$thousands_sep$2/;
        print "$_";
    }
    print "\n";









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38.5.10 I18N::Langinfo



Another interface for querying locale-dependent information is the
I18N::Langinfo::langinfo() function, available at least in Unix-like
systems and VMS.



The following example will import the langinfo() function itself and
three constants to be used as arguments to langinfo(): a constant for
the abbreviated first day of the week (the numbering starts from
Sunday = 1) and two more constants for the affirmative and negative
answers for a yes/no question in the current locale.



    use I18N::Langinfo qw(langinfo ABDAY_1 YESSTR NOSTR);

    my ($abday_1, $yesstr, $nostr)
                = map { langinfo } qw(ABDAY_1 YESSTR NOSTR);

    print "$abday_1? [$yesstr/$nostr] ";



In other words, in the "C" (or English) locale the above will probably
print something like:



    Sun? [yes/no]



See (I18N-Langinfo) for more information.









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38.6 LOCALE CATEGORIES



The following subsections describe basic locale categories.  Beyond these,
some combination categories allow manipulation of more than one
basic category at a time.  See ENVIRONMENT for a discussion of these.





• perllocale Category LC_COLLATE: Collation 1:  



• perllocale Category LC_CTYPE: Character Types 1:  



• perllocale Category LC_NUMERIC: Numeric Formatting:  



• perllocale Category LC_MONETARY: Formatting of monetary amounts 1:  



• perllocale LC_TIME:  



• perllocale Other categories:  












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38.6.1 Category LC_COLLATE: Collation



In the scope of use locale (but not a
use locale ':not_characters'), Perl looks to the LC_COLLATE
environment variable to determine the application’s notions on collation
(ordering) of characters.  For example, "b" follows "a" in Latin
alphabets, but where do "á" and "å" belong?  And while
"color" follows "chocolate" in English, what about in Spanish?



The following collations all make sense and you may meet any of them
if you "use locale".



        A B C D E a b c d e
        A a B b C c D d E e
        a A b B c C d D e E
        a b c d e A B C D E



Here is a code snippet to tell what "word"
characters are in the current locale, in that locale’s order:



        use locale;
        print +(sort grep /\w/, map { chr } 0..255), "\n";



Compare this with the characters that you see and their order if you
state explicitly that the locale should be ignored:



        no locale;
        print +(sort grep /\w/, map { chr } 0..255), "\n";



This machine-native collation (which is what you get unless use locale has appeared earlier in the same block) must be used for
sorting raw binary data, whereas the locale-dependent collation of the
first example is useful for natural text.



As noted in USING LOCALES, cmp compares according to the current
collation locale when use locale is in effect, but falls back to a
char-by-char comparison for strings that the locale says are equal. You
can use POSIX::strcoll() if you don’t want this fall-back:



        use POSIX qw(strcoll);
        $equal_in_locale =
            !strcoll("space and case ignored", "SpaceAndCaseIgnored");



$equal_in_locale will be true if the collation locale specifies a
dictionary-like ordering that ignores space characters completely and
which folds case.



If you have a single string that you want to check for "equality in
locale" against several others, you might think you could gain a little
efficiency by using POSIX::strxfrm() in conjunction with eq:



        use POSIX qw(strxfrm);
        $xfrm_string = strxfrm("Mixed-case string");
        print "locale collation ignores spaces\n"
            if $xfrm_string eq strxfrm("Mixed-casestring");
        print "locale collation ignores hyphens\n"
            if $xfrm_string eq strxfrm("Mixedcase string");
        print "locale collation ignores case\n"
            if $xfrm_string eq strxfrm("mixed-case string");



strxfrm() takes a string and maps it into a transformed string for use
in char-by-char comparisons against other transformed strings during
collation.  "Under the hood", locale-affected Perl comparison operators
call strxfrm() for both operands, then do a char-by-char
comparison of the transformed strings.  By calling strxfrm() explicitly
and using a non locale-affected comparison, the example attempts to save
a couple of transformations.  But in fact, it doesn’t save anything: Perl
magic (see perlguts Magic Variables) creates the transformed version of a
string the first time it’s needed in a comparison, then keeps this version around
in case it’s needed again.  An example rewritten the easy way with
cmp runs just about as fast.  It also copes with null characters
embedded in strings; if you call strxfrm() directly, it treats the first
null it finds as a terminator.  don’t expect the transformed strings
it produces to be portable across systems–or even from one revision
of your operating system to the next.  In short, don’t call strxfrm()
directly: let Perl do it for you.



Note: use locale isn’t shown in some of these examples because it isn’t
needed: strcoll() and strxfrm() exist only to generate locale-dependent
results, and so always obey the current LC_COLLATE locale.









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38.6.2 Category LC_CTYPE: Character Types



In the scope of use locale (but not a
use locale ':not_characters'), Perl obeys the LC_CTYPE locale
setting.  This controls the application’s notion of which characters are
alphabetic.  This affects Perl’s \w regular expression metanotation,
which stands for alphanumeric characters–that is, alphabetic,
numeric, and including other special characters such as the underscore or
hyphen.  (Consult (perlre) for more information about
regular expressions.)  Thanks to LC_CTYPE, depending on your locale
setting, characters like "æ", "ð", "ß", and
"ø" may be understood as \w characters.



The LC_CTYPE locale also provides the map used in transliterating
characters between lower and uppercase.  This affects the case-mapping
functions–lc(), lcfirst, uc(), and ucfirst(); case-mapping
interpolation with \l, \L, \u, or \U in double-quoted strings
and s/// substitutions; and case-independent regular expression
pattern matching using the i modifier.



Finally, LC_CTYPE affects the POSIX character-class test
functions–isalpha(), islower(), and so on.  For example, if you move
from the "C" locale to a 7-bit Scandinavian one, you may find–possibly
to your surprise–that "|" moves from the ispunct() class to isalpha().
Unfortunately, this creates big problems for regular expressions. "|" still
means alternation even though it matches \w.



Note: A broken or malicious LC_CTYPE locale definition may result
in clearly ineligible characters being considered to be alphanumeric by
your application.  For strict matching of (mundane) ASCII letters and
digits–for example, in command strings–locale-aware applications
should use \w with the /a regular expression modifier.  See SECURITY.









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38.6.3 Category LC_NUMERIC: Numeric Formatting



After a proper POSIX::setlocale() call, Perl obeys the LC_NUMERIC
locale information, which controls an application’s idea of how numbers
should be formatted for human readability by the printf(), sprintf(), and
write() functions. String-to-numeric conversion by the POSIX::strtod()
function is also affected.  In most implementations the only effect is to
change the character used for the decimal point–perhaps from "."  to ",".
These functions aren’t aware of such niceties as thousands separation and
so on. (See The localeconv function if you care about these things.)



Output produced by print() is also affected by the current locale: it
corresponds to what you’d get from printf() in the "C" locale.  The
same is true for Perl’s internal conversions between numeric and
string formats:



        use POSIX qw(strtod setlocale LC_NUMERIC);

        setlocale LC_NUMERIC, "";

        $n = 5/2;   # Assign numeric 2.5 to $n

        $a = " $n"; # Locale-dependent conversion to string

        print "half five is $n\n";       # Locale-dependent output

        printf "half five is %g\n", $n;  # Locale-dependent output

        print "DECIMAL POINT IS COMMA\n"
            if $n == (strtod("2,5"))[0]; # Locale-dependent conversion



See also (I18N-Langinfo) and RADIXCHAR.









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38.6.4 Category LC_MONETARY: Formatting of monetary amounts



The C standard defines the LC_MONETARY category, but not a function
that is affected by its contents.  (Those with experience of standards
committees will recognize that the working group decided to punt on the
issue.)  Consequently, Perl takes no notice of it.  If you really want
to use LC_MONETARY, you can query its contents–see
The localeconv function–and use the information that it returns in your
application’s own formatting of currency amounts.  However, you may well
find that the information, voluminous and complex though it may be, still
does not quite meet your requirements: currency formatting is a hard nut
to crack.



See also (I18N-Langinfo) and CRNCYSTR.









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38.6.5 LC_TIME



Output produced by POSIX::strftime(), which builds a formatted
human-readable date/time string, is affected by the current LC_TIME
locale.  Thus, in a French locale, the output produced by the %B
format element (full month name) for the first month of the year would
be "janvier".  Here’s how to get a list of long month names in the
current locale:



        use POSIX qw(strftime);
        for (0..11) {
            $long_month_name[$_] =
                strftime("%B", 0, 0, 0, 1, $_, 96);
        }



Note: use locale isn’t needed in this example: as a function that
exists only to generate locale-dependent results, strftime() always
obeys the current LC_TIME locale.



See also (I18N-Langinfo) and ABDAY_1..ABDAY_7, DAY_1..DAY_7,
ABMON_1..ABMON_12, and ABMON_1..ABMON_12.









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38.6.6 Other categories



The remaining locale category, LC_MESSAGES (possibly supplemented
by others in particular implementations) is not currently used by
Perl–except possibly to affect the behavior of library functions
called by extensions outside the standard Perl distribution and by the
operating system and its utilities.  Note especially that the string
value of $! and the error messages given by external utilities may
be changed by LC_MESSAGES.  If you want to have portable error
codes, use %!.  See (Errno).









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38.7 SECURITY



Although the main discussion of Perl security issues can be found in
perlsec NAME, a discussion of Perl’s locale handling would be incomplete
if it did not draw your attention to locale-dependent security issues.
Locales–particularly on systems that allow unprivileged users to
build their own locales–are untrustworthy.  A malicious (or just plain
broken) locale can make a locale-aware application give unexpected
results.  Here are a few possibilities:





Such dangers are not peculiar to the locale system: any aspect of an
application’s environment which may be modified maliciously presents
similar challenges.  Similarly, they are not specific to Perl: any
programming language that allows you to write programs that take
account of their environment exposes you to these issues.



Perl cannot protect you from all possibilities shown in the
examples–there is no substitute for your own vigilance–but, when
use locale is in effect, Perl uses the tainting mechanism (see
perlsec NAME) to mark string results that become locale-dependent, and
which may be untrustworthy in consequence.  Here is a summary of the
tainting behavior of operators and functions that may be affected by
the locale:





Three examples illustrate locale-dependent tainting.
The first program, which ignores its locale, won’t run: a value taken
directly from the command line may not be used to name an output file
when taint checks are enabled.



        #/usr/local/bin/perl -T
        # Run with taint checking

        # Command line sanity check omitted...
        $tainted_output_file = shift;

        open(F, ">$tainted_output_file")
            or warn "Open of $untainted_output_file failed: $!\n";



The program can be made to run by "laundering" the tainted value through
a regular expression: the second example–which still ignores locale
information–runs, creating the file named on its command line
if it can.



        #/usr/local/bin/perl -T

        $tainted_output_file = shift;
        $tainted_output_file =~ m%[\w/]+%;
        $untainted_output_file = $&;

        open(F, ">$untainted_output_file")
            or warn "Open of $untainted_output_file failed: $!\n";



Compare this with a similar but locale-aware program:



        #/usr/local/bin/perl -T

        $tainted_output_file = shift;
        use locale;
        $tainted_output_file =~ m%[\w/]+%;
        $localized_output_file = $&;

        open(F, ">$localized_output_file")
            or warn "Open of $localized_output_file failed: $!\n";



This third program fails to run because $& is tainted: it is the result
of a match involving \w while use locale is in effect.









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38.8 ENVIRONMENT





PERL_BADLANG




A string that can suppress Perl’s warning about failed locale settings
at startup.  Failure can occur if the locale support in the operating
system is lacking (broken) in some way–or if you mistyped the name of
a locale when you set up your environment.  If this environment
variable is absent, or has a value that does not evaluate to integer
zero–that is, "0" or ""– Perl will complain about locale setting
failures.



NOTE: PERL_BADLANG only gives you a way to hide the warning message.
The message tells about some problem in your system’s locale support,
and you should investigate what the problem is.








The following environment variables are not specific to Perl: They are
part of the standardized (ISO C, XPG4, POSIX 1.c) setlocale() method
for controlling an application’s opinion on data.





LC_ALL




LC_ALL is the "override-all" locale environment variable. If
set, it overrides all the rest of the locale environment variables.





LANGUAGE




NOTE: LANGUAGE is a GNU extension, it affects you only if you
are using the GNU libc.  This is the case if you are using e.g. Linux.
If you are using "commercial" Unixes you are most probably not
using GNU libc and you can ignore LANGUAGE.



However, in the case you are using LANGUAGE: it affects the
language of informational, warning, and error messages output by
commands (in other words, it’s like LC_MESSAGES) but it has higher
priority than LC_ALL.  Moreover, it’s not a single value but
instead a "path" (":"-separated list) of languages (not locales).
See the GNU gettext library documentation for more information.





LC_CTYPE




In the absence of LC_ALL, LC_CTYPE chooses the character type
locale.  In the absence of both LC_ALL and LC_CTYPE, LANG
chooses the character type locale.





LC_COLLATE




In the absence of LC_ALL, LC_COLLATE chooses the collation
(sorting) locale.  In the absence of both LC_ALL and LC_COLLATE,
LANG chooses the collation locale.





LC_MONETARY




In the absence of LC_ALL, LC_MONETARY chooses the monetary
formatting locale.  In the absence of both LC_ALL and LC_MONETARY,
LANG chooses the monetary formatting locale.





LC_NUMERIC




In the absence of LC_ALL, LC_NUMERIC chooses the numeric format
locale.  In the absence of both LC_ALL and LC_NUMERIC, LANG
chooses the numeric format.





LC_TIME




In the absence of LC_ALL, LC_TIME chooses the date and time
formatting locale.  In the absence of both LC_ALL and LC_TIME,
LANG chooses the date and time formatting locale.





LANG




LANG is the "catch-all" locale environment variable. If it is set, it
is used as the last resort after the overall LC_ALL and the
category-specific LC_....










• perllocale Examples:  












Up: perllocale ENVIRONMENT   [Contents]





38.8.1 Examples



The LC_NUMERIC controls the numeric output:



   use locale;
   use POSIX qw(locale_h); # Imports setlocale() and the LC_ constants.
   setlocale(LC_NUMERIC, "fr_FR") or die "Pardon";
   printf "%g\n", 1.23; # If the "fr_FR" succeeded, probably shows 1,23.



and also how strings are parsed by POSIX::strtod() as numbers:



   use locale;
   use POSIX qw(locale_h strtod);
   setlocale(LC_NUMERIC, "de_DE") or die "Entschuldigung";
   my $x = strtod("2,34") + 5;
   print $x, "\n"; # Probably shows 7,34.









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38.9 NOTES





• perllocale Backward compatibility:  



• perllocale I18N:Collate obsolete:  



• perllocale Sort speed and memory use impacts:  



• perllocale write() and LC_NUMERIC:  



• perllocale Freely available locale definitions:  



• perllocale I18n and l10n:  



• perllocale An imperfect standard:  












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38.9.1 Backward compatibility



Versions of Perl prior to 5.004 mostly ignored locale information,
generally behaving as if something similar to the "C" locale were
always in force, even if the program environment suggested otherwise
(see The setlocale function).  By default, Perl still behaves this
way for backward compatibility.  If you want a Perl application to pay
attention to locale information, you must use the use locale
pragma (see The use locale pragma) or, in the unlikely event
that you want to do so for just pattern matching, the
/l regular expression modifier (see (perlre)Character set
modifiers) to instruct it to do so.



Versions of Perl from 5.002 to 5.003 did use the LC_CTYPE
information if available; that is, \w did understand what
were the letters according to the locale environment variables.
The problem was that the user had no control over the feature:
if the C library supported locales, Perl used them.









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38.9.2 I18N:Collate obsolete



In versions of Perl prior to 5.004, per-locale collation was possible
using the I18N::Collate library module.  This module is now mildly
obsolete and should be avoided in new applications.  The LC_COLLATE
functionality is now integrated into the Perl core language: One can
use locale-specific scalar data completely normally with use locale,
so there is no longer any need to juggle with the scalar references of
I18N::Collate.









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38.9.3 Sort speed and memory use impacts



Comparing and sorting by locale is usually slower than the default
sorting; slow-downs of two to four times have been observed.  It will
also consume more memory: once a Perl scalar variable has participated
in any string comparison or sorting operation obeying the locale
collation rules, it will take 3-15 times more memory than before.  (The
exact multiplier depends on the string’s contents, the operating system
and the locale.) These downsides are dictated more by the operating
system’s implementation of the locale system than by Perl.









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38.9.4 write() and LC_NUMERIC



If a program’s environment specifies an LC_NUMERIC locale and use
locale is in effect when the format is declared, the locale is used
to specify the decimal point character in formatted output.  Formatted
output cannot be controlled by use locale at the time when write()
is called.









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38.9.5 Freely available locale definitions



The Unicode CLDR project extracts the POSIX portion of many of its
locales, available at



  http://unicode.org/Public/cldr/latest/



There is a large collection of locale definitions at:



  http://std.dkuug.dk/i18n/WG15-collection/locales/



You should be aware that it is
unsupported, and is not claimed to be fit for any purpose.  If your
system allows installation of arbitrary locales, you may find the
definitions useful as they are, or as a basis for the development of
your own locales.









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38.9.6 I18n and l10n



"Internationalization" is often abbreviated as i18n because its first
and last letters are separated by eighteen others.  (You may guess why
the internalin ... internaliti ... i18n tends to get abbreviated.)  In
the same way, "localization" is often abbreviated to l10n.









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38.9.7 An imperfect standard



Internationalization, as defined in the C and POSIX standards, can be
criticized as incomplete, ungainly, and having too large a granularity.
(Locales apply to a whole process, when it would arguably be more useful
to have them apply to a single thread, window group, or whatever.)  They
also have a tendency, like standards groups, to divide the world into
nations, when we all know that the world can equally well be divided
into bankers, bikers, gamers, and so on.









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38.10 Unicode and UTF-8



The support of Unicode is new starting from Perl version v5.6, and more fully
implemented in version v5.8 and later.  See perluniintro NAME.  It is
strongly recommended that when combining Unicode and locale (starting in
v5.16), you use



    use locale ':not_characters';



When this form of the pragma is used, only the non-character portions of
locales are used by Perl, for example LC_NUMERIC.  Perl assumes that
you have translated all the characters it is to operate on into Unicode
(actually the platform’s native character set (ASCII or EBCDIC) plus
Unicode).  For data in files, this can conveniently be done by also
specifying



    use open ':locale';



This pragma arranges for all inputs from files to be translated into
Unicode from the current locale as specified in the environment (see
ENVIRONMENT), and all outputs to files to be translated back
into the locale.  (See (open)).  On a per-filehandle basis, you can
instead use the (PerlIO-locale) module, or the (Encode-Locale)
module, both available from CPAN.  The latter module also has methods to
ease the handling of ARGV and environment variables, and can be used
on individual strings.  Also, if you know that all your locales will be
UTF-8, as many are these days, you can use the ‘-C’
command line switch.



This form of the pragma allows essentially seamless handling of locales
with Unicode.  The collation order will be Unicode’s.  It is strongly
recommended that when you need to order and sort strings that you use
the standard module (Unicode-Collate) which gives much better results
in many instances than you can get with the old-style locale handling.



For pre-v5.16 Perls, or if you use the locale pragma without the
:not_characters parameter, Perl tries to work with both Unicode and
locales–but there are problems.



Perl does not handle multi-byte locales in this case, such as have been
used for various
Asian languages, such as Big5 or Shift JIS.  However, the increasingly
common multi-byte UTF-8 locales, if properly implemented, may work
reasonably well (depending on your C library implementation) in this
form of the locale pragma, simply because both
they and Perl store characters that take up multiple bytes the same way.
However, some, if not most, C library implementations may not process
the characters in the upper half of the Latin-1 range (128 - 255)
properly under LC_CTYPE.  To see if a character is a particular type
under a locale, Perl uses the functions like isalnum().  Your C
library may not work for UTF-8 locales with those functions, instead
only working under the newer wide library functions like iswalnum().



Perl generally takes the tack to use locale rules on code points that can fit
in a single byte, and Unicode rules for those that can’t (though this
isn’t uniformly applied, see the note at the end of this section).  This
prevents many problems in locales that aren’t UTF-8.  Suppose the locale
is ISO8859-7, Greek.  The character at 0xD7 there is a capital Chi. But
in the ISO8859-1 locale, Latin1, it is a multiplication sign.  The POSIX
regular expression character class [[:alpha:]] will magically match
0xD7 in the Greek locale but not in the Latin one.



However, there are places where this breaks down.  Certain constructs are
for Unicode only, such as \p{Alpha}.  They assume that 0xD7 always has its
Unicode meaning (or the equivalent on EBCDIC platforms).  Since Latin1 is a
subset of Unicode and 0xD7 is the multiplication sign in both Latin1 and
Unicode, \p{Alpha} will never match it, regardless of locale.  A similar
issue occurs with \N{...}.  It is therefore a bad idea to use \p{} or
\N{} under plain use locale–unless you can guarantee that the
locale will be a ISO8859-1.  Use POSIX character classes instead.



Another problem with this approach is that operations that cross the
single byte/multiple byte boundary are not well-defined, and so are
disallowed.  (This boundary is between the codepoints at 255/256.).
For example, lower casing LATIN CAPITAL LETTER Y WITH DIAERESIS (U+0178)
should return LATIN SMALL LETTER Y WITH DIAERESIS (U+00FF).  But in the
Greek locale, for example, there is no character at 0xFF, and Perl
has no way of knowing what the character at 0xFF is really supposed to
represent.  Thus it disallows the operation.  In this mode, the
lowercase of U+0178 is itself.



The same problems ensue if you enable automatic UTF-8-ification of your
standard file handles, default open() layer, and @ARGV on non-ISO8859-1,
non-UTF-8 locales (by using either the -C command line switch or the
PERL_UNICODE environment variable; see perlrun NAME).
Things are read in as UTF-8, which would normally imply a Unicode
interpretation, but the presence of a locale causes them to be interpreted
in that locale instead.  For example, a 0xD7 code point in the Unicode
input, which should mean the multiplication sign, won’t be interpreted by
Perl that way under the Greek locale.  This is not a problem
provided you make certain that all locales will always and only be either
an ISO8859-1, or, if you don’t have a deficient C library, a UTF-8 locale.



Vendor locales are notoriously buggy, and it is difficult for Perl to test
its locale-handling code because this interacts with code that Perl has no
control over; therefore the locale-handling code in Perl may be buggy as
well.  (However, the Unicode-supplied locales should be better, and
there is a feed back mechanism to correct any problems.  See
Freely available locale definitions.)



If you have Perl v5.16, the problems mentioned above go away if you use
the :not_characters parameter to the locale pragma (except for vendor
bugs in the non-character portions).  If you don’t have v5.16, and you
do have locales that work, using them may be worthwhile for certain
specific purposes, as long as you keep in mind the gotchas already
mentioned.  For example, if the collation for your locales works, it
runs faster under locales than under (Unicode-Collate); and you gain
access to such things as the local currency symbol and the names of the
months and days of the week.  (But to hammer home the point, in v5.16,
you get this access without the downsides of locales by using the
:not_characters form of the pragma.)



Note: The policy of using locale rules for code points that can fit in a
byte, and Unicode rules for those that can’t is not uniformly applied.
Pre-v5.12, it was somewhat haphazard; in v5.12 it was applied fairly
consistently to regular expression matching except for bracketed
character classes; in v5.14 it was extended to all regex matches; and in
v5.16 to the casing operations such as "\L" and uc().  For
collation, in all releases, the system’s strxfrm() function is called,
and whatever it does is what you get.









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38.11 BUGS





• perllocale Broken systems:  












Up: perllocale BUGS   [Contents]





38.11.1 Broken systems



In certain systems, the operating system’s locale support
is broken and cannot be fixed or used by Perl.  Such deficiencies can
and will result in mysterious hangs and/or Perl core dumps when
use locale is in effect.  When confronted with such a system,
please report in excruciating detail to <perlbug@perl.org>, and
also contact your vendor: bug fixes may exist for these problems
in your operating system.  Sometimes such bug fixes are called an
operating system upgrade.









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38.12 SEE ALSO



(I18N-Langinfo), perluniintro NAME, perlunicode NAME, (open),
(POSIX)isalnum, (POSIX)isalpha,
(POSIX)isdigit, (POSIX)isgraph, (POSIX)islower,
(POSIX)isprint, (POSIX)ispunct, (POSIX)isspace,
(POSIX)isupper, (POSIX)isxdigit, (POSIX)localeconv,
(POSIX)setlocale, (POSIX)strcoll, (POSIX)strftime,
(POSIX)strtod, (POSIX)strxfrm.









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38.13 HISTORY



Jarkko Hietaniemi’s original perli18n.pod heavily hacked by Dominic
Dunlop, assisted by the perl5-porters.  Prose worked over a bit by
Tom Christiansen, and updated by Perl 5 porters.









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39 perllol





• perllol NAME:  



• perllol DESCRIPTION:  



• perllol SEE ALSO:  



• perllol AUTHOR:  












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39.1 NAME



perllol - Manipulating Arrays of Arrays in Perl









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39.2 DESCRIPTION





• perllol Declaration and Access of Arrays of Arrays:  



• perllol Growing Your Own:  



• perllol Access and Printing:  



• perllol Slices:  












Next: , Up: perllol DESCRIPTION   [Contents]





39.2.1 Declaration and Access of Arrays of Arrays



The simplest two-level data structure to build in Perl is an array of
arrays, sometimes casually called a list of lists.  It’s reasonably easy to
understand, and almost everything that applies here will also be applicable
later on with the fancier data structures.



An array of an array is just a regular old array @AoA that you can
get at with two subscripts, like $AoA[3][2].  Here’s a declaration
of the array:



    use 5.010;  # so we can use say()

    # assign to our array, an array of array references
    @AoA = (
           [ "fred", "barney", "pebbles", "bambam", "dino", ],
           [ "george", "jane", "elroy", "judy", ],
           [ "homer", "bart", "marge", "maggie", ],
    );
    say $AoA[2][1];
  bart



Now you should be very careful that the outer bracket type
is a round one, that is, a parenthesis.  That’s because you’re assigning to
an @array, so you need parentheses.  If you wanted there not to be an @AoA,
but rather just a reference to it, you could do something more like this:



    # assign a reference to array of array references
    $ref_to_AoA = [
        [ "fred", "barney", "pebbles", "bambam", "dino", ],
        [ "george", "jane", "elroy", "judy", ],
        [ "homer", "bart", "marge", "maggie", ],
    ];
    say $ref_to_AoA->[2][1];
  bart



Notice that the outer bracket type has changed, and so our access syntax
has also changed.  That’s because unlike C, in perl you can’t freely
interchange arrays and references thereto.  $ref_to_AoA is a reference to an
array, whereas @AoA is an array proper.  Likewise, $AoA[2] is not an
array, but an array ref.  So how come you can write these:



    $AoA[2][2]
    $ref_to_AoA->[2][2]



instead of having to write these:



    $AoA[2]->[2]
    $ref_to_AoA->[2]->[2]



Well, that’s because the rule is that on adjacent brackets only (whether
square or curly), you are free to omit the pointer dereferencing arrow.
But you cannot do so for the very first one if it’s a scalar containing
a reference, which means that $ref_to_AoA always needs it.









Next: , Previous: perllol Declaration and Access of Arrays of Arrays, Up: perllol DESCRIPTION   [Contents]





39.2.2 Growing Your Own



That’s all well and good for declaration of a fixed data structure,
but what if you wanted to add new elements on the fly, or build
it up entirely from scratch?



First, let’s look at reading it in from a file.  This is something like
adding a row at a time.  We’ll assume that there’s a flat file in which
each line is a row and each word an element.  If you’re trying to develop an
@AoA array containing all these, here’s the right way to do that:



    while (<>) {
        @tmp = split;
        push @AoA, [ @tmp ];
    }



You might also have loaded that from a function:



    for $i ( 1 .. 10 ) {
        $AoA[$i] = [ somefunc($i) ];
    }



Or you might have had a temporary variable sitting around with the
array in it.



    for $i ( 1 .. 10 ) {
        @tmp = somefunc($i);
        $AoA[$i] = [ @tmp ];
    }



It’s important you make sure to use the [ ] array reference
constructor.  That’s because this wouldn’t work:



    $AoA[$i] = @tmp;   # WRONG!



The reason that doesn’t do what you want is because assigning a
named array like that to a scalar is taking an array in scalar
context, which means just counts the number of elements in @tmp.



If you are running under use strict (and if you aren’t, why in
the world aren’t you?), you’ll have to add some declarations to
make it happy:



    use strict;
    my(@AoA, @tmp);
    while (<>) {
        @tmp = split;
        push @AoA, [ @tmp ];
    }



Of course, you don’t need the temporary array to have a name at all:



    while (<>) {
        push @AoA, [ split ];
    }



You also don’t have to use push().  You could just make a direct assignment
if you knew where you wanted to put it:



    my (@AoA, $i, $line);
    for $i ( 0 .. 10 ) {
        $line = <>;
        $AoA[$i] = [ split " ", $line ];
    }



or even just



    my (@AoA, $i);
    for $i ( 0 .. 10 ) {
        $AoA[$i] = [ split " ", <> ];
    }



You should in general be leery of using functions that could
potentially return lists in scalar context without explicitly stating
such.  This would be clearer to the casual reader:



    my (@AoA, $i);
    for $i ( 0 .. 10 ) {
        $AoA[$i] = [ split " ", scalar(<>) ];
    }



If you wanted to have a $ref_to_AoA variable as a reference to an array,
you’d have to do something like this:



    while (<>) {
        push @$ref_to_AoA, [ split ];
    }



Now you can add new rows.  What about adding new columns?  If you’re
dealing with just matrices, it’s often easiest to use simple assignment:



    for $x (1 .. 10) {
        for $y (1 .. 10) {
            $AoA[$x][$y] = func($x, $y);
        }
    }

    for $x ( 3, 7, 9 ) {
        $AoA[$x][20] += func2($x);
    }



It doesn’t matter whether those elements are already
there or not: it’ll gladly create them for you, setting
intervening elements to undef as need be.



If you wanted just to append to a row, you’d have
to do something a bit funnier looking:



    # add new columns to an existing row
    push @{ $AoA[0] }, "wilma", "betty";   # explicit deref



Prior to Perl 5.14, this wouldn’t even compile:



    push $AoA[0], "wilma", "betty";        # implicit deref



How come?  Because once upon a time, the argument to push() had to be a
real array, not just a reference to one. That’s no longer true.  In fact,
the line marked "implicit deref" above works just fine–in this
instance–to do what the one that says explicit deref did.



The reason I said "in this instance" is because that only works
because $AoA[0] already held an array reference.  If you try that on an
undefined variable, you’ll take an exception.  That’s because the implicit
derefererence will never autovivify an undefined variable the way @{ }
always will:



    my $aref = undef;
    push $aref,  qw(some more values);  # WRONG!
    push @$aref, qw(a few more);        # ok



If you want to take advantage of this new implicit dereferencing behavior,
go right ahead: it makes code easier on the eye and wrist.  Just understand
that older releases will choke on it during compilation.  Whenever you make
use of something that works only in some given release of Perl and later,
but not earlier, you should place a prominent



    use v5.14;   # needed for implicit deref of array refs by array ops



directive at the top of the file that needs it.  That way when somebody
tries to run the new code under an old perl, rather than getting an error like



    Type of arg 1 to push must be array (not array element) at /tmp/a line 8, near ""betty";"
    Execution of /tmp/a aborted due to compilation errors.



they’ll be politely informed that



    Perl v5.14.0 required--this is only v5.12.3, stopped at /tmp/a line 1.
    BEGIN failed--compilation aborted at /tmp/a line 1.









Next: , Previous: perllol Growing Your Own, Up: perllol DESCRIPTION   [Contents]





39.2.3 Access and Printing



Now it’s time to print your data structure out.  How
are you going to do that?  Well, if you want only one
of the elements, it’s trivial:



    print $AoA[0][0];



If you want to print the whole thing, though, you can’t
say



    print @AoA;         # WRONG



because you’ll get just references listed, and perl will never
automatically dereference things for you.  Instead, you have to
roll yourself a loop or two.  This prints the whole structure,
using the shell-style for() construct to loop across the outer
set of subscripts.



    for $aref ( @AoA ) {
        say "\t [ @$aref ],";
    }



If you wanted to keep track of subscripts, you might do this:



    for $i ( 0 .. $#AoA ) {
        say "\t elt $i is [ @{$AoA[$i]} ],";
    }



or maybe even this.  Notice the inner loop.



    for $i ( 0 .. $#AoA ) {
        for $j ( 0 .. $#{$AoA[$i]} ) {
            say "elt $i $j is $AoA[$i][$j]";
        }
    }



As you can see, it’s getting a bit complicated.  That’s why
sometimes is easier to take a temporary on your way through:



    for $i ( 0 .. $#AoA ) {
        $aref = $AoA[$i];
        for $j ( 0 .. $#{$aref} ) {
            say "elt $i $j is $AoA[$i][$j]";
        }
    }



Hmm... that’s still a bit ugly.  How about this:



    for $i ( 0 .. $#AoA ) {
        $aref = $AoA[$i];
        $n = @$aref - 1;
        for $j ( 0 .. $n ) {
            say "elt $i $j is $AoA[$i][$j]";
        }
    }



When you get tired of writing a custom print for your data structures,
you might look at the standard (Dumpvalue) or (Data-Dumper) modules.
The former is what the Perl debugger uses, while the latter generates
parsable Perl code.  For example:



    use v5.14;     # using the + prototype, new to v5.14

    sub show(+) {
        require Dumpvalue;
        state $prettily = new Dumpvalue::
                            tick        => q("),
                            compactDump => 1,  # comment these two lines out
                            veryCompact => 1,  # if you want a bigger dump
                        ;
        dumpValue $prettily @_;
    }

    # Assign a list of array references to an array.
    my @AoA = (
           [ "fred", "barney" ],
           [ "george", "jane", "elroy" ],
           [ "homer", "marge", "bart" ],
    );
    push $AoA[0], "wilma", "betty";
    show @AoA;



will print out:



    0  0..3  "fred" "barney" "wilma" "betty"
    1  0..2  "george" "jane" "elroy"
    2  0..2  "homer" "marge" "bart"



Whereas if you comment out the two lines I said you might wish to,
then it shows it to you this way instead:



    0  ARRAY(0x8031d0)
       0  "fred"
       1  "barney"
       2  "wilma"
       3  "betty"
    1  ARRAY(0x803d40)
       0  "george"
       1  "jane"
       2  "elroy"
    2  ARRAY(0x803e10)
       0  "homer"
       1  "marge"
       2  "bart"









Previous: perllol Access and Printing, Up: perllol DESCRIPTION   [Contents]





39.2.4 Slices



If you want to get at a slice (part of a row) in a multidimensional
array, you’re going to have to do some fancy subscripting.  That’s
because while we have a nice synonym for single elements via the
pointer arrow for dereferencing, no such convenience exists for slices.



Here’s how to do one operation using a loop.  We’ll assume an @AoA
variable as before.



    @part = ();
    $x = 4;
    for ($y = 7; $y < 13; $y++) {
        push @part, $AoA[$x][$y];
    }



That same loop could be replaced with a slice operation:



    @part = @{$AoA[4]}[7..12];



or spaced out a bit:



    @part = @{ $AoA[4] } [ 7..12 ];



But as you might well imagine, this can get pretty rough on the reader.



Ah, but what if you wanted a two-dimensional slice, such as having
$x run from 4..8 and $y run from 7 to 12?  Hmm... here’s the simple way:



    @newAoA = ();
    for ($startx = $x = 4; $x <= 8; $x++) {
        for ($starty = $y = 7; $y <= 12; $y++) {
            $newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y];
        }
    }



We can reduce some of the looping through slices



    for ($x = 4; $x <= 8; $x++) {
        push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ];
    }



If you were into Schwartzian Transforms, you would probably
have selected map for that



    @newAoA = map { [ @{ $AoA[$_] } [ 7..12 ] ] } 4 .. 8;



Although if your manager accused you of seeking job security (or rapid
insecurity) through inscrutable code, it would be hard to argue. :-)
If I were you, I’d put that in a function:



    @newAoA = splice_2D( \@AoA, 4 => 8, 7 => 12 );
    sub splice_2D {
        my $lrr = shift;        # ref to array of array refs!
        my ($x_lo, $x_hi,
            $y_lo, $y_hi) = @_;

        return map {
            [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ]
        } $x_lo .. $x_hi;
    }









Next: , Previous: perllol DESCRIPTION, Up: perllol   [Contents]





39.3 SEE ALSO



perldata NAME, (perlref), (perldsc)









Previous: perllol SEE ALSO, Up: perllol   [Contents]





39.4 AUTHOR



Tom Christiansen <tchrist@perl.com>



Last update: Tue Apr 26 18:30:55 MDT 2011









Next: , Previous: perllol, Up: Top   [Contents]





40 perlmod





• perlmod NAME:  



• perlmod DESCRIPTION:  



• perlmod SEE ALSO:  












Next: , Up: perlmod   [Contents]





40.1 NAME



perlmod - Perl modules (packages and symbol tables)









Next: , Previous: perlmod NAME, Up: perlmod   [Contents]





40.2 DESCRIPTION





• perlmod Packages:  



• perlmod Symbol Tables:  



• perlmod BEGIN, UNITCHECK, CHECK, INIT and END:  



• perlmod Perl Classes:  



• perlmod Perl Modules:  



• perlmod Making your module threadsafe:  












Next: , Up: perlmod DESCRIPTION   [Contents]





40.2.1 Packages








Perl provides a mechanism for alternative namespaces to protect
packages from stomping on each other’s variables.  In fact, there’s
really no such thing as a global variable in Perl.  The package
statement declares the compilation unit as being in the given
namespace.  The scope of the package declaration is from the
declaration itself through the end of the enclosing block, eval,
or file, whichever comes first (the same scope as the my() and
local() operators).  Unqualified dynamic identifiers will be in
this namespace, except for those few identifiers that if unqualified,
default to the main package instead of the current one as described
below.  A package statement affects only dynamic variables–including
those you’ve used local() on–but not lexical variables created
with my().  Typically it would be the first declaration in a file
included by the do, require, or use operators.  You can
switch into a package in more than one place; it merely influences
which symbol table is used by the compiler for the rest of that
block.  You can refer to variables and filehandles in other packages
by prefixing the identifier with the package name and a double
colon: $Package::Variable.  If the package name is null, the
main package is assumed.  That is, $::sail is equivalent to
$main::sail.





The old package delimiter was a single quote, but double colon is now the
preferred delimiter, in part because it’s more readable to humans, and
in part because it’s more readable to emacs macros.  It also makes C++
programmers feel like they know what’s going on–as opposed to using the
single quote as separator, which was there to make Ada programmers feel
like they knew what was going on.  Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string like
"This is $owner's house", you’ll be accessing $owner::s; that is,
the $s variable in package owner, which is probably not what you meant.
Use braces to disambiguate, as in "This is ${owner}'s house".




Packages may themselves contain package separators, as in
$OUTER::INNER::var.  This implies nothing about the order of
name lookups, however.  There are no relative packages: all symbols
are either local to the current package, or must be fully qualified
from the outer package name down.  For instance, there is nowhere
within package OUTER that $INNER::var refers to
$OUTER::INNER::var.  INNER refers to a totally
separate global package.




Only identifiers starting with letters (or underscore) are stored
in a package’s symbol table.  All other symbols are kept in package
main, including all punctuation variables, like $_.  In addition,
when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
ARGVOUT, ENV, INC, and SIG are forced to be in package main,
even when used for other purposes than their built-in ones.  If you
have a package called m, s, or y, then you can’t use the
qualified form of an identifier because it would be instead interpreted
as a pattern match, a substitution, or a transliteration.




Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
However, variables and functions named with a single _, such as
$_ and sub _, are still forced into the package main.  See also
perlvar The Syntax of Variable Names.



evaled strings are compiled in the package in which the eval() was
compiled.  (Assignments to $SIG{}, however, assume the signal
handler specified is in the main package.  Qualify the signal handler
name if you wish to have a signal handler in a package.)  For an
example, examine perldb.pl in the Perl library.  It initially switches
to the DB package so that the debugger doesn’t interfere with variables
in the program you are trying to debug.  At various points, however, it
temporarily switches back to the main package to evaluate various
expressions in the context of the main package (or wherever you came
from).  See (perldebug).



The special symbol __PACKAGE__ contains the current package, but cannot
(easily) be used to construct variable names.



See (perlsub) for other scoping issues related to my() and local(),
and (perlref) regarding closures.









Next: , Previous: perlmod Packages, Up: perlmod DESCRIPTION   [Contents]





40.2.2 Symbol Tables










The symbol table for a package happens to be stored in the hash of that
name with two colons appended.  The main symbol table’s name is thus
%main::, or %:: for short.  Likewise the symbol table for the nested
package mentioned earlier is named %OUTER::INNER::.



The value in each entry of the hash is what you are referring to when you
use the *name typeglob notation.



    local *main::foo    = *main::bar;



You can use this to print out all the variables in a package, for
instance.  The standard but antiquated dumpvar.pl library and
the CPAN module Devel::Symdump make use of this.



The results of creating new symbol table entries directly or modifying any
entries that are not already typeglobs are undefined and subject to change
between releases of perl.



Assignment to a typeglob performs an aliasing operation, i.e.,



    *dick = *richard;



causes variables, subroutines, formats, and file and directory handles
accessible via the identifier richard also to be accessible via the
identifier dick.  If you want to alias only a particular variable or
subroutine, assign a reference instead:



    *dick = \$richard;



Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays.  Tricky, eh?



There is one subtle difference between the following statements:



    *foo = *bar;
    *foo = \$bar;



*foo = *bar makes the typeglobs themselves synonymous while
*foo = \$bar makes the SCALAR portions of two distinct typeglobs
refer to the same scalar value. This means that the following code:



    $bar = 1;
    *foo = \$bar;       # Make $foo an alias for $bar

    {
        local $bar = 2; # Restrict changes to block
        print $foo;     # Prints '1'!
    }



Would print ’1’, because $foo holds a reference to the original
$bar. The one that was stuffed away by local() and which will be
restored when the block ends. Because variables are accessed through the
typeglob, you can use *foo = *bar to create an alias which can be
localized. (But be aware that this means you can’t have a separate
@foo and @bar, etc.)



What makes all of this important is that the Exporter module uses glob
aliasing as the import/export mechanism. Whether or not you can properly
localize a variable that has been exported from a module depends on how
it was exported:



    @EXPORT = qw($FOO); # Usual form, can't be localized
    @EXPORT = qw(*FOO); # Can be localized



You can work around the first case by using the fully qualified name
($Package::FOO) where you need a local value, or by overriding it
by saying *FOO = *Package::FOO in your script.



The *x = \$y mechanism may be used to pass and return cheap references
into or from subroutines if you don’t want to copy the whole
thing.  It only works when assigning to dynamic variables, not
lexicals.



    %some_hash = ();                    # can't be my()
    *some_hash = fn( \%another_hash );
    sub fn {
        local *hashsym = shift;
        # now use %hashsym normally, and you
        # will affect the caller's %another_hash
        my %nhash = (); # do what you want
        return \%nhash;
    }



On return, the reference will overwrite the hash slot in the
symbol table specified by the *some_hash typeglob.  This
is a somewhat tricky way of passing around references cheaply
when you don’t want to have to remember to dereference variables
explicitly.





Another use of symbol tables is for making "constant" scalars.




    *PI = \3.14159265358979;



Now you cannot alter $PI, which is probably a good thing all in all.
This isn’t the same as a constant subroutine, which is subject to
optimization at compile-time.  A constant subroutine is one prototyped
to take no arguments and to return a constant expression.  See
(perlsub) for details on these.  The use constant pragma is a
convenient shorthand for these.



You can say *foo{PACKAGE} and *foo{NAME} to find out what name and
package the *foo symbol table entry comes from.  This may be useful
in a subroutine that gets passed typeglobs as arguments:



    sub identify_typeglob {
        my $glob = shift;
        print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
    }
    identify_typeglob *foo;
    identify_typeglob *bar::baz;



This prints



    You gave me main::foo
    You gave me bar::baz



The *foo{THING} notation can also be used to obtain references to the
individual elements of *foo.  See (perlref).



Subroutine definitions (and declarations, for that matter) need
not necessarily be situated in the package whose symbol table they
occupy.  You can define a subroutine outside its package by
explicitly qualifying the name of the subroutine:



    package main;
    sub Some_package::foo { ... }   # &foo defined in Some_package



This is just a shorthand for a typeglob assignment at compile time:



    BEGIN { *Some_package::foo = sub { ... } }



and is not the same as writing:



    {
        package Some_package;
        sub foo { ... }
    }



In the first two versions, the body of the subroutine is
lexically in the main package, not in Some_package. So
something like this:



    package main;

    $Some_package::name = "fred";
    $main::name = "barney";

    sub Some_package::foo {
        print "in ", __PACKAGE__, ": \$name is '$name'\n";
    }

    Some_package::foo();



prints:



    in main: $name is 'barney'



rather than:



    in Some_package: $name is 'fred'



This also has implications for the use of the SUPER:: qualifier
(see (perlobj)).









Next: , Previous: perlmod Symbol Tables, Up: perlmod DESCRIPTION   [Contents]





40.2.3 BEGIN, UNITCHECK, CHECK, INIT and END








Five specially named code blocks are executed at the beginning and at
the end of a running Perl program.  These are the BEGIN,
UNITCHECK, CHECK, INIT, and END blocks.



These code blocks can be prefixed with sub to give the appearance of a
subroutine (although this is not considered good style).  One should note
that these code blocks don’t really exist as named subroutines (despite
their appearance). The thing that gives this away is the fact that you can
have more than one of these code blocks in a program, and they will get
all executed at the appropriate moment.  So you can’t execute any of
these code blocks by name.



A BEGIN code block is executed as soon as possible, that is, the moment
it is completely defined, even before the rest of the containing file (or
string) is parsed.  You may have multiple BEGIN blocks within a file (or
eval’ed string); they will execute in order of definition.  Because a BEGIN
code block executes immediately, it can pull in definitions of subroutines
and such from other files in time to be visible to the rest of the compile
and run time.  Once a BEGIN has run, it is immediately undefined and any
code it used is returned to Perl’s memory pool.



An END code block is executed as late as possible, that is, after
perl has finished running the program and just before the interpreter
is being exited, even if it is exiting as a result of a die() function.
(But not if it’s morphing into another program via exec, or
being blown out of the water by a signal–you have to trap that yourself
(if you can).)  You may have multiple END blocks within a file–they
will execute in reverse order of definition; that is: last in, first
out (LIFO).  END blocks are not executed when you run perl with the
-c switch, or if compilation fails.



Note that END code blocks are not executed at the end of a string
eval(): if any END code blocks are created in a string eval(),
they will be executed just as any other END code block of that package
in LIFO order just before the interpreter is being exited.




Inside an END code block, $? contains the value that the program is
going to pass to exit().  You can modify $? to change the exit
value of the program.  Beware of changing $? by accident (e.g. by
running something via system).




Inside of a END block, the value of ${^GLOBAL_PHASE} will be
"END".



UNITCHECK, CHECK and INIT code blocks are useful to catch the
transition between the compilation phase and the execution phase of
the main program.



UNITCHECK blocks are run just after the unit which defined them has
been compiled.  The main program file and each module it loads are
compilation units, as are string evals, code compiled using the
(?{ }) construct in a regex, calls to do FILE, require FILE,
and code after the -e switch on the command line.



BEGIN and UNITCHECK blocks are not directly related to the phase of
the interpreter.  They can be created and executed during any phase.



CHECK code blocks are run just after the initial Perl compile phase ends
and before the run time begins, in LIFO order.  CHECK code blocks are used
in the Perl compiler suite to save the compiled state of the program.



Inside of a CHECK block, the value of ${^GLOBAL_PHASE} will be
"CHECK".



INIT blocks are run just before the Perl runtime begins execution, in
"first in, first out" (FIFO) order.



Inside of an INIT block, the value of ${^GLOBAL_PHASE} will be "INIT".



The CHECK and INIT blocks in code compiled by require, string do,
or string eval will not be executed if they occur after the end of the
main compilation phase; that can be a problem in mod_perl and other persistent
environments which use those functions to load code at runtime.



When you use the -n and -p switches to Perl, BEGIN and
END work just as they do in awk, as a degenerate case.
Both BEGIN and CHECK blocks are run when you use the -c
switch for a compile-only syntax check, although your main code
is not.



The begincheck program makes it all clear, eventually:



  #!/usr/bin/perl

  # begincheck

  print         "10. Ordinary code runs at runtime.\n";

  END { print   "16.   So this is the end of the tale.\n" }
  INIT { print  " 7. INIT blocks run FIFO just before runtime.\n" }
  UNITCHECK {
    print       " 4.   And therefore before any CHECK blocks.\n"
  }
  CHECK { print " 6.   So this is the sixth line.\n" }

  print         "11.   It runs in order, of course.\n";

  BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
  END { print   "15.   Read perlmod for the rest of the story.\n" }
  CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
  INIT { print  " 8.   Run this again, using Perl's -c switch.\n" }

  print         "12.   This is anti-obfuscated code.\n";

  END { print   "14. END blocks run LIFO at quitting time.\n" }
  BEGIN { print " 2.   So this line comes out second.\n" }
  UNITCHECK {
   print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
  }
  INIT { print  " 9.   You'll see the difference right away.\n" }

  print         "13.   It merely _looks_ like it should be confusing.\n";

  __END__









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40.2.4 Perl Classes





There is no special class syntax in Perl, but a package may act
as a class if it provides subroutines to act as methods.  Such a
package may also derive some of its methods from another class (package)
by listing the other package name(s) in its global @ISA array (which
must be a package global, not a lexical).



For more on this, see perlootut NAME and (perlobj).









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40.2.5 Perl Modules




A module is just a set of related functions in a library file, i.e.,
a Perl package with the same name as the file.  It is specifically
designed to be reusable by other modules or programs.  It may do this
by providing a mechanism for exporting some of its symbols into the
symbol table of any package using it, or it may function as a class
definition and make its semantics available implicitly through
method calls on the class and its objects, without explicitly
exporting anything.  Or it can do a little of both.



For example, to start a traditional, non-OO module called Some::Module,
create a file called Some/Module.pm and start with this template:



    package Some::Module;  # assumes Some/Module.pm

    use strict;
    use warnings;

    BEGIN {
        require Exporter;

        # set the version for version checking
        our $VERSION     = 1.00;

        # Inherit from Exporter to export functions and variables
        our @ISA         = qw(Exporter);

        # Functions and variables which are exported by default
        our @EXPORT      = qw(func1 func2);

        # Functions and variables which can be optionally exported
        our @EXPORT_OK   = qw($Var1 %Hashit func3);
    }

    # exported package globals go here
    our $Var1    = '';
    our %Hashit  = ();

    # non-exported package globals go here
    # (they are still accessible as $Some::Module::stuff)
    our @more    = ();
    our $stuff   = '';

    # file-private lexicals go here, before any functions which use them
    my $priv_var    = '';
    my %secret_hash = ();

    # here's a file-private function as a closure,
    # callable as $priv_func->();
    my $priv_func = sub {
        ...
    };

    # make all your functions, whether exported or not;
    # remember to put something interesting in the {} stubs
    sub func1      { ... }
    sub func2      { ... }

    # this one isn't exported, but could be called directly
    # as Some::Module::func3()
    sub func3      { ... }

    END { ... }       # module clean-up code here (global destructor)

    1;  # don't forget to return a true value from the file



Then go on to declare and use your variables in functions without
any qualifications.  See (Exporter) and the (perlmodlib) for
details on mechanics and style issues in module creation.



Perl modules are included into your program by saying



    use Module;



or



    use Module LIST;



This is exactly equivalent to



    BEGIN { require 'Module.pm'; 'Module'->import; }



or



    BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }



As a special case



    use Module ();



is exactly equivalent to



    BEGIN { require 'Module.pm'; }



All Perl module files have the extension .pm.  The use operator
assumes this so you don’t have to spell out "Module.pm" in quotes.
This also helps to differentiate new modules from old .pl and
.ph files.  Module names are also capitalized unless they’re
functioning as pragmas; pragmas are in effect compiler directives,
and are sometimes called "pragmatic modules" (or even "pragmata"
if you’re a classicist).



The two statements:



    require SomeModule;
    require "SomeModule.pm";



differ from each other in two ways.  In the first case, any double
colons in the module name, such as Some::Module, are translated
into your system’s directory separator, usually "/".   The second
case does not, and would have to be specified literally.  The other
difference is that seeing the first require clues in the compiler
that uses of indirect object notation involving "SomeModule", as
in $ob = purge SomeModule, are method calls, not function calls.
(Yes, this really can make a difference.)



Because the use statement implies a BEGIN block, the importing
of semantics happens as soon as the use statement is compiled,
before the rest of the file is compiled.  This is how it is able
to function as a pragma mechanism, and also how modules are able to
declare subroutines that are then visible as list or unary operators for
the rest of the current file.  This will not work if you use require
instead of use.  With require you can get into this problem:



    require Cwd;                # make Cwd:: accessible
    $here = Cwd::getcwd();

    use Cwd;                    # import names from Cwd::
    $here = getcwd();

    require Cwd;                # make Cwd:: accessible
    $here = getcwd();           # oops! no main::getcwd()



In general, use Module () is recommended over require Module,
because it determines module availability at compile time, not in the
middle of your program’s execution.  An exception would be if two modules
each tried to use each other, and each also called a function from
that other module.  In that case, it’s easy to use require instead.



Perl packages may be nested inside other package names, so we can have
package names containing ::.  But if we used that package name
directly as a filename it would make for unwieldy or impossible
filenames on some systems.  Therefore, if a module’s name is, say,
Text::Soundex, then its definition is actually found in the library
file Text/Soundex.pm.



Perl modules always have a .pm file, but there may also be
dynamically linked executables (often ending in .so) or autoloaded
subroutine definitions (often ending in .al) associated with the
module.  If so, these will be entirely transparent to the user of
the module.  It is the responsibility of the .pm file to load
(or arrange to autoload) any additional functionality.  For example,
although the POSIX module happens to do both dynamic loading and
autoloading, the user can say just use POSIX to get it all.









Previous: perlmod Perl Modules, Up: perlmod DESCRIPTION   [Contents]





40.2.6 Making your module threadsafe












Since 5.6.0, Perl has had support for a new type of threads called
interpreter threads (ithreads). These threads can be used explicitly
and implicitly.



Ithreads work by cloning the data tree so that no data is shared
between different threads. These threads can be used by using the threads
module or by doing fork() on win32 (fake fork() support). When a
thread is cloned all Perl data is cloned, however non-Perl data cannot
be cloned automatically.  Perl after 5.7.2 has support for the CLONE
special subroutine.  In CLONE you can do whatever
you need to do,
like for example handle the cloning of non-Perl data, if necessary.
CLONE will be called once as a class method for every package that has it
defined (or inherits it).  It will be called in the context of the new thread,
so all modifications are made in the new area.  Currently CLONE is called with
no parameters other than the invocant package name, but code should not assume
that this will remain unchanged, as it is likely that in future extra parameters
will be passed in to give more information about the state of cloning.



If you want to CLONE all objects you will need to keep track of them per
package. This is simply done using a hash and Scalar::Util::weaken().



Perl after 5.8.7 has support for the CLONE_SKIP special subroutine.
Like CLONE, CLONE_SKIP is called once per package; however, it is
called just before cloning starts, and in the context of the parent
thread. If it returns a true value, then no objects of that class will
be cloned; or rather, they will be copied as unblessed, undef values.
For example: if in the parent there are two references to a single blessed
hash, then in the child there will be two references to a single undefined
scalar value instead.
This provides a simple mechanism for making a module threadsafe; just add
sub CLONE_SKIP { 1 } at the top of the class, and DESTROY() will
now only be called once per object. Of course, if the child thread needs
to make use of the objects, then a more sophisticated approach is
needed.



Like CLONE, CLONE_SKIP is currently called with no parameters other
than the invocant package name, although that may change. Similarly, to
allow for future expansion, the return value should be a single 0 or
1 value.









Previous: perlmod DESCRIPTION, Up: perlmod   [Contents]





40.3 SEE ALSO



See (perlmodlib) for general style issues related to building Perl
modules and classes, as well as descriptions of the standard library
and CPAN, (Exporter) for how Perl’s standard import/export mechanism
works, perlootut NAME and (perlobj) for in-depth information on
creating classes, (perlobj) for a hard-core reference document on
objects, (perlsub) for an explanation of functions and scoping,
and (perlxstut) and perlguts NAME for more information on writing
extension modules.









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41 perlmodinstall





• perlmodinstall NAME:  



• perlmodinstall DESCRIPTION:  



• perlmodinstall PORTABILITY:  



• perlmodinstall HEY:  



• perlmodinstall AUTHOR:  



• perlmodinstall COPYRIGHT:  












Next: , Up: perlmodinstall   [Contents]





41.1 NAME



perlmodinstall - Installing CPAN Modules









Next: , Previous: perlmodinstall NAME, Up: perlmodinstall   [Contents]





41.2 DESCRIPTION



You can think of a module as the fundamental unit of reusable Perl
code; see perlmod NAME for details.  Whenever anyone creates a chunk of
Perl code that they think will be useful to the world, they register
as a Perl developer at http://www.cpan.org/modules/04pause.html
so that they can then upload their code to the CPAN.  The CPAN is the
Comprehensive Perl Archive Network and can be accessed at
http://www.cpan.org/ , and searched at http://search.cpan.org/ .



This documentation is for people who want to download CPAN modules
and install them on their own computer.





• perlmodinstall PREAMBLE:  












Up: perlmodinstall DESCRIPTION   [Contents]





41.2.1 PREAMBLE



First, are you sure that the module isn’t already on your system?  Try
perl -MFoo -e 1.  (Replace "Foo" with the name of the module; for
instance, perl -MCGI::Carp -e 1.



If you don’t see an error message, you have the module.  (If you do
see an error message, it’s still possible you have the module, but
that it’s not in your path, which you can display with perl -e
"print qq(@INC)".)  For the remainder of this document, we’ll assume
that you really honestly truly lack an installed module, but have
found it on the CPAN.



So now you have a file ending in .tar.gz (or, less often, .zip).  You
know there’s a tasty module inside.  There are four steps you must now
take:





DECOMPRESS the file






UNPACK the file into a directory






BUILD the module (sometimes unnecessary)






INSTALL the module.









Here’s how to perform each step for each operating system.  This is
<not> a substitute for reading the README and INSTALL files that
might have come with your module!



Also note that these instructions are tailored for installing the
module into your system’s repository of Perl modules, but you can
install modules into any directory you wish.  For instance, where I
say perl Makefile.PL, you can substitute perl Makefile.PL
PREFIX=/my/perl_directory to install the modules into
/my/perl_directory.  Then you can use the modules from your Perl
programs with use lib "/my/perl_directory/lib/site_perl"; or
sometimes just use "/my/perl_directory";.  If you’re on a system
that requires superuser/root access to install modules into the
directories you see when you type perl -e "print qq(@INC)", you’ll
want to install them into a local directory (such as your home
directory) and use this approach.











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41.3 PORTABILITY



Note that not all modules will work with on all platforms.
See perlport NAME for more information on portability issues.
Read the documentation to see if the module will work on your
system.  There are basically three categories
of modules that will not work "out of the box" with all
platforms (with some possibility of overlap):





Check the CPAN Testers if a module should work with your platform
but it doesn’t behave as you’d expect, or you aren’t sure whether or
not a module will work under your platform.  If the module you want
isn’t listed there, you can test it yourself and let CPAN Testers know,
you can join CPAN Testers, or you can request it be tested.



    http://testers.cpan.org/









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41.4 HEY



If you have any suggested changes for this page, let me know.  Please
don’t send me mail asking for help on how to install your modules.
There are too many modules, and too few Orwants, for me to be able to
answer or even acknowledge all your questions.  Contact the module
author instead, or post to comp.lang.perl.modules, or ask someone
familiar with Perl on your operating system.









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41.5 AUTHOR



Jon Orwant



orwant@medita.mit.edu



with invaluable help from Chris Nandor, and valuable help from Brandon
Allbery, Charles Bailey, Graham Barr, Dominic Dunlop, Jarkko
Hietaniemi, Ben Holzman, Tom Horsley, Nick Ing-Simmons, Tuomas
J. Lukka, Laszlo Molnar, Alan Olsen, Peter Prymmer, Gurusamy Sarathy,
Christoph Spalinger, Dan Sugalski, Larry Virden, and Ilya Zakharevich.



First version July 22, 1998; last revised November 21, 2001.









Previous: perlmodinstall AUTHOR, Up: perlmodinstall   [Contents]





41.6 COPYRIGHT



Copyright (C) 1998, 2002, 2003 Jon Orwant.  All Rights Reserved.



This document may be distributed under the same terms as Perl itself.









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42 perlmodstyle





• perlmodstyle NAME:  



• perlmodstyle INTRODUCTION:  



• perlmodstyle QUICK CHECKLIST:  



• perlmodstyle BEFORE YOU START WRITING A MODULE:  



• perlmodstyle DESIGNING AND WRITING YOUR MODULE:  



• perlmodstyle DOCUMENTING YOUR MODULE:  



• perlmodstyle RELEASE CONSIDERATIONS:  



• perlmodstyle COMMON PITFALLS:  



• perlmodstyle SEE ALSO:  



• perlmodstyle AUTHOR:  












Next: , Up: perlmodstyle   [Contents]





42.1 NAME



perlmodstyle - Perl module style guide









Next: , Previous: perlmodstyle NAME, Up: perlmodstyle   [Contents]





42.2 INTRODUCTION



This document attempts to describe the Perl Community’s "best practice"
for writing Perl modules.  It extends the recommendations found in 
perlstyle NAME , which should be considered required reading
before reading this document.



While this document is intended to be useful to all module authors, it is
particularly aimed at authors who wish to publish their modules on CPAN.



The focus is on elements of style which are visible to the users of a 
module, rather than those parts which are only seen by the module’s 
developers.  However, many of the guidelines presented in this document
can be extrapolated and applied successfully to a module’s internals.



This document differs from perlnewmod NAME in that it is a style guide
rather than a tutorial on creating CPAN modules.  It provides a
checklist against which modules can be compared to determine whether
they conform to best practice, without necessarily describing in detail
how to achieve this.  



All the advice contained in this document has been gleaned from
extensive conversations with experienced CPAN authors and users.  Every
piece of advice given here is the result of previous mistakes.  This
information is here to help you avoid the same mistakes and the extra
work that would inevitably be required to fix them.



The first section of this document provides an itemized checklist; 
subsequent sections provide a more detailed discussion of the items on 
the list.  The final section, "Common Pitfalls", describes some of the 
most popular mistakes made by CPAN authors.









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42.3 QUICK CHECKLIST



For more detail on each item in this checklist, see below.





• perlmodstyle Before you start:  



• perlmodstyle The API:  



• perlmodstyle Stability:  



• perlmodstyle Documentation:  



• perlmodstyle Release considerations:  












Next: , Up: perlmodstyle QUICK CHECKLIST   [Contents]





42.3.1 Before you start











Next: , Previous: perlmodstyle Before you start, Up: perlmodstyle QUICK CHECKLIST   [Contents]





42.3.2 The API











Next: , Previous: perlmodstyle The API, Up: perlmodstyle QUICK CHECKLIST   [Contents]





42.3.3 Stability











Next: , Previous: perlmodstyle Stability, Up: perlmodstyle QUICK CHECKLIST   [Contents]





42.3.4 Documentation











Previous: perlmodstyle Documentation, Up: perlmodstyle QUICK CHECKLIST   [Contents]





42.3.5 Release considerations











Next: , Previous: perlmodstyle QUICK CHECKLIST, Up: perlmodstyle   [Contents]





42.4 BEFORE YOU START WRITING A MODULE



Try not to launch headlong into developing your module without spending
some time thinking first.  A little forethought may save you a vast
amount of effort later on.





• perlmodstyle Has it been done before?:  



• perlmodstyle Do one thing and do it well:  



• perlmodstyle What's in a name?:  












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42.4.1 Has it been done before?



You may not even need to write the module.  Check whether it’s already 
been done in Perl, and avoid re-inventing the wheel unless you have a 
good reason.



Good places to look for pre-existing modules include
http://search.cpan.org/ and asking on modules@perl.org



If an existing module almost does what you want, consider writing a
patch, writing a subclass, or otherwise extending the existing module
rather than rewriting it.









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42.4.2 Do one thing and do it well



At the risk of stating the obvious, modules are intended to be modular.
A Perl developer should be able to use modules to put together the
building blocks of their application.  However, it’s important that the
blocks are the right shape, and that the developer shouldn’t have to use
a big block when all they need is a small one.



Your module should have a clearly defined scope which is no longer than
a single sentence.  Can your module be broken down into a family of
related modules?



Bad example:



"FooBar.pm provides an implementation of the FOO protocol and the
related BAR standard."



Good example:



"Foo.pm provides an implementation of the FOO protocol.  Bar.pm
implements the related BAR protocol."



This means that if a developer only needs a module for the BAR standard,
they should not be forced to install libraries for FOO as well.









Previous: perlmodstyle Do one thing and do it well, Up: perlmodstyle BEFORE YOU START WRITING A MODULE   [Contents]





42.4.3 What’s in a name?



Make sure you choose an appropriate name for your module early on.  This
will help people find and remember your module, and make programming
with your module more intuitive.



When naming your module, consider the following:





You should contact modules@perl.org to ask them about your module name
before publishing your module.  You should also try to ask people who 
are already familiar with the module’s application domain and the CPAN
naming system.  Authors of similar modules, or modules with similar
names, may be a good place to start.









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42.5 DESIGNING AND WRITING YOUR MODULE



Considerations for module design and coding:





• perlmodstyle To OO or not to OO?:  



• perlmodstyle Designing your API:  



• perlmodstyle Strictness and warnings:  



• perlmodstyle Backwards compatibility:  



• perlmodstyle Error handling and messages:  












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42.5.1 To OO or not to OO?



Your module may be object oriented (OO) or not, or it may have both kinds 
of interfaces available.  There are pros and cons of each technique, which 
should be considered when you design your API.



In Perl Best Practices (copyright 2004, Published by O’Reilly Media, Inc.),
Damian Conway provides a list of criteria to use when deciding if OO is the
right fit for your problem:





Think carefully about whether OO is appropriate for your module.
Gratuitous object orientation results in complex APIs which are
difficult for the average module user to understand or use.









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42.5.2 Designing your API



Your interfaces should be understandable by an average Perl programmer.  
The following guidelines may help you judge whether your API is
sufficiently straightforward:





Write simple routines to do simple things.




It’s better to have numerous simple routines than a few monolithic ones.
If your routine changes its behaviour significantly based on its
arguments, it’s a sign that you should have two (or more) separate
routines.





Separate functionality from output.




Return your results in the most generic form possible and allow the user 
to choose how to use them.  The most generic form possible is usually a
Perl data structure which can then be used to generate a text report,
HTML, XML, a database query, or whatever else your users require.



If your routine iterates through some kind of list (such as a list of
files, or records in a database) you may consider providing a callback
so that users can manipulate each element of the list in turn.
File::Find provides an example of this with its 
find(\&wanted, $dir) syntax.





Provide sensible shortcuts and defaults.




Don’t require every module user to jump through the same hoops to achieve a
simple result.  You can always include optional parameters or routines for 
more complex or non-standard behaviour.  If most of your users have to
type a few almost identical lines of code when they start using your
module, it’s a sign that you should have made that behaviour a default.
Another good indicator that you should use defaults is if most of your 
users call your routines with the same arguments.





Naming conventions




Your naming should be consistent.  For instance, it’s better to have:



        display_day();
        display_week();
        display_year();



than



        display_day();
        week_display();
        show_year();



This applies equally to method names, parameter names, and anything else
which is visible to the user (and most things that aren’t!)





Parameter passing




Use named parameters. It’s easier to use a hash like this:



    $obj->do_something(
            name => "wibble",
            type => "text",
            size => 1024,
    );



... than to have a long list of unnamed parameters like this:



    $obj->do_something("wibble", "text", 1024);



While the list of arguments might work fine for one, two or even three
arguments, any more arguments become hard for the module user to
remember, and hard for the module author to manage.  If you want to add
a new parameter you will have to add it to the end of the list for
backward compatibility, and this will probably make your list order
unintuitive.  Also, if many elements may be undefined you may see the
following unattractive method calls:



    $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);



Provide sensible defaults for parameters which have them.  Don’t make
your users specify parameters which will almost always be the same.



The issue of whether to pass the arguments in a hash or a hashref is
largely a matter of personal style. 



The use of hash keys starting with a hyphen (-name) or entirely in 
upper case (NAME) is a relic of older versions of Perl in which
ordinary lower case strings were not handled correctly by the =>
operator.  While some modules retain uppercase or hyphenated argument
keys for historical reasons or as a matter of personal style, most new
modules should use simple lower case keys.  Whatever you choose, be
consistent!














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42.5.3 Strictness and warnings



Your module should run successfully under the strict pragma and should
run without generating any warnings.  Your module should also handle 
taint-checking where appropriate, though this can cause difficulties in
many cases.









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42.5.4 Backwards compatibility



Modules which are "stable" should not break backwards compatibility
without at least a long transition phase and a major change in version
number.









Previous: perlmodstyle Backwards compatibility, Up: perlmodstyle DESIGNING AND WRITING YOUR MODULE   [Contents]





42.5.5 Error handling and messages



When your module encounters an error it should do one or more of:





Configurable error handling can be very useful to your users.  Consider
offering a choice of levels for warning and debug messages, an option to
send messages to a separate file, a way to specify an error-handling
routine, or other such features.  Be sure to default all these options
to the commonest use.









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42.6 DOCUMENTING YOUR MODULE





• perlmodstyle POD:  



• perlmodstyle README, INSTALL, release notes, changelogs:  












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42.6.1 POD



Your module should include documentation aimed at Perl developers.
You should use Perl’s "plain old documentation" (POD) for your general 
technical documentation, though you may wish to write additional
documentation (white papers, tutorials, etc) in some other format.  
You need to cover the following subjects:





The level of detail in Perl module documentation generally goes from
less detailed to more detailed.  Your SYNOPSIS section should contain a
minimal example of use (perhaps as little as one line of code; skip the
unusual use cases or anything not needed by most users); the
DESCRIPTION should describe your module in broad terms, generally in
just a few paragraphs; more detail of the module’s routines or methods,
lengthy code examples, or other in-depth material should be given in 
subsequent sections.



Ideally, someone who’s slightly familiar with your module should be able
to refresh their memory without hitting "page down".  As your reader
continues through the document, they should receive a progressively
greater amount of knowledge.



The recommended order of sections in Perl module documentation is:





Keep your documentation near the code it documents ("inline"
documentation).  Include POD for a given method right above that 
method’s subroutine.  This makes it easier to keep the documentation up
to date, and avoids having to document each piece of code twice (once in
POD and once in comments).









Previous: perlmodstyle POD, Up: perlmodstyle DOCUMENTING YOUR MODULE   [Contents]





42.6.2 README, INSTALL, release notes, changelogs



Your module should also include a README file describing the module and
giving pointers to further information (website, author email).  



An INSTALL file should be included, and should contain simple installation 
instructions. When using ExtUtils::MakeMaker this will usually be:





perl Makefile.PL






make






make test






make install









When using Module::Build, this will usually be:





perl Build.PL






perl Build






perl Build test






perl Build install









Release notes or changelogs should be produced for each release of your
software describing user-visible changes to your module, in terms
relevant to the user.









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42.7 RELEASE CONSIDERATIONS





• perlmodstyle Version numbering:  



• perlmodstyle Pre-requisites:  



• perlmodstyle Testing:  



• perlmodstyle Packaging:  



• perlmodstyle Licensing:  












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42.7.1 Version numbering



Version numbers should indicate at least major and minor releases, and
possibly sub-minor releases.  A major release is one in which most of
the functionality has changed, or in which major new functionality is
added.  A minor release is one in which a small amount of functionality
has been added or changed.  Sub-minor version numbers are usually used
for changes which do not affect functionality, such as documentation
patches.



The most common CPAN version numbering scheme looks like this:



    1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32



A correct CPAN version number is a floating point number with at least 
2 digits after the decimal. You can test whether it conforms to CPAN by 
using



    perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'



If you want to release a ’beta’ or ’alpha’ version of a module but
don’t want CPAN.pm to list it as most recent use an ’_’ after the
regular version number followed by at least 2 digits, eg. 1.20_01. If
you do this, the following idiom is recommended:



  $VERSION = "1.12_01";
  $XS_VERSION = $VERSION; # only needed if you have XS code
  $VERSION = eval $VERSION;



With that trick MakeMaker will only read the first line and thus read
the underscore, while the perl interpreter will evaluate the $VERSION
and convert the string into a number. Later operations that treat
$VERSION as a number will then be able to do so without provoking a
warning about $VERSION not being a number.



Never release anything (even a one-word documentation patch) without
incrementing the number.  Even a one-word documentation patch should
result in a change in version at the sub-minor level.









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42.7.2 Pre-requisites



Module authors should carefully consider whether to rely on other
modules, and which modules to rely on.



Most importantly, choose modules which are as stable as possible.  In
order of preference: 





Specify version requirements for other Perl modules in the
pre-requisites in your Makefile.PL or Build.PL.



Be sure to specify Perl version requirements both in Makefile.PL or
Build.PL and with require 5.6.1 or similar. See the section on
use VERSION of (perlfunc)require for details.









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42.7.3 Testing



All modules should be tested before distribution (using "make disttest"),
and the tests should also be available to people installing the modules 
(using "make test").  
For Module::Build you would use the make test equivalent perl Build test.



The importance of these tests is proportional to the alleged stability of a 
module. A module which purports to be stable or which hopes to achieve wide 
use should adhere to as strict a testing regime as possible.



Useful modules to help you write tests (with minimum impact on your 
development process or your time) include Test::Simple, Carp::Assert 
and Test::Inline.
For more sophisticated test suites there are Test::More and Test::MockObject.









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42.7.4 Packaging



Modules should be packaged using one of the standard packaging tools.
Currently you have the choice between ExtUtils::MakeMaker and the
more platform independent Module::Build, allowing modules to be installed in a
consistent manner.
When using ExtUtils::MakeMaker, you can use "make dist" to create your
package. Tools exist to help you to build your module in a MakeMaker-friendly
style. These include ExtUtils::ModuleMaker and h2xs.  See also perlnewmod NAME.









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42.7.5 Licensing



Make sure that your module has a license, and that the full text of it
is included in the distribution (unless it’s a common one and the terms
of the license don’t require you to include it).



If you don’t know what license to use, dual licensing under the GPL
and Artistic licenses (the same as Perl itself) is a good idea.
See perlgpl NAME and perlartistic NAME.









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42.8 COMMON PITFALLS





• perlmodstyle Reinventing the wheel:  



• perlmodstyle Trying to do too much:  



• perlmodstyle Inappropriate documentation:  












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42.8.1 Reinventing the wheel



There are certain application spaces which are already very, very well
served by CPAN.  One example is templating systems, another is date and
time modules, and there are many more.  While it is a rite of passage to
write your own version of these things, please consider carefully
whether the Perl world really needs you to publish it.









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42.8.2 Trying to do too much



Your module will be part of a developer’s toolkit.  It will not, in
itself, form the entire toolkit.  It’s tempting to add extra features
until your code is a monolithic system rather than a set of modular
building blocks.









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42.8.3 Inappropriate documentation



Don’t fall into the trap of writing for the wrong audience.  Your
primary audience is a reasonably experienced developer with at least 
a moderate understanding of your module’s application domain, who’s just 
downloaded your module and wants to start using it as quickly as possible.



Tutorials, end-user documentation, research papers, FAQs etc are not 
appropriate in a module’s main documentation.  If you really want to 
write these, include them as sub-documents such as My::Module::Tutorial or
My::Module::FAQ and provide a link in the SEE ALSO section of the
main documentation.  









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42.9 SEE ALSO





perlstyle NAME




General Perl style guide





perlnewmod NAME




How to create a new module





(perlpod)




POD documentation





(podchecker)




Verifies your POD’s correctness





Packaging Tools




(ExtUtils-MakeMaker), (Module-Build)





Testing tools




(Test-Simple), (Test-Inline), (Carp-Assert), (Test-More), (Test-MockObject)





http://pause.perl.org/




Perl Authors Upload Server.  Contains links to information for module
authors.





Any good book on software engineering















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42.10 AUTHOR



Kirrily "Skud" Robert <skud@cpan.org>









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43 perlmroapi





• perlmroapi NAME:  



• perlmroapi DESCRIPTION:  



• perlmroapi Callbacks:  



• perlmroapi Caching:  



• perlmroapi Examples:  



• perlmroapi AUTHORS:  












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43.1 NAME



perlmroapi - Perl method resolution plugin interface









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43.2 DESCRIPTION



As of Perl 5.10.1 there is a new interface for plugging and using method
resolution orders other than the default (linear depth first search).
The C3 method resolution order added in 5.10.0 has been re-implemented as
a plugin, without changing its Perl-space interface.



Each plugin should register itself by providing
the following structure



    struct mro_alg {
        AV *(*resolve)(pTHX_ HV *stash, U32 level);
        const char *name;
        U16 length;
        U16 kflags;
        U32 hash;
    };



and calling Perl_mro_register:



    Perl_mro_register(aTHX_ &my_mro_alg);





resolve




Pointer to the linearisation function, described below.





name




Name of the MRO, either in ISO-8859-1 or UTF-8.





length




Length of the name.





kflags




If the name is given in UTF-8, set this to HVhek_UTF8. The value is passed
direct as the parameter kflags to hv_common().





hash




A precomputed hash value for the MRO’s name, or 0.














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43.3 Callbacks



The resolve function is called to generate a linearised ISA for the
given stash, using this MRO. It is called with a pointer to the stash, and
a level of 0. The core always sets level to 0 when it calls your
function - the parameter is provided to allow your implementation to track
depth if it needs to recurse.



The function should return a reference to an array containing the parent
classes in order. The names of the classes should be the result of calling
HvENAME() on the stash. In those cases where HvENAME() returns null,
HvNAME() should be used instead.



The caller is responsible for incrementing the reference count of the array
returned if it wants to keep the structure. Hence, if you have created a
temporary value that you keep no pointer to, sv_2mortal() to ensure that
it is disposed of correctly. If you have cached your return value, then
return a pointer to it without changing the reference count.









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43.4 Caching



Computing MROs can be expensive. The implementation provides a cache, in
which you can store a single SV *, or anything that can be cast to
SV *, such as AV *. To read your private value, use the macro
MRO_GET_PRIVATE_DATA(), passing it the mro_meta structure from the
stash, and a pointer to your mro_alg structure:



    meta = HvMROMETA(stash);
    private_sv = MRO_GET_PRIVATE_DATA(meta, &my_mro_alg);



To set your private value, call Perl_mro_set_private_data():



    Perl_mro_set_private_data(aTHX_ meta, &c3_alg, private_sv);



The private data cache will take ownership of a reference to private_sv,
much the same way that hv_store() takes ownership of a reference to the
value that you pass it.









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43.5 Examples



For examples of MRO implementations, see S_mro_get_linear_isa_c3()
and the BOOT: section of mro/mro.xs, and S_mro_get_linear_isa_dfs()
in mro.c









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43.6 AUTHORS



The implementation of the C3 MRO and switchable MROs within the perl core was
written by Brandon L Black. Nicholas Clark created the pluggable interface, 
refactored Brandon’s implementation to work with it, and wrote this document.









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44 perlnewmod





• perlnewmod NAME:  



• perlnewmod DESCRIPTION:  



• perlnewmod AUTHOR:  



• perlnewmod SEE ALSO:  












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44.1 NAME



perlnewmod - preparing a new module for distribution









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44.2 DESCRIPTION



This document gives you some suggestions about how to go about writing
Perl modules, preparing them for distribution, and making them available
via CPAN.



One of the things that makes Perl really powerful is the fact that Perl
hackers tend to want to share the solutions to problems they’ve faced,
so you and I don’t have to battle with the same problem again.



The main way they do this is by abstracting the solution into a Perl
module. If you don’t know what one of these is, the rest of this
document isn’t going to be much use to you. You’re also missing out on
an awful lot of useful code; consider having a look at perlmod NAME,
(perlmodlib) and perlmodinstall NAME before coming back here.



When you’ve found that there isn’t a module available for what you’re
trying to do, and you’ve had to write the code yourself, consider
packaging up the solution into a module and uploading it to CPAN so that
others can benefit.





• perlnewmod Warning:  



• perlnewmod What should I make into a module?:  



• perlnewmod Step-by-step: Preparing the ground:  



• perlnewmod Step-by-step: Making the module:  



• perlnewmod Step-by-step: Distributing your module:  












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44.2.1 Warning



We’re going to primarily concentrate on Perl-only modules here, rather
than XS modules. XS modules serve a rather different purpose, and
you should consider different things before distributing them - the
popularity of the library you are gluing, the portability to other
operating systems, and so on. However, the notes on preparing the Perl
side of the module and packaging and distributing it will apply equally
well to an XS module as a pure-Perl one.









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44.2.2 What should I make into a module?



You should make a module out of any code that you think is going to be
useful to others. Anything that’s likely to fill a hole in the communal
library and which someone else can slot directly into their program. Any
part of your code which you can isolate and extract and plug into
something else is a likely candidate.



Let’s take an example. Suppose you’re reading in data from a local
format into a hash-of-hashes in Perl, turning that into a tree, walking
the tree and then piping each node to an Acme Transmogrifier Server.



Now, quite a few people have the Acme Transmogrifier, and you’ve had to
write something to talk the protocol from scratch - you’d almost
certainly want to make that into a module. The level at which you pitch
it is up to you: you might want protocol-level modules analogous to
(Net-SMTP)Net::SMTP which then talk to higher level modules analogous
to (Mail-Send)Mail::Send. The choice is yours, but you do want to get
a module out for that server protocol.



Nobody else on the planet is going to talk your local data format, so we
can ignore that. But what about the thing in the middle? Building tree
structures from Perl variables and then traversing them is a nice,
general problem, and if nobody’s already written a module that does
that, you might want to modularise that code too.



So hopefully you’ve now got a few ideas about what’s good to modularise.
Let’s now see how it’s done.









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44.2.3 Step-by-step: Preparing the ground



Before we even start scraping out the code, there are a few things we’ll
want to do in advance.





Look around




Dig into a bunch of modules to see how they’re written. I’d suggest
starting with (Text-Tabs)Text::Tabs, since it’s in the standard
library and is nice and simple, and then looking at something a little
more complex like (File-Copy)File::Copy.  For object oriented
code, WWW::Mechanize or the Email::* modules provide some good
examples.



These should give you an overall feel for how modules are laid out and
written.





Check it’s new




There are a lot of modules on CPAN, and it’s easy to miss one that’s
similar to what you’re planning on contributing. Have a good plough
through the http://search.cpan.org and make sure you’re not the one
reinventing the wheel!





Discuss the need




You might love it. You might feel that everyone else needs it. But there
might not actually be any real demand for it out there. If you’re unsure
about the demand your module will have, consider sending out feelers
on the comp.lang.perl.modules newsgroup, or as a last resort, ask the
modules list at modules@perl.org. Remember that this is a closed list
with a very long turn-around time - be prepared to wait a good while for
a response from them.





Choose a name




Perl modules included on CPAN have a naming hierarchy you should try to
fit in with. See (perlmodlib) for more details on how this works, and
browse around CPAN and the modules list to get a feel of it. At the very
least, remember this: modules should be title capitalised, (This::Thing)
fit in with a category, and explain their purpose succinctly.





Check again




While you’re doing that, make really sure you haven’t missed a module
similar to the one you’re about to write.



When you’ve got your name sorted out and you’re sure that your module is
wanted and not currently available, it’s time to start coding.














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44.2.4 Step-by-step: Making the module





Start with module-starter or h2xs




The module-starter utility is distributed as part of the
(Module-Starter)Module::Starter CPAN package.  It creates a directory
with stubs of all the necessary files to start a new module, according
to recent "best practice" for module development, and is invoked from
the command line, thus:



    module-starter --module=Foo::Bar \
       --author="Your Name" --email=yourname@cpan.org



If you do not wish to install the (Module-Starter)Module::Starter
package from CPAN, h2xs is an older tool, originally intended for the
development of XS modules, which comes packaged with the Perl
distribution. 



A typical invocation of (h2xs)h2xs for a pure Perl module is:



    h2xs -AX --skip-exporter --use-new-tests -n Foo::Bar 



The -A omits the Autoloader code, -X omits XS elements,
--skip-exporter omits the Exporter code, --use-new-tests sets up a
modern testing environment, and -n specifies the name of the module.





Use (strict)strict and (warnings)warnings




A module’s code has to be warning and strict-clean, since you can’t
guarantee the conditions that it’ll be used under. Besides, you wouldn’t
want to distribute code that wasn’t warning or strict-clean anyway,
right?





Use (Carp)Carp




The (Carp)Carp module allows you to present your error messages from
the caller’s perspective; this gives you a way to signal a problem with
the caller and not your module. For instance, if you say this:



    warn "No hostname given";



the user will see something like this:



    No hostname given at /usr/local/lib/perl5/site_perl/5.6.0/Net/Acme.pm
    line 123.



which looks like your module is doing something wrong. Instead, you want
to put the blame on the user, and say this:



    No hostname given at bad_code, line 10.



You do this by using (Carp)Carp and replacing your warns with
carps. If you need to die, say croak instead. However, keep
warn and die in place for your sanity checks - where it really is
your module at fault.





Use (Exporter)Exporter - wisely!




(Exporter)Exporter gives you a standard way of exporting symbols and
subroutines from your module into the caller’s namespace. For instance,
saying use Net::Acme qw(&frob) would import the frob subroutine.



The package variable @EXPORT will determine which symbols will get
exported when the caller simply says use Net::Acme - you will hardly
ever want to put anything in there. @EXPORT_OK, on the other hand,
specifies which symbols you’re willing to export. If you do want to
export a bunch of symbols, use the %EXPORT_TAGS and define a standard
export set - look at (Exporter) for more details.





Use (perlpod)plain old documentation




The work isn’t over until the paperwork is done, and you’re going to
need to put in some time writing some documentation for your module.
module-starter or h2xs will provide a stub for you to fill in; if
you’re not sure about the format, look at (perlpod) for an
introduction. Provide a good synopsis of how your module is used in
code, a description, and then notes on the syntax and function of the
individual subroutines or methods. Use Perl comments for developer notes
and POD for end-user notes.





Write tests




You’re encouraged to create self-tests for your module to ensure it’s
working as intended on the myriad platforms Perl supports; if you upload
your module to CPAN, a host of testers will build your module and send
you the results of the tests. Again, module-starter and h2xs
provide a test framework which you can extend - you should do something
more than just checking your module will compile.
(Test-Simple)Test::Simple and (Test-More)Test::More are good
places to start when writing a test suite.





Write the README




If you’re uploading to CPAN, the automated gremlins will extract the
README file and place that in your CPAN directory. It’ll also appear in
the main by-module and by-category directories if you make it onto
the modules list. It’s a good idea to put here what the module actually
does in detail, and the user-visible changes since the last release.














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44.2.5 Step-by-step: Distributing your module





Get a CPAN user ID




Every developer publishing modules on CPAN needs a CPAN ID.  Visit
http://pause.perl.org/, select "Request PAUSE Account", and wait for
your request to be approved by the PAUSE administrators.





perl Makefile.PL; make test; make dist




Once again, module-starter or h2xs has done all the work for you.
They produce the standard Makefile.PL you see when you download and
install modules, and this produces a Makefile with a dist target.



Once you’ve ensured that your module passes its own tests - always a
good thing to make sure - you can make dist, and the Makefile will
hopefully produce you a nice tarball of your module, ready for upload.





Upload the tarball




The email you got when you received your CPAN ID will tell you how to
log in to PAUSE, the Perl Authors Upload SErver. From the menus there,
you can upload your module to CPAN.





Announce to the modules list




Once uploaded, it’ll sit unnoticed in your author directory. If you want
it connected to the rest of the CPAN, you’ll need to go to "Register
Namespace" on PAUSE.  Once registered, your module will appear in the
by-module and by-category listings on CPAN.





Announce to clpa




If you have a burning desire to tell the world about your release, post
an announcement to the moderated comp.lang.perl.announce newsgroup.





Fix bugs!




Once you start accumulating users, they’ll send you bug reports. If
you’re lucky, they’ll even send you patches. Welcome to the joys of
maintaining a software project...














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44.3 AUTHOR



Simon Cozens, simon@cpan.org



Updated by Kirrily "Skud" Robert, skud@cpan.org









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44.4 SEE ALSO



perlmod NAME, (perlmodlib), perlmodinstall NAME, (h2xs), (strict),
(Carp), (Exporter), (perlpod), (Test-Simple), (Test-More)
(ExtUtils-MakeMaker), (Module-Build), (Module-Starter)
http://www.cpan.org/ , Ken Williams’s tutorial on building your own
module at http://mathforum.org/~ken/perl_modules.html









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45 perlnumber





• perlnumber NAME:  



• perlnumber SYNOPSIS:  



• perlnumber DESCRIPTION:  



• perlnumber Storing numbers:  



• perlnumber Numeric operators and numeric conversions:  



• perlnumber Flavors of Perl numeric operations:  



• perlnumber AUTHOR:  



• perlnumber SEE ALSO:  












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45.1 NAME



perlnumber - semantics of numbers and numeric operations in Perl









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45.2 SYNOPSIS



    $n = 1234;              # decimal integer
    $n = 0b1110011;         # binary integer
    $n = 01234;             # octal integer
    $n = 0x1234;            # hexadecimal integer
    $n = 12.34e-56;         # exponential notation
    $n = "-12.34e56";       # number specified as a string
    $n = "1234";            # number specified as a string









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45.3 DESCRIPTION



This document describes how Perl internally handles numeric values.



Perl’s operator overloading facility is completely ignored here.  Operator
overloading allows user-defined behaviors for numbers, such as operations
over arbitrarily large integers, floating points numbers with arbitrary
precision, operations over "exotic" numbers such as modular arithmetic or
p-adic arithmetic, and so on.  See (overload) for details.









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45.4 Storing numbers



Perl can internally represent numbers in 3 different ways: as native
integers, as native floating point numbers, and as decimal strings.
Decimal strings may have an exponential notation part, as in "12.34e-56".
Native here means "a format supported by the C compiler which was used
to build perl".



The term "native" does not mean quite as much when we talk about native
integers, as it does when native floating point numbers are involved.
The only implication of the term "native" on integers is that the limits for
the maximal and the minimal supported true integral quantities are close to
powers of 2.  However, "native" floats have a most fundamental
restriction: they may represent only those numbers which have a relatively
"short" representation when converted to a binary fraction.  For example,
0.9 cannot be represented by a native float, since the binary fraction
for 0.9 is infinite:



  binary0.1110011001100...



with the sequence 1100 repeating again and again.  In addition to this
limitation,  the exponent of the binary number is also restricted when it
is represented as a floating point number.  On typical hardware, floating
point values can store numbers with up to 53 binary digits, and with binary
exponents between -1024 and 1024.  In decimal representation this is close
to 16 decimal digits and decimal exponents in the range of -304..304.
The upshot of all this is that Perl cannot store a number like
12345678901234567 as a floating point number on such architectures without
loss of information.



Similarly, decimal strings can represent only those numbers which have a
finite decimal expansion.  Being strings, and thus of arbitrary length, there
is no practical limit for the exponent or number of decimal digits for these
numbers.  (But realize that what we are discussing the rules for just the
storage of these numbers.  The fact that you can store such "large" numbers
does not mean that the operations over these numbers will use all
of the significant digits.
See Numeric operators and numeric conversions for details.)



In fact numbers stored in the native integer format may be stored either
in the signed native form, or in the unsigned native form.  Thus the limits
for Perl numbers stored as native integers would typically be -2**31..2**32-1,
with appropriate modifications in the case of 64-bit integers.  Again, this
does not mean that Perl can do operations only over integers in this range:
it is possible to store many more integers in floating point format.



Summing up, Perl numeric values can store only those numbers which have
a finite decimal expansion or a "short" binary expansion.









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45.5 Numeric operators and numeric conversions



As mentioned earlier, Perl can store a number in any one of three formats,
but most operators typically understand only one of those formats.  When
a numeric value is passed as an argument to such an operator, it will be
converted to the format understood by the operator.



Six such conversions are possible:



  native integer        --> native floating point       (*)
  native integer        --> decimal string
  native floating_point --> native integer              (*)
  native floating_point --> decimal string              (*)
  decimal string        --> native integer
  decimal string        --> native floating point       (*)



These conversions are governed by the following general rules:





RESTRICTION: The conversions marked with (*) above involve steps
performed by the C compiler.  In particular, bugs/features of the compiler
used may lead to breakage of some of the above rules.









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45.6 Flavors of Perl numeric operations



Perl operations which take a numeric argument treat that argument in one
of four different ways: they may force it to one of the integer/floating/
string formats, or they may behave differently depending on the format of
the operand.  Forcing a numeric value to a particular format does not
change the number stored in the value.



All the operators which need an argument in the integer format treat the
argument as in modular arithmetic, e.g., mod 2**32 on a 32-bit
architecture.  sprintf "%u", -1 therefore provides the same result as
sprintf "%u", ~0.





Arithmetic operators




The binary operators + - * / % == != > <
>= <= and the unary operators - abs and -- will
attempt to convert arguments to integers.  If both conversions are possible
without loss of precision, and the operation can be performed without
loss of precision then the integer result is used.  Otherwise arguments are
converted to floating point format and the floating point result is used.
The caching of conversions (as described above) means that the integer
conversion does not throw away fractional parts on floating point numbers.





++




++ behaves as the other operators above, except that if it is a string
matching the format /^[a-zA-Z]*[0-9]*\z/ the string increment described
in (perlop) is used.





Arithmetic operators during use integer




In scopes where use integer; is in force, nearly all the operators listed
above will force their argument(s) into integer format, and return an integer
result.  The exceptions, abs, ++ and --, do not change their
behavior with use integer;





Other mathematical operators




Operators such as **, sin and exp force arguments to floating point
format.





Bitwise operators




Arguments are forced into the integer format if not strings.





Bitwise operators during use integer




forces arguments to integer format. Also shift operations internally use
signed integers rather than the default unsigned.





Operators which expect an integer




force the argument into the integer format.  This is applicable
to the third and fourth arguments of sysread, for example.





Operators which expect a string




force the argument into the string format.  For example, this is
applicable to printf "%s", $value.








Though forcing an argument into a particular form does not change the
stored number, Perl remembers the result of such conversions.  In
particular, though the first such conversion may be time-consuming,
repeated operations will not need to redo the conversion.









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45.7 AUTHOR



Ilya Zakharevich ilya@math.ohio-state.edu



Editorial adjustments by Gurusamy Sarathy <gsar@ActiveState.com>



Updates for 5.8.0 by Nicholas Clark <nick@ccl4.org>









Previous: perlnumber AUTHOR, Up: perlnumber   [Contents]





45.8 SEE ALSO



(overload), (perlop)









Next: , Previous: perlnumber, Up: Top   [Contents]





46 NAME object OOP





• NAME object OOP NAME:  



• NAME object OOP DESCRIPTION:  



• NAME object OOP SEE ALSO:  












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46.1 NAME





perlobj - Perl object reference









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46.2 DESCRIPTION



This document provides a reference for Perl’s object orientation
features. If you’re looking for an introduction to object-oriented
programming in Perl, please see perlootut NAME.



In order to understand Perl objects, you first need to understand
references in Perl. See (perlref) for details.



This document describes all of Perl’s object-oriented (OO) features
from the ground up. If you’re just looking to write some
object-oriented code of your own, you are probably better served by
using one of the object systems from CPAN described in perlootut NAME.



If you’re looking to write your own object system, or you need to
maintain code which implements objects from scratch then this document
will help you understand exactly how Perl does object orientation.



There are a few basic principles which define object oriented Perl:



    

  1.  An object is simply a data structure that knows to which class it
belongs.


  2.  A class is simply a package. A class provides methods that expect to
operate on objects.


  3.  A method is simply a subroutine that expects a reference to an object
(or a package name, for class methods) as the first argument.





Let’s look at each of these principles in depth.





• NAME object OOP An Object is Simply a Data Structure:  



• NAME object OOP A Class is Simply a Package:  



• NAME object OOP A Method is Simply a Subroutine:  



• NAME object OOP Method Invocation:  



• NAME object OOP Inheritance:  



• NAME object OOP Writing Constructors:  



• NAME object OOP Attributes:  



• NAME object OOP An Aside About Smarter and Safer Code:  



• NAME object OOP Method Call Variations:  



• NAME object OOP Invoking Class Methods:  



• NAME object OOP bless, blessed, and ref:  



• NAME object OOP The UNIVERSAL Class:  



• NAME object OOP AUTOLOAD:  



• NAME object OOP Destructors:  



• NAME object OOP Non-Hash Objects:  



• NAME object OOP Inside-Out objects:  



• NAME object OOP Pseudo-hashes:  












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46.2.1 An Object is Simply a Data Structure







Unlike many other languages which support object orientation, Perl does
not provide any special syntax for constructing an object. Objects are
merely Perl data structures (hashes, arrays, scalars, filehandles,
etc.) that have been explicitly associated with a particular class.



That explicit association is created by the built-in bless function,
which is typically used within the constructor subroutine of the
class.



Here is a simple constructor:



  package File;

  sub new {
      my $class = shift;

      return bless {}, $class;
  }



The name new isn’t special. We could name our constructor something
else:



  package File;

  sub load {
      my $class = shift;

      return bless {}, $class;
  }



The modern convention for OO modules is to always use new as the
name for the constructor, but there is no requirement to do so. Any
subroutine that blesses a data structure into a class is a valid
constructor in Perl.



In the previous examples, the {} code creates a reference to an
empty anonymous hash. The bless function then takes that reference
and associates the hash with the class in $class. In the simplest
case, the $class variable will end up containing the string "File".



We can also use a variable to store a reference to the data structure
that is being blessed as our object:



  sub new {
      my $class = shift;

      my $self = {};
      bless $self, $class;

      return $self;
  }



Once we’ve blessed the hash referred to by $self we can start
calling methods on it. This is useful if you want to put object
initialization in its own separate method:



  sub new {
      my $class = shift;

      my $self = {};
      bless $self, $class;

      $self->_initialize();

      return $self;
  }



Since the object is also a hash, you can treat it as one, using it to
store data associated with the object. Typically, code inside the class
can treat the hash as an accessible data structure, while code outside
the class should always treat the object as opaque. This is called
encapsulation. Encapsulation means that the user of an object does
not have to know how it is implemented. The user simply calls
documented methods on the object.



Note, however, that (unlike most other OO languages) Perl does not
ensure or enforce encapsulation in any way. If you want objects to
actually be opaque you need to arrange for that yourself. This can
be done in a varierty of ways, including using Inside-Out objects
or modules from CPAN.





• NAME object OOP Objects Are Blessed; Variables Are Not:  












Up: NAME object OOP An Object is Simply a Data Structure   [Contents]





46.2.1.1 Objects Are Blessed; Variables Are Not



When we bless something, we are not blessing the variable which
contains a reference to that thing, nor are we blessing the reference
that the variable stores; we are blessing the thing that the variable
refers to (sometimes known as the referent). This is best
demonstrated with this code:



  use Scalar::Util 'blessed';

  my $foo = {};
  my $bar = $foo;

  bless $foo, 'Class';
  print blessed( $bar );      # prints "Class"

  $bar = "some other value";
  print blessed( $bar );      # prints undef



When we call bless on a variable, we are actually blessing the
underlying data structure that the variable refers to. We are not
blessing the reference itself, nor the variable that contains that
reference. That’s why the second call to blessed( $bar ) returns
false. At that point $bar is no longer storing a reference to an
object.



You will sometimes see older books or documentation mention "blessing a
reference" or describe an object as a "blessed reference", but this is
incorrect. It isn’t the reference that is blessed as an object; it’s
the thing the reference refers to (i.e. the referent).









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46.2.2 A Class is Simply a Package







Perl does not provide any special syntax for class definitions. A
package is simply a namespace containing variables and subroutines. The
only difference is that in a class, the subroutines may expect a
reference to an object or the name of a class as the first argument.
This is purely a matter of convention, so a class may contain both
methods and subroutines which don’t operate on an object or class.



Each package contains a special array called @ISA. The @ISA array
contains a list of that class’s parent classes, if any. This array is
examined when Perl does method resolution, which we will cover later.



It is possible to manually set @ISA, and you may see this in older
Perl code. Much older code also uses the (base) pragma. For new code,
we recommend that you use the (parent) pragma to declare your parents.
This pragma will take care of setting @ISA.  It will also load the
parent classes and make sure that the package doesn’t inherit from
itself.



However the parent classes are set, the package’s @ISA variable will
contain a list of those parents. This is simply a list of scalars, each
of which is a string that corresponds to a package name.



All classes inherit from the (UNIVERSAL) class implicitly. The
(UNIVERSAL) class is implemented by the Perl core, and provides
several default methods, such as isa(), can(), and VERSION().
The UNIVERSAL class will never appear in a package’s @ISA
variable.



Perl only provides method inheritance as a built-in feature.
Attribute inheritance is left up the class to implement. See the
Writing Accessors section for details.









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46.2.3 A Method is Simply a Subroutine




Perl does not provide any special syntax for defining a method. A
method is simply a regular subroutine, and is declared with sub.
What makes a method special is that it expects to receive either an
object or a class name as its first argument.



Perl does provide special syntax for method invocation, the -> operator. We will cover this in more detail later.



Most methods you write will expect to operate on objects:



  sub save {
      my $self = shift;

      open my $fh, '>', $self->path() or die $!;
      print {$fh} $self->data()       or die $!;
      close $fh                       or die $!;
  }









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46.2.4 Method Invocation







Calling a method on an object is written as $object->method.



The left hand side of the method invocation (or arrow) operator is the
object (or class name), and the right hand side is the method name.



  my $pod = File->new( 'perlobj.pod', $data );
  $pod->save();



The -> syntax is also used when dereferencing a reference.  It
looks like the same operator, but these are two different operations.



When you call a method, the thing on the left side of the arrow is
passed as the first argument to the method. That means when we call Critter->new(), the new() method receives the string "Critter"
as its first argument. When we call $fred->speak(), the $fred
variable is passed as the first argument to speak().



Just as with any Perl subroutine, all of the arguments passed in @_
are aliases to the original argument. This includes the object itself.
If you assign directly to $_[0] you will change the contents of the
variable that holds the reference to the object. We recommend that you
don’t do this unless you know exactly what you’re doing.



Perl knows what package the method is in by looking at the left side of
the arrow. If the left hand side is a package name, it looks for the
method in that package. If the left hand side is an object, then Perl
looks for the method in the package that the object has been blessed
into.



If the left hand side is neither a package name nor an object, then the
method call will cause an error, but see the section on Method Call
Variations for more nuances.









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46.2.5 Inheritance




We already talked about the special @ISA array and the (parent)
pragma.



When a class inherits from another class, any methods defined in the
parent class are available to the child class. If you attempt to call a
method on an object that isn’t defined in its own class, Perl will also
look for that method in any parent classes it may have.



  package File::MP3;
  use parent 'File';    # sets @File::MP3::ISA = ('File');

  my $mp3 = File::MP3->new( 'Andvari.mp3', $data );
  $mp3->save();



Since we didn’t define a save() method in the File::MP3 class,
Perl will look at the File::MP3 class’s parent classes to find the
save() method. If Perl cannot find a save() method anywhere in
the inheritance hierarchy, it will die.



In this case, it finds a save() method in the File class. Note
that the object passed to save() in this case is still a
File::MP3 object, even though the method is found in the File
class.



We can override a parent’s method in a child class. When we do so, we
can still call the parent class’s method with the SUPER
pseudo-class.



  sub save {
      my $self = shift;

      say 'Prepare to rock';
      $self->SUPER::save();
  }



The SUPER modifier can only be used for method calls. You can’t
use it for regular subroutine calls or class methods:



  SUPER::save($thing);     # FAIL: looks for save() sub in package SUPER

  SUPER->save($thing);     # FAIL: looks for save() method in class
                           #       SUPER

  $thing->SUPER::save();   # Okay: looks for save() method in parent
                           #       classes





• NAME object OOP How SUPER is Resolved:  



• NAME object OOP Multiple Inheritance:  



• NAME object OOP Method Resolution Order:  



• NAME object OOP Method Resolution Caching:  












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46.2.5.1 How SUPER is Resolved




The SUPER pseudo-class is resolved from the package where the call
is made. It is not resolved based on the object’s class. This is
important, because it lets methods at different levels within a deep
inheritance hierarchy each correctly call their respective parent
methods.



  package A;

  sub new {
      return bless {}, shift;
  }

  sub speak {
      my $self = shift;

      $self->SUPER::speak();

      say 'A';
  }

  package B;

  use parent 'A';

  sub speak {
      my $self = shift;

      $self->SUPER::speak();

      say 'B';
  }

  package C;

  use parent 'B';

  sub speak {
      my $self = shift;

      $self->SUPER::speak();

      say 'C';
  }

  my $c = C->new();
  $c->speak();



In this example, we will get the following output:



  A
  B
  C



This demonstrates how SUPER is resolved. Even though the object is
blessed into the C class, the speak() method in the B class
can still call SUPER::speak() and expect it to correctly look in the
parent class of B (i.e the class the method call is in), not in the
parent class of C (i.e. the class the object belongs to).



There are rare cases where this package-based resolution can be a
problem. If you copy a subroutine from one package to another, SUPER
resolution will be done based on the original package.









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46.2.5.2 Multiple Inheritance




Multiple inheritance often indicates a design problem, but Perl always
gives you enough rope to hang yourself with if you ask for it.



To declare multiple parents, you simply need to pass multiple class
names to use parent:



  package MultiChild;

  use parent 'Parent1', 'Parent2';









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46.2.5.3 Method Resolution Order





Method resolution order only matters in the case of multiple
inheritance. In the case of single inheritance, Perl simply looks up
the inheritance chain to find a method:



  Grandparent
    |
  Parent
    |
  Child



If we call a method on a Child object and that method is not defined
in the Child class, Perl will look for that method in the Parent
class and then, if necessary, in the Grandparent class.



If Perl cannot find the method in any of these classes, it will die
with an error message.



When a class has multiple parents, the method lookup order becomes more
complicated.



By default, Perl does a depth-first left-to-right search for a method.
That means it starts with the first parent in the @ISA array, and
then searches all of its parents, grandparents, etc. If it fails to
find the method, it then goes to the next parent in the original
class’s @ISA array and searches from there.



            SharedGreatGrandParent
            /                    \
  PaternalGrandparent       MaternalGrandparent
            \                    /
             Father        Mother
                   \      /
                    Child



So given the diagram above, Perl will search Child, Father,
PaternalGrandparent, SharedGreatGrandParent, Mother, and
finally MaternalGrandparent. This may be a problem because now we’re
looking in SharedGreatGrandParent before we’ve checked all its
derived classes (i.e. before we tried Mother and
MaternalGrandparent).



It is possible to ask for a different method resolution order with the
(mro) pragma.



  package Child;

  use mro 'c3';
  use parent 'Father', 'Mother';



This pragma lets you switch to the "C3" resolution order. In simple
terms, "C3" order ensures that shared parent classes are never searched
before child classes, so Perl will now search: Child, Father,
PaternalGrandparent, Mother MaternalGrandparent, and finally
SharedGreatGrandParent. Note however that this is not
"breadth-first" searching: All the Father ancestors (except the
common ancestor) are searched before any of the Mother ancestors are
considered.



The C3 order also lets you call methods in sibling classes with the
next pseudo-class. See the (mro) documentation for more details on
this feature.









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46.2.5.4 Method Resolution Caching



When Perl searches for a method, it caches the lookup so that future
calls to the method do not need to search for it again. Changing a
class’s parent class or adding subroutines to a class will invalidate
the cache for that class.



The (mro) pragma provides some functions for manipulating the method
cache directly.









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46.2.6 Writing Constructors




As we mentioned earlier, Perl provides no special constructor syntax.
This means that a class must implement its own constructor. A
constructor is simply a class method that returns a reference to a new
object.



The constructor can also accept additional parameters that define the
object. Let’s write a real constructor for the File class we used
earlier:



  package File;

  sub new {
      my $class = shift;
      my ( $path, $data ) = @_;

      my $self = bless {
          path => $path,
          data => $data,
      }, $class;

      return $self;
  }



As you can see, we’ve stored the path and file data in the object
itself. Remember, under the hood, this object is still just a hash.
Later, we’ll write accessors to manipulate this data.



For our File::MP3 class, we can check to make sure that the path we’re
given ends with ".mp3":



  package File::MP3;

  sub new {
      my $class = shift;
      my ( $path, $data ) = @_;

      die "You cannot create a File::MP3 without an mp3 extension\n"
          unless $path =~ /\.mp3\z/;

      return $class->SUPER::new(@_);
  }



This constructor lets its parent class do the actual object
construction.









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46.2.7 Attributes




An attribute is a piece of data belonging to a particular object.
Unlike most object-oriented languages, Perl provides no special syntax
or support for declaring and manipulating attributes.



Attributes are often stored in the object itself. For example, if the
object is an anonymous hash, we can store the attribute values in the
hash using the attribute name as the key.



While it’s possible to refer directly to these hash keys outside of the
class, it’s considered a best practice to wrap all access to the
attribute with accessor methods.



This has several advantages. Accessors make it easier to change the
implementation of an object later while still preserving the original
API.



An accessor lets you add additional code around attribute access. For
example, you could apply a default to an attribute that wasn’t set in
the constructor, or you could validate that a new value for the
attribute is acceptable.



Finally, using accessors makes inheritance much simpler. Subclasses can
use the accessors rather than having to know how a parent class is
implemented internally.





• NAME object OOP Writing Accessors:  












Up: NAME object OOP Attributes   [Contents]





46.2.7.1 Writing Accessors




As with constructors, Perl provides no special accessor declaration
syntax, so classes must provide explicitly written accessor methods.
There are two common types of accessors, read-only and read-write.



A simple read-only accessor simply gets the value of a single
attribute:



  sub path {
      my $self = shift;

      return $self->{path};
  }



A read-write accessor will allow the caller to set the value as well as
get it:



  sub path {
      my $self = shift;

      if (@_) {
          $self->{path} = shift;
      }

      return $self->{path};
  }









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46.2.8 An Aside About Smarter and Safer Code



Our constructor and accessors are not very smart. They don’t check that
a $path is defined, nor do they check that a $path is a valid
filesystem path.



Doing these checks by hand can quickly become tedious. Writing a bunch
of accessors by hand is also incredibly tedious. There are a lot of
modules on CPAN that can help you write safer and more concise code,
including the modules we recommend in perlootut NAME.









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46.2.9 Method Call Variations




Perl supports several other ways to call methods besides the $object->method() usage we’ve seen so far.





• NAME object OOP Method Names as Strings:  



• NAME object OOP Class Names as Strings:  



• NAME object OOP Subroutine References as Methods:  



• NAME object OOP Deferencing Method Call:  



• NAME object OOP Method Calls on Filehandles:  












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46.2.9.1 Method Names as Strings



Perl lets you use a scalar variable containing a string as a method
name:



  my $file = File->new( $path, $data );

  my $method = 'save';
  $file->$method();



This works exactly like calling $file->save(). This can be very
useful for writing dynamic code. For example, it allows you to pass a
method name to be called as a parameter to another method.









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46.2.9.2 Class Names as Strings



Perl also lets you use a scalar containing a string as a class name:



  my $class = 'File';

  my $file = $class->new( $path, $data );



Again, this allows for very dynamic code.









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46.2.9.3 Subroutine References as Methods



You can also use a subroutine reference as a method:



  my $sub = sub {
      my $self = shift;

      $self->save();
  };

  $file->$sub();



This is exactly equivalent to writing $sub->($file). You may see
this idiom in the wild combined with a call to can:



  if ( my $meth = $object->can('foo') ) {
      $object->$meth();
  }









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46.2.9.4 Deferencing Method Call



Perl also lets you use a dereferenced scalar reference in a method
call. That’s a mouthful, so let’s look at some code:



  $file->${ \'save' };
  $file->${ returns_scalar_ref() };
  $file->${ \( returns_scalar() ) };
  $file->${ returns_sub_ref() };



This works if the dereference produces a string or a subroutine
reference.









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46.2.9.5 Method Calls on Filehandles



Under the hood, Perl filehandles are instances of the IO::Handle or
IO::File class. Once you have an open filehandle, you can call
methods on it. Additionally, you can call methods on the STDIN,
STDOUT, and STDERR filehandles.



  open my $fh, '>', 'path/to/file';
  $fh->autoflush();
  $fh->print('content');

  STDOUT->autoflush();









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46.2.10 Invoking Class Methods




Because Perl allows you to use barewords for package names and
subroutine names, it sometimes interprets a bareword’s meaning
incorrectly. For example, the construct Class->new() can be
interpreted as either 'Class'->new() or Class()->new().
In English, that second interpretation reads as "call a subroutine
named Class(), then call new() as a method on the return value of
Class()". If there is a subroutine named Class() in the current
namespace, Perl will always interpret Class->new() as the second
alternative: a call to new() on the object  returned by a call to
Class()



You can force Perl to use the first interpretation (i.e. as a method
call on the class named "Class") in two ways. First, you can append a
:: to the class name:



    Class::->new()



Perl will always interpret this as a method call.



Alternatively, you can quote the class name:



    'Class'->new()



Of course, if the class name is in a scalar Perl will do the right
thing as well:



    my $class = 'Class';
    $class->new();





• NAME object OOP Indirect Object Syntax:  












Up: NAME object OOP Invoking Class Methods   [Contents]





46.2.10.1 Indirect Object Syntax




Outside of the file handle case, use of this syntax is discouraged,
as it can confuse the Perl interpreter. See below for more details.



Perl suports another method invocation syntax called "indirect object"
notation. This syntax is called "indirect" because the method comes
before the object it is being invoked on.



This syntax can be used with any class or object method:



    my $file = new File $path, $data;
    save $file;



We recommend that you avoid this syntax, for several reasons.



First, it can be confusing to read. In the above example, it’s not
clear if save is a method provided by the File class or simply a
subroutine that expects a file object as its first argument.



When used with class methods, the problem is even worse. Because Perl
allows subroutine names to be written as barewords, Perl has to guess
whether the bareword after the method is a class name or subroutine
name. In other words, Perl can resolve the syntax as either File->new( $path, $data ) or new( File( $path, $data ) ).



To parse this code, Perl uses a heuristic based on what package names
it has seen, what subroutines exist in the current package, what
barewords it has previously seen, and other input. Needless to say,
heuristics can produce very surprising results!



Older documentation (and some CPAN modules) encouraged this syntax,
particularly for constructors, so you may still find it in the wild.
However, we encourage you to avoid using it in new code.



You can force Perl to interpret the bareword as a class name by
appending "::" to it, like we saw earlier:



  my $file = new File:: $path, $data;









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46.2.11 bless, blessed, and ref



As we saw earlier, an object is simply a data structure that has been
blessed into a class via the bless function. The bless function
can take either one or two arguments:



  my $object = bless {}, $class;
  my $object = bless {};



In the first form, the anonymous hash is being blessed into the class
in $class. In the second form, the anonymous hash is blessed into
the current package.



The second form is strongly discouraged, because it breaks the ability
of a subclass to reuse the parent’s constructor, but you may still run
across it in existing code.



If you want to know whether a particular scalar refers to an object,
you can use the blessed function exported by (Scalar-Util), which
is shipped with the Perl core.



  use Scalar::Util 'blessed';

  if ( defined blessed($thing) ) { ... }



If $thing refers to an object, then this function returns the name
of the package the object has been blessed into. If $thing doesn’t
contain a reference to a blessed object, the blessed function
returns undef.



Note that blessed($thing) will also return false if $thing has
been blessed into a class named "0". This is a possible, but quite
pathological. Don’t create a class named "0" unless you know what
you’re doing.



Similarly, Perl’s built-in ref function treats a reference to a
blessed object specially. If you call ref($thing) and $thing
holds a reference to an object, it will return the name of the class
that the object has been blessed into.



If you simply want to check that a variable contains an object
reference, we recommend that you use defined blessed($object), since
ref returns true values for all references, not just objects.









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46.2.12 The UNIVERSAL Class




All classes automatically inherit from the (UNIVERSAL) class, which is
built-in to the Perl core. This class provides a number of methods, all
of which can be called on either a class or an object. You can also
choose to override some of these methods in your class. If you do so,
we recommend that you follow the built-in semantics described below.





isa($class)





The isa method returns true if the object is a member of the
class in $class, or a member of a subclass of $class.



If you override this method, it should never throw an exception.





DOES($role)





The DOES method returns true if its object claims to perform the
role $role. By default, this is equivalent to isa. This method is
provided for use by object system extensions that implement roles, like
Moose and Role::Tiny.



You can also override DOES directly in your own classes. If you
override this method, it should never throw an exception.





can($method)





The can method checks to see if the class or object it was called on
has a method named $method. This checks for the method in the class
and all of its parents. If the method exists, then a reference to the
subroutine is returned. If it does not then undef is returned.



If your class responds to method calls via AUTOLOAD, you may want to
overload can to return a subroutine reference for methods which your
AUTOLOAD method handles.



If you override this method, it should never throw an exception.





VERSION($need)





The VERSION method returns the version number of the class
(package).



If the $need argument is given then it will check that the current
version (as defined by the $VERSION variable in the package) is greater
than or equal to $need; it will die if this is not the case. This
method is called automatically by the VERSION form of use.



    use Package 1.2 qw(some imported subs);
    # implies:
    Package->VERSION(1.2);



We recommend that you use this method to access another package’s
version, rather than looking directly at $Package::VERSION. The
package you are looking at could have overridden the VERSION method.



We also recommend using this method to check whether a module has a
sufficient version. The internal implementation uses the (version)
module to make sure that different types of version numbers are
compared correctly.














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46.2.13 AUTOLOAD




If you call a method that doesn’t exist in a class, Perl will throw an
error. However, if that class or any of its parent classes defines an
AUTOLOAD method, that AUTOLOAD method is called instead.



AUTOLOAD is called as a regular method, and the caller will not know
the difference. Whatever value your AUTOLOAD method returns is
returned to the caller.



The fully qualified method name that was called is available in the
$AUTOLOAD package global for your class. Since this is a global, if
you want to refer to do it without a package name prefix under strict
'vars', you need to declare it.



  # XXX - this is a terrible way to implement accessors, but it makes
  # for a simple example.
  our $AUTOLOAD;
  sub AUTOLOAD {
      my $self = shift;

      # Remove qualifier from original method name...
      my $called =  $AUTOLOAD =~ s/.*:://r;

      # Is there an attribute of that name?
      die "No such attribute: $called"
          unless exists $self->{$called};

      # If so, return it...
      return $self->{$called};
  }

  sub DESTROY { } # see below



Without the our $AUTOLOAD declaration, this code will not compile
under the (strict) pragma.



As the comment says, this is not a good way to implement accessors. 
It’s slow and too clever by far. However, you may see this as a way to
provide accessors in older Perl code. See perlootut NAME for
recommendations on OO coding in Perl.



If your class does have an AUTOLOAD method, we strongly recommend
that you override can in your class as well. Your overridden can
method should return a subroutine reference for any method that your
AUTOLOAD responds to.









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46.2.14 Destructors





When the last reference to an object goes away, the object is
destroyed. If you only have one reference to an object stored in a
lexical scalar, the object is destroyed when that scalar goes out of
scope. If you store the object in a package global, that object may not
go out of scope until the program exits.



If you want to do something when the object is destroyed, you can
define a DESTROY method in your class. This method will always be
called by Perl at the appropriate time, unless the method is empty.



This is called just like any other method, with the object as the first
argument. It does not receive any additional arguments. However, the
$_[0] variable will be read-only in the destructor, so you cannot
assign a value to it.



If your DESTROY method throws an error, this error will be ignored.
It will not be sent to STDERR and it will not cause the program to
die. However, if your destructor is running inside an eval {} block,
then the error will change the value of $@.



Because DESTROY methods can be called at any time, you should
localize any global variables you might update in your DESTROY. In
particular, if you use eval {} you should localize $@, and if you
use system or backticks, you should localize $?.



If you define an AUTOLOAD in your class, then Perl will call your
AUTOLOAD to handle the DESTROY method. You can prevent this by
defining an empty DESTROY, like we did in the autoloading example.
You can also check the value of $AUTOLOAD and return without doing
anything when called to handle DESTROY.





• NAME object OOP Global Destruction:  












Up: NAME object OOP Destructors   [Contents]





46.2.14.1 Global Destruction



The order in which objects are destroyed during the global destruction
before the program exits is unpredictable. This means that any objects
contained by your object may already have been destroyed. You should
check that a contained object is defined before calling a method on it:



  sub DESTROY {
      my $self = shift;

      $self->{handle}->close() if $self->{handle};
  }



You can use the ${^GLOBAL_PHASE} variable to detect if you are
currently in the global destruction phase:



  sub DESTROY {
      my $self = shift;

      return if ${^GLOBAL_PHASE} eq 'DESTRUCT';

      $self->{handle}->close();
  }



Note that this variable was added in Perl 5.14.0. If you want to detect
the global destruction phase on older versions of Perl, you can use the
Devel::GlobalDestruction module on CPAN.



If your DESTROY method issues a warning during global destruction,
the Perl interpreter will append the string " during global
destruction" the warning.



During global destruction, Perl will always garbage collect objects
before unblessed references. See perlhacktips PERL_DESTRUCT_LEVEL
for more information about global destruction.









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46.2.15 Non-Hash Objects



All the examples so far have shown objects based on a blessed hash.
However, it’s possible to bless any type of data structure or referent,
including scalars, globs, and subroutines. You may see this sort of
thing when looking at code in the wild.



Here’s an example of a module as a blessed scalar:



  package Time;

  use strict;
  use warnings;

  sub new {
      my $class = shift;

      my $time = time;
      return bless \$time, $class;
  }

  sub epoch {
      my $self = shift;
      return ${ $self };
  }

  my $time = Time->new();
  print $time->epoch();









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46.2.16 Inside-Out objects



In the past, the Perl community experimented with a technique called
"inside-out objects". An inside-out object stores its data outside of
the object’s reference, indexed on a unique property of the object,
such as its memory address, rather than in the object itself. This has
the advantage of enforcing the encapsulation of object attributes,
since their data is not stored in the object itself.



This technique was popular for a while (and was recommended in Damian
Conway’s Perl Best Practices), but never achieved universal
adoption. The (Object-InsideOut) module on CPAN provides a
comprehensive implementation of this technique, and you may see it or
other inside-out modules in the wild.



Here is a simple example of the technique, using the
(Hash-Util-FieldHash) core module. This module was added to the core
to support inside-out object implementations.



  package Time;

  use strict;
  use warnings;

  use Hash::Util::FieldHash 'fieldhash';

  fieldhash my %time_for;

  sub new {
      my $class = shift;

      my $self = bless \( my $object ), $class;

      $time_for{$self} = time;

      return $self;
  }

  sub epoch {
      my $self = shift;

      return $time_for{$self};
  }

  my $time = Time->new;
  print $time->epoch;









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46.2.17 Pseudo-hashes



The pseudo-hash feature was an experimental feature introduced in
earlier versions of Perl and removed in 5.10.0. A pseudo-hash is an
array reference which can be accessed using named keys like a hash. You
may run in to some code in the wild which uses it. See the (fields)
pragma for more information.









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46.3 SEE ALSO



A kinder, gentler tutorial on object-oriented programming in Perl can
be found in perlootut NAME. You should also check out (perlmodlib) for
some style guides on constructing both modules and classes.









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47 perlootut





• perlootut NAME:  



• perlootut DATE:  



• perlootut DESCRIPTION:  



• perlootut OBJECT-ORIENTED FUNDAMENTALS:  



• perlootut PERL OO SYSTEMS:  



• perlootut CONCLUSION:  












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47.1 NAME



perlootut - Object-Oriented Programming in Perl Tutorial









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47.2 DATE



This document was created in February, 2011.









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47.3 DESCRIPTION



This document provides an introduction to object-oriented programming
in Perl. It begins with a brief overview of the concepts behind object
oriented design. Then it introduces several different OO systems from
CPAN which build on top of what Perl
provides.



By default, Perl’s built-in OO system is very minimal, leaving you to
do most of the work. This minimalism made a lot of sense in 1994, but
in the years since Perl 5.0 we’ve seen a number of common patterns
emerge in Perl OO. Fortunately, Perl’s flexibility has allowed a rich
ecosystem of Perl OO systems to flourish.



If you want to know how Perl OO works under the hood, the (perlobj)
document explains the nitty gritty details.



This document assumes that you already understand the basics of Perl
syntax, variable types, operators, and subroutine calls. If you don’t
understand these concepts yet, please read perlintro NAME first. You
should also read the (perlsyn), (perlop), and (perlsub) documents.









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47.4 OBJECT-ORIENTED FUNDAMENTALS



Most object systems share a number of common concepts. You’ve probably
heard terms like "class", "object, "method", and "attribute" before.
Understanding the concepts will make it much easier to read and write
object-oriented code. If you’re already familiar with these terms, you
should still skim this section, since it explains each concept in terms
of Perl’s OO implementation.



Perl’s OO system is class-based. Class-based OO is fairly common. It’s
used by Java, C++, C#, Python, Ruby, and many other languages. There
are other object orientation paradigms as well. JavaScript is the most
popular language to use another paradigm. JavaScript’s OO system is
prototype-based.





• perlootut Object:  



• perlootut Class:  



• perlootut Methods:  



• perlootut Attributes:  



• perlootut Polymorphism:  



• perlootut Inheritance:  



• perlootut Encapsulation:  



• perlootut Composition:  



• perlootut Roles:  



• perlootut When to Use OO:  












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47.4.1 Object



An object is a data structure that bundles together data and
subroutines which operate on that data. An object’s data is called
attributes, and its subroutines are called methods. An object can
be thought of as a noun (a person, a web service, a computer).



An object represents a single discrete thing. For example, an object
might represent a file. The attributes for a file object might include
its path, content, and last modification time. If we created an object
to represent /etc/hostname on a machine named "foo.example.com",
that object’s path would be "/etc/hostname", its content would be
"foo\n", and it’s last modification time would be 1304974868 seconds
since the beginning of the epoch.



The methods associated with a file might include rename() and
write().



In Perl most objects are hashes, but the OO systems we recommend keep
you from having to worry about this. In practice, it’s best to consider
an object’s internal data structure opaque.









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47.4.2 Class



A class defines the behavior of a category of objects. A class is a
name for a category (like "File"), and a class also defines the
behavior of objects in that category.



All objects belong to a specific class. For example, our
/etc/hostname object belongs to the File class. When we want to
create a specific object, we start with its class, and construct or
instantiate an object. A specific object is often referred to as an
instance of a class.



In Perl, any package can be a class. The difference between a package
which is a class and one which isn’t is based on how the package is
used. Here’s our "class declaration" for the File class:



  package File;



In Perl, there is no special keyword for constructing an object.
However, most OO modules on CPAN use a method named new() to
construct a new object:



  my $hostname = File->new(
      path          => '/etc/hostname',
      content       => "foo\n",
      last_mod_time => 1304974868,
  );



(Don’t worry about that -> operator, it will be explained
later.)





• perlootut Blessing:  



• perlootut Constructor:  












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47.4.2.1 Blessing



As we said earlier, most Perl objects are hashes, but an object can be
an instance of any Perl data type (scalar, array, etc.). Turning a
plain data structure into an object is done by blessing that data
structure using Perl’s bless function.



While we strongly suggest you don’t build your objects from scratch,
you should know the term bless. A blessed data structure (aka "a
referent") is an object. We sometimes say that an object has been
"blessed into a class".



Once a referent has been blessed, the blessed function from the
(Scalar-Util) core module can tell us its class name. This subroutine
returns an object’s class when passed an object, and false otherwise.



  use Scalar::Util 'blessed';

  print blessed($hash);      # undef
  print blessed($hostname);  # File









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47.4.2.2 Constructor



A constructor creates a new object. In Perl, a class’s constructor
is just another method, unlike some other languages, which provide
syntax for constructors. Most Perl classes use new as the name for
their constructor:



  my $file = File->new(...);









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47.4.3 Methods



You already learned that a method is a subroutine that operates on
an object. You can think of a method as the things that an object can
do. If an object is a noun, then methods are its verbs (save, print,
open).



In Perl, methods are simply subroutines that live in a class’s package.
Methods are always written to receive the object as their first
argument:



  sub print_info {
      my $self = shift;

      print "This file is at ", $self->path, "\n";
  }

  $file->print_info;
  # The file is at /etc/hostname



What makes a method special is how it’s called. The arrow operator
(->) tells Perl that we are calling a method.



When we make a method call, Perl arranges for the method’s invocant
to be passed as the first argument. Invocant is a fancy name for the
thing on the left side of the arrow. The invocant can either be a class
name or an object. We can also pass additional arguments to the method:



  sub print_info {
      my $self   = shift;
      my $prefix = shift // "This file is at ";

      print $prefix, ", ", $self->path, "\n";
  }

  $file->print_info("The file is located at ");
  # The file is located at /etc/hostname









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47.4.4 Attributes



Each class can define its attributes. When we instantiate an object,
we assign values to those attributes. For example, every File object
has a path. Attributes are sometimes called properties.



Perl has no special syntax for attributes. Under the hood, attributes
are often stored as keys in the object’s underlying hash, but don’t
worry about this.



We recommend that you only access attributes via accessor methods.
These are methods that can get or set the value of each attribute. We
saw this earlier in the print_info() example, which calls $self->path.



You might also see the terms getter and setter. These are two
types of accessors. A getter gets the attribute’s value, while a setter
sets it. Another term for a setter is mutator



Attributes are typically defined as read-only or read-write. Read-only
attributes can only be set when the object is first created, while
read-write attributes can be altered at any time.



The value of an attribute may itself be another object. For example,
instead of returning its last mod time as a number, the File class
could return a (DateTime) object representing that value.



It’s possible to have a class that does not expose any publicly
settable attributes. Not every class has attributes and methods.









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47.4.5 Polymorphism



Polymorphism is a fancy way of saying that objects from two
different classes share an API. For example, we could have File and
WebPage classes which both have a print_content() method. This
method might produce different output for each class, but they share a
common interface.



While the two classes may differ in many ways, when it comes to the
print_content() method, they are the same. This means that we can
try to call the print_content() method on an object of either class,
and we don’t have to know what class the object belongs to!



Polymorphism is one of the key concepts of object-oriented design.









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47.4.6 Inheritance



Inheritance lets you create a specialized version of an existing
class. Inheritance lets the new class to reuse the methods and
attributes of another class.



For example, we could create an File::MP3 class which inherits
from File. An File::MP3 is-a more specific type of File.
All mp3 files are files, but not all files are mp3 files.



We often refer to inheritance relationships as parent-child or
superclass/subclass relationships. Sometimes we say that the child
has an is-a relationship with its parent class.



File is a superclass of File::MP3, and File::MP3 is a
subclass of File.



  package File::MP3;

  use parent 'File';



The (parent) module is one of several ways that Perl lets you define
inheritance relationships.



Perl allows multiple inheritance, which means that a class can inherit
from multiple parents. While this is possible, we strongly recommend
against it. Generally, you can use roles to do everything you can do
with multiple inheritance, but in a cleaner way.



Note that there’s nothing wrong with defining multiple subclasses of a
given class. This is both common and safe. For example, we might define
File::MP3::FixedBitrate and File::MP3::VariableBitrate classes to
distinguish between different types of mp3 file.





• perlootut Overriding methods and method resolution:  












Up: perlootut Inheritance   [Contents]





47.4.6.1 Overriding methods and method resolution



Inheritance allows two classes to share code. By default, every method
in the parent class is also available in the child. The child can
explicitly override a parent’s method to provide its own
implementation. For example, if we have an File::MP3 object, it has
the print_info() method from File:



  my $cage = File::MP3->new(
      path          => 'mp3s/My-Body-Is-a-Cage.mp3',
      content       => $mp3_data,
      last_mod_time => 1304974868,
      title         => 'My Body Is a Cage',
  );

  $cage->print_info;
  # The file is at mp3s/My-Body-Is-a-Cage.mp3



If we wanted to include the mp3’s title in the greeting, we could
override the method:



  package File::MP3;

  use parent 'File';

  sub print_info {
      my $self = shift;

      print "This file is at ", $self->path, "\n";
      print "Its title is ", $self->title, "\n";
  }

  $cage->print_info;
  # The file is at mp3s/My-Body-Is-a-Cage.mp3
  # Its title is My Body Is a Cage



The process of determining what method should be used is called
method resolution. What Perl does is look at the object’s class
first (File::MP3 in this case). If that class defines the method,
then that class’s version of the method is called. If not, Perl looks
at each parent class in turn. For File::MP3, its only parent is
File. If File::MP3 does not define the method, but File does,
then Perl calls the method in File.



If File inherited from DataSource, which inherited from Thing,
then Perl would keep looking "up the chain" if necessary.



It is possible to explicitly call a parent method from a child:



  package File::MP3;

  use parent 'File';

  sub print_info {
      my $self = shift;

      $self->SUPER::print_info();
      print "Its title is ", $self->title, "\n";
  }



The SUPER:: bit tells Perl to look for the print_info() in the
File::MP3 class’s inheritance chain. When it finds the parent class
that implements this method, the method is called.



We mentioned multiple inheritance earlier. The main problem with
multiple inheritance is that it greatly complicates method resolution.
See (perlobj) for more details.









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47.4.7 Encapsulation



Encapsulation is the idea that an object is opaque. When another
developer uses your class, they don’t need to know how it is
implemented, they just need to know what it does.



Encapsulation is important for several reasons. First, it allows you to
separate the public API from the private implementation. This means you
can change that implementation without breaking the API.



Second, when classes are well encapsulated, they become easier to
subclass. Ideally, a subclass uses the same APIs to access object data
that its parent class uses. In reality, subclassing sometimes involves
violating encapsulation, but a good API can minimize the need to do
this.



We mentioned earlier that most Perl objects are implemented as hashes
under the hood. The principle of encapsulation tells us that we should
not rely on this. Instead, we should use accessor methods to access the
data in that hash. The object systems that we recommend below all
automate the generation of accessor methods. If you use one of them,
you should never have to access the object as a hash directly.









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47.4.8 Composition



In object-oriented code, we often find that one object references
another object. This is called composition, or a has-a
relationship.



Earlier, we mentioned that the File class’s last_mod_time
accessor could return a (DateTime) object. This is a perfect example
of composition. We could go even further, and make the path and
content accessors return objects as well. The File class would
then be composed of several other objects.









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47.4.9 Roles



Roles are something that a class does, rather than something that
it is. Roles are relatively new to Perl, but have become rather
popular. Roles are applied to classes. Sometimes we say that classes
consume roles.



Roles are an alternative to inheritance for providing polymorphism.
Let’s assume we have two classes, Radio and Computer. Both of
these things have on/off switches. We want to model that in our class
definitions.



We could have both classes inherit from a common parent, like
Machine, but not all machines have on/off switches. We could create
a parent class called HasOnOffSwitch, but that is very artificial.
Radios and computers are not specializations of this parent. This
parent is really a rather ridiculous creation.



This is where roles come in. It makes a lot of sense to create a
HasOnOffSwitch role and apply it to both classes. This role would
define a known API like providing turn_on() and turn_off()
methods.



Perl does not have any built-in way to express roles. In the past,
people just bit the bullet and used multiple inheritance. Nowadays,
there are several good choices on CPAN for using roles.









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47.4.10 When to Use OO



Object Orientation is not the best solution to every problem. In Perl
Best Practices (copyright 2004, Published by O’Reilly Media, Inc.),
Damian Conway provides a list of criteria to use when deciding if OO is
the right fit for your problem:











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47.5 PERL OO SYSTEMS



As we mentioned before, Perl’s built-in OO system is very minimal, but
also quite flexible. Over the years, many people have developed systems
which build on top of Perl’s built-in system to provide more features
and convenience.



We strongly recommend that you use one of these systems. Even the most
minimal of them eliminates a lot of repetitive boilerplate. There’s
really no good reason to write your classes from scratch in Perl.



If you are interested in the guts underlying these systems, check out
(perlobj).





• perlootut Moose:  



• perlootut Class::Accessor:  



• perlootut Object::Tiny:  



• perlootut Role::Tiny:  



• perlootut OO System Summary:  



• perlootut Other OO Systems:  












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47.5.1 Moose



(Moose) bills itself as a "postmodern object system for Perl 5". Don’t
be scared, the "postmodern" label is a callback to Larry’s description
of Perl as "the first postmodern computer language".



Moose provides a complete, modern OO system. Its biggest influence
is the Common Lisp Object System, but it also borrows ideas from
Smalltalk and several other languages. Moose was created by Stevan
Little, and draws heavily from his work on the Perl 6 OO design.



Here is our File class using Moose:



  package File;
  use Moose;

  has path          => ( is => 'ro' );
  has content       => ( is => 'ro' );
  has last_mod_time => ( is => 'ro' );

  sub print_info {
      my $self = shift;

      print "This file is at ", $self->path, "\n";
  }



Moose provides a number of features:





Of course, Moose isn’t perfect.



Moose can make your code slower to load. Moose itself is not
small, and it does a lot of code generation when you define your
class. This code generation means that your runtime code is as fast as
it can be, but you pay for this when your modules are first loaded.



This load time hit can be a problem when startup speed is important,
such as with a command-line script or a "plain vanilla" CGI script that
must be loaded each time it is executed.



Before you panic, know that many people do use Moose for
command-line tools and other startup-sensitive code. We encourage you
to try Moose out first before worrying about startup speed.



Moose also has several dependencies on other modules. Most of these
are small stand-alone modules, a number of which have been spun off
from Moose. Moose itself, and some of its dependencies, require a
compiler. If you need to install your software on a system without a
compiler, or if having any dependencies is a problem, then Moose
may not be right for you.





• perlootut Mouse:  












Up: perlootut Moose   [Contents]





47.5.1.1 Mouse



If you try Moose and find that one of these issues is preventing you
from using Moose, we encourage you to consider (Mouse) next.
Mouse implements a subset of Moose’s functionality in a simpler
package. For all features that it does implement, the end-user API is
identical to Moose, meaning you can switch from Mouse to
Moose quite easily.



Mouse does not implement most of Moose’s introspection API, so
it’s often faster when loading your modules. Additionally, all of its
required dependencies ship with the Perl core, and it can run
without a compiler. If you do have a compiler, Mouse will use it to
compile some of its code for a speed boost.



Finally, it ships with a Mouse::Tiny module that takes most of
Mouse’s features and bundles them up in a single module file. You
can copy this module file into your application’s library directory for
easy bundling.



The Moose authors hope that one day Mouse can be made obsolete by
improving Moose enough, but for now it provides a worthwhile
alternative to Moose.









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47.5.2 Class::Accessor



(Class-Accessor) is the polar opposite of Moose. It provides very
few features, nor is it self-hosting.



It is, however, very simple, pure Perl, and it has no non-core
dependencies. It also provides a "Moose-like" API on demand for the
features it supports.



Even though it doesn’t do much, it is still preferable to writing your
own classes from scratch.



Here’s our File class with Class::Accessor:



  package File;
  use Class::Accessor 'antlers';

  has path          => ( is => 'ro' );
  has content       => ( is => 'ro' );
  has last_mod_time => ( is => 'ro' );

  sub print_info {
      my $self = shift;

      print "This file is at ", $self->path, "\n";
  }



The antlers import flag tells Class::Accessor that you want to
define your attributes using Moose-like syntax. The only parameter
that you can pass to has is is. We recommend that you use this
Moose-like syntax if you choose Class::Accessor since it means you
will have a smoother upgrade path if you later decide to move to
Moose.



Like Moose, Class::Accessor generates accessor methods and a
constructor for your class.









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47.5.3 Object::Tiny



Finally, we have (Object-Tiny). This module truly lives up to its
name. It has an incredibly minimal API and absolutely no dependencies
(core or not). Still, we think it’s a lot easier to use than writing
your own OO code from scratch.



Here’s our File class once more:



  package File;
  use Object::Tiny qw( path content last_mod_time );

  sub print_info {
      my $self = shift;

      print "This file is at ", $self->path, "\n";
  }



That’s it!



With Object::Tiny, all accessors are read-only. It generates a
constructor for you, as well as the accessors you define.









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47.5.4 Role::Tiny



As we mentioned before, roles provide an alternative to inheritance,
but Perl does not have any built-in role support. If you choose to use
Moose, it comes with a full-fledged role implementation. However, if
you use one of our other recommended OO modules, you can still use
roles with (Role-Tiny)



Role::Tiny provides some of the same features as Moose’s role
system, but in a much smaller package. Most notably, it doesn’t support
any sort of attribute declaration, so you have to do that by hand.
Still, it’s useful, and works well with Class::Accessor and
Object::Tiny









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47.5.5 OO System Summary



Here’s a brief recap of the options we covered:











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47.5.6 Other OO Systems



There are literally dozens of other OO-related modules on CPAN besides
those covered here, and you’re likely to run across one or more of them
if you work with other people’s code.



In addition, plenty of code in the wild does all of its OO "by hand",
using just the Perl built-in OO features. If you need to maintain such
code, you should read (perlobj) to understand exactly how Perl’s
built-in OO works.









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47.6 CONCLUSION



As we said before, Perl’s minimal OO system has led to a profusion of
OO systems on CPAN. While you can still drop down to the bare metal and
write your classes by hand, there’s really no reason to do that with
modern Perl.



For small systems, (Object-Tiny) and (Class-Accessor) both provide
minimal object systems that take care of basic boilerplate for you.



For bigger projects, (Moose) provides a rich set of features that will
let you focus on implementing your business logic.



We encourage you to play with and evaluate (Moose),
(Class-Accessor), and (Object-Tiny) to see which OO system is right
for you.









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48 NAME operator





• NAME operator NAME:  



• NAME operator DESCRIPTION:  












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48.1 NAME




perlop - Perl operators and precedence









Previous: NAME operator NAME, Up: NAME operator   [Contents]





48.2 DESCRIPTION





• NAME operator Operator Precedence and Associativity:  



• NAME operator Terms and List Operators (Leftward):  



• NAME operator The Arrow Operator:  



• NAME operator Auto-increment and Auto-decrement:  



• NAME operator Exponentiation:  



• NAME operator Symbolic Unary Operators:  



• NAME operator Binding Operators:  



• NAME operator Multiplicative Operators:  



• NAME operator Additive Operators:  



• NAME operator Shift Operators:  



• NAME operator Named Unary Operators:  



• NAME operator Relational Operators:  



• NAME operator Equality Operators:  



• NAME operator Smartmatch Operator:  



• NAME operator Bitwise And:  



• NAME operator Bitwise Or and Exclusive Or:  



• NAME operator C-style Logical And:  



• NAME operator C-style Logical Or:  



• NAME operator Logical Defined-Or:  



• NAME operator Range Operators:  



• NAME operator Conditional Operator:  



• NAME operator Assignment Operators:  



• NAME operator Comma Operator:  



• NAME operator List Operators (Rightward):  



• NAME operator Logical Not:  



• NAME operator Logical And:  



• NAME operator Logical or and Exclusive Or:  



• NAME operator C Operators Missing From Perl:  



• NAME operator Quote and Quote-like Operators:  



• NAME operator Regexp Quote-Like Operators:  



• NAME operator Quote-Like Operators:  



• NAME operator Gory details of parsing quoted constructs:  



• NAME operator I/O Operators:  



• NAME operator Constant Folding:  



• NAME operator No-ops:  



• NAME operator Bitwise String Operators:  



• NAME operator Integer Arithmetic:  



• NAME operator Floating-point Arithmetic:  



• NAME operator Bigger Numbers:  












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48.2.1 Operator Precedence and Associativity






Operator precedence and associativity work in Perl more or less like
they do in mathematics.



Operator precedence means some operators are evaluated before
others.  For example, in 2 + 4 * 5, the multiplication has higher
precedence so 4 * 5 is evaluated first yielding 2 + 20 ==
22 and not 6 * 5 == 30.



Operator associativity defines what happens if a sequence of the
same operators is used one after another: whether the evaluator will
evaluate the left operations first or the right.  For example, in 8
- 4 - 2, subtraction is left associative so Perl evaluates the
expression left to right.  8 - 4 is evaluated first making the
expression 4 - 2 == 2 and not 8 - 2 == 6.



Perl operators have the following associativity and precedence,
listed from highest precedence to lowest.  Operators borrowed from
C keep the same precedence relationship with each other, even where
C’s precedence is slightly screwy.  (This makes learning Perl easier
for C folks.)  With very few exceptions, these all operate on scalar
values only, not array values.



    left        terms and list operators (leftward)
    left        ->
    nonassoc    ++ --
    right       **
    right       ! ~ \ and unary + and -
    left        =~ !~
    left        * / % x
    left        + - .
    left        << >>
    nonassoc    named unary operators
    nonassoc    < > <= >= lt gt le ge
    nonassoc    == != <=> eq ne cmp ~~
    left        &
    left        | ^
    left        &&
    left        || //
    nonassoc    ..  ...
    right       ?:
    right       = += -= *= etc.
    left        , =>
    nonassoc    list operators (rightward)
    right       not
    left        and
    left        or xor



In the following sections, these operators are covered in precedence order.



Many operators can be overloaded for objects.  See (overload).









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48.2.2 Terms and List Operators (Leftward)






A TERM has the highest precedence in Perl.  They include variables,
quote and quote-like operators, any expression in parentheses,
and any function whose arguments are parenthesized.  Actually, there
aren’t really functions in this sense, just list operators and unary
operators behaving as functions because you put parentheses around
the arguments.  These are all documented in (perlfunc).



If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.



In the absence of parentheses, the precedence of list operators such as
print, sort, or chmod is either very high or very low depending on
whether you are looking at the left side or the right side of the operator.
For example, in



    @ary = (1, 3, sort 4, 2);
    print @ary;         # prints 1324



the commas on the right of the sort are evaluated before the sort,
but the commas on the left are evaluated after.  In other words,
list operators tend to gobble up all arguments that follow, and
then act like a simple TERM with regard to the preceding expression.
Be careful with parentheses:



    # These evaluate exit before doing the print:
    print($foo, exit);  # Obviously not what you want.
    print $foo, exit;   # Nor is this.

    # These do the print before evaluating exit:
    (print $foo), exit; # This is what you want.
    print($foo), exit;  # Or this.
    print ($foo), exit; # Or even this.



Also note that



    print ($foo & 255) + 1, "\n";



probably doesn’t do what you expect at first glance.  The parentheses
enclose the argument list for print which is evaluated (printing
the result of $foo & 255).  Then one is added to the return value
of print (usually 1).  The result is something like this:



    1 + 1, "\n";    # Obviously not what you meant.



To do what you meant properly, you must write:



    print(($foo & 255) + 1, "\n");



See Named Unary Operators for more discussion of this.



Also parsed as terms are the do {} and eval {} constructs, as
well as subroutine and method calls, and the anonymous
constructors [] and {}.



See also Quote and Quote-like Operators toward the end of this section,
as well as I/O Operators.









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48.2.3 The Arrow Operator






"->" is an infix dereference operator, just as it is in C
and C++.  If the right side is either a [...], {...}, or a
(...) subscript, then the left side must be either a hard or
symbolic reference to an array, a hash, or a subroutine respectively.
(Or technically speaking, a location capable of holding a hard
reference, if it’s an array or hash reference being used for
assignment.)  See perlreftut NAME and (perlref).



Otherwise, the right side is a method name or a simple scalar
variable containing either the method name or a subroutine reference,
and the left side must be either an object (a blessed reference)
or a class name (that is, a package name).  See (perlobj).









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48.2.4 Auto-increment and Auto-decrement









"++" and "–" work as in C.  That is, if placed before a variable,
they increment or decrement the variable by one before returning the
value, and if placed after, increment or decrement after returning the
value.



    $i = 0;  $j = 0;
    print $i++;  # prints 0
    print ++$j;  # prints 1



Note that just as in C, Perl doesn’t define when the variable is
incremented or decremented. You just know it will be done sometime
before or after the value is returned. This also means that modifying
a variable twice in the same statement will lead to undefined behavior.
Avoid statements like:



    $i = $i ++;
    print ++ $i + $i ++;



Perl will not guarantee what the result of the above statements is.



The auto-increment operator has a little extra builtin magic to it.  If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment.  If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
/^[a-zA-Z]*[0-9]*\z/, the increment is done as a string, preserving each
character within its range, with carry:



    print ++($foo = "99");      # prints "100"
    print ++($foo = "a0");      # prints "a1"
    print ++($foo = "Az");      # prints "Ba"
    print ++($foo = "zz");      # prints "aaa"



undef is always treated as numeric, and in particular is changed
to 0 before incrementing (so that a post-increment of an undef value
will return 0 rather than undef).



The auto-decrement operator is not magical.









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48.2.5 Exponentiation






Binary "**" is the exponentiation operator.  It binds even more
tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
implemented using C’s pow(3) function, which actually works on doubles
internally.)









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48.2.6 Symbolic Unary Operators






Unary "!" performs logical negation, that is, "not".  See also not for a lower
precedence version of this.






Unary "-" performs arithmetic negation if the operand is numeric,
including any string that looks like a number.  If the operand is
an identifier, a string consisting of a minus sign concatenated
with the identifier is returned.  Otherwise, if the string starts
with a plus or minus, a string starting with the opposite sign is
returned.  One effect of these rules is that -bareword is equivalent
to the string "-bareword".  If, however, the string begins with a
non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
the string to a numeric and the arithmetic negation is performed. If the
string cannot be cleanly converted to a numeric, Perl will give the warning
Argument "the string" isn’t numeric in negation (-) at ....






Unary "~" performs bitwise negation, that is, 1’s complement.  For
example, 0666 & ~027 is 0640.  (See also Integer Arithmetic and
Bitwise String Operators.)  Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
width, remember to use the "&" operator to mask off the excess bits.




When complementing strings, if all characters have ordinal values under
256, then their complements will, also.  But if they do not, all
characters will be in either 32- or 64-bit complements, depending on your
architecture.  So for example, ~"\x{3B1}" is "\x{FFFF_FC4E}" on
32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.




Unary "+" has no effect whatsoever, even on strings.  It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
arguments.  (See examples above under Terms and List Operators (Leftward).)







Unary "\" creates a reference to whatever follows it.  See perlreftut NAME
and (perlref).  Do not confuse this behavior with the behavior of
backslash within a string, although both forms do convey the notion
of protecting the next thing from interpolation.










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48.2.7 Binding Operators







Binary "=~" binds a scalar expression to a pattern match.  Certain operations
search or modify the string $_ by default.  This operator makes that kind
of operation work on some other string.  The right argument is a search
pattern, substitution, or transliteration.  The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
$_.  When used in scalar context, the return value generally indicates the
success of the operation.  The exceptions are substitution (s///)
and transliteration (y///) with the /r (non-destructive) option,
which cause the return value to be the result of the substitution.
Behavior in list context depends on the particular operator.
See Regexp Quote-Like Operators for details and perlretut NAME for
examples using these operators.



If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
time. Note that this means that its contents will be interpolated twice, so



    '\\' =~ q'\\';



is not ok, as the regex engine will end up trying to compile the
pattern \, which it will consider a syntax error.



Binary "!~" is just like "=~" except the return value is negated in
the logical sense.



Binary "!~" with a non-destructive substitution (s///r) or transliteration
(y///r) is a syntax error.









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48.2.8 Multiplicative Operators





Binary "*" multiplies two numbers.






Binary "/" divides two numbers.








Binary "%" is the modulo operator, which computes the division
remainder of its first argument with respect to its second argument.
Given integer
operands $a and $b: If $b is positive, then $a % $b is
$a minus the largest multiple of $b less than or equal to
$a.  If $b is negative, then $a % $b is $a minus the
smallest multiple of $b that is not less than $a (that is, the
result will be less than or equal to zero).  If the operands
$a and $b are floating point values and the absolute value of
$b (that is abs($b)) is less than (UV_MAX + 1), only
the integer portion of $a and $b will be used in the operation
(Note: here UV_MAX means the maximum of the unsigned integer type).
If the absolute value of the right operand (abs($b)) is greater than
or equal to (UV_MAX + 1), "%" computes the floating-point remainder
$r in the equation ($r = $a - $i*$b) where $i is a certain
integer that makes $r have the same sign as the right operand
$b (not as the left operand $a like C function fmod())
and the absolute value less than that of $b.
Note that when use integer is in scope, "%" gives you direct access
to the modulo operator as implemented by your C compiler.  This
operator is not as well defined for negative operands, but it will
execute faster.





Binary "x" is the repetition operator.  In scalar context or if the left
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand.  In list context, if the left operand is enclosed in
parentheses or is a list formed by qw/STRING/, it repeats the list.
If the right operand is zero or negative, it returns an empty string
or an empty list, depending on the context.




    print '-' x 80;             # print row of dashes

    print "\t" x ($tab/8), ' ' x ($tab%8);      # tab over

    @ones = (1) x 80;           # a list of 80 1's
    @ones = (5) x @ones;        # set all elements to 5









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48.2.9 Additive Operators





Binary + returns the sum of two numbers.





Binary - returns the difference of two numbers.










Binary . concatenates two strings.










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48.2.10 Shift Operators














Binary << returns the value of its left argument shifted left by the
number of bits specified by the right argument.  Arguments should be
integers.  (See also Integer Arithmetic.)



Binary >> returns the value of its left argument shifted right by
the number of bits specified by the right argument.  Arguments should
be integers.  (See also Integer Arithmetic.)



Note that both << and >> in Perl are implemented directly using
<< and >>  in C.  If use integer (see Integer Arithmetic) is
in force then signed C integers are used, else unsigned C integers are
used.  Either way, the implementation isn’t going to generate results
larger than the size of the integer type Perl was built with (32 bits
or 64 bits).



The result of overflowing the range of the integers is undefined
because it is undefined also in C.  In other words, using 32-bit
integers, 1 << 32 is undefined.  Shifting by a negative number
of bits is also undefined.



If you get tired of being subject to your platform’s native integers,
the use bigint pragma neatly sidesteps the issue altogether:



    print 20 << 20;  # 20971520
    print 20 << 40;  # 5120 on 32-bit machines, 
                     # 21990232555520 on 64-bit machines
    use bigint;
    print 20 << 100; # 25353012004564588029934064107520









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48.2.11 Named Unary Operators




The various named unary operators are treated as functions with one
argument, with optional parentheses.



If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.  For example,
because named unary operators are higher precedence than ||:



    chdir $foo    || die;       # (chdir $foo) || die
    chdir($foo)   || die;       # (chdir $foo) || die
    chdir ($foo)  || die;       # (chdir $foo) || die
    chdir +($foo) || die;       # (chdir $foo) || die



but, because * is higher precedence than named operators:



    chdir $foo * 20;    # chdir ($foo * 20)
    chdir($foo) * 20;   # (chdir $foo) * 20
    chdir ($foo) * 20;  # (chdir $foo) * 20
    chdir +($foo) * 20; # chdir ($foo * 20)

    rand 10 * 20;       # rand (10 * 20)
    rand(10) * 20;      # (rand 10) * 20
    rand (10) * 20;     # (rand 10) * 20
    rand +(10) * 20;    # rand (10 * 20)






Regarding precedence, the filetest operators, like -f, -M, etc. are
treated like named unary operators, but they don’t follow this functional
parenthesis rule.  That means, for example, that -f($file).".bak" is
equivalent to -f "$file.bak".




See also Terms and List Operators (Leftward).









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48.2.12 Relational Operators





Perl operators that return true or false generally return values 
that can be safely used as numbers.  For example, the relational
operators in this section and the equality operators in the next
one return 1 for true and a special version of the defined empty
string, "", which counts as a zero but is exempt from warnings
about improper numeric conversions, just as "0 but true" is.




Binary "<" returns true if the left argument is numerically less than
the right argument.





Binary ">" returns true if the left argument is numerically greater
than the right argument.





Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.





Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.





Binary "lt" returns true if the left argument is stringwise less than
the right argument.





Binary "gt" returns true if the left argument is stringwise greater
than the right argument.





Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.





Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.










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48.2.13 Equality Operators








Binary "==" returns true if the left argument is numerically equal to
the right argument.





Binary "!=" returns true if the left argument is numerically not equal
to the right argument.






Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument.  If your platform supports NaNs (not-a-numbers) as numeric
values, using them with "<=>" returns undef.  NaN is not "<", "==", ">",
"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
returns true, as does NaN != anything else. If your platform doesn’t
support NaNs then NaN is just a string with numeric value 0.




    $ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
    $ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'



(Note that the (bigint), (bigrat), and (bignum) pragmas all 
support "NaN".)




Binary "eq" returns true if the left argument is stringwise equal to
the right argument.





Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.





Binary "cmp" returns -1, 0, or 1 depending on whether the left
argument is stringwise less than, equal to, or greater than the right
argument.





Binary "~~" does a smartmatch between its arguments.  Smart matching
is described in the next section.




"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
by the current locale if a legacy use locale (but not
use locale ':not_characters') is in effect.  See
perllocale NAME.  Do not mix these with Unicode, only with legacy binary
encodings.  The standard (Unicode-Collate) and
(Unicode-Collate-Locale) modules offer much more powerful solutions to
collation issues.









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48.2.14 Smartmatch Operator



First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
binary ~~ does a "smartmatch" between its arguments.  This is mostly
used implicitly in the when construct described in (perlsyn), although
not all when clauses call the smartmatch operator.  Unique among all of
Perl’s operators, the smartmatch operator can recurse.



It is also unique in that all other Perl operators impose a context
(usually string or numeric context) on their operands, autoconverting
those operands to those imposed contexts.  In contrast, smartmatch
infers contexts from the actual types of its operands and uses that
type information to select a suitable comparison mechanism.



The ~~ operator compares its operands "polymorphically", determining how
to compare them according to their actual types (numeric, string, array,
hash, etc.)  Like the equality operators with which it shares the same
precedence, ~~ returns 1 for true and "" for false.  It is often best
read aloud as "in", "inside of", or "is contained in", because the left
operand is often looked for inside the right operand.  That makes the
order of the operands to the smartmatch operand often opposite that of
the regular match operator.  In other words, the "smaller" thing is usually
placed in the left operand and the larger one in the right.



The behavior of a smartmatch depends on what type of things its arguments
are, as determined by the following table.  The first row of the table
whose types apply determines the smartmatch behavior.  Because what
actually happens is mostly determined by the type of the second operand,
the table is sorted on the right operand instead of on the left.



 Left      Right      Description and pseudocode                               
 ===============================================================
 Any       undef      check whether Any is undefined                    
                like: !defined Any

 Any       Object     invoke ~~ overloading on Object, or die

 Right operand is an ARRAY:

 Left      Right      Description and pseudocode                               
 ===============================================================
 ARRAY1    ARRAY2     recurse on paired elements of ARRAY1 and ARRAY2[2]
                like: (ARRAY1[0] ~~ ARRAY2[0])
                        && (ARRAY1[1] ~~ ARRAY2[1]) && ...
 HASH      ARRAY      any ARRAY elements exist as HASH keys             
                like: grep { exists HASH->{$_} } ARRAY
 Regexp    ARRAY      any ARRAY elements pattern match Regexp
                like: grep { /Regexp/ } ARRAY
 undef     ARRAY      undef in ARRAY                                    
                like: grep { !defined } ARRAY
 Any       ARRAY      smartmatch each ARRAY element[3]                   
                like: grep { Any ~~ $_ } ARRAY

 Right operand is a HASH:

 Left      Right      Description and pseudocode                               
 ===============================================================
 HASH1     HASH2      all same keys in both HASHes                      
                like: keys HASH1 ==
                         grep { exists HASH2->{$_} } keys HASH1
 ARRAY     HASH       any ARRAY elements exist as HASH keys             
                like: grep { exists HASH->{$_} } ARRAY
 Regexp    HASH       any HASH keys pattern match Regexp                
                like: grep { /Regexp/ } keys HASH
 undef     HASH       always false (undef can't be a key)               
                like: 0 == 1
 Any       HASH       HASH key existence                                
                like: exists HASH->{Any}

 Right operand is CODE:
   
 Left      Right      Description and pseudocode                               
 ===============================================================
 ARRAY     CODE       sub returns true on all ARRAY elements[1]
                like: !grep { !CODE->($_) } ARRAY
 HASH      CODE       sub returns true on all HASH keys[1]
                like: !grep { !CODE->($_) } keys HASH
 Any       CODE       sub passed Any returns true              
                like: CODE->(Any)



Right operand is a Regexp:



 Left      Right      Description and pseudocode                               
 ===============================================================
 ARRAY     Regexp     any ARRAY elements match Regexp                   
                like: grep { /Regexp/ } ARRAY
 HASH      Regexp     any HASH keys match Regexp                        
                like: grep { /Regexp/ } keys HASH
 Any       Regexp     pattern match                                     
                like: Any =~ /Regexp/

 Other:

 Left      Right      Description and pseudocode                               
 ===============================================================
 Object    Any        invoke ~~ overloading on Object,
                      or fall back to...

 Any       Num        numeric equality                                  
                 like: Any == Num
 Num       nummy[4]    numeric equality
                 like: Num == nummy
 undef     Any        check whether undefined
                 like: !defined(Any)
 Any       Any        string equality                                   
                 like: Any eq Any



Notes:





1. Empty hashes or arrays match.






2. That is, each element smartmatches the element of the same index in the other array.[3]






3. If a circular reference is found, fall back to referential equality.






4. Either an actual number, or a string that looks like one.









The smartmatch implicitly dereferences any non-blessed hash or array
reference, so the HASH and ARRAY entries apply in those cases.
For blessed references, the Object entries apply.  Smartmatches
involving hashes only consider hash keys, never hash values.



The "like" code entry is not always an exact rendition.  For example, the
smartmatch operator short-circuits whenever possible, but grep does
not.  Also, grep in scalar context returns the number of matches, but
~~ returns only true or false.



Unlike most operators, the smartmatch operator knows to treat undef
specially:



    use v5.10.1;
    @array = (1, 2, 3, undef, 4, 5);
    say "some elements undefined" if undef ~~ @array;



Each operand is considered in a modified scalar context, the modification
being that array and hash variables are passed by reference to the
operator, which implicitly dereferences them.  Both elements
of each pair are the same:



    use v5.10.1;

    my %hash = (red    => 1, blue   => 2, green  => 3,
                orange => 4, yellow => 5, purple => 6,
                black  => 7, grey   => 8, white  => 9);

    my @array = qw(red blue green);

    say "some array elements in hash keys" if  @array ~~  %hash;
    say "some array elements in hash keys" if \@array ~~ \%hash;

    say "red in array" if "red" ~~  @array;
    say "red in array" if "red" ~~ \@array;

    say "some keys end in e" if /e$/ ~~  %hash;
    say "some keys end in e" if /e$/ ~~ \%hash;



Two arrays smartmatch if each element in the first array smartmatches
(that is, is "in") the corresponding element in the second array,
recursively.



    use v5.10.1;
    my @little = qw(red blue green);
    my @bigger = ("red", "blue", [ "orange", "green" ] );
    if (@little ~~ @bigger) {  # true!
        say "little is contained in bigger";
    } 



Because the smartmatch operator recurses on nested arrays, this
will still report that "red" is in the array.



    use v5.10.1;
    my @array = qw(red blue green);
    my $nested_array = [[[[[[[ @array ]]]]]]];
    say "red in array" if "red" ~~ $nested_array;



If two arrays smartmatch each other, then they are deep
copies of each others’ values, as this example reports:



    use v5.12.0;
    my @a = (0, 1, 2, [3, [4, 5], 6], 7); 
    my @b = (0, 1, 2, [3, [4, 5], 6], 7); 

    if (@a ~~ @b && @b ~~ @a) {
        say "a and b are deep copies of each other";
    } 
    elsif (@a ~~ @b) {
        say "a smartmatches in b";
    } 
    elsif (@b ~~ @a) {
        say "b smartmatches in a";
    } 
    else {
        say "a and b don't smartmatch each other at all";
    } 



If you were to set $b[3] = 4, then instead of reporting that "a and b
are deep copies of each other", it now reports that "b smartmatches in a".
That because the corresponding position in @a contains an array that
(eventually) has a 4 in it.



Smartmatching one hash against another reports whether both contain the
same keys, no more and no less. This could be used to see whether two
records have the same field names, without caring what values those fields
might have.  For example:



    use v5.10.1;
    sub make_dogtag {
        state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };

        my ($class, $init_fields) = @_;

        die "Must supply (only) name, rank, and serial number"
            unless $init_fields ~~ $REQUIRED_FIELDS;

        ...
    }



or, if other non-required fields are allowed, use ARRAY ~~ HASH:



    use v5.10.1;
    sub make_dogtag {
        state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };

        my ($class, $init_fields) = @_;

        die "Must supply (at least) name, rank, and serial number"
            unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;

        ...
    }



The smartmatch operator is most often used as the implicit operator of a
when clause.  See the section on "Switch Statements" in (perlsyn).





• NAME operator Smartmatching of Objects:  












Up: NAME operator Smartmatch Operator   [Contents]





48.2.14.1 Smartmatching of Objects



To avoid relying on an object’s underlying representation, if the
smartmatch’s right operand is an object that doesn’t overload ~~,
it raises the exception "Smartmatching a non-overloaded object
breaks encapsulation". That’s because one has no business digging
around to see whether something is "in" an object. These are all
illegal on objects without a ~~ overload:



    %hash ~~ $object
       42 ~~ $object
   "fred" ~~ $object



However, you can change the way an object is smartmatched by overloading
the ~~ operator. This is allowed to extend the usual smartmatch semantics.
For objects that do have an ~~ overload, see (overload).



Using an object as the left operand is allowed, although not very useful.
Smartmatching rules take precedence over overloading, so even if the
object in the left operand has smartmatch overloading, this will be
ignored.  A left operand that is a non-overloaded object falls back on a
string or numeric comparison of whatever the ref operator returns.  That
means that



    $object ~~ X



does not invoke the overload method with X as an argument.
Instead the above table is consulted as normal, and based on the type of
X, overloading may or may not be invoked.  For simple strings or
numbers, in becomes equivalent to this:



    $object ~~ $number          ref($object) == $number
    $object ~~ $string          ref($object) eq $string 



For example, this reports that the handle smells IOish
(but please don’t really do this!):



    use IO::Handle;
    my $fh = IO::Handle->new();
    if ($fh ~~ /\bIO\b/) {
        say "handle smells IOish";
    } 



That’s because it treats $fh as a string like
"IO::Handle=GLOB(0x8039e0)", then pattern matches against that.









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48.2.15 Bitwise And






Binary "&" returns its operands ANDed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)



Note that "&" has lower priority than relational operators, so for example
the parentheses are essential in a test like



    print "Even\n" if ($x & 1) == 0;









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48.2.16 Bitwise Or and Exclusive Or









Binary "|" returns its operands ORed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)



Binary "^" returns its operands XORed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)



Note that "|" and "^" have lower priority than relational operators, so
for example the brackets are essential in a test like



    print "false\n" if (8 | 2) != 10;









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48.2.17 C-style Logical And






Binary "&&" performs a short-circuit logical AND operation.  That is,
if the left operand is false, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.









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48.2.18 C-style Logical Or





Binary "||" performs a short-circuit logical OR operation.  That is,
if the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.









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48.2.19 Logical Defined-Or





Although it has no direct equivalent in C, Perl’s // operator is related
to its C-style or.  In fact, it’s exactly the same as ||, except that it
tests the left hand side’s definedness instead of its truth.  Thus,
EXPR1 // EXPR2 returns the value of EXPR1 if it’s defined,
otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated
in scalar context, EXPR2 in the context of // itself). Usually,
this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that
the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2
cannot). This is very useful for
providing default values for variables.  If you actually want to test if
at least one of $a and $b is defined, use defined($a // $b).



The ||, // and && operators return the last value evaluated
(unlike C’s || and &&, which return 0 or 1). Thus, a reasonably
portable way to find out the home directory might be:



    $home =  $ENV{HOME}
          // $ENV{LOGDIR}
          // (getpwuid($<))[7]
          // die "You're homeless!\n";



In particular, this means that you shouldn’t use this
for selecting between two aggregates for assignment:



    @a = @b || @c;              # this is wrong
    @a = scalar(@b) || @c;      # really meant this
    @a = @b ? @b : @c;          # this works fine, though



As alternatives to && and || when used for
control flow, Perl provides the and and or operators (see below).
The short-circuit behavior is identical.  The precedence of "and"
and "or" is much lower, however, so that you can safely use them after a
list operator without the need for parentheses:



    unlink "alpha", "beta", "gamma"
            or gripe(), next LINE;



With the C-style operators that would have been written like this:



    unlink("alpha", "beta", "gamma")
            || (gripe(), next LINE);



It would be even more readable to write that this way:



    unless(unlink("alpha", "beta", "gamma")) {
        gripe();
        next LINE;
    } 



Using "or" for assignment is unlikely to do what you want; see below.









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48.2.20 Range Operators







Binary ".." is the range operator, which is really two different
operators depending on the context.  In list context, it returns a
list of values counting (up by ones) from the left value to the right
value.  If the left value is greater than the right value then it
returns the empty list.  The range operator is useful for writing
foreach (1..10) loops and for doing slice operations on arrays. In
the current implementation, no temporary array is created when the
range operator is used as the expression in foreach loops, but older
versions of Perl might burn a lot of memory when you write something
like this:



    for (1 .. 1_000_000) {
        # code
    }



The range operator also works on strings, using the magical
auto-increment, see below.



In scalar context, ".." returns a boolean value.  The operator is
bistable, like a flip-flop, and emulates the line-range (comma)
operator of sed, awk, and various editors. Each ".." operator
maintains its own boolean state, even across calls to a subroutine
that contains it. It is false as long as its left operand is false.
Once the left operand is true, the range operator stays true until the
right operand is true, AFTER which the range operator becomes false
again.  It doesn’t become false till the next time the range operator
is evaluated.  It can test the right operand and become false on the
same evaluation it became true (as in awk), but it still returns
true once. If you don’t want it to test the right operand until the
next evaluation, as in sed, just use three dots ("...") instead of
two.  In all other regards, "..." behaves just like ".." does.



The right operand is not evaluated while the operator is in the
"false" state, and the left operand is not evaluated while the
operator is in the "true" state.  The precedence is a little lower
than || and &&.  The value returned is either the empty string for
false, or a sequence number (beginning with 1) for true.  The sequence
number is reset for each range encountered.  The final sequence number
in a range has the string "E0" appended to it, which doesn’t affect
its numeric value, but gives you something to search for if you want
to exclude the endpoint.  You can exclude the beginning point by
waiting for the sequence number to be greater than 1.



If either operand of scalar ".." is a constant expression,
that operand is considered true if it is equal (==) to the current
input line number (the $. variable).



To be pedantic, the comparison is actually int(EXPR) == int(EXPR),
but that is only an issue if you use a floating point expression; when
implicitly using $. as described in the previous paragraph, the
comparison is int(EXPR) == int($.) which is only an issue when $.
is set to a floating point value and you are not reading from a file.
Furthermore, "span" .. "spat" or 2.18 .. 3.14 will not do what
you want in scalar context because each of the operands are evaluated
using their integer representation.



Examples:



As a scalar operator:



    if (101 .. 200) { print; } # print 2nd hundred lines, short for
                               #  if ($. == 101 .. $. == 200) { print; }

    next LINE if (1 .. /^$/);  # skip header lines, short for
                               #   next LINE if ($. == 1 .. /^$/);
                               # (typically in a loop labeled LINE)

    s/^/> / if (/^$/ .. eof());  # quote body

    # parse mail messages
    while (<>) {
        $in_header =   1  .. /^$/;
        $in_body   = /^$/ .. eof;
        if ($in_header) {
            # do something
        } else { # in body
            # do something else
        }
    } continue {
        close ARGV if eof;             # reset $. each file
    }



Here’s a simple example to illustrate the difference between
the two range operators:



    @lines = ("   - Foo",
              "01 - Bar",
              "1  - Baz",
              "   - Quux");

    foreach (@lines) {
        if (/0/ .. /1/) {
            print "$_\n";
        }
    }



This program will print only the line containing "Bar". If
the range operator is changed to ..., it will also print the
"Baz" line.



And now some examples as a list operator:



    for (101 .. 200) { print }      # print $_ 100 times
    @foo = @foo[0 .. $#foo];        # an expensive no-op
    @foo = @foo[$#foo-4 .. $#foo];  # slice last 5 items



The range operator (in list context) makes use of the magical
auto-increment algorithm if the operands are strings.  You
can say



    @alphabet = ("A" .. "Z");



to get all normal letters of the English alphabet, or



    $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];



to get a hexadecimal digit, or



    @z2 = ("01" .. "31");
    print $z2[$mday];



to get dates with leading zeros.



If the final value specified is not in the sequence that the magical
increment would produce, the sequence goes until the next value would
be longer than the final value specified.



If the initial value specified isn’t part of a magical increment
sequence (that is, a non-empty string matching /^[a-zA-Z]*[0-9]*\z/),
only the initial value will be returned.  So the following will only
return an alpha:



    use charnames "greek";
    my @greek_small =  ("\N{alpha}" .. "\N{omega}");



To get the 25 traditional lowercase Greek letters, including both sigmas,
you could use this instead:



    use charnames "greek";
    my @greek_small =  map { chr } ( ord("\N{alpha}") 
                                        ..
                                     ord("\N{omega}") 
                                   );



However, because there are many other lowercase Greek characters than
just those, to match lowercase Greek characters in a regular expression,
you would use the pattern /(?:(?=\p{Greek})\p{Lower})+/.



Because each operand is evaluated in integer form, 2.18 .. 3.14 will
return two elements in list context.



    @list = (2.18 .. 3.14); # same as @list = (2 .. 3);









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48.2.21 Conditional Operator







Ternary "?:" is the conditional operator, just as in C.  It works much
like an if-then-else.  If the argument before the ? is true, the
argument before the : is returned, otherwise the argument after the :
is returned.  For example:



    printf "I have %d dog%s.\n", $n,
            ($n == 1) ? "" : "s";



Scalar or list context propagates downward into the 2nd
or 3rd argument, whichever is selected.



    $a = $ok ? $b : $c;  # get a scalar
    @a = $ok ? @b : @c;  # get an array
    $a = $ok ? @b : @c;  # oops, that's just a count!



The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):



    ($a_or_b ? $a : $b) = $c;



Because this operator produces an assignable result, using assignments
without parentheses will get you in trouble.  For example, this:



    $a % 2 ? $a += 10 : $a += 2



Really means this:



    (($a % 2) ? ($a += 10) : $a) += 2



Rather than this:



    ($a % 2) ? ($a += 10) : ($a += 2)



That should probably be written more simply as:



    $a += ($a % 2) ? 10 : 2;









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48.2.22 Assignment Operators






















"=" is the ordinary assignment operator.



Assignment operators work as in C.  That is,



    $a += 2;



is equivalent to



    $a = $a + 2;



although without duplicating any side effects that dereferencing the lvalue
might trigger, such as from tie().  Other assignment operators work similarly.
The following are recognized:



    **=    +=    *=    &=    <<=    &&=
           -=    /=    |=    >>=    ||=
           .=    %=    ^=           //=
                 x=



Although these are grouped by family, they all have the precedence
of assignment.



Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and
then modifying the variable that was assigned to.  This is useful
for modifying a copy of something, like this:



    ($tmp = $global) =~ tr/13579/24680/;



Although as of 5.14, that can be also be accomplished this way:



    use v5.14;
    $tmp = ($global =~  tr/13579/24680/r);



Likewise,



    ($a += 2) *= 3;



is equivalent to



    $a += 2;
    $a *= 3;



Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.









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48.2.23 Comma Operator






Binary "," is the comma operator.  In scalar context it evaluates
its left argument, throws that value away, then evaluates its right
argument and returns that value.  This is just like C’s comma operator.



In list context, it’s just the list argument separator, and inserts
both its arguments into the list.  These arguments are also evaluated
from left to right.



The => operator is a synonym for the comma except that it causes a
word on its left to be interpreted as a string if it begins with a letter
or underscore and is composed only of letters, digits and underscores.
This includes operands that might otherwise be interpreted as operators,
constants, single number v-strings or function calls. If in doubt about
this behavior, the left operand can be quoted explicitly.



Otherwise, the => operator behaves exactly as the comma operator
or list argument separator, according to context.



For example:



    use constant FOO => "something";

    my %h = ( FOO => 23 );



is equivalent to:



    my %h = ("FOO", 23);



It is NOT:



    my %h = ("something", 23);



The => operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.



    %hash = ( $key => $value );
    login( $username => $password );



The special quoting behavior ignores precedence, and hence may apply to
part of the left operand:



    print time.shift => "bbb";



That example prints something like "1314363215shiftbbb", because the
=> implicitly quotes the shift immediately on its left, ignoring
the fact that time.shift is the entire left operand.









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48.2.24 List Operators (Rightward)





On the right side of a list operator, the comma has very low precedence,
such that it controls all comma-separated expressions found there.
The only operators with lower precedence are the logical operators
"and", "or", and "not", which may be used to evaluate calls to list
operators without the need for parentheses:



    open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";



However, some people find that code harder to read than writing
it with parentheses:



    open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";



in which case you might as well just use the more customary "||" operator:



    open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";



See also discussion of list operators in Terms and List Operators (Leftward).









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48.2.25 Logical Not





Unary "not" returns the logical negation of the expression to its right.
It’s the equivalent of "!" except for the very low precedence.









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48.2.26 Logical And





Binary "and" returns the logical conjunction of the two surrounding
expressions.  It’s equivalent to && except for the very low
precedence.  This means that it short-circuits: the right
expression is evaluated only if the left expression is true.









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48.2.27 Logical or and Exclusive Or








Binary "or" returns the logical disjunction of the two surrounding
expressions.  It’s equivalent to || except for the very low precedence.
This makes it useful for control flow:



    print FH $data              or die "Can't write to FH: $!";



This means that it short-circuits: the right expression is evaluated
only if the left expression is false.  Due to its precedence, you must
be careful to avoid using it as replacement for the || operator.
It usually works out better for flow control than in assignments:



    $a = $b or $c;              # bug: this is wrong
    ($a = $b) or $c;            # really means this
    $a = $b || $c;              # better written this way



However, when it’s a list-context assignment and you’re trying to use
|| for control flow, you probably need "or" so that the assignment
takes higher precedence.



    @info = stat($file) || die;     # oops, scalar sense of stat!
    @info = stat($file) or die;     # better, now @info gets its due



Then again, you could always use parentheses.



Binary xor returns the exclusive-OR of the two surrounding expressions.
It cannot short-circuit (of course).



There is no low precedence operator for defined-OR.









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48.2.28 C Operators Missing From Perl








Here is what C has that Perl doesn’t:





unary &




Address-of operator.  (But see the "\" operator for taking a reference.)





unary *




Dereference-address operator. (Perl’s prefix dereferencing
operators are typed: $, @, %, and &.)





(TYPE)




Type-casting operator.














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48.2.29 Quote and Quote-like Operators























While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities.  Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them.  In the following table, a {} represents
any pair of delimiters you choose.



    Customary  Generic        Meaning        Interpolates
        ''       q{}          Literal             no
        ""      qq{}          Literal             yes
        ``      qx{}          Command             yes*
                qw{}         Word list            no
        //       m{}       Pattern match          yes*
                qr{}          Pattern             yes*
                 s{}{}      Substitution          yes*
                tr{}{}    Transliteration         no (but see below)
                 y{}{}    Transliteration         no (but see below)
        <<EOF                 here-doc            yes*

        * unless the delimiter is ''.



Non-bracketing delimiters use the same character fore and aft, but the four
sorts of ASCII brackets (round, angle, square, curly) all nest, which means
that



    q{foo{bar}baz}



is the same as



    'foo{bar}baz'



Note, however, that this does not always work for quoting Perl code:



    $s = q{ if($a eq "}") ... }; # WRONG



is a syntax error. The Text::Balanced module (standard as of v5.8,
and from CPAN before then) is able to do this properly.



There can be whitespace between the operator and the quoting
characters, except when # is being used as the quoting character.
q#foo# is parsed as the string foo, while q #foo# is the
operator q followed by a comment.  Its argument will be taken
from the next line.  This allows you to write:



    s {foo}  # Replace foo
      {bar}  # with bar.
















The following escape sequences are available in constructs that interpolate,
and in transliterations:




    Sequence     Note  Description
    \t                  tab               (HT, TAB)
    \n                  newline           (NL)
    \r                  return            (CR)
    \f                  form feed         (FF)
    \b                  backspace         (BS)
    \a                  alarm (bell)      (BEL)
    \e                  escape            (ESC)
    \x{263A}     [1,8]  hex char          (example: SMILEY)
    \x1b         [2,8]  restricted range hex char (example: ESC)
    \N{name}     [3]    named Unicode character or character sequence
    \N{U+263D}   [4,8]  Unicode character (example: FIRST QUARTER MOON)
    \c[          [5]    control char      (example: chr(27))
    \o{23072}    [6,8]  octal char        (example: SMILEY)
    \033         [7,8]  restricted range octal char  (example: ESC)





[1]




The result is the character specified by the hexadecimal number between
the braces.  See [8] below for details on which character.



Only hexadecimal digits are valid between the braces. If an invalid
character is encountered, a warning will be issued and the invalid
character and all subsequent characters (valid or invalid) within the
braces will be discarded.



If there are no valid digits between the braces, the generated character is
the NULL character (\x{00}).  However, an explicit empty brace (\x{})
will not cause a warning (currently).





[2]




The result is the character specified by the hexadecimal number in the range
0x00 to 0xFF.  See [8] below for details on which character.



Only hexadecimal digits are valid following \x.  When \x is followed
by fewer than two valid digits, any valid digits will be zero-padded.  This
means that \x7 will be interpreted as \x07, and a lone <\x> will be
interpreted as \x00.  Except at the end of a string, having fewer than
two valid digits will result in a warning.  Note that although the warning
says the illegal character is ignored, it is only ignored as part of the
escape and will still be used as the subsequent character in the string.
For example:



  Original    Result    Warns?
  "\x7"       "\x07"    no
  "\x"        "\x00"    no
  "\x7q"      "\x07q"   yes
  "\xq"       "\x00q"   yes





[3]




The result is the Unicode character or character sequence given by name.
See (charnames).





[4]




\N{U+hexadecimal number} means the Unicode character whose Unicode code
point is hexadecimal number.





[5]




The character following \c is mapped to some other character as shown in the
table:



 Sequence   Value
   \c@      chr(0)
   \cA      chr(1)
   \ca      chr(1)
   \cB      chr(2)
   \cb      chr(2)
   ...
   \cZ      chr(26)
   \cz      chr(26)
   \c[      chr(27)
   \c]      chr(29)
   \c^      chr(30)
   \c?      chr(127)



In other words, it’s the character whose code point has had 64 xor’d with
its uppercase.  \c? is DELETE because ord("@") ^ 64 is 127, and
\c@ is NULL because the ord of "@" is 64, so xor’ing 64 itself produces 0.



Also, \c\X yields  chr(28) . "X" for any X, but cannot come at the
end of a string, because the backslash would be parsed as escaping the end
quote.



On ASCII platforms, the resulting characters from the list above are the
complete set of ASCII controls.  This isn’t the case on EBCDIC platforms; see
perlebcdic OPERATOR DIFFERENCES for the complete list of what these
sequences mean on both ASCII and EBCDIC platforms.



Use of any other character following the "c" besides those listed above is
discouraged, and some are deprecated with the intention of removing
those in a later Perl version.  What happens for any of these
other characters currently though, is that the value is derived by xor’ing
with the seventh bit, which is 64.



To get platform independent controls, you can use \N{...}.





[6]




The result is the character specified by the octal number between the braces.
See [8] below for details on which character.



If a character that isn’t an octal digit is encountered, a warning is raised,
and the value is based on the octal digits before it, discarding it and all
following characters up to the closing brace.  It is a fatal error if there are
no octal digits at all.





[7]




The result is the character specified by the three-digit octal number in the
range 000 to 777 (but best to not use above 077, see next paragraph).  See
[8] below for details on which character.



Some contexts allow 2 or even 1 digit, but any usage without exactly
three digits, the first being a zero, may give unintended results.  (For
example, in a regular expression it may be confused with a backreference;
see perlrebackslash Octal escapes.)  Starting in Perl 5.14, you may
use \o{} instead, which avoids all these problems.  Otherwise, it is best to
use this construct only for ordinals \077 and below, remembering to pad to
the left with zeros to make three digits.  For larger ordinals, either use
\o{}, or convert to something else, such as to hex and use \x{}
instead.



Having fewer than 3 digits may lead to a misleading warning message that says
that what follows is ignored.  For example, "\128" in the ASCII character set
is equivalent to the two characters "\n8", but the warning Illegal octal
digit '8' ignored will be thrown.  If "\n8" is what you want, you can
avoid this warning by padding your octal number with 0’s: "\0128".





[8]




Several constructs above specify a character by a number.  That number
gives the character’s position in the character set encoding (indexed from 0).
This is called synonymously its ordinal, code position, or code point.  Perl
works on platforms that have a native encoding currently of either ASCII/Latin1
or EBCDIC, each of which allow specification of 256 characters.  In general, if
the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform’s
native encoding.  If the number is 256 (0x100, 0400) or above, Perl interprets
it as a Unicode code point and the result is the corresponding Unicode
character.  For example \x{50} and \o{120} both are the number 80 in
decimal, which is less than 256, so the number is interpreted in the native
character set encoding.  In ASCII the character in the 80th position (indexed
from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
\x{100} and \o{400} are both 256 in decimal, so the number is interpreted
as a Unicode code point no matter what the native encoding is.  The name of the
character in the 256th position (indexed by 0) in Unicode is
LATIN CAPITAL LETTER A WITH MACRON.



There are a couple of exceptions to the above rule.  \N{U+hex number} is
always interpreted as a Unicode code point, so that \N{U+0050} is "P" even
on EBCDIC platforms.  And if (encoding)use encoding is in effect, the
number is considered to be in that encoding, and is translated from that into
the platform’s native encoding if there is a corresponding native character;
otherwise to Unicode.








NOTE: Unlike C and other languages, Perl has no \v escape sequence for
the vertical tab (VT - ASCII 11), but you may use \ck or \x0b.  (\v
does have meaning in regular expression patterns in Perl, see (perlre).)










The following escape sequences are available in constructs that interpolate,
but not in transliterations.




    \l          lowercase next character only
    \u          titlecase (not uppercase!) next character only
    \L          lowercase all characters till \E or end of string
    \U          uppercase all characters till \E or end of string
    \F          foldcase all characters till \E or end of string
    \Q          quote (disable) pattern metacharacters till \E or
                end of string
    \E          end either case modification or quoted section
                (whichever was last seen)



See (perlfunc)quotemeta for the exact definition of characters that
are quoted by \Q.



\L, \U, \F, and \Q can stack, in which case you need one
\E for each.  For example:



 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
 This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?



If use locale is in effect (but not use locale ':not_characters'),
the case map used by \l, \L,
\u, and \U is taken from the current locale.  See perllocale NAME.
If Unicode (for example, \N{} or code points of 0x100 or
beyond) is being used, the case map used by \l, \L, \u, and
\U is as defined by Unicode.  That means that case-mapping
a single character can sometimes produce several characters.
Under use locale, \F produces the same results as \L.










All systems use the virtual "\n" to represent a line terminator,
called a "newline".  There is no such thing as an unvarying, physical
newline character.  It is only an illusion that the operating system,
device drivers, C libraries, and Perl all conspire to preserve.  Not all
systems read "\r" as ASCII CR and "\n" as ASCII LF.  For example,
on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
and on systems without line terminator,
printing "\n" might emit no actual data.  In general, use "\n" when
you mean a "newline" for your system, but use the literal ASCII when you
need an exact character.  For example, most networking protocols expect
and prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators,
and although they often accept just "\012", they seldom tolerate just
"\015".  If you get in the habit of using "\n" for networking,
you may be burned some day.




For constructs that do interpolate, variables beginning with "$"
or "@" are interpolated.  Subscripted variables such as $a[3] or
$href->{key}[0] are also interpolated, as are array and hash slices.
But method calls such as $obj->meth are not.



Interpolating an array or slice interpolates the elements in order,
separated by the value of $", so is equivalent to interpolating
join $", @array.  "Punctuation" arrays such as @* are usually
interpolated only if the name is enclosed in braces @{*}, but the
arrays @_, @+, and @- are interpolated even without braces.



For double-quoted strings, the quoting from \Q is applied after
interpolation and escapes are processed.



    "abc\Qfoo\tbar$s\Exyz"



is equivalent to



    "abc" . quotemeta("foo\tbar$s") . "xyz"



For the pattern of regex operators (qr//, m// and s///),
the quoting from \Q is applied after interpolation is processed,
but before escapes are processed. This allows the pattern to match
literally (except for $ and @). For example, the following matches:



    '\s\t' =~ /\Q\s\t/



Because $ or @ trigger interpolation, you’ll need to use something
like /\Quser\E\@\Qhost/ to match them literally.



Patterns are subject to an additional level of interpretation as a
regular expression.  This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables.  If this is not what you want, use \Q to
interpolate a variable literally.



Apart from the behavior described above, Perl does not expand
multiple levels of interpolation.  In particular, contrary to the
expectations of shell programmers, back-quotes do NOT interpolate
within double quotes, nor do single quotes impede evaluation of
variables when used within double quotes.









Next: , Previous: NAME operator Quote and Quote-like Operators, Up: NAME operator DESCRIPTION   [Contents]





48.2.30 Regexp Quote-Like Operators




Here are the quote-like operators that apply to pattern
matching and related activities.





qr/STRING/msixpodual











This operator quotes (and possibly compiles) its STRING as a regular
expression.  STRING is interpolated the same way as PATTERN
in m/PATTERN/.  If "’" is used as the delimiter, no interpolation
is done.  Returns a Perl value which may be used instead of the
corresponding /STRING/msixpodual expression. The returned value is a
normalized version of the original pattern. It magically differs from
a string containing the same characters: ref(qr/x/) returns "Regexp";
however, dereferencing it is not well defined (you currently get the 
normalized version of the original pattern, but this may change).



For example,



    $rex = qr/my.STRING/is;
    print $rex;                 # prints (?si-xm:my.STRING)
    s/$rex/foo/;



is equivalent to



    s/my.STRING/foo/is;



The result may be used as a subpattern in a match:



    $re = qr/$pattern/;
    $string =~ /foo${re}bar/;   # can be interpolated in other patterns
    $string =~ $re;             # or used standalone
    $string =~ /$re/;           # or this way



Since Perl may compile the pattern at the moment of execution of the qr()
operator, using qr() may have speed advantages in some situations,
notably if the result of qr() is used standalone:



    sub match {
        my $patterns = shift;
        my @compiled = map qr/$_/i, @$patterns;
        grep {
            my $success = 0;
            foreach my $pat (@compiled) {
                $success = 1, last if /$pat/;
            }
            $success;
        } @_;
    }



Precompilation of the pattern into an internal representation at
the moment of qr() avoids a need to recompile the pattern every
time a match /$pat/ is attempted.  (Perl has many other internal
optimizations, but none would be triggered in the above example if
we did not use qr() operator.)



Options (specified by the following modifiers) are:



    m   Treat string as multiple lines.
    s   Treat string as single line. (Make . match a newline)
    i   Do case-insensitive pattern matching.
    x   Use extended regular expressions.
    p   When matching preserve a copy of the matched string so
        that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
    o   Compile pattern only once.
    a   ASCII-restrict: Use ASCII for \d, \s, \w; specifying two a's
        further restricts /i matching so that no ASCII character will
        match a non-ASCII one
    l   Use the locale
    u   Use Unicode rules
    d   Use Unicode or native charset, as in 5.12 and earlier



If a precompiled pattern is embedded in a larger pattern then the effect
of "msixpluad" will be propagated appropriately.  The effect the "o"
modifier has is not propagated, being restricted to those patterns
explicitly using it.



The last four modifiers listed above, added in Perl 5.14,
control the character set semantics, but /a is the only one you are likely
to want to specify explicitly; the other three are selected
automatically by various pragmas.



See (perlre) for additional information on valid syntax for STRING, and
for a detailed look at the semantics of regular expressions.  In
particular, all modifiers except the largely obsolete /o are further
explained in (perlre)Modifiers.  /o is described in the next section.





m/PATTERN/msixpodualgc




















/PATTERN/msixpodualgc




Searches a string for a pattern match, and in scalar context returns
true if it succeeds, false if it fails.  If no string is specified
via the =~ or !~ operator, the $_ string is searched.  (The
string specified with =~ need not be an lvalue–it may be the
result of an expression evaluation, but remember the =~ binds
rather tightly.)  See also (perlre).



Options are as described in qr// above; in addition, the following match
process modifiers are available:



 g  Match globally, i.e., find all occurrences.
 c  Do not reset search position on a failed match when /g is in effect.



If "/" is the delimiter then the initial m is optional.  With the m
you can use any pair of non-whitespace (ASCII) characters
as delimiters.  This is particularly useful for matching path names
that contain "/", to avoid LTS (leaning toothpick syndrome).  If "?" is
the delimiter, then a match-only-once rule applies,
described in m?PATTERN? below.
If "’" is the delimiter, no interpolation is performed on the PATTERN.
When using a character valid in an identifier, whitespace is required
after the m.



PATTERN may contain variables, which will be interpolated
every time the pattern search is evaluated, except
for when the delimiter is a single quote.  (Note that $(, $), and
$| are not interpolated because they look like end-of-string tests.)
Perl will not recompile the pattern unless an interpolated
variable that it contains changes.  You can force Perl to skip the
test and never recompile by adding a /o (which stands for "once")
after the trailing delimiter.
Once upon a time, Perl would recompile regular expressions
unnecessarily, and this modifier was useful to tell it not to do so, in the
interests of speed.  But now, the only reasons to use /o are either:



    

  1.  The variables are thousands of characters long and you know that they
don’t change, and you need to wring out the last little bit of speed by
having Perl skip testing for that.  (There is a maintenance penalty for
doing this, as mentioning /o constitutes a promise that you won’t
change the variables in the pattern.  If you do change them, Perl won’t
even notice.)


  2.  you want the pattern to use the initial values of the variables
regardless of whether they change or not.  (But there are saner ways
of accomplishing this than using /o.)





The bottom line is that using /o is almost never a good idea.





The empty pattern //




If the PATTERN evaluates to the empty string, the last
successfully matched regular expression is used instead. In this
case, only the g and c flags on the empty pattern are honored;
the other flags are taken from the original pattern. If no match has
previously succeeded, this will (silently) act instead as a genuine
empty pattern (which will always match).



Note that it’s possible to confuse Perl into thinking // (the empty
regex) is really // (the defined-or operator).  Perl is usually pretty
good about this, but some pathological cases might trigger this, such as
$a/// (is that ($a) / (//) or $a // /?) and print $fh //
(print $fh(// or print($fh //?).  In all of these examples, Perl
will assume you meant defined-or.  If you meant the empty regex, just
use parentheses or spaces to disambiguate, or even prefix the empty
regex with an m (so // becomes m//).





Matching in list context




If the /g option is not used, m// in list context returns a
list consisting of the subexpressions matched by the parentheses in the
pattern, that is, ($1, $2, $3...).  (Note that here $1 etc. are
also set, and that this differs from Perl 4’s behavior.)  When there are
no parentheses in the pattern, the return value is the list (1) for
success.  With or without parentheses, an empty list is returned upon
failure.



Examples:



    open(TTY, "+</dev/tty")
        || die "can't access /dev/tty: $!";

    <TTY> =~ /^y/i && foo();    # do foo if desired

    if (/Version: *([0-9.]*)/) { $version = $1; }

    next if m#^/usr/spool/uucp#;

    # poor man's grep
    $arg = shift;
    while (<>) {
        print if /$arg/o;       # compile only once (no longer needed!)
    }

    if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))



This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2, and
$Etc.  The conditional is true if any variables were assigned; that is,
if the pattern matched.



The /g modifier specifies global pattern matching–that is,
matching as many times as possible within the string. How it behaves
depends on the context. In list context, it returns a list of the
substrings matched by any capturing parentheses in the regular
expression. If there are no parentheses, it returns a list of all
the matched strings, as if there were parentheses around the whole
pattern.



In scalar context, each execution of m//g finds the next match,
returning true if it matches, and false if there is no further match.
The position after the last match can be read or set using the pos()
function; see (perlfunc)pos. A failed match normally resets the
search position to the beginning of the string, but you can avoid that
by adding the /c modifier (for example, m//gc). Modifying the target
string also resets the search position.





\G assertion




You can intermix m//g matches with m/\G.../g, where \G is a
zero-width assertion that matches the exact position where the
previous m//g, if any, left off. Without the /g modifier, the
\G assertion still anchors at pos() as it was at the start of
the operation (see (perlfunc)pos), but the match is of course only
attempted once. Using \G without /g on a target string that has
not previously had a /g match applied to it is the same as using
the \A assertion to match the beginning of the string.  Note also
that, currently, \G is only properly supported when anchored at the
very beginning of the pattern.



Examples:



    # list context
    ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

    # scalar context
    local $/ = "";
    while ($paragraph = <>) {
        while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
            $sentences++;
        }
    }
    say $sentences;



Here’s another way to check for sentences in a paragraph:



    my $sentence_rx = qr{
        (?: (?<= ^ ) | (?<= \s ) )  # after start-of-string or whitespace
        \p{Lu}                      # capital letter
        .*?                         # a bunch of anything
        (?<= \S )                   # that ends in non-whitespace
        (?<! \b [DMS]r  )           # but isn't a common abbreviation
        (?<! \b Mrs )
        (?<! \b Sra )
        (?<! \b St  )
        [.?!]                       # followed by a sentence ender
        (?= $ | \s )                # in front of end-of-string or whitespace
    }sx;
    local $/ = "";
    while (my $paragraph = <>) {
        say "NEW PARAGRAPH";
        my $count = 0;
        while ($paragraph =~ /($sentence_rx)/g) {
            printf "\tgot sentence %d: <%s>\n", ++$count, $1;
        }
    }



Here’s how to use m//gc with \G:



    $_ = "ppooqppqq";
    while ($i++ < 2) {
        print "1: '";
        print $1 while /(o)/gc; print "', pos=", pos, "\n";
        print "2: '";
        print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
        print "3: '";
        print $1 while /(p)/gc; print "', pos=", pos, "\n";
    }
    print "Final: '$1', pos=",pos,"\n" if /\G(.)/;



The last example should print:



    1: 'oo', pos=4
    2: 'q', pos=5
    3: 'pp', pos=7
    1: '', pos=7
    2: 'q', pos=8
    3: '', pos=8
    Final: 'q', pos=8



Notice that the final match matched q instead of p, which a match
without the \G anchor would have done. Also note that the final match
did not update pos. pos is only updated on a /g match. If the
final match did indeed match p, it’s a good bet that you’re running a
very old (pre-5.6.0) version of Perl.



A useful idiom for lex-like scanners is /\G.../gc.  You can
combine several regexps like this to process a string part-by-part,
doing different actions depending on which regexp matched.  Each
regexp tries to match where the previous one leaves off.



 $_ = <<'EOL';
    $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
 EOL

 LOOP: {
     print(" digits"),       redo LOOP if /\G\d+\b[,.;]?\s*/gc;
     print(" lowercase"),    redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
     print(" UPPERCASE"),    redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
     print(" Capitalized"),  redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
     print(" MiXeD"),        redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
     print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
     print(" line-noise"),   redo LOOP if /\G\W+/gc;
     print ". That's all!\n";
 }



Here is the output (split into several lines):



    line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
    line-noise lowercase line-noise lowercase line-noise lowercase
    lowercase line-noise lowercase lowercase line-noise lowercase
    lowercase line-noise MiXeD line-noise. That's all!





m?PATTERN?msixpodualgc








?PATTERN?msixpodualgc




This is just like the m/PATTERN/ search, except that it matches
only once between calls to the reset() operator.  This is a useful
optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance.  Only m??
patterns local to the current package are reset.



    while (<>) {
        if (m?^$?) {
                            # blank line between header and body
        }
    } continue {
        reset if eof;       # clear m?? status for next file
    }



Another example switched the first "latin1" encoding it finds
to "utf8" in a pod file:



    s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;



The match-once behavior is controlled by the match delimiter being
?; with any other delimiter this is the normal m// operator.  



For historical reasons, the leading m in m?PATTERN? is optional,
but the resulting ?PATTERN? syntax is deprecated, will warn on
usage and might be removed from a future stable release of Perl (without
further notice!).





s/PATTERN/REPLACEMENT/msixpodualgcer



















Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
made.  Otherwise it returns false (specifically, the empty string).



If the /r (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning the
number of substitutions, it returns the copy whether or not a
substitution occurred.  The original string is never changed when
/r is used.  The copy will always be a plain string, even if the
input is an object or a tied variable.



If no string is specified via the =~ or !~ operator, the $_
variable is searched and modified.  Unless the /r option is used,
the string specified must be a scalar variable, an array element, a
hash element, or an assignment to one of those; that is, some sort of
scalar lvalue.



If the delimiter chosen is a single quote, no interpolation is
done on either the PATTERN or the REPLACEMENT.  Otherwise, if the
PATTERN contains a $ that looks like a variable rather than an
end-of-string test, the variable will be interpolated into the pattern
at run-time.  If you want the pattern compiled only once the first time
the variable is interpolated, use the /o option.  If the pattern
evaluates to the empty string, the last successfully executed regular
expression is used instead.  See (perlre) for further explanation on these.



Options are as with m// with the addition of the following replacement
specific options:



    e   Evaluate the right side as an expression.
    ee  Evaluate the right side as a string then eval the result.
    r   Return substitution and leave the original string untouched.



Any non-whitespace delimiter may replace the slashes.  Add space after
the s when using a character allowed in identifiers.  If single quotes
are used, no interpretation is done on the replacement string (the /e
modifier overrides this, however).  Unlike Perl 4, Perl 5 treats backticks
as normal delimiters; the replacement text is not evaluated as a command.
If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
its own pair of quotes, which may or may not be bracketing quotes, for example,
s(foo)(bar) or s<foo>/bar/.  A /e will cause the
replacement portion to be treated as a full-fledged Perl expression
and evaluated right then and there.  It is, however, syntax checked at
compile-time. A second e modifier will cause the replacement portion
to be evaled before being run as a Perl expression.



Examples:



    s/\bgreen\b/mauve/g;                # don't change wintergreen

    $path =~ s|/usr/bin|/usr/local/bin|;

    s/Login: $foo/Login: $bar/; # run-time pattern

    ($foo = $bar) =~ s/this/that/;      # copy first, then change
    ($foo = "$bar") =~ s/this/that/;    # convert to string, copy, then change
    $foo = $bar =~ s/this/that/r;       # Same as above using /r
    $foo = $bar =~ s/this/that/r
                =~ s/that/the other/r;  # Chained substitutes using /r
    @foo = map { s/this/that/r } @bar   # /r is very useful in maps

    $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

    $_ = 'abc123xyz';
    s/\d+/$&*2/e;               # yields 'abc246xyz'
    s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
    s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

    s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
    s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
    s/^=(\w+)/pod($1)/ge;       # use function call

    $_ = 'abc123xyz';
    $a = s/abc/def/r;           # $a is 'def123xyz' and
                                # $_ remains 'abc123xyz'.

    # expand variables in $_, but dynamics only, using
    # symbolic dereferencing
    s/\$(\w+)/${$1}/g;

    # Add one to the value of any numbers in the string
    s/(\d+)/1 + $1/eg;

    # Titlecase words in the last 30 characters only
    substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;

    # This will expand any embedded scalar variable
    # (including lexicals) in $_ : First $1 is interpolated
    # to the variable name, and then evaluated
    s/(\$\w+)/$1/eeg;

    # Delete (most) C comments.
    $program =~ s {
        /\*     # Match the opening delimiter.
        .*?     # Match a minimal number of characters.
        \*/     # Match the closing delimiter.
    } []gsx;

    s/^\s*(.*?)\s*$/$1/;        # trim whitespace in $_, expensively

    for ($variable) {           # trim whitespace in $variable, cheap
        s/^\s+//;
        s/\s+$//;
    }

    s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields



Note the use of $ instead of \ in the last example.  Unlike
sed, we use the \<digit> form in only the left hand side.
Anywhere else it’s $<digit>.



Occasionally, you can’t use just a /g to get all the changes
to occur that you might want.  Here are two common cases:



    # put commas in the right places in an integer
    1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

    # expand tabs to 8-column spacing
    1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;



s///le is treated as a substitution followed by the le operator, not
the /le flags.  This may change in a future version of Perl.  It
produces a warning if warnings are enabled.  To disambiguate, use a space
or change the order of the flags:



    s/foo/bar/ le 5;  # "le" infix operator
    s/foo/bar/el;     # "e" and "l" flags














Next: , Previous: NAME operator Regexp Quote-Like Operators, Up: NAME operator DESCRIPTION   [Contents]





48.2.31 Quote-Like Operators






q/STRING/










’STRING’




A single-quoted, literal string.  A backslash represents a backslash
unless followed by the delimiter or another backslash, in which case
the delimiter or backslash is interpolated.



    $foo = q!I said, "You said, 'She said it.'"!;
    $bar = q('This is it.');
    $baz = '\n';                # a two-character string





qq/STRING/










"STRING"




A double-quoted, interpolated string.



    $_ .= qq
     (*** The previous line contains the naughty word "$1".\n)
                if /\b(tcl|java|python)\b/i;      # :-)
    $baz = "\n";                # a one-character string





qx/STRING/










‘STRING‘




A string which is (possibly) interpolated and then executed as a
system command with /bin/sh or its equivalent.  Shell wildcards,
pipes, and redirections will be honored.  The collected standard
output of the command is returned; standard error is unaffected.  In
scalar context, it comes back as a single (potentially multi-line)
string, or undef if the command failed.  In list context, returns a
list of lines (however you’ve defined lines with $/ or
$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.



Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
To capture a command’s STDERR and STDOUT together:



    $output = `cmd 2>&1`;



To capture a command’s STDOUT but discard its STDERR:



    $output = `cmd 2>/dev/null`;



To capture a command’s STDERR but discard its STDOUT (ordering is
important here):



    $output = `cmd 2>&1 1>/dev/null`;



To exchange a command’s STDOUT and STDERR in order to capture the STDERR
but leave its STDOUT to come out the old STDERR:



    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;



To read both a command’s STDOUT and its STDERR separately, it’s easiest
to redirect them separately to files, and then read from those files
when the program is done:



    system("program args 1>program.stdout 2>program.stderr");



The STDIN filehandle used by the command is inherited from Perl’s STDIN.
For example:



    open(SPLAT, "stuff")   || die "can't open stuff: $!";
    open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
    print STDOUT `sort`;



will print the sorted contents of the file named "stuff".



Using single-quote as a delimiter protects the command from Perl’s
double-quote interpolation, passing it on to the shell instead:



    $perl_info  = qx(ps $$);            # that's Perl's $$
    $shell_info = qx'ps $$';            # that's the new shell's $$



How that string gets evaluated is entirely subject to the command
interpreter on your system.  On most platforms, you will have to protect
shell metacharacters if you want them treated literally.  This is in
practice difficult to do, as it’s unclear how to escape which characters.
See perlsec NAME for a clean and safe example of a manual fork() and exec()
to emulate backticks safely.



On some platforms (notably DOS-like ones), the shell may not be
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want.  You may be able to evaluate
multiple commands in a single line by separating them with the command
separator character, if your shell supports that (for example, ; on 
many Unix shells and & on the Windows NT cmd shell).



Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before starting the child process, but this may not be supported
on some platforms (see perlport NAME).  To be safe, you may need to set
$| ($AUTOFLUSH in English) or call the autoflush() method of
IO::Handle on any open handles.



Beware that some command shells may place restrictions on the length
of the command line.  You must ensure your strings don’t exceed this
limit after any necessary interpolations.  See the platform-specific
release notes for more details about your particular environment.



Using this operator can lead to programs that are difficult to port,
because the shell commands called vary between systems, and may in
fact not be present at all.  As one example, the type command under
the POSIX shell is very different from the type command under DOS.
That doesn’t mean you should go out of your way to avoid backticks
when they’re the right way to get something done.  Perl was made to be
a glue language, and one of the things it glues together is commands.
Just understand what you’re getting yourself into.



See I/O Operators for more discussion.





qw/STRING/







Evaluates to a list of the words extracted out of STRING, using embedded
whitespace as the word delimiters.  It can be understood as being roughly
equivalent to:



    split(" ", q/STRING/);



the differences being that it generates a real list at compile time, and
in scalar context it returns the last element in the list.  So
this expression:



    qw(foo bar baz)



is semantically equivalent to the list:



    "foo", "bar", "baz"



Some frequently seen examples:



    use POSIX qw( setlocale localeconv )
    @EXPORT = qw( foo bar baz );



A common mistake is to try to separate the words with comma or to
put comments into a multi-line qw-string.  For this reason, the
use warnings pragma and the -w switch (that is, the $^W variable)
produces warnings if the STRING contains the "," or the "#" character.





tr/SEARCHLIST/REPLACEMENTLIST/cdsr












y/SEARCHLIST/REPLACEMENTLIST/cdsr




Transliterates all occurrences of the characters found in the search list
with the corresponding character in the replacement list.  It returns
the number of characters replaced or deleted.  If no string is
specified via the =~ or !~ operator, the $_ string is transliterated.



If the /r (non-destructive) option is present, a new copy of the string
is made and its characters transliterated, and this copy is returned no
matter whether it was modified or not: the original string is always
left unchanged.  The new copy is always a plain string, even if the input
string is an object or a tied variable.



Unless the /r option is used, the string specified with =~ must be a
scalar variable, an array element, a hash element, or an assignment to one
of those; in other words, an lvalue.



A character range may be specified with a hyphen, so tr/A-J/0-9/
does the same replacement as tr/ACEGIBDFHJ/0246813579/.
For sed devotees, y is provided as a synonym for tr.  If the
SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
its own pair of quotes, which may or may not be bracketing quotes;
for example, tr[aeiouy][yuoiea] or tr(+\-*/)/ABCD/.



Note that tr does not do regular expression character classes such as
\d or \pL.  The tr operator is not equivalent to the tr(1)
utility.  If you want to map strings between lower/upper cases, see
(perlfunc)lc and (perlfunc)uc, and in general consider using the s
operator if you need regular expressions.  The \U, \u, \L, and
\l string-interpolation escapes on the right side of a substitution
operator will perform correct case-mappings, but tr[a-z][A-Z] will not
(except sometimes on legacy 7-bit data).



Note also that the whole range idea is rather unportable between
character sets–and even within character sets they may cause results
you probably didn’t expect.  A sound principle is to use only ranges
that begin from and end at either alphabets of equal case (a-e, A-E),
or digits (0-4).  Anything else is unsafe.  If in doubt, spell out the
character sets in full.



Options:



    c   Complement the SEARCHLIST.
    d   Delete found but unreplaced characters.
    s   Squash duplicate replaced characters.
    r   Return the modified string and leave the original string
        untouched.



If the /c modifier is specified, the SEARCHLIST character set
is complemented.  If the /d modifier is specified, any characters
specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
(Note that this is slightly more flexible than the behavior of some
tr programs, which delete anything they find in the SEARCHLIST,
period.) If the /s modifier is specified, sequences of characters
that were transliterated to the same character are squashed down
to a single instance of the character.



If the /d modifier is used, the REPLACEMENTLIST is always interpreted
exactly as specified.  Otherwise, if the REPLACEMENTLIST is shorter
than the SEARCHLIST, the final character is replicated till it is long
enough.  If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
This latter is useful for counting characters in a class or for
squashing character sequences in a class.



Examples:



    $ARGV[1] =~ tr/A-Z/a-z/;    # canonicalize to lower case ASCII

    $cnt = tr/*/*/;             # count the stars in $_

    $cnt = $sky =~ tr/*/*/;     # count the stars in $sky

    $cnt = tr/0-9//;            # count the digits in $_

    tr/a-zA-Z//s;               # bookkeeper -> bokeper

    ($HOST = $host) =~ tr/a-z/A-Z/;
     $HOST = $host  =~ tr/a-z/A-Z/r;   # same thing

    $HOST = $host =~ tr/a-z/A-Z/r    # chained with s///r
                  =~ s/:/ -p/r;

    tr/a-zA-Z/ /cs;             # change non-alphas to single space

    @stripped = map tr/a-zA-Z/ /csr, @original;
                                # /r with map

    tr [\200-\377]
       [\000-\177];             # wickedly delete 8th bit



If multiple transliterations are given for a character, only the
first one is used:



    tr/AAA/XYZ/



will transliterate any A to X.



Because the transliteration table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
interpolation.  That means that if you want to use variables, you
must use an eval():



    eval "tr/$oldlist/$newlist/";
    die $@ if $@;

    eval "tr/$oldlist/$newlist/, 1" or die $@;





<<EOF








A line-oriented form of quoting is based on the shell "here-document"
syntax.  Following a << you specify a string to terminate
the quoted material, and all lines following the current line down to
the terminating string are the value of the item.



The terminating string may be either an identifier (a word), or some
quoted text.  An unquoted identifier works like double quotes.
There may not be a space between the << and the identifier,
unless the identifier is explicitly quoted.  (If you put a space it
will be treated as a null identifier, which is valid, and matches the
first empty line.)  The terminating string must appear by itself
(unquoted and with no surrounding whitespace) on the terminating line.



If the terminating string is quoted, the type of quotes used determine
the treatment of the text.





Double Quotes




Double quotes indicate that the text will be interpolated using exactly
the same rules as normal double quoted strings.



       print <<EOF;
    The price is $Price.
    EOF

       print << "EOF"; # same as above
    The price is $Price.
    EOF





Single Quotes




Single quotes indicate the text is to be treated literally with no
interpolation of its content. This is similar to single quoted
strings except that backslashes have no special meaning, with \\
being treated as two backslashes and not one as they would in every
other quoting construct.



Just as in the shell, a backslashed bareword following the <<
means the same thing as a single-quoted string does:



        $cost = <<'VISTA';  # hasta la ...
    That'll be $10 please, ma'am.
    VISTA

        $cost = <<\VISTA;   # Same thing!
    That'll be $10 please, ma'am.
    VISTA



This is the only form of quoting in perl where there is no need
to worry about escaping content, something that code generators
can and do make good use of.





Backticks




The content of the here doc is treated just as it would be if the
string were embedded in backticks. Thus the content is interpolated
as though it were double quoted and then executed via the shell, with
the results of the execution returned.



       print << `EOC`; # execute command and get results
    echo hi there
    EOC








It is possible to stack multiple here-docs in a row:



       print <<"foo", <<"bar"; # you can stack them
    I said foo.
    foo
    I said bar.
    bar

       myfunc(<< "THIS", 23, <<'THAT');
    Here's a line
    or two.
    THIS
    and here's another.
    THAT



Just don’t forget that you have to put a semicolon on the end
to finish the statement, as Perl doesn’t know you’re not going to
try to do this:



       print <<ABC
    179231
    ABC
       + 20;



If you want to remove the line terminator from your here-docs,
use chomp().



    chomp($string = <<'END');
    This is a string.
    END



If you want your here-docs to be indented with the rest of the code,
you’ll need to remove leading whitespace from each line manually:



    ($quote = <<'FINIS') =~ s/^\s+//gm;
       The Road goes ever on and on,
       down from the door where it began.
    FINIS



If you use a here-doc within a delimited construct, such as in s///eg,
the quoted material must come on the lines following the final delimiter.
So instead of



    s/this/<<E . 'that'
    the other
    E
     . 'more '/eg;



you have to write



    s/this/<<E . 'that'
     . 'more '/eg;
    the other
    E



If the terminating identifier is on the last line of the program, you
must be sure there is a newline after it; otherwise, Perl will give the
warning Can’t find string terminator "END" anywhere before EOF....



Additionally, quoting rules for the end-of-string identifier are
unrelated to Perl’s quoting rules. q(), qq(), and the like are not
supported in place of '' and "", and the only interpolation is for
backslashing the quoting character:



    print << "abc\"def";
    testing...
    abc"def



Finally, quoted strings cannot span multiple lines.  The general rule is
that the identifier must be a string literal.  Stick with that, and you
should be safe.














Next: , Previous: NAME operator Quote-Like Operators, Up: NAME operator DESCRIPTION   [Contents]





48.2.32 Gory details of parsing quoted constructs




When presented with something that might have several different
interpretations, Perl uses the DWIM (that’s "Do What I Mean")
principle to pick the most probable interpretation.  This strategy
is so successful that Perl programmers often do not suspect the
ambivalence of what they write.  But from time to time, Perl’s
notions differ substantially from what the author honestly meant.



This section hopes to clarify how Perl handles quoted constructs.
Although the most common reason to learn this is to unravel labyrinthine
regular expressions, because the initial steps of parsing are the
same for all quoting operators, they are all discussed together.



The most important Perl parsing rule is the first one discussed
below: when processing a quoted construct, Perl first finds the end
of that construct, then interprets its contents.  If you understand
this rule, you may skip the rest of this section on the first
reading.  The other rules are likely to contradict the user’s
expectations much less frequently than this first one.



Some passes discussed below are performed concurrently, but because
their results are the same, we consider them individually.  For different
quoting constructs, Perl performs different numbers of passes, from
one to four, but these passes are always performed in the same order.





Finding the end




The first pass is finding the end of the quoted construct, where
the information about the delimiters is used in parsing.
During this search, text between the starting and ending delimiters
is copied to a safe location. The text copied gets delimiter-independent.



If the construct is a here-doc, the ending delimiter is a line
that has a terminating string as the content. Therefore <<EOF is
terminated by EOF immediately followed by "\n" and starting
from the first column of the terminating line.
When searching for the terminating line of a here-doc, nothing
is skipped. In other words, lines after the here-doc syntax
are compared with the terminating string line by line.



For the constructs except here-docs, single characters are used as starting
and ending delimiters. If the starting delimiter is an opening punctuation
(that is (, [, {, or <), the ending delimiter is the
corresponding closing punctuation (that is ), ], }, or >).
If the starting delimiter is an unpaired character like / or a closing
punctuation, the ending delimiter is same as the starting delimiter.
Therefore a / terminates a qq// construct, while a ] terminates
qq[] and qq]] constructs.



When searching for single-character delimiters, escaped delimiters
and \\ are skipped.  For example, while searching for terminating /,
combinations of \\ and \/ are skipped.  If the delimiters are
bracketing, nested pairs are also skipped.  For example, while searching
for closing ] paired with the opening [, combinations of \\, \],
and \[ are all skipped, and nested [ and ] are skipped as well.
However, when backslashes are used as the delimiters (like qq\\ and
tr\\\), nothing is skipped.
During the search for the end, backslashes that escape delimiters or
backslashes are removed (exactly speaking, they are not copied to the
safe location).



For constructs with three-part delimiters (s///, y///, and
tr///), the search is repeated once more.
If the first delimiter is not an opening punctuation, three delimiters must
be same such as s!!! and tr))), in which case the second delimiter
terminates the left part and starts the right part at once.
If the left part is delimited by bracketing punctuation (that is (),
[], {}, or <>), the right part needs another pair of
delimiters such as s(){} and tr[]//.  In these cases, whitespace
and comments are allowed between both parts, though the comment must follow
at least one whitespace character; otherwise a character expected as the 
start of the comment may be regarded as the starting delimiter of the right part.



During this search no attention is paid to the semantics of the construct.
Thus:



    "$hash{"$foo/$bar"}"



or:



    m/
      bar       # NOT a comment, this slash / terminated m//!
     /x



do not form legal quoted expressions.   The quoted part ends on the
first " and /, and the rest happens to be a syntax error.
Because the slash that terminated m// was followed by a SPACE,
the example above is not m//x, but rather m// with no /x
modifier.  So the embedded # is interpreted as a literal #.



Also no attention is paid to \c\ (multichar control char syntax) during
this search. Thus the second \ in qq/\c\/ is interpreted as a part
of \/, and the following / is not recognized as a delimiter.
Instead, use \034 or \x1c at the end of quoted constructs.





Interpolation





The next step is interpolation in the text obtained, which is now
delimiter-independent.  There are multiple cases.





<<'EOF'




No interpolation is performed.
Note that the combination \\ is left intact, since escaped delimiters
are not available for here-docs.





m'', the pattern of s'''




No interpolation is performed at this stage.
Any backslashed sequences including \\ are treated at the stage
to parsing regular expressions.





'', q//, tr''', y''', the replacement of s'''




The only interpolation is removal of \ from pairs of \\.
Therefore - in tr''' and y''' is treated literally
as a hyphen and no character range is available.
\1 in the replacement of s''' does not work as $1.





tr///, y///




No variable interpolation occurs.  String modifying combinations for
case and quoting such as \Q, \U, and \E are not recognized.
The other escape sequences such as \200 and \t and backslashed
characters such as \\ and \- are converted to appropriate literals.
The character - is treated specially and therefore \- is treated
as a literal -.





"", ``, qq//, qx//, <file*glob>, <<"EOF"




\Q, \U, \u, \L, \l, \F (possibly paired with \E) are
converted to corresponding Perl constructs.  Thus, "$foo\Qbaz$bar"
is converted to $foo . (quotemeta("baz" . $bar)) internally.
The other escape sequences such as \200 and \t and backslashed
characters such as \\ and \- are replaced with appropriate
expansions.



Let it be stressed that whatever falls between \Q and \E
is interpolated in the usual way.  Something like "\Q\\E" has
no \E inside.  instead, it has \Q, \\, and E, so the
result is the same as for "\\\\E".  As a general rule, backslashes
between \Q and \E may lead to counterintuitive results.  So,
"\Q\t\E" is converted to quotemeta("\t"), which is the same
as "\\\t" (since TAB is not alphanumeric).  Note also that:



  $str = '\t';
  return "\Q$str";



may be closer to the conjectural intention of the writer of "\Q\t\E".



Interpolated scalars and arrays are converted internally to the join and
. catenation operations.  Thus, "$foo XXX '@arr'" becomes:



  $foo . " XXX '" . (join $", @arr) . "'";



All operations above are performed simultaneously, left to right.



Because the result of "\Q STRING \E" has all metacharacters
quoted, there is no way to insert a literal $ or @ inside a
\Q\E pair.  If protected by \, $ will be quoted to became
"\\\$"; if not, it is interpreted as the start of an interpolated
scalar.



Note also that the interpolation code needs to make a decision on
where the interpolated scalar ends.  For instance, whether
"a $b -> {c}" really means:



  "a " . $b . " -> {c}";



or:



  "a " . $b -> {c};



Most of the time, the longest possible text that does not include
spaces between components and which contains matching braces or
brackets.  because the outcome may be determined by voting based
on heuristic estimators, the result is not strictly predictable.
Fortunately, it’s usually correct for ambiguous cases.





the replacement of s///




Processing of \Q, \U, \u, \L, \l, \F and interpolation
happens as with qq// constructs.



It is at this step that \1 is begrudgingly converted to $1 in
the replacement text of s///, in order to correct the incorrigible
sed hackers who haven’t picked up the saner idiom yet.  A warning
is emitted if the use warnings pragma or the -w command-line flag
(that is, the $^W variable) was set.





RE in ?RE?, /RE/, m/RE/, s/RE/foo/,




Processing of \Q, \U, \u, \L, \l, \F, \E,
and interpolation happens (almost) as with qq// constructs.



Processing of \N{...} is also done here, and compiled into an intermediate
form for the regex compiler.  (This is because, as mentioned below, the regex
compilation may be done at execution time, and \N{...} is a compile-time
construct.)



However any other combinations of \ followed by a character
are not substituted but only skipped, in order to parse them
as regular expressions at the following step.
As \c is skipped at this step, @ of \c@ in RE is possibly
treated as an array symbol (for example @foo),
even though the same text in qq// gives interpolation of \c@.



Moreover, inside (?{BLOCK}), (?# comment ), and
a #-comment in a //x-regular expression, no processing is
performed whatsoever.  This is the first step at which the presence
of the //x modifier is relevant.



Interpolation in patterns has several quirks: $|, $(, $), @+
and @- are not interpolated, and constructs $var[SOMETHING] are
voted (by several different estimators) to be either an array element
or $var followed by an RE alternative.  This is where the notation
${arr[$bar]} comes handy: /${arr[0-9]}/ is interpreted as
array element -9, not as a regular expression from the variable
$arr followed by a digit, which would be the interpretation of
/$arr[0-9]/.  Since voting among different estimators may occur,
the result is not predictable.



The lack of processing of \\ creates specific restrictions on
the post-processed text.  If the delimiter is /, one cannot get
the combination \/ into the result of this step.  / will
finish the regular expression, \/ will be stripped to / on
the previous step, and \\/ will be left as is.  Because / is
equivalent to \/ inside a regular expression, this does not
matter unless the delimiter happens to be character special to the
RE engine, such as in s*foo*bar*, m[foo], or ?foo?; or an
alphanumeric char, as in:



  m m ^ a \s* b mmx;



In the RE above, which is intentionally obfuscated for illustration, the
delimiter is m, the modifier is mx, and after delimiter-removal the
RE is the same as for m/ ^ a \s* b /mx.  There’s more than one
reason you’re encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.








This step is the last one for all constructs except regular expressions,
which are processed further.





parsing regular expressions





Previous steps were performed during the compilation of Perl code,
but this one happens at run time, although it may be optimized to
be calculated at compile time if appropriate.  After preprocessing
described above, and possibly after evaluation if concatenation,
joining, casing translation, or metaquoting are involved, the
resulting string is passed to the RE engine for compilation.



Whatever happens in the RE engine might be better discussed in (perlre),
but for the sake of continuity, we shall do so here.



This is another step where the presence of the //x modifier is
relevant.  The RE engine scans the string from left to right and
converts it to a finite automaton.



Backslashed characters are either replaced with corresponding
literal strings (as with \{), or else they generate special nodes
in the finite automaton (as with \b).  Characters special to the
RE engine (such as |) generate corresponding nodes or groups of
nodes.  (?#...) comments are ignored.  All the rest is either
converted to literal strings to match, or else is ignored (as is
whitespace and #-style comments if //x is present).



Parsing of the bracketed character class construct, [...], is
rather different than the rule used for the rest of the pattern.
The terminator of this construct is found using the same rules as
for finding the terminator of a {}-delimited construct, the only
exception being that ] immediately following [ is treated as
though preceded by a backslash.  Similarly, the terminator of
(?{...}) is found using the same rules as for finding the
terminator of a {}-delimited construct.



It is possible to inspect both the string given to RE engine and the
resulting finite automaton.  See the arguments debug/debugcolor
in the use (re) pragma, as well as Perl’s -Dr command-line
switch documented in perlrun Command Switches.





Optimization of regular expressions





This step is listed for completeness only.  Since it does not change
semantics, details of this step are not documented and are subject
to change without notice.  This step is performed over the finite
automaton that was generated during the previous pass.



It is at this stage that split() silently optimizes /^/ to
mean /^/m.














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48.2.33 I/O Operators










There are several I/O operators you should know about.








A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation.  It is then interpreted as an external
command, and the output of that command is the value of the
backtick string, like in a shell.  In scalar context, a single string
consisting of all output is returned.  In list context, a list of
values is returned, one per line of output.  (You can set $/ to use
a different line terminator.)  The command is executed each time the
pseudo-literal is evaluated.  The status value of the command is
returned in $? (see perlvar NAME for the interpretation of $?).
Unlike in csh, no translation is done on the return data–newlines
remain newlines.  Unlike in any of the shells, single quotes do not
hide variable names in the command from interpretation.  To pass a
literal dollar-sign through to the shell you need to hide it with a
backslash.  The generalized form of backticks is qx//.  (Because
backticks always undergo shell expansion as well, see perlsec NAME for
security concerns.)




In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
undef at end-of-file or on error.  When $/ is set to undef
(sometimes known as file-slurp mode) and the file is empty, it
returns '' the first time, followed by undef subsequently.



Ordinarily you must assign the returned value to a variable, but
there is one situation where an automatic assignment happens.  If
and only if the input symbol is the only thing inside the conditional
of a while statement (even if disguised as a for(;;) loop),
the value is automatically assigned to the global variable $_,
destroying whatever was there previously.  (This may seem like an
odd thing to you, but you’ll use the construct in almost every Perl
script you write.)  The $_ variable is not implicitly localized.
You’ll have to put a local $_; before the loop if you want that
to happen.



The following lines are equivalent:



    while (defined($_ = <STDIN>)) { print; }
    while ($_ = <STDIN>) { print; }
    while (<STDIN>) { print; }
    for (;<STDIN>;) { print; }
    print while defined($_ = <STDIN>);
    print while ($_ = <STDIN>);
    print while <STDIN>;



This also behaves similarly, but assigns to a lexical variable 
instead of to $_:



    while (my $line = <STDIN>) { print $line }



In these loop constructs, the assigned value (whether assignment
is automatic or explicit) is then tested to see whether it is
defined.  The defined test avoids problems where the line has a string
value that would be treated as false by Perl; for example a "" or
a "0" with no trailing newline.  If you really mean for such values
to terminate the loop, they should be tested for explicitly:



    while (($_ = <STDIN>) ne '0') { ... }
    while (<STDIN>) { last unless $_; ... }



In other boolean contexts, <FILEHANDLE> without an
explicit defined test or comparison elicits a warning if the
use warnings pragma or the -w
command-line switch (the $^W variable) is in effect.






The filehandles STDIN, STDOUT, and STDERR are predefined.  (The
filehandles stdin, stdout, and stderr will also work except
in packages, where they would be interpreted as local identifiers
rather than global.)  Additional filehandles may be created with
the open() function, amongst others.  See perlopentut NAME and
(perlfunc)open for details on this.




If a <FILEHANDLE> is used in a context that is looking for
a list, a list comprising all input lines is returned, one line per
list element.  It’s easy to grow to a rather large data space this
way, so use with care.



<FILEHANDLE> may also be spelled readline(*FILEHANDLE).
See (perlfunc)readline.



The null filehandle <> is special: it can be used to emulate the
behavior of sed and awk, and any other Unix filter program
that takes a list of filenames, doing the same to each line
of input from all of them.  Input from <> comes either from
standard input, or from each file listed on the command line.  Here’s
how it works: the first time <> is evaluated, the @ARGV array is
checked, and if it is empty, $ARGV[0] is set to "-", which when opened
gives you standard input.  The @ARGV array is then processed as a list
of filenames.  The loop



    while (<>) {
        ...                     # code for each line
    }



is equivalent to the following Perl-like pseudo code:



    unshift(@ARGV, '-') unless @ARGV;
    while ($ARGV = shift) {
        open(ARGV, $ARGV);
        while (<ARGV>) {
            ...         # code for each line
        }
    }



except that it isn’t so cumbersome to say, and will actually work.
It really does shift the @ARGV array and put the current filename
into the $ARGV variable.  It also uses filehandle ARGV
internally. <> is just a synonym for <ARGV>, which
is magical.  (The pseudo code above doesn’t work because it treats
<ARGV> as non-magical.)



Since the null filehandle uses the two argument form of (perlfunc)open
it interprets special characters, so if you have a script like this:



    while (<>) {
        print;
    }



and call it with perl dangerous.pl 'rm -rfv *|', it actually opens a
pipe, executes the rm command and reads rm’s output from that pipe.
If you want all items in @ARGV to be interpreted as file names, you
can use the module ARGV::readonly from CPAN.



You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want.  Line numbers ($.)
continue as though the input were one big happy file.  See the example
in (perlfunc)eof for how to reset line numbers on each file.



If you want to set @ARGV to your own list of files, go right ahead.
This sets @ARGV to all plain text files if no @ARGV was given:



    @ARGV = grep { -f && -T } glob('*') unless @ARGV;



You can even set them to pipe commands.  For example, this automatically
filters compressed arguments through gzip:



    @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;



If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:



    while ($_ = $ARGV[0], /^-/) {
        shift;
        last if /^--$/;
        if (/^-D(.*)/) { $debug = $1 }
        if (/^-v/)     { $verbose++  }
        # ...           # other switches
    }

    while (<>) {
        # ...           # code for each line
    }



The <> symbol will return undef for end-of-file only once.
If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven’t set @ARGV, will read input from STDIN.



If what the angle brackets contain is a simple scalar variable (for example,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same.  For example:



    $fh = \*STDIN;
    $line = <$fh>;



If what’s within the angle brackets is neither a filehandle nor a simple
scalar variable containing a filehandle name, typeglob, or typeglob
reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned,
depending on context.  This distinction is determined on syntactic
grounds alone.  That means <$x> is always a readline() from
an indirect handle, but <$hash{key}> is always a glob().
That’s because $x is a simple scalar variable, but $hash{key} is
not–it’s a hash element.  Even <$x > (note the extra space)
is treated as glob("$x "), not readline($x).



One level of double-quote interpretation is done first, but you can’t
say <$foo> because that’s an indirect filehandle as explained
in the previous paragraph.  (In older versions of Perl, programmers
would insert curly brackets to force interpretation as a filename glob:
<${foo}>.  These days, it’s considered cleaner to call the
internal function directly as glob($foo), which is probably the right
way to have done it in the first place.)  For example:



    while (<*.c>) {
        chmod 0644, $_;
    }



is roughly equivalent to:



    open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
    while (<FOO>) {
        chomp;
        chmod 0644, $_;
    }



except that the globbing is actually done internally using the standard
File::Glob extension.  Of course, the shortest way to do the above is:



    chmod 0644, <*.c>;



A (file)glob evaluates its (embedded) argument only when it is
starting a new list.  All values must be read before it will start
over.  In list context, this isn’t important because you automatically
get them all anyway.  However, in scalar context the operator returns
the next value each time it’s called, or undef when the list has
run out.  As with filehandle reads, an automatic defined is
generated when the glob occurs in the test part of a while,
because legal glob returns (for example,
a file called 0) would otherwise
terminate the loop.  Again, undef is returned only once.  So if
you’re expecting a single value from a glob, it is much better to
say



    ($file) = <blurch*>;



than



    $file = <blurch*>;



because the latter will alternate between returning a filename and
returning false.



If you’re trying to do variable interpolation, it’s definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.



    @files = glob("$dir/*.[ch]");
    @files = glob($files[$i]);









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48.2.34 Constant Folding





Like C, Perl does a certain amount of expression evaluation at
compile time whenever it determines that all arguments to an
operator are static and have no side effects.  In particular, string
concatenation happens at compile time between literals that don’t do
variable substitution.  Backslash interpolation also happens at
compile time.  You can say



      'Now is the time for all'
    . "\n" 
    .  'good men to come to.'



and this all reduces to one string internally.  Likewise, if
you say



    foreach $file (@filenames) {
        if (-s $file > 5 + 100 * 2**16) {  }
    }



the compiler precomputes the number which that expression
represents so that the interpreter won’t have to.









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48.2.35 No-ops





Perl doesn’t officially have a no-op operator, but the bare constants
0 and 1 are special-cased not to produce a warning in void
context, so you can for example safely do



    1 while foo();









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48.2.36 Bitwise String Operators




Bitstrings of any size may be manipulated by the bitwise operators
(~ | & ^).



If the operands to a binary bitwise op are strings of different
sizes, | and ^ ops act as though the shorter operand had
additional zero bits on the right, while the & op acts as though
the longer operand were truncated to the length of the shorter.
The granularity for such extension or truncation is one or more
bytes.



    # ASCII-based examples
    print "j p \n" ^ " a h";            # prints "JAPH\n"
    print "JA" | "  ph\n";              # prints "japh\n"
    print "japh\nJunk" & '_____';       # prints "JAPH\n";
    print 'p N$' ^ " E<H\n";            # prints "Perl\n";



If you are intending to manipulate bitstrings, be certain that
you’re supplying bitstrings: If an operand is a number, that will imply
a numeric bitwise operation.  You may explicitly show which type of
operation you intend by using "" or 0+, as in the examples below.



    $foo =  150  |  105;        # yields 255  (0x96 | 0x69 is 0xFF)
    $foo = '150' |  105;        # yields 255
    $foo =  150  | '105';       # yields 255
    $foo = '150' | '105';       # yields string '155' (under ASCII)

    $baz = 0+$foo & 0+$bar;     # both ops explicitly numeric
    $biz = "$foo" ^ "$bar";     # both ops explicitly stringy



See (perlfunc)vec for information on how to manipulate individual bits
in a bit vector.









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48.2.37 Integer Arithmetic




By default, Perl assumes that it must do most of its arithmetic in
floating point.  But by saying



    use integer;



you may tell the compiler to use integer operations
(see (integer) for a detailed explanation) from here to the end of
the enclosing BLOCK.  An inner BLOCK may countermand this by saying



    no integer;



which lasts until the end of that BLOCK.  Note that this doesn’t
mean everything is an integer, merely that Perl will use integer
operations for arithmetic, comparison, and bitwise operators.  For
example, even under use integer, if you take the sqrt(2), you’ll
still get 1.4142135623731 or so.



Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
and ">>") always produce integral results.  (But see also
Bitwise String Operators.)  However, use integer still has meaning for
them.  By default, their results are interpreted as unsigned integers, but
if use integer is in effect, their results are interpreted
as signed integers.  For example, ~0 usually evaluates to a large
integral value.  However, use integer; ~0 is -1 on two’s-complement
machines.









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48.2.38 Floating-point Arithmetic









While use integer provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
certain number of decimal places.  For rounding to a certain number
of digits, sprintf() or printf() is usually the easiest route.
See (perlfaq4).



Floating-point numbers are only approximations to what a mathematician
would call real numbers.  There are infinitely more reals than floats,
so some corners must be cut.  For example:



    printf "%.20g\n", 123456789123456789;
    #        produces 123456789123456784



Testing for exact floating-point equality or inequality is not a
good idea.  Here’s a (relatively expensive) work-around to compare
whether two floating-point numbers are equal to a particular number of
decimal places.  See Knuth, volume II, for a more robust treatment of
this topic.



    sub fp_equal {
        my ($X, $Y, $POINTS) = @_;
        my ($tX, $tY);
        $tX = sprintf("%.${POINTS}g", $X);
        $tY = sprintf("%.${POINTS}g", $Y);
        return $tX eq $tY;
    }



The POSIX module (part of the standard perl distribution) implements
ceil(), floor(), and other mathematical and trigonometric functions.
The Math::Complex module (part of the standard perl distribution)
defines mathematical functions that work on both the reals and the
imaginary numbers.  Math::Complex not as efficient as POSIX, but
POSIX can’t work with complex numbers.



Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely.  In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.









Previous: NAME operator Floating-point Arithmetic, Up: NAME operator DESCRIPTION   [Contents]





48.2.39 Bigger Numbers




The standard Math::BigInt, Math::BigRat, and Math::BigFloat modules,
along with the bigint, bigrat, and bitfloat pragmas, provide
variable-precision arithmetic and overloaded operators, although
they’re currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.



        use 5.010;
        use bigint;  # easy interface to Math::BigInt
        $x = 123456789123456789;
        say $x * $x;
    +15241578780673678515622620750190521



Or with rationals:



        use 5.010;
        use bigrat;
        $a = 3/22;
        $b = 4/6;
        say "a/b is ", $a/$b;
        say "a*b is ", $a*$b;
    a/b is 9/44
    a*b is 1/11



Several modules let you calculate with (bound only by memory and CPU time)
unlimited or fixed precision. There are also some non-standard modules that
provide faster implementations via external C libraries.



Here is a short, but incomplete summary:



  Math::Fraction         big, unlimited fractions like 9973 / 12967
  Math::String           treat string sequences like numbers
  Math::FixedPrecision   calculate with a fixed precision
  Math::Currency         for currency calculations
  Bit::Vector            manipulate bit vectors fast (uses C)
  Math::BigIntFast       Bit::Vector wrapper for big numbers
  Math::Pari             provides access to the Pari C library
  Math::BigInteger       uses an external C library
  Math::Cephes           uses external Cephes C library (no big numbers)
  Math::Cephes::Fraction fractions via the Cephes library
  Math::GMP              another one using an external C library



Choose wisely.









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49 perlopentut





• perlopentut NAME:  



• perlopentut DESCRIPTION:  



• perlopentut Open à la shell:  



• perlopentut Open à la C:  



• perlopentut Obscure Open Tricks:  



• perlopentut Other I/O Issues:  



• perlopentut SEE ALSO:  



• perlopentut AUTHOR and COPYRIGHT:  



• perlopentut HISTORY:  












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49.1 NAME



perlopentut - tutorial on opening things in Perl









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49.2 DESCRIPTION



Perl has two simple, built-in ways to open files: the shell way for
convenience, and the C way for precision.  The shell way also has 2- and
3-argument forms, which have different semantics for handling the filename.
The choice is yours.









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49.3 Open à la shell



Perl’s open function was designed to mimic the way command-line
redirection in the shell works.  Here are some basic examples
from the shell:



    $ myprogram file1 file2 file3
    $ myprogram    <  inputfile
    $ myprogram    >  outputfile
    $ myprogram    >> outputfile
    $ myprogram    |  otherprogram 
    $ otherprogram |  myprogram



And here are some more advanced examples:



    $ otherprogram      | myprogram f1 - f2
    $ otherprogram 2>&1 | myprogram -
    $ myprogram     <&3
    $ myprogram     >&4



Programmers accustomed to constructs like those above can take comfort
in learning that Perl directly supports these familiar constructs using
virtually the same syntax as the shell.





• perlopentut Simple Opens:  



• perlopentut Indirect Filehandles:  



• perlopentut Pipe Opens:  



• perlopentut The Minus File:  



• perlopentut Mixing Reads and Writes:  



• perlopentut Filters:  












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49.3.1 Simple Opens



The open function takes two arguments: the first is a filehandle,
and the second is a single string comprising both what to open and how
to open it.  open returns true when it works, and when it fails,
returns a false value and sets the special variable $! to reflect
the system error.  If the filehandle was previously opened, it will
be implicitly closed first.



For example:



    open(INFO,      "datafile") || die("can't open datafile: $!");
    open(INFO,   "<  datafile") || die("can't open datafile: $!");
    open(RESULTS,">  runstats") || die("can't open runstats: $!");
    open(LOG,    ">> logfile ") || die("can't open logfile:  $!");



If you prefer the low-punctuation version, you could write that this way:



    open INFO,   "<  datafile"  or die "can't open datafile: $!";
    open RESULTS,">  runstats"  or die "can't open runstats: $!";
    open LOG,    ">> logfile "  or die "can't open logfile:  $!";



A few things to notice.  First, the leading < is optional.
If omitted, Perl assumes that you want to open the file for reading.



Note also that the first example uses the || logical operator, and the
second uses or, which has lower precedence.  Using || in the latter
examples would effectively mean



    open INFO, ( "<  datafile"  || die "can't open datafile: $!" );



which is definitely not what you want.



The other important thing to notice is that, just as in the shell,
any whitespace before or after the filename is ignored.  This is good,
because you wouldn’t want these to do different things:



    open INFO,   "<datafile"   
    open INFO,   "< datafile" 
    open INFO,   "<  datafile"



Ignoring surrounding whitespace also helps for when you read a filename
in from a different file, and forget to trim it before opening:



    $filename = <INFO>;         # oops, \n still there
    open(EXTRA, "< $filename") || die "can't open $filename: $!";



This is not a bug, but a feature.  Because open mimics the shell in
its style of using redirection arrows to specify how to open the file, it
also does so with respect to extra whitespace around the filename itself
as well.  For accessing files with naughty names, see 
Dispelling the Dweomer.



There is also a 3-argument version of open, which lets you put the
special redirection characters into their own argument:



    open( INFO, ">", $datafile ) || die "Can't create $datafile: $!";



In this case, the filename to open is the actual string in $datafile,
so you don’t have to worry about $datafile containing characters
that might influence the open mode, or whitespace at the beginning of
the filename that would be absorbed in the 2-argument version.  Also,
any reduction of unnecessary string interpolation is a good thing.









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49.3.2 Indirect Filehandles



open’s first argument can be a reference to a filehandle.  As of
perl 5.6.0, if the argument is uninitialized, Perl will automatically
create a filehandle and put a reference to it in the first argument,
like so:



    open( my $in, $infile )   or die "Couldn't read $infile: $!";
    while ( <$in> ) {
        # do something with $_
    }
    close $in;



Indirect filehandles make namespace management easier.  Since filehandles
are global to the current package, two subroutines trying to open
INFILE will clash.  With two functions opening indirect filehandles
like my $infile, there’s no clash and no need to worry about future
conflicts.



Another convenient behavior is that an indirect filehandle automatically
closes when there are no more references to it:



    sub firstline {
        open( my $in, shift ) && return scalar <$in>;
        # no close() required
    }



Indirect filehandles also make it easy to pass filehandles to and return
filehandles from subroutines:



    for my $file ( qw(this.conf that.conf) ) {
        my $fin = open_or_throw('<', $file);
        process_conf( $fin );
        # no close() needed
    }

    use Carp;
    sub open_or_throw {
        my ($mode, $filename) = @_;
        open my $h, $mode, $filename
            or croak "Could not open '$filename': $!";
        return $h;
    }









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49.3.3 Pipe Opens



In C, when you want to open a file using the standard I/O library,
you use the fopen function, but when opening a pipe, you use the
popen function.  But in the shell, you just use a different redirection
character.  That’s also the case for Perl.  The open call 
remains the same–just its argument differs.  



If the leading character is a pipe symbol, open starts up a new
command and opens a write-only filehandle leading into that command.
This lets you write into that handle and have what you write show up on
that command’s standard input.  For example:



    open(PRINTER, "| lpr -Plp1")    || die "can't run lpr: $!";
    print PRINTER "stuff\n";
    close(PRINTER)                  || die "can't close lpr: $!";



If the trailing character is a pipe, you start up a new command and open a
read-only filehandle leading out of that command.  This lets whatever that
command writes to its standard output show up on your handle for reading.
For example:



    open(NET, "netstat -i -n |")    || die "can't fork netstat: $!";
    while (<NET>) { }               # do something with input
    close(NET)                      || die "can't close netstat: $!";



What happens if you try to open a pipe to or from a non-existent
command?  If possible, Perl will detect the failure and set $! as
usual.  But if the command contains special shell characters, such as
> or *, called ’metacharacters’, Perl does not execute the
command directly.  Instead, Perl runs the shell, which then tries to
run the command.  This means that it’s the shell that gets the error
indication.  In such a case, the open call will only indicate
failure if Perl can’t even run the shell.  See (perlfaq8)How can I
capture STDERR from an external command? to see how to cope with
this.  There’s also an explanation in perlipc NAME.



If you would like to open a bidirectional pipe, the IPC::Open2
library will handle this for you.  Check out 
perlipc Bidirectional Communication with Another Process



perl-5.6.x introduced a version of piped open that executes a process
based on its command line arguments without relying on the shell. (Similar
to the system(@LIST) notation.) This is safer and faster than executing
a single argument pipe-command, but does not allow special shell
constructs. (It is also not supported on Microsoft Windows, Mac OS Classic
or RISC OS.)



Here’s an example of open '-|', which prints a random Unix
fortune cookie as uppercase:



    my $collection = shift(@ARGV);
    open my $fortune, '-|', 'fortune', $collection
        or die "Could not find fortune - $!";
    while (<$fortune>)
    {
        print uc($_);
    }
    close($fortune);



And this open '|-' pipes into lpr:



    open my $printer, '|-', 'lpr', '-Plp1'
        or die "can't run lpr: $!";
    print {$printer} "stuff\n";
    close($printer)
        or die "can't close lpr: $!";









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49.3.4 The Minus File



Again following the lead of the standard shell utilities, Perl’s
open function treats a file whose name is a single minus, "-", in a
special way.  If you open minus for reading, it really means to access
the standard input.  If you open minus for writing, it really means to
access the standard output.



If minus can be used as the default input or default output, what happens
if you open a pipe into or out of minus?  What’s the default command it
would run?  The same script as you’re currently running!  This is actually
a stealth fork hidden inside an open call.  See 
perlipc Safe Pipe Opens for details.









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49.3.5 Mixing Reads and Writes



It is possible to specify both read and write access.  All you do is
add a "+" symbol in front of the redirection.  But as in the shell,
using a less-than on a file never creates a new file; it only opens an
existing one.  On the other hand, using a greater-than always clobbers
(truncates to zero length) an existing file, or creates a brand-new one
if there isn’t an old one.  Adding a "+" for read-write doesn’t affect
whether it only works on existing files or always clobbers existing ones.



    open(WTMP, "+< /usr/adm/wtmp") 
        || die "can't open /usr/adm/wtmp: $!";

    open(SCREEN, "+> lkscreen")
        || die "can't open lkscreen: $!";

    open(LOGFILE, "+>> /var/log/applog")
        || die "can't open /var/log/applog: $!";



The first one won’t create a new file, and the second one will always
clobber an old one.  The third one will create a new file if necessary
and not clobber an old one, and it will allow you to read at any point
in the file, but all writes will always go to the end.  In short,
the first case is substantially more common than the second and third
cases, which are almost always wrong.  (If you know C, the plus in
Perl’s open is historically derived from the one in C’s fopen(3S),
which it ultimately calls.)



In fact, when it comes to updating a file, unless you’re working on
a binary file as in the WTMP case above, you probably don’t want to
use this approach for updating.  Instead, Perl’s -i flag comes to
the rescue.  The following command takes all the C, C++, or yacc source
or header files and changes all their foo’s to bar’s, leaving
the old version in the original filename with a ".orig" tacked
on the end:



    $ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]



This is a short cut for some renaming games that are really
the best way to update textfiles.  See the second question in 
(perlfaq5) for more details.









Previous: perlopentut Mixing Reads and Writes, Up: perlopentut Open à la shell   [Contents]





49.3.6 Filters



One of the most common uses for open is one you never
even notice.  When you process the ARGV filehandle using
<ARGV>, Perl actually does an implicit open 
on each file in @ARGV.  Thus a program called like this:



    $ myprogram file1 file2 file3



can have all its files opened and processed one at a time
using a construct no more complex than:



    while (<>) {
        # do something with $_
    } 



If @ARGV is empty when the loop first begins, Perl pretends you’ve opened
up minus, that is, the standard input.  In fact, $ARGV, the currently
open file during <ARGV> processing, is even set to "-"
in these circumstances.



You are welcome to pre-process your @ARGV before starting the loop to
make sure it’s to your liking.  One reason to do this might be to remove
command options beginning with a minus.  While you can always roll the
simple ones by hand, the Getopts modules are good for this:



    use Getopt::Std;

    # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
    getopts("vDo:");            

    # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
    getopts("vDo:", \%args);    



Or the standard Getopt::Long module to permit named arguments:



    use Getopt::Long;
    GetOptions( "verbose"  => \$verbose,        # --verbose
                "Debug"    => \$debug,          # --Debug
                "output=s" => \$output );       
            # --output=somestring or --output somestring



Another reason for preprocessing arguments is to make an empty
argument list default to all files:



    @ARGV = glob("*") unless @ARGV;



You could even filter out all but plain, text files.  This is a bit
silent, of course, and you might prefer to mention them on the way.



    @ARGV = grep { -f && -T } @ARGV;



If you’re using the -n or -p command-line options, you
should put changes to @ARGV in a BEGIN{} block.



Remember that a normal open has special properties, in that it might
call fopen(3S) or it might called popen(3S), depending on what its
argument looks like; that’s why it’s sometimes called "magic open".
Here’s an example:



    $pwdinfo = `domainname` =~ /^(\(none\))?$/
                    ? '< /etc/passwd'
                    : 'ypcat passwd |';

    open(PWD, $pwdinfo)                 
                or die "can't open $pwdinfo: $!";



This sort of thing also comes into play in filter processing.  Because
<ARGV> processing employs the normal, shell-style Perl open,
it respects all the special things we’ve already seen:



    $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile



That program will read from the file f1, the process cmd1, standard
input (tmpfile in this case), the f2 file, the cmd2 command,
and finally the f3 file.



Yes, this also means that if you have files named "-" (and so on) in
your directory, they won’t be processed as literal files by open.
You’ll need to pass them as "./-", much as you would for the rm program,
or you could use sysopen as described below.



One of the more interesting applications is to change files of a certain
name into pipes.  For example, to autoprocess gzipped or compressed
files by decompressing them with gzip:



    @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc $_ |" : $_  } @ARGV;



Or, if you have the GET program installed from LWP,
you can fetch URLs before processing them:



    @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;



It’s not for nothing that this is called magic <ARGV>.
Pretty nifty, eh?









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49.4 Open à la C



If you want the convenience of the shell, then Perl’s open is
definitely the way to go.  On the other hand, if you want finer precision
than C’s simplistic fopen(3S) provides you should look to Perl’s
sysopen, which is a direct hook into the open(2) system call.
That does mean it’s a bit more involved, but that’s the price of 
precision.



sysopen takes 3 (or 4) arguments.



    sysopen HANDLE, PATH, FLAGS, [MASK]



The HANDLE argument is a filehandle just as with open.  The PATH is
a literal path, one that doesn’t pay attention to any greater-thans or
less-thans or pipes or minuses, nor ignore whitespace.  If it’s there,
it’s part of the path.  The FLAGS argument contains one or more values
derived from the Fcntl module that have been or’d together using the
bitwise "|" operator.  The final argument, the MASK, is optional; if
present, it is combined with the user’s current umask for the creation
mode of the file.  You should usually omit this.



Although the traditional values of read-only, write-only, and read-write
are 0, 1, and 2 respectively, this is known not to hold true on some
systems.  Instead, it’s best to load in the appropriate constants first
from the Fcntl module, which supplies the following standard flags:



    O_RDONLY            Read only
    O_WRONLY            Write only
    O_RDWR              Read and write
    O_CREAT             Create the file if it doesn't exist
    O_EXCL              Fail if the file already exists
    O_APPEND            Append to the file
    O_TRUNC             Truncate the file
    O_NONBLOCK          Non-blocking access



Less common flags that are sometimes available on some operating
systems include O_BINARY, O_TEXT, O_SHLOCK, O_EXLOCK,
O_DEFER, O_SYNC, O_ASYNC, O_DSYNC, O_RSYNC,
O_NOCTTY, O_NDELAY and O_LARGEFILE.  Consult your open(2)
manpage or its local equivalent for details.  (Note: starting from
Perl release 5.6 the O_LARGEFILE flag, if available, is automatically
added to the sysopen() flags because large files are the default.)



Here’s how to use sysopen to emulate the simple open calls we had
before.  We’ll omit the || die $! checks for clarity, but make sure
you always check the return values in real code.  These aren’t quite
the same, since open will trim leading and trailing whitespace,
but you’ll get the idea.



To open a file for reading:



    open(FH, "< $path");
    sysopen(FH, $path, O_RDONLY);



To open a file for writing, creating a new file if needed or else truncating
an old file:



    open(FH, "> $path");
    sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);



To open a file for appending, creating one if necessary:



    open(FH, ">> $path");
    sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);



To open a file for update, where the file must already exist:



    open(FH, "+< $path");
    sysopen(FH, $path, O_RDWR);



And here are things you can do with sysopen that you cannot do with
a regular open.  As you’ll see, it’s just a matter of controlling the
flags in the third argument.



To open a file for writing, creating a new file which must not previously
exist:



    sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);



To open a file for appending, where that file must already exist:



    sysopen(FH, $path, O_WRONLY | O_APPEND);



To open a file for update, creating a new file if necessary:



    sysopen(FH, $path, O_RDWR | O_CREAT);



To open a file for update, where that file must not already exist:



    sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);



To open a file without blocking, creating one if necessary:



    sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);





• perlopentut Permissions à la mode:  












Up: perlopentut Open à la C   [Contents]





49.4.1 Permissions à la mode



If you omit the MASK argument to sysopen, Perl uses the octal value
0666.  The normal MASK to use for executables and directories should
be 0777, and for anything else, 0666.



Why so permissive?  Well, it isn’t really.  The MASK will be modified
by your process’s current umask.  A umask is a number representing
disabled permissions bits; that is, bits that will not be turned on
in the created file’s permissions field.



For example, if your umask were 027, then the 020 part would
disable the group from writing, and the 007 part would disable others
from reading, writing, or executing.  Under these conditions, passing
sysopen 0666 would create a file with mode 0640, since 0666 & ~027
is 0640.



You should seldom use the MASK argument to sysopen().  That takes
away the user’s freedom to choose what permission new files will have.
Denying choice is almost always a bad thing.  One exception would be for
cases where sensitive or private data is being stored, such as with mail
folders, cookie files, and internal temporary files.









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49.5 Obscure Open Tricks





• perlopentut Re-Opening Files (dups):  



• perlopentut Dispelling the Dweomer:  



• perlopentut Paths as Opens:  



• perlopentut Single Argument Open:  



• perlopentut Playing with STDIN and STDOUT:  












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49.5.1 Re-Opening Files (dups)



Sometimes you already have a filehandle open, and want to make another
handle that’s a duplicate of the first one.  In the shell, we place an
ampersand in front of a file descriptor number when doing redirections.
For example, 2>&1 makes descriptor 2 (that’s STDERR in Perl)
be redirected into descriptor 1 (which is usually Perl’s STDOUT).
The same is essentially true in Perl: a filename that begins with an
ampersand is treated instead as a file descriptor if a number, or as a
filehandle if a string.



    open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
    open(MHCONTEXT, "<&4")     || die "couldn't dup fd4: $!";



That means that if a function is expecting a filename, but you don’t
want to give it a filename because you already have the file open, you
can just pass the filehandle with a leading ampersand.  It’s best to
use a fully qualified handle though, just in case the function happens
to be in a different package:



    somefunction("&main::LOGFILE");



This way if somefunction() is planning on opening its argument, it can
just use the already opened handle.  This differs from passing a handle,
because with a handle, you don’t open the file.  Here you have something
you can pass to open.



If you have one of those tricky, newfangled I/O objects that the C++
folks are raving about, then this doesn’t work because those aren’t a
proper filehandle in the native Perl sense.  You’ll have to use fileno()
to pull out the proper descriptor number, assuming you can:



    use IO::Socket;
    $handle = IO::Socket::INET->new("www.perl.com:80");
    $fd = $handle->fileno;
    somefunction("&$fd");  # not an indirect function call



It can be easier (and certainly will be faster) just to use real
filehandles though:



    use IO::Socket;
    local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
    die "can't connect" unless defined(fileno(REMOTE));
    somefunction("&main::REMOTE");



If the filehandle or descriptor number is preceded not just with a simple
"&" but rather with a "&=" combination, then Perl will not create a
completely new descriptor opened to the same place using the dup(2)
system call.  Instead, it will just make something of an alias to the
existing one using the fdopen(3S) library call.  This is slightly more
parsimonious of systems resources, although this is less a concern
these days.  Here’s an example of that:



    $fd = $ENV{"MHCONTEXTFD"};
    open(MHCONTEXT, "<&=$fd")   or die "couldn't fdopen $fd: $!";



If you’re using magic <ARGV>, you could even pass in as a
command line argument in @ARGV something like "<&=$MHCONTEXTFD",
but we’ve never seen anyone actually do this.









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49.5.2 Dispelling the Dweomer



Perl is more of a DWIMmer language than something like Java–where DWIM
is an acronym for "do what I mean".  But this principle sometimes leads
to more hidden magic than one knows what to do with.  In this way, Perl
is also filled with dweomer, an obscure word meaning an enchantment.
Sometimes, Perl’s DWIMmer is just too much like dweomer for comfort.



If magic open is a bit too magical for you, you don’t have to turn
to sysopen.  To open a file with arbitrary weird characters in
it, it’s necessary to protect any leading and trailing whitespace.
Leading whitespace is protected by inserting a "./" in front of a
filename that starts with whitespace.  Trailing whitespace is protected
by appending an ASCII NUL byte ("\0") at the end of the string.



    $file =~ s#^(\s)#./$1#;
    open(FH, "< $file\0")   || die "can't open $file: $!";



This assumes, of course, that your system considers dot the current
working directory, slash the directory separator, and disallows ASCII
NULs within a valid filename.  Most systems follow these conventions,
including all POSIX systems as well as proprietary Microsoft systems.
The only vaguely popular system that doesn’t work this way is the
"Classic" Macintosh system, which uses a colon where the rest of us
use a slash.  Maybe sysopen isn’t such a bad idea after all.



If you want to use <ARGV> processing in a totally boring
and non-magical way, you could do this first:



    #   "Sam sat on the ground and put his head in his hands.  
    #   'I wish I had never come here, and I don't want to see 
    #   no more magic,' he said, and fell silent."
    for (@ARGV) { 
        s#^([^./])#./$1#;
        $_ .= "\0";
    } 
    while (<>) {  
        # now process $_
    } 



But be warned that users will not appreciate being unable to use "-"
to mean standard input, per the standard convention.









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49.5.3 Paths as Opens



You’ve probably noticed how Perl’s warn and die functions can
produce messages like:



    Some warning at scriptname line 29, <FH> line 7.



That’s because you opened a filehandle FH, and had read in seven records
from it.  But what was the name of the file, rather than the handle?



If you aren’t running with strict refs, or if you’ve turned them off
temporarily, then all you have to do is this:



    open($path, "< $path") || die "can't open $path: $!";
    while (<$path>) {
        # whatever
    } 



Since you’re using the pathname of the file as its handle,
you’ll get warnings more like



    Some warning at scriptname line 29, </etc/motd> line 7.









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49.5.4 Single Argument Open



Remember how we said that Perl’s open took two arguments?  That was a
passive prevarication.  You see, it can also take just one argument.
If and only if the variable is a global variable, not a lexical, you
can pass open just one argument, the filehandle, and it will 
get the path from the global scalar variable of the same name.



    $FILE = "/etc/motd";
    open FILE or die "can't open $FILE: $!";
    while (<FILE>) {
        # whatever
    } 



Why is this here?  Someone has to cater to the hysterical porpoises.
It’s something that’s been in Perl since the very beginning, if not
before.









Previous: perlopentut Single Argument Open, Up: perlopentut Obscure Open Tricks   [Contents]





49.5.5 Playing with STDIN and STDOUT



One clever move with STDOUT is to explicitly close it when you’re done
with the program.



    END { close(STDOUT) || die "can't close stdout: $!" }



If you don’t do this, and your program fills up the disk partition due
to a command line redirection, it won’t report the error exit with a
failure status.



You don’t have to accept the STDIN and STDOUT you were given.  You are
welcome to reopen them if you’d like.



    open(STDIN, "< datafile")
        || die "can't open datafile: $!";

    open(STDOUT, "> output")
        || die "can't open output: $!";



And then these can be accessed directly or passed on to subprocesses.
This makes it look as though the program were initially invoked
with those redirections from the command line.



It’s probably more interesting to connect these to pipes.  For example:



    $pager = $ENV{PAGER} || "(less || more)";
    open(STDOUT, "| $pager")
        || die "can't fork a pager: $!";



This makes it appear as though your program were called with its stdout
already piped into your pager.  You can also use this kind of thing
in conjunction with an implicit fork to yourself.  You might do this
if you would rather handle the post processing in your own program,
just in a different process:



    head(100);
    while (<>) {
        print;
    } 

    sub head {
        my $lines = shift || 20;
        return if $pid = open(STDOUT, "|-");       # return if parent
        die "cannot fork: $!" unless defined $pid;
        while (<STDIN>) {
            last if --$lines < 0;
            print;
        } 
        exit;
    } 



This technique can be applied to repeatedly push as many filters on your
output stream as you wish.









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49.6 Other I/O Issues



These topics aren’t really arguments related to open or sysopen,
but they do affect what you do with your open files.





• perlopentut Opening Non-File Files:  



• perlopentut Opening Named Pipes:  



• perlopentut Opening Sockets:  



• perlopentut Binary Files:  



• perlopentut File Locking:  



• perlopentut IO Layers:  












Next: , Up: perlopentut Other I/O Issues   [Contents]





49.6.1 Opening Non-File Files



When is a file not a file?  Well, you could say when it exists but
isn’t a plain file.   We’ll check whether it’s a symbolic link first,
just in case.



    if (-l $file || ! -f _) {
        print "$file is not a plain file\n";
    } 



What other kinds of files are there than, well, files?  Directories,
symbolic links, named pipes, Unix-domain sockets, and block and character
devices.  Those are all files, too–just not plain files.  This isn’t
the same issue as being a text file. Not all text files are plain files.
Not all plain files are text files.  That’s why there are separate -f
and -T file tests.



To open a directory, you should use the opendir function, then
process it with readdir, carefully restoring the directory 
name if necessary:



    opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
    while (defined($file = readdir(DIR))) {
        # do something with "$dirname/$file"
    }
    closedir(DIR);



If you want to process directories recursively, it’s better to use the
File::Find module.  For example, this prints out all files recursively
and adds a slash to their names if the file is a directory.



    @ARGV = qw(.) unless @ARGV;
    use File::Find;
    find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;



This finds all bogus symbolic links beneath a particular directory:



    find sub { print "$File::Find::name\n" if -l && !-e }, $dir;



As you see, with symbolic links, you can just pretend that it is
what it points to.  Or, if you want to know what it points to, then
readlink is called for:



    if (-l $file) {
        if (defined($whither = readlink($file))) {
            print "$file points to $whither\n";
        } else {
            print "$file points nowhere: $!\n";
        } 
    } 









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49.6.2 Opening Named Pipes



Named pipes are a different matter.  You pretend they’re regular files,
but their opens will normally block until there is both a reader and
a writer.  You can read more about them in perlipc Named Pipes.
Unix-domain sockets are rather different beasts as well; they’re
described in perlipc Unix-Domain TCP Clients and Servers.



When it comes to opening devices, it can be easy and it can be tricky.
We’ll assume that if you’re opening up a block device, you know what
you’re doing.  The character devices are more interesting.  These are
typically used for modems, mice, and some kinds of printers.  This is
described in (perlfaq8)How do I read and write the serial port?
It’s often enough to open them carefully:



    sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
                # (O_NOCTTY no longer needed on POSIX systems)
        or die "can't open /dev/ttyS1: $!";
    open(TTYOUT, "+>&TTYIN")
        or die "can't dup TTYIN: $!";

    $ofh = select(TTYOUT); $| = 1; select($ofh);

    print TTYOUT "+++at\015";
    $answer = <TTYIN>;



With descriptors that you haven’t opened using sysopen, such as
sockets, you can set them to be non-blocking using fcntl:



    use Fcntl;
    my $old_flags = fcntl($handle, F_GETFL, 0) 
        or die "can't get flags: $!";
    fcntl($handle, F_SETFL, $old_flags | O_NONBLOCK) 
        or die "can't set non blocking: $!";



Rather than losing yourself in a morass of twisting, turning ioctls,
all dissimilar, if you’re going to manipulate ttys, it’s best to
make calls out to the stty(1) program if you have it, or else use the
portable POSIX interface.  To figure this all out, you’ll need to read the
termios(3) manpage, which describes the POSIX interface to tty devices,
and then (POSIX), which describes Perl’s interface to POSIX.  There are
also some high-level modules on CPAN that can help you with these games.
Check out Term::ReadKey and Term::ReadLine.









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49.6.3 Opening Sockets



What else can you open?  To open a connection using sockets, you won’t use
one of Perl’s two open functions.  See 
perlipc Sockets: Client/Server Communication for that.  Here’s an 
example.  Once you have it, you can use FH as a bidirectional filehandle.



    use IO::Socket;
    local *FH = IO::Socket::INET->new("www.perl.com:80");



For opening up a URL, the LWP modules from CPAN are just what
the doctor ordered.  There’s no filehandle interface, but
it’s still easy to get the contents of a document:



    use LWP::Simple;
    $doc = get('http://www.cpan.org/');









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49.6.4 Binary Files



On certain legacy systems with what could charitably be called terminally
convoluted (some would say broken) I/O models, a file isn’t a file–at
least, not with respect to the C standard I/O library.  On these old
systems whose libraries (but not kernels) distinguish between text and
binary streams, to get files to behave properly you’ll have to bend over
backwards to avoid nasty problems.  On such infelicitous systems, sockets
and pipes are already opened in binary mode, and there is currently no
way to turn that off.  With files, you have more options.



Another option is to use the binmode function on the appropriate
handles before doing regular I/O on them:



    binmode(STDIN);
    binmode(STDOUT);
    while (<STDIN>) { print } 



Passing sysopen a non-standard flag option will also open the file in
binary mode on those systems that support it.  This is the equivalent of
opening the file normally, then calling binmode on the handle.



    sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
        || die "can't open records.data: $!";



Now you can use read and print on that handle without worrying
about the non-standard system I/O library breaking your data.  It’s not
a pretty picture, but then, legacy systems seldom are.  CP/M will be
with us until the end of days, and after.



On systems with exotic I/O systems, it turns out that, astonishingly
enough, even unbuffered I/O using sysread and syswrite might do
sneaky data mutilation behind your back.



    while (sysread(WHENCE, $buf, 1024)) {
        syswrite(WHITHER, $buf, length($buf));
    } 



Depending on the vicissitudes of your runtime system, even these calls
may need binmode or O_BINARY first.  Systems known to be free of
such difficulties include Unix, the Mac OS, Plan 9, and Inferno.









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49.6.5 File Locking



In a multitasking environment, you may need to be careful not to collide
with other processes who want to do I/O on the same files as you
are working on.  You’ll often need shared or exclusive locks
on files for reading and writing respectively.  You might just
pretend that only exclusive locks exist.



Never use the existence of a file -e $file as a locking indication,
because there is a race condition between the test for the existence of
the file and its creation.  It’s possible for another process to create
a file in the slice of time between your existence check and your attempt
to create the file.  Atomicity is critical.



Perl’s most portable locking interface is via the flock function,
whose simplicity is emulated on systems that don’t directly support it
such as SysV or Windows.  The underlying semantics may affect how
it all works, so you should learn how flock is implemented on your
system’s port of Perl.



File locking does not lock out another process that would like to
do I/O.  A file lock only locks out others trying to get a lock, not
processes trying to do I/O.  Because locks are advisory, if one process
uses locking and another doesn’t, all bets are off.



By default, the flock call will block until a lock is granted.
A request for a shared lock will be granted as soon as there is no
exclusive locker.  A request for an exclusive lock will be granted as
soon as there is no locker of any kind.  Locks are on file descriptors,
not file names.  You can’t lock a file until you open it, and you can’t
hold on to a lock once the file has been closed.



Here’s how to get a blocking shared lock on a file, typically used
for reading:



    use 5.004;
    use Fcntl qw(:DEFAULT :flock);
    open(FH, "< filename")  or die "can't open filename: $!";
    flock(FH, LOCK_SH)      or die "can't lock filename: $!";
    # now read from FH



You can get a non-blocking lock by using LOCK_NB.



    flock(FH, LOCK_SH | LOCK_NB)
        or die "can't lock filename: $!";



This can be useful for producing more user-friendly behaviour by warning
if you’re going to be blocking:



    use 5.004;
    use Fcntl qw(:DEFAULT :flock);
    open(FH, "< filename")  or die "can't open filename: $!";
    unless (flock(FH, LOCK_SH | LOCK_NB)) {
        $| = 1;
        print "Waiting for lock...";
        flock(FH, LOCK_SH)  or die "can't lock filename: $!";
        print "got it.\n"
    } 
    # now read from FH



To get an exclusive lock, typically used for writing, you have to be
careful.  We sysopen the file so it can be locked before it gets
emptied.  You can get a nonblocking version using LOCK_EX | LOCK_NB.



    use 5.004;
    use Fcntl qw(:DEFAULT :flock);
    sysopen(FH, "filename", O_WRONLY | O_CREAT)
        or die "can't open filename: $!";
    flock(FH, LOCK_EX)
        or die "can't lock filename: $!";
    truncate(FH, 0)
        or die "can't truncate filename: $!";
    # now write to FH



Finally, due to the uncounted millions who cannot be dissuaded from
wasting cycles on useless vanity devices called hit counters, here’s
how to increment a number in a file safely:



    use Fcntl qw(:DEFAULT :flock);

    sysopen(FH, "numfile", O_RDWR | O_CREAT)
        or die "can't open numfile: $!";
    # autoflush FH
    $ofh = select(FH); $| = 1; select ($ofh);
    flock(FH, LOCK_EX)
        or die "can't write-lock numfile: $!";

    $num = <FH> || 0;
    seek(FH, 0, 0)
        or die "can't rewind numfile : $!";
    print FH $num+1, "\n"
        or die "can't write numfile: $!";

    truncate(FH, tell(FH))
        or die "can't truncate numfile: $!";
    close(FH)
        or die "can't close numfile: $!";









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49.6.6 IO Layers



In Perl 5.8.0 a new I/O framework called "PerlIO" was introduced.
This is a new "plumbing" for all the I/O happening in Perl; for the
most part everything will work just as it did, but PerlIO also brought
in some new features such as the ability to think of I/O as "layers".
One I/O layer may in addition to just moving the data also do
transformations on the data.  Such transformations may include
compression and decompression, encryption and decryption, and transforming
between various character encodings.



Full discussion about the features of PerlIO is out of scope for this
tutorial, but here is how to recognize the layers being used:





For more detailed discussion about PerlIO see (PerlIO);
for more detailed discussion about Unicode and I/O see perluniintro NAME.









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49.7 SEE ALSO



The open and sysopen functions in perlfunc(1);
the system open(2), dup(2), fopen(3), and fdopen(3) manpages;
the POSIX documentation.









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49.8 AUTHOR and COPYRIGHT



Copyright 1998 Tom Christiansen.  



This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.



Irrespective of its distribution, all code examples in these files are
hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun or for profit
as you see fit.  A simple comment in the code giving credit would be
courteous but is not required.









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49.9 HISTORY



First release: Sat Jan  9 08:09:11 MST 1999









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50 perlpacktut





• perlpacktut NAME:  



• perlpacktut DESCRIPTION:  



• perlpacktut The Basic Principle:  



• perlpacktut Packing Text:  



• perlpacktut Packing Numbers:  



• perlpacktut Exotic Templates:  



• perlpacktut Template Grouping:  



• perlpacktut Lengths and Widths:  



• perlpacktut Packing and Unpacking C Structures:  



• perlpacktut Pack Recipes:  



• perlpacktut Funnies Section:  



• perlpacktut Authors:  












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50.1 NAME



perlpacktut - tutorial on pack and unpack









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50.2 DESCRIPTION



pack and unpack are two functions for transforming data according
to a user-defined template, between the guarded way Perl stores values
and some well-defined representation as might be required in the 
environment of a Perl program. Unfortunately, they’re also two of 
the most misunderstood and most often overlooked functions that Perl
provides. This tutorial will demystify them for you.









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50.3 The Basic Principle



Most programming languages don’t shelter the memory where variables are
stored. In C, for instance, you can take the address of some variable,
and the sizeof operator tells you how many bytes are allocated to
the variable. Using the address and the size, you may access the storage
to your heart’s content.



In Perl, you just can’t access memory at random, but the structural and
representational conversion provided by pack and unpack is an
excellent alternative. The pack function converts values to a byte
sequence containing representations according to a given specification,
the so-called "template" argument. unpack is the reverse process,
deriving some values from the contents of a string of bytes. (Be cautioned,
however, that not all that has been packed together can be neatly unpacked - 
a very common experience as seasoned travellers are likely to confirm.)



Why, you may ask, would you need a chunk of memory containing some values
in binary representation? One good reason is input and output accessing
some file, a device, or a network connection, whereby this binary
representation is either forced on you or will give you some benefit
in processing. Another cause is passing data to some system call that
is not available as a Perl function: syscall requires you to provide
parameters stored in the way it happens in a C program. Even text processing 
(as shown in the next section) may be simplified with judicious usage 
of these two functions.



To see how (un)packing works, we’ll start with a simple template
code where the conversion is in low gear: between the contents of a byte
sequence and a string of hexadecimal digits. Let’s use unpack, since
this is likely to remind you of a dump program, or some desperate last
message unfortunate programs are wont to throw at you before they expire
into the wild blue yonder. Assuming that the variable $mem holds a 
sequence of bytes that we’d like to inspect without assuming anything 
about its meaning, we can write



   my( $hex ) = unpack( 'H*', $mem );
   print "$hex\n";



whereupon we might see something like this, with each pair of hex digits
corresponding to a byte:



   41204d414e204120504c414e20412043414e414c2050414e414d41



What was in this chunk of memory? Numbers, characters, or a mixture of
both? Assuming that we’re on a computer where ASCII (or some similar)
encoding is used: hexadecimal values in the range 0x40 - 0x5A
indicate an uppercase letter, and 0x20 encodes a space. So we might
assume it is a piece of text, which some are able to read like a tabloid;
but others will have to get hold of an ASCII table and relive that
firstgrader feeling. Not caring too much about which way to read this,
we note that unpack with the template code H converts the contents
of a sequence of bytes into the customary hexadecimal notation. Since
"a sequence of" is a pretty vague indication of quantity, H has been
defined to convert just a single hexadecimal digit unless it is followed
by a repeat count. An asterisk for the repeat count means to use whatever
remains.



The inverse operation - packing byte contents from a string of hexadecimal
digits - is just as easily written. For instance:



   my $s = pack( 'H2' x 10, 30..39 );
   print "$s\n";



Since we feed a list of ten 2-digit hexadecimal strings to pack, the
pack template should contain ten pack codes. If this is run on a computer
with ASCII character coding, it will print 0123456789.









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50.4 Packing Text



Let’s suppose you’ve got to read in a data file like this:



    Date      |Description                | Income|Expenditure
    01/24/2001 Ahmed's Camel Emporium                  1147.99
    01/28/2001 Flea spray                                24.99
    01/29/2001 Camel rides to tourists      235.00



How do we do it? You might think first to use split; however, since
split collapses blank fields, you’ll never know whether a record was
income or expenditure. Oops. Well, you could always use substr:



    while (<>) { 
        my $date   = substr($_,  0, 11);
        my $desc   = substr($_, 12, 27);
        my $income = substr($_, 40,  7);
        my $expend = substr($_, 52,  7);
        ...
    }



It’s not really a barrel of laughs, is it? In fact, it’s worse than it
may seem; the eagle-eyed may notice that the first field should only be
10 characters wide, and the error has propagated right through the other
numbers - which we’ve had to count by hand. So it’s error-prone as well
as horribly unfriendly.



Or maybe we could use regular expressions:



    while (<>) { 
        my($date, $desc, $income, $expend) = 
            m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
        ...
    }



Urgh. Well, it’s a bit better, but - well, would you want to maintain
that?



Hey, isn’t Perl supposed to make this sort of thing easy? Well, it does,
if you use the right tools. pack and unpack are designed to help
you out when dealing with fixed-width data like the above. Let’s have a
look at a solution with unpack:



    while (<>) { 
        my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
        ...
    }



That looks a bit nicer; but we’ve got to take apart that weird template.
Where did I pull that out of? 



OK, let’s have a look at some of our data again; in fact, we’ll include
the headers, and a handy ruler so we can keep track of where we are.



             1         2         3         4         5        
    1234567890123456789012345678901234567890123456789012345678
    Date      |Description                | Income|Expenditure
    01/28/2001 Flea spray                                24.99
    01/29/2001 Camel rides to tourists      235.00



From this, we can see that the date column stretches from column 1 to
column 10 - ten characters wide. The pack-ese for "character" is
A, and ten of them are A10. So if we just wanted to extract the
dates, we could say this:



    my($date) = unpack("A10", $_);



OK, what’s next? Between the date and the description is a blank column;
we want to skip over that. The x template means "skip forward", so we
want one of those. Next, we have another batch of characters, from 12 to
38. That’s 27 more characters, hence A27. (Don’t make the fencepost
error - there are 27 characters between 12 and 38, not 26. Count ’em!)



Now we skip another character and pick up the next 7 characters:



    my($date,$description,$income) = unpack("A10xA27xA7", $_);



Now comes the clever bit. Lines in our ledger which are just income and
not expenditure might end at column 46. Hence, we don’t want to tell our
unpack pattern that we need to find another 12 characters; we’ll
just say "if there’s anything left, take it". As you might guess from
regular expressions, that’s what the * means: "use everything
remaining".





Hence, putting it all together:



    my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);



Now, that’s our data parsed. I suppose what we might want to do now is
total up our income and expenditure, and add another line to the end of
our ledger - in the same format - saying how much we’ve brought in and
how much we’ve spent:



    while (<>) {
        my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
        $tot_income += $income;
        $tot_expend += $expend;
    }

    $tot_income = sprintf("%.2f", $tot_income); # Get them into 
    $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format

    $date = POSIX::strftime("%m/%d/%Y", localtime); 

    # OK, let's go:

    print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);



Oh, hmm. That didn’t quite work. Let’s see what happened:



    01/24/2001 Ahmed's Camel Emporium                   1147.99
    01/28/2001 Flea spray                                 24.99
    01/29/2001 Camel rides to tourists     1235.00
    03/23/2001Totals                     1235.001172.98



OK, it’s a start, but what happened to the spaces? We put x, didn’t
we? Shouldn’t it skip forward? Let’s look at what (perlfunc)pack says:



    x   A null byte.



Urgh. No wonder. There’s a big difference between "a null byte",
character zero, and "a space", character 32. Perl’s put something
between the date and the description - but unfortunately, we can’t see
it! 



What we actually need to do is expand the width of the fields. The A
format pads any non-existent characters with spaces, so we can use the
additional spaces to line up our fields, like this:



    print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);



(Note that you can put spaces in the template to make it more readable,
but they don’t translate to spaces in the output.) Here’s what we got
this time:



    01/24/2001 Ahmed's Camel Emporium                   1147.99
    01/28/2001 Flea spray                                 24.99
    01/29/2001 Camel rides to tourists     1235.00
    03/23/2001 Totals                      1235.00 1172.98



That’s a bit better, but we still have that last column which needs to
be moved further over. There’s an easy way to fix this up:
unfortunately, we can’t get pack to right-justify our fields, but we
can get sprintf to do it:



    $tot_income = sprintf("%.2f", $tot_income); 
    $tot_expend = sprintf("%12.2f", $tot_expend);
    $date = POSIX::strftime("%m/%d/%Y", localtime); 
    print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);



This time we get the right answer:



    01/28/2001 Flea spray                                 24.99
    01/29/2001 Camel rides to tourists     1235.00
    03/23/2001 Totals                      1235.00      1172.98



So that’s how we consume and produce fixed-width data. Let’s recap what
we’ve seen of pack and unpack so far:











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50.5 Packing Numbers



So much for textual data. Let’s get onto the meaty stuff that pack
and unpack are best at: handling binary formats for numbers. There is,
of course, not just one binary format  - life would be too simple - but
Perl will do all the finicky labor for you.





• perlpacktut Integers:  



• perlpacktut Unpacking a Stack Frame:  



• perlpacktut How to Eat an Egg on a Net:  



• perlpacktut Byte-order modifiers:  



• perlpacktut Floating point Numbers:  












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50.5.1 Integers



Packing and unpacking numbers implies conversion to and from some
specific binary representation. Leaving floating point numbers
aside for the moment, the salient properties of any such representation
are:





So, for instance, to pack 20302 to a signed 16 bit integer in your
computer’s representation you write



   my $ps = pack( 's', 20302 );



Again, the result is a string, now containing 2 bytes. If you print 
this string (which is, generally, not recommended) you might see
ON or NO (depending on your system’s byte ordering) - or something
entirely different if your computer doesn’t use ASCII character encoding.
Unpacking $ps with the same template returns the original integer value:



   my( $s ) = unpack( 's', $ps );



This is true for all numeric template codes. But don’t expect miracles:
if the packed value exceeds the allotted byte capacity, high order bits
are silently discarded, and unpack certainly won’t be able to pull them
back out of some magic hat. And, when you pack using a signed template
code such as s, an excess value may result in the sign bit
getting set, and unpacking this will smartly return a negative value.



16 bits won’t get you too far with integers, but there is l and L
for signed and unsigned 32-bit integers. And if this is not enough and
your system supports 64 bit integers you can push the limits much closer
to infinity with pack codes q and Q. A notable exception is provided
by pack codes i and I for signed and unsigned integers of the 
"local custom" variety: Such an integer will take up as many bytes as
a local C compiler returns for sizeof(int), but it’ll use at least
32 bits.



Each of the integer pack codes sSlLqQ results in a fixed number of bytes,
no matter where you execute your program. This may be useful for some 
applications, but it does not provide for a portable way to pass data 
structures between Perl and C programs (bound to happen when you call 
XS extensions or the Perl function syscall), or when you read or
write binary files. What you’ll need in this case are template codes that
depend on what your local C compiler compiles when you code short or
unsigned long, for instance. These codes and their corresponding
byte lengths are shown in the table below.  Since the C standard leaves
much leeway with respect to the relative sizes of these data types, actual
values may vary, and that’s why the values are given as expressions in
C and Perl. (If you’d like to use values from %Config in your program
you have to import it with use Config.)



   signed unsigned  byte length in C   byte length in Perl       
     s!     S!      sizeof(short)      $Config{shortsize}
     i!     I!      sizeof(int)        $Config{intsize}
     l!     L!      sizeof(long)       $Config{longsize}
     q!     Q!      sizeof(long long)  $Config{longlongsize}



The i! and I! codes aren’t different from i and I; they are
tolerated for completeness’ sake.









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50.5.2 Unpacking a Stack Frame



Requesting a particular byte ordering may be necessary when you work with
binary data coming from some specific architecture whereas your program could
run on a totally different system. As an example, assume you have 24 bytes
containing a stack frame as it happens on an Intel 8086:



      +---------+        +----+----+               +---------+
 TOS: |   IP    |  TOS+4:| FL | FH | FLAGS  TOS+14:|   SI    |
      +---------+        +----+----+               +---------+
      |   CS    |        | AL | AH | AX            |   DI    |
      +---------+        +----+----+               +---------+
                         | BL | BH | BX            |   BP    |
                         +----+----+               +---------+
                         | CL | CH | CX            |   DS    |
                         +----+----+               +---------+
                         | DL | DH | DX            |   ES    |
                         +----+----+               +---------+



First, we note that this time-honored 16-bit CPU uses little-endian order,
and that’s why the low order byte is stored at the lower address. To
unpack such a (unsigned) short we’ll have to use code v. A repeat
count unpacks all 12 shorts:



   my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
     unpack( 'v12', $frame );



Alternatively, we could have used C to unpack the individually
accessible byte registers FL, FH, AL, AH, etc.:



   my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
     unpack( 'C10', substr( $frame, 4, 10 ) );



It would be nice if we could do this in one fell swoop: unpack a short,
back up a little, and then unpack 2 bytes. Since Perl is nice, it
proffers the template code X to back up one byte. Putting this all
together, we may now write:



   my( $ip, $cs,
       $flags,$fl,$fh,
       $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh, 
       $si, $di, $bp, $ds, $es ) =
   unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );



(The clumsy construction of the template can be avoided - just read on!)  



We’ve taken some pains to construct the template so that it matches
the contents of our frame buffer. Otherwise we’d either get undefined values,
or unpack could not unpack all. If pack runs out of items, it will
supply null strings (which are coerced into zeroes whenever the pack code
says so).









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50.5.3 How to Eat an Egg on a Net



The pack code for big-endian (high order byte at the lowest address) is
n for 16 bit and N for 32 bit integers. You use these codes
if you know that your data comes from a compliant architecture, but,
surprisingly enough, you should also use these pack codes if you
exchange binary data, across the network, with some system that you
know next to nothing about. The simple reason is that this
order has been chosen as the network order, and all standard-fearing
programs ought to follow this convention. (This is, of course, a stern
backing for one of the Lilliputian parties and may well influence the
political development there.) So, if the protocol expects you to send
a message by sending the length first, followed by just so many bytes,
you could write:



   my $buf = pack( 'N', length( $msg ) ) . $msg;



or even:



   my $buf = pack( 'NA*', length( $msg ), $msg );



and pass $buf to your send routine. Some protocols demand that the
count should include the length of the count itself: then just add 4
to the data length. (But make sure to read Lengths and Widths before
you really code this!)









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50.5.4 Byte-order modifiers



In the previous sections we’ve learned how to use n, N, v and
V to pack and unpack integers with big- or little-endian byte-order.
While this is nice, it’s still rather limited because it leaves out all
kinds of signed integers as well as 64-bit integers. For example, if you
wanted to unpack a sequence of signed big-endian 16-bit integers in a
platform-independent way, you would have to write:



   my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;



This is ugly. As of Perl 5.9.2, there’s a much nicer way to express your
desire for a certain byte-order: the > and < modifiers.
> is the big-endian modifier, while < is the little-endian
modifier. Using them, we could rewrite the above code as:



   my @data = unpack 's>*', $buf;



As you can see, the "big end" of the arrow touches the s, which is a
nice way to remember that > is the big-endian modifier. The same
obviously works for <, where the "little end" touches the code.



You will probably find these modifiers even more useful if you have
to deal with big- or little-endian C structures. Be sure to read
Packing and Unpacking C Structures for more on that.









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50.5.5 Floating point Numbers



For packing floating point numbers you have the choice between the
pack codes f, d, F and D. f and d pack into (or unpack
from) single-precision or double-precision representation as it is provided
by your system. If your systems supports it, D can be used to pack and
unpack extended-precision floating point values (long double), which
can offer even more resolution than f or d. F packs an NV,
which is the floating point type used by Perl internally. (There
is no such thing as a network representation for reals, so if you want
to send your real numbers across computer boundaries, you’d better stick
to ASCII representation, unless you’re absolutely sure what’s on the other
end of the line. For the even more adventuresome, you can use the byte-order
modifiers from the previous section also on floating point codes.)









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50.6 Exotic Templates





• perlpacktut Bit Strings:  



• perlpacktut Uuencoding:  



• perlpacktut Doing Sums:  



• perlpacktut Unicode:  



• perlpacktut Another Portable Binary Encoding:  












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50.6.1 Bit Strings



Bits are the atoms in the memory world. Access to individual bits may
have to be used either as a last resort or because it is the most
convenient way to handle your data. Bit string (un)packing converts
between strings containing a series of 0 and 1 characters and
a sequence of bytes each containing a group of 8 bits. This is almost
as simple as it sounds, except that there are two ways the contents of
a byte may be written as a bit string. Let’s have a look at an annotated
byte:



     7 6 5 4 3 2 1 0
   +-----------------+
   | 1 0 0 0 1 1 0 0 |
   +-----------------+
    MSB           LSB



It’s egg-eating all over again: Some think that as a bit string this should
be written "10001100" i.e. beginning with the most significant bit, others
insist on "00110001". Well, Perl isn’t biased, so that’s why we have two bit
string codes:



   $byte = pack( 'B8', '10001100' ); # start with MSB
   $byte = pack( 'b8', '00110001' ); # start with LSB



It is not possible to pack or unpack bit fields - just integral bytes.
pack always starts at the next byte boundary and "rounds up" to the
next multiple of 8 by adding zero bits as required. (If you do want bit
fields, there is (perlfunc)vec. Or you could implement bit field 
handling at the character string level, using split, substr, and
concatenation on unpacked bit strings.)



To illustrate unpacking for bit strings, we’ll decompose a simple
status register (a "-" stands for a "reserved" bit):



   +-----------------+-----------------+
   | S Z - A - P - C | - - - - O D I T |
   +-----------------+-----------------+
    MSB           LSB MSB           LSB



Converting these two bytes to a string can be done with the unpack 
template 'b16'. To obtain the individual bit values from the bit
string we use split with the "empty" separator pattern which dissects
into individual characters. Bit values from the "reserved" positions are
simply assigned to undef, a convenient notation for "I don’t care where
this goes".



   ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
    $trace, $interrupt, $direction, $overflow) =
      split( //, unpack( 'b16', $status ) );



We could have used an unpack template 'b12' just as well, since the
last 4 bits can be ignored anyway. 









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50.6.2 Uuencoding



Another odd-man-out in the template alphabet is u, which packs an
"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
you won’t ever need this encoding technique which was invented to overcome
the shortcomings of old-fashioned transmission mediums that do not support
other than simple ASCII data. The essential recipe is simple: Take three 
bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to 
each. Repeat until all of the data is blended. Fold groups of 4 bytes into 
lines no longer than 60 and garnish them in front with the original byte count 
(incremented by 0x20) and a "\n" at the end. - The pack chef will
prepare this for you, a la minute, when you select pack code u on the menu:



   my $uubuf = pack( 'u', $bindat );



A repeat count after u sets the number of bytes to put into an
uuencoded line, which is the maximum of 45 by default, but could be
set to some (smaller) integer multiple of three. unpack simply ignores
the repeat count.









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50.6.3 Doing Sums



An even stranger template code is %<number>. First, because 
it’s used as a prefix to some other template code. Second, because it
cannot be used in pack at all, and third, in unpack, doesn’t return the
data as defined by the template code it precedes. Instead it’ll give you an
integer of number bits that is computed from the data value by 
doing sums. For numeric unpack codes, no big feat is achieved:



    my $buf = pack( 'iii', 100, 20, 3 );
    print unpack( '%32i3', $buf ), "\n";  # prints 123



For string values, % returns the sum of the byte values saving
you the trouble of a sum loop with substr and ord:



    print unpack( '%32A*', "\x01\x10" ), "\n";  # prints 17



Although the % code is documented as returning a "checksum":
don’t put your trust in such values! Even when applied to a small number
of bytes, they won’t guarantee a noticeable Hamming distance.



In connection with b or B, % simply adds bits, and this can be put
to good use to count set bits efficiently:



    my $bitcount = unpack( '%32b*', $mask );



And an even parity bit can be determined like this:



    my $evenparity = unpack( '%1b*', $mask );









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50.6.4 Unicode



Unicode is a character set that can represent most characters in most of
the world’s languages, providing room for over one million different
characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
characters that are used in several European languages is in the next
range, up to 255. After some more Latin extensions we find the character
sets from languages using non-Roman alphabets, interspersed with a
variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
(You might want to visit http://www.unicode.org/ for a look at some of
them - my personal favourites are Telugu and Kannada.)



The Unicode character sets associates characters with integers. Encoding
these numbers in an equal number of bytes would more than double the
requirements for storing texts written in Latin alphabets.
The UTF-8 encoding avoids this by storing the most common (from a western
point of view) characters in a single byte while encoding the rarer
ones in three or more bytes.



Perl uses UTF-8, internally, for most Unicode strings.



So what has this got to do with pack? Well, if you want to compose a
Unicode string (that is internally encoded as UTF-8), you can do so by
using template code U. As an example, let’s produce the Euro currency
symbol (code number 0x20AC):



   $UTF8{Euro} = pack( 'U', 0x20AC );
   # Equivalent to: $UTF8{Euro} = "\x{20ac}";



Inspecting $UTF8{Euro} shows that it contains 3 bytes:
"\xe2\x82\xac". However, it contains only 1 character, number 0x20AC.
The round trip can be completed with unpack:



   $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );



Unpacking using the U template code also works on UTF-8 encoded byte
strings.



Usually you’ll want to pack or unpack UTF-8 strings:



   # pack and unpack the Hebrew alphabet
   my $alefbet = pack( 'U*', 0x05d0..0x05ea );
   my @hebrew = unpack( 'U*', $utf );



Please note: in the general case, you’re better off using
Encode::decode_utf8 to decode a UTF-8 encoded byte string to a Perl
Unicode string, and Encode::encode_utf8 to encode a Perl Unicode string
to UTF-8 bytes. These functions provide means of handling invalid byte
sequences and generally have a friendlier interface.









Previous: perlpacktut Unicode, Up: perlpacktut Exotic Templates   [Contents]





50.6.5 Another Portable Binary Encoding



The pack code w has been added to support a portable binary data
encoding scheme that goes way beyond simple integers. (Details can
be found at http://Casbah.org/, the Scarab project.)  A BER (Binary Encoded
Representation) compressed unsigned integer stores base 128
digits, most significant digit first, with as few digits as possible.
Bit eight (the high bit) is set on each byte except the last. There
is no size limit to BER encoding, but Perl won’t go to extremes.



   my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );



A hex dump of $berbuf, with spaces inserted at the right places,
shows 01 8100 8101 81807F. Since the last byte is always less than
128, unpack knows where to stop.









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50.7 Template Grouping



Prior to Perl 5.8, repetitions of templates had to be made by
x-multiplication of template strings. Now there is a better way as
we may use the pack codes ( and ) combined with a repeat count.
The unpack template from the Stack Frame example can simply
be written like this:



   unpack( 'v2 (vXXCC)5 v5', $frame )



Let’s explore this feature a little more. We’ll begin with the equivalent of



   join( '', map( substr( $_, 0, 1 ), @str ) )



which returns a string consisting of the first character from each string.
Using pack, we can write



   pack( '(A)'.@str, @str )



or, because a repeat count * means "repeat as often as required",
simply



   pack( '(A)*', @str )



(Note that the template A* would only have packed $str[0] in full
length.)



To pack dates stored as triplets ( day, month, year ) in an array @dates
into a sequence of byte, byte, short integer we can write



   $pd = pack( '(CCS)*', map( @$_, @dates ) );



To swap pairs of characters in a string (with even length) one could use
several techniques. First, let’s use x and X to skip forward and back:



   $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );



We can also use @ to jump to an offset, with 0 being the position where
we were when the last ( was encountered:



   $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );



Finally, there is also an entirely different approach by unpacking big
endian shorts and packing them in the reverse byte order:



   $s = pack( '(v)*', unpack( '(n)*', $s );









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50.8 Lengths and Widths





• perlpacktut String Lengths:  



• perlpacktut Dynamic Templates:  



• perlpacktut Counting Repetitions:  



• perlpacktut Intel HEX:  












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50.8.1 String Lengths



In the previous section we’ve seen a network message that was constructed
by prefixing the binary message length to the actual message. You’ll find
that packing a length followed by so many bytes of data is a 
frequently used recipe since appending a null byte won’t work
if a null byte may be part of the data. Here is an example where both
techniques are used: after two null terminated strings with source and
destination address, a Short Message (to a mobile phone) is sent after
a length byte:



   my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );



Unpacking this message can be done with the same template:



   ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );



There’s a subtle trap lurking in the offing: Adding another field after
the Short Message (in variable $sm) is all right when packing, but this
cannot be unpacked naively:



   # pack a message
   my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );

   # unpack fails - $prio remains undefined!
   ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );



The pack code A* gobbles up all remaining bytes, and $prio remains
undefined! Before we let disappointment dampen the morale: Perl’s got
the trump card to make this trick too, just a little further up the sleeve.
Watch this:



   # pack a message: ASCIIZ, ASCIIZ, length/string, byte
   my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );

   # unpack
   ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );



Combining two pack codes with a slash (/) associates them with a single
value from the argument list. In pack, the length of the argument is
taken and packed according to the first code while the argument itself
is added after being converted with the template code after the slash.
This saves us the trouble of inserting the length call, but it is 
in unpack where we really score: The value of the length byte marks the
end of the string to be taken from the buffer. Since this combination
doesn’t make sense except when the second pack code isn’t a*, A*
or Z*, Perl won’t let you.



The pack code preceding / may be anything that’s fit to represent a
number: All the numeric binary pack codes, and even text codes such as
A4 or Z*:



   # pack/unpack a string preceded by its length in ASCII
   my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
   # unpack $buf: '13  Humpty-Dumpty'
   my $txt = unpack( 'A4/A*', $buf );



/ is not implemented in Perls before 5.6, so if your code is required to
work on older Perls you’ll need to unpack( 'Z* Z* C') to get the length,
then use it to make a new unpack string. For example



   # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
   my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );

   # unpack
   ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
   ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );



But that second unpack is rushing ahead. It isn’t using a simple literal
string for the template. So maybe we should introduce...









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50.8.2 Dynamic Templates



So far, we’ve seen literals used as templates. If the list of pack
items doesn’t have fixed length, an expression constructing the
template is required (whenever, for some reason, ()* cannot be used).
Here’s an example: To store named string values in a way that can be
conveniently parsed by a C program, we create a sequence of names and
null terminated ASCII strings, with = between the name and the value,
followed by an additional delimiting null byte. Here’s how:



   my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
                   map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );



Let’s examine the cogs of this byte mill, one by one. There’s the map
call, creating the items we intend to stuff into the $env buffer:
to each key (in $_) it adds the = separator and the hash entry value.
Each triplet is packed with the template code sequence A*A*Z* that
is repeated according to the number of keys. (Yes, that’s what the keys
function returns in scalar context.) To get the very last null byte,
we add a 0 at the end of the pack list, to be packed with C.
(Attentive readers may have noticed that we could have omitted the 0.)



For the reverse operation, we’ll have to determine the number of items
in the buffer before we can let unpack rip it apart:



   my $n = $env =~ tr/\0// - 1;
   my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );



The tr counts the null bytes. The unpack call returns a list of
name-value pairs each of which is taken apart in the map block. 









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50.8.3 Counting Repetitions



Rather than storing a sentinel at the end of a data item (or a list of items),
we could precede the data with a count. Again, we pack keys and values of
a hash, preceding each with an unsigned short length count, and up front
we store the number of pairs:



   my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );



This simplifies the reverse operation as the number of repetitions can be
unpacked with the / code:



   my %env = unpack( 'S/(S/A* S/A*)', $env );



Note that this is one of the rare cases where you cannot use the same
template for pack and unpack because pack can’t determine
a repeat count for a ()-group.









Previous: perlpacktut Counting Repetitions, Up: perlpacktut Lengths and Widths   [Contents]





50.8.4 Intel HEX



Intel HEX is a file format for representing binary data, mostly for
programming various chips, as a text file. (See
http://en.wikipedia.org/wiki/.hex for a detailed description, and
http://en.wikipedia.org/wiki/SREC_(file_format) for the Motorola
S-record format, which can be unravelled using the same technique.)
Each line begins with a colon (’:’) and is followed by a sequence of
hexadecimal characters, specifying a byte count n (8 bit),
an address (16 bit, big endian), a record type (8 bit), n data bytes
and a checksum (8 bit) computed as the least significant byte of the two’s
complement sum of the preceding bytes. Example: :0300300002337A1E.



The first step of processing such a line is the conversion, to binary,
of the hexadecimal data, to obtain the four fields, while checking the
checksum. No surprise here: we’ll start with a simple pack call to 
convert everything to binary:



   my $binrec = pack( 'H*', substr( $hexrec, 1 ) );



The resulting byte sequence is most convenient for checking the checksum.
Don’t slow your program down with a for loop adding the ord values
of this string’s bytes - the unpack code % is the thing to use
for computing the 8-bit sum of all bytes, which must be equal to zero:



   die unless unpack( "%8C*", $binrec ) == 0;



Finally, let’s get those four fields. By now, you shouldn’t have any
problems with the first three fields - but how can we use the byte count
of the data in the first field as a length for the data field? Here
the codes x and X come to the rescue, as they permit jumping
back and forth in the string to unpack.



   my( $addr, $type, $data ) = unpack( "x n C X4 C x3 /a", $bin ); 



Code x skips a byte, since we don’t need the count yet. Code n takes
care of the 16-bit big-endian integer address, and C unpacks the
record type. Being at offset 4, where the data begins, we need the count.
X4 brings us back to square one, which is the byte at offset 0.
Now we pick up the count, and zoom forth to offset 4, where we are
now fully furnished to extract the exact number of data bytes, leaving
the trailing checksum byte alone.









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50.9 Packing and Unpacking C Structures



In previous sections we have seen how to pack numbers and character
strings. If it were not for a couple of snags we could conclude this
section right away with the terse remark that C structures don’t
contain anything else, and therefore you already know all there is to it.
Sorry, no: read on, please.



If you have to deal with a lot of C structures, and don’t want to
hack all your template strings manually, you’ll probably want to have
a look at the CPAN module Convert::Binary::C. Not only can it parse
your C source directly, but it also has built-in support for all the
odds and ends described further on in this section.





• perlpacktut The Alignment Pit:  



• perlpacktut Dealing with Endian-ness:  



• perlpacktut Alignment, Take 2:  



• perlpacktut Alignment, Take 3:  



• perlpacktut Pointers for How to Use Them:  












Next: , Up: perlpacktut Packing and Unpacking C Structures   [Contents]





50.9.1 The Alignment Pit



In the consideration of speed against memory requirements the balance
has been tilted in favor of faster execution. This has influenced the
way C compilers allocate memory for structures: On architectures
where a 16-bit or 32-bit operand can be moved faster between places in
memory, or to or from a CPU register, if it is aligned at an even or 
multiple-of-four or even at a multiple-of eight address, a C compiler
will give you this speed benefit by stuffing extra bytes into structures.
If you don’t cross the C shoreline this is not likely to cause you any
grief (although you should care when you design large data structures,
or you want your code to be portable between architectures (you do want
that, don’t you?)).



To see how this affects pack and unpack, we’ll compare these two
C structures:



   typedef struct {
     char     c1;
     short    s;
     char     c2;
     long     l;
   } gappy_t;

   typedef struct {
     long     l;
     short    s;
     char     c1;
     char     c2;
   } dense_t;



Typically, a C compiler allocates 12 bytes to a gappy_t variable, but
requires only 8 bytes for a dense_t. After investigating this further,
we can draw memory maps, showing where the extra 4 bytes are hidden:



   0           +4          +8          +12
   +--+--+--+--+--+--+--+--+--+--+--+--+
   |c1|xx|  s  |c2|xx|xx|xx|     l     |    xx = fill byte
   +--+--+--+--+--+--+--+--+--+--+--+--+
   gappy_t

   0           +4          +8
   +--+--+--+--+--+--+--+--+
   |     l     |  h  |c1|c2|
   +--+--+--+--+--+--+--+--+
   dense_t



And that’s where the first quirk strikes: pack and unpack
templates have to be stuffed with x codes to get those extra fill bytes.



The natural question: "Why can’t Perl compensate for the gaps?" warrants
an answer. One good reason is that C compilers might provide (non-ANSI)
extensions permitting all sorts of fancy control over the way structures
are aligned, even at the level of an individual structure field. And, if
this were not enough, there is an insidious thing called union where
the amount of fill bytes cannot be derived from the alignment of the next
item alone.



OK, so let’s bite the bullet. Here’s one way to get the alignment right
by inserting template codes x, which don’t take a corresponding item 
from the list:



  my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );



Note the ! after l: We want to make sure that we pack a long
integer as it is compiled by our C compiler. And even now, it will only
work for the platforms where the compiler aligns things as above.
And somebody somewhere has a platform where it doesn’t.
[Probably a Cray, where shorts, ints and longs are all 8 bytes. :-)]



Counting bytes and watching alignments in lengthy structures is bound to 
be a drag. Isn’t there a way we can create the template with a simple
program? Here’s a C program that does the trick:



   #include <stdio.h>
   #include <stddef.h>

   typedef struct {
     char     fc1;
     short    fs;
     char     fc2;
     long     fl;
   } gappy_t;

   #define Pt(struct,field,tchar) \
     printf( "@%d%s ", offsetof(struct,field), # tchar );

   int main() {
     Pt( gappy_t, fc1, c  );
     Pt( gappy_t, fs,  s! );
     Pt( gappy_t, fc2, c  );
     Pt( gappy_t, fl,  l! );
     printf( "\n" );
   }



The output line can be used as a template in a pack or unpack call:



  my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );



Gee, yet another template code - as if we hadn’t plenty. But 
@ saves our day by enabling us to specify the offset from the beginning
of the pack buffer to the next item: This is just the value
the offsetof macro (defined in <stddef.h>) returns when
given a struct type and one of its field names ("member-designator" in 
C standardese).



Neither using offsets nor adding x’s to bridge the gaps is satisfactory.
(Just imagine what happens if the structure changes.) What we really need
is a way of saying "skip as many bytes as required to the next multiple of N".
In fluent Templatese, you say this with x!N where N is replaced by the
appropriate value. Here’s the next version of our struct packaging:



  my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );



That’s certainly better, but we still have to know how long all the
integers are, and portability is far away. Rather than 2,
for instance, we want to say "however long a short is". But this can be
done by enclosing the appropriate pack code in brackets: [s]. So, here’s
the very best we can do:



  my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );









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50.9.2 Dealing with Endian-ness



Now, imagine that we want to pack the data for a machine with a
different byte-order. First, we’ll have to figure out how big the data
types on the target machine really are. Let’s assume that the longs are
32 bits wide and the shorts are 16 bits wide. You can then rewrite the
template as:



  my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l );



If the target machine is little-endian, we could write:



  my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );



This forces the short and the long members to be little-endian, and is
just fine if you don’t have too many struct members. But we could also
use the byte-order modifier on a group and write the following:



  my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l );



This is not as short as before, but it makes it more obvious that we
intend to have little-endian byte-order for a whole group, not only
for individual template codes. It can also be more readable and easier
to maintain.









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50.9.3 Alignment, Take 2



I’m afraid that we’re not quite through with the alignment catch yet. The
hydra raises another ugly head when you pack arrays of structures:



   typedef struct {
     short    count;
     char     glyph;
   } cell_t;

   typedef cell_t buffer_t[BUFLEN];



Where’s the catch? Padding is neither required before the first field count,
nor between this and the next field glyph, so why can’t we simply pack
like this:



   # something goes wrong here:
   pack( 's!a' x @buffer,
         map{ ( $_->{count}, $_->{glyph} ) } @buffer );



This packs 3*@buffer bytes, but it turns out that the size of 
buffer_t is four times BUFLEN! The moral of the story is that
the required alignment of a structure or array is propagated to the
next higher level where we have to consider padding at the end
of each component as well. Thus the correct template is:



   pack( 's!ax' x @buffer,
         map{ ( $_->{count}, $_->{glyph} ) } @buffer );









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50.9.4 Alignment, Take 3



And even if you take all the above into account, ANSI still lets this:



   typedef struct {
     char     foo[2];
   } foo_t;



vary in size. The alignment constraint of the structure can be greater than
any of its elements. [And if you think that this doesn’t affect anything
common, dismember the next cellphone that you see. Many have ARM cores, and
the ARM structure rules make sizeof (foo_t) == 4]









Previous: perlpacktut Alignment, Take 3, Up: perlpacktut Packing and Unpacking C Structures   [Contents]





50.9.5 Pointers for How to Use Them



The title of this section indicates the second problem you may run into
sooner or later when you pack C structures. If the function you intend
to call expects a, say, void * value, you cannot simply take
a reference to a Perl variable. (Although that value certainly is a
memory address, it’s not the address where the variable’s contents are
stored.)



Template code P promises to pack a "pointer to a fixed length string".
Isn’t this what we want? Let’s try:



    # allocate some storage and pack a pointer to it
    my $memory = "\x00" x $size;
    my $memptr = pack( 'P', $memory );



But wait: doesn’t pack just return a sequence of bytes? How can we pass this
string of bytes to some C code expecting a pointer which is, after all,
nothing but a number? The answer is simple: We have to obtain the numeric
address from the bytes returned by pack.



    my $ptr = unpack( 'L!', $memptr );



Obviously this assumes that it is possible to typecast a pointer
to an unsigned long and vice versa, which frequently works but should not
be taken as a universal law. - Now that we have this pointer the next question
is: How can we put it to good use? We need a call to some C function
where a pointer is expected. The read(2) system call comes to mind:



    ssize_t read(int fd, void *buf, size_t count);



After reading (perlfunc) explaining how to use syscall we can write
this Perl function copying a file to standard output:



    require 'syscall.ph';
    sub cat($){
        my $path = shift();
        my $size = -s $path;
        my $memory = "\x00" x $size;  # allocate some memory
        my $ptr = unpack( 'L', pack( 'P', $memory ) );
        open( F, $path ) || die( "$path: cannot open ($!)\n" );
        my $fd = fileno(F);
        my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
        print $memory;
        close( F );
    }



This is neither a specimen of simplicity nor a paragon of portability but
it illustrates the point: We are able to sneak behind the scenes and
access Perl’s otherwise well-guarded memory! (Important note: Perl’s
syscall does not require you to construct pointers in this roundabout
way. You simply pass a string variable, and Perl forwards the address.) 



How does unpack with P work? Imagine some pointer in the buffer
about to be unpacked: If it isn’t the null pointer (which will smartly
produce the undef value) we have a start address - but then what?
Perl has no way of knowing how long this "fixed length string" is, so
it’s up to you to specify the actual size as an explicit length after P.



   my $mem = "abcdefghijklmn";
   print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"



As a consequence, pack ignores any number or * after P.



Now that we have seen P at work, we might as well give p a whirl.
Why do we need a second template code for packing pointers at all? The 
answer lies behind the simple fact that an unpack with p promises
a null-terminated string starting at the address taken from the buffer,
and that implies a length for the data item to be returned:



   my $buf = pack( 'p', "abc\x00efhijklmn" );
   print unpack( 'p', $buf );    # prints "abc"



Albeit this is apt to be confusing: As a consequence of the length being
implied by the string’s length, a number after pack code p is a repeat
count, not a length as after P. 



Using pack(..., $x) with P or p to get the address where $x is
actually stored must be used with circumspection. Perl’s internal machinery
considers the relation between a variable and that address as its very own 
private matter and doesn’t really care that we have obtained a copy. Therefore:





It’s safe, however, to P- or p-pack a string literal, because Perl simply
allocates an anonymous variable.









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50.10 Pack Recipes



Here are a collection of (possibly) useful canned recipes for pack
and unpack:



    # Convert IP address for socket functions
    pack( "C4", split /\./, "123.4.5.6" ); 

    # Count the bits in a chunk of memory (e.g. a select vector)
    unpack( '%32b*', $mask );

    # Determine the endianness of your system
    $is_little_endian = unpack( 'c', pack( 's', 1 ) );
    $is_big_endian = unpack( 'xc', pack( 's', 1 ) );

    # Determine the number of bits in a native integer
    $bits = unpack( '%32I!', ~0 );

    # Prepare argument for the nanosleep system call
    my $timespec = pack( 'L!L!', $secs, $nanosecs );



For a simple memory dump we unpack some bytes into just as 
many pairs of hex digits, and use map to handle the traditional
spacing - 16 bytes to a line:



    my $i;
    print map( ++$i % 16 ? "$_ " : "$_\n",
               unpack( 'H2' x length( $mem ), $mem ) ),
          length( $mem ) % 16 ? "\n" : '';









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50.11 Funnies Section



    # Pulling digits out of nowhere...
    print unpack( 'C', pack( 'x' ) ),
          unpack( '%B*', pack( 'A' ) ),
          unpack( 'H', pack( 'A' ) ),
          unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";

    # One for the road ;-)
    my $advice = pack( 'all u can in a van' );









Previous: perlpacktut Funnies Section, Up: perlpacktut   [Contents]





50.12 Authors



Simon Cozens and Wolfgang Laun.









Next: , Previous: perlpacktut, Up: Top   [Contents]





51 perlperf





• perlperf NAME:  



• perlperf DESCRIPTION:  



• perlperf OVERVIEW:  



• perlperf GENERAL GUIDELINES:  



• perlperf BENCHMARKS:  



• perlperf PROFILING TOOLS:  



• perlperf SORTING:  



• perlperf LOGGING:  



• perlperf POSTSCRIPT:  



• perlperf SEE ALSO:  



• perlperf AUTHOR:  












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51.1 NAME



perlperf - Perl Performance and Optimization Techniques









Next: , Previous: perlperf NAME, Up: perlperf   [Contents]





51.2 DESCRIPTION



This is an introduction to the use of performance and optimization techniques
which can be used with particular reference to perl programs.  While many perl
developers have come from other languages, and can use their prior knowledge
where appropriate, there are many other people who might benefit from a few
perl specific pointers.  If you want the condensed version, perhaps the best
advice comes from the renowned Japanese Samurai, Miyamoto Musashi, who said:



    "Do Not Engage in Useless Activity"



in 1645.









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51.3 OVERVIEW



Perhaps the most common mistake programmers make is to attempt to optimize
their code before a program actually does anything useful - this is a bad idea.
There’s no point in having an extremely fast program that doesn’t work.  The
first job is to get a program to correctly do something useful, (not to
mention ensuring the test suite is fully functional), and only then to consider
optimizing it.  Having decided to optimize existing working code, there are
several simple but essential steps to consider which are intrinsic to any
optimization process.





• perlperf ONE STEP SIDEWAYS:  



• perlperf ONE STEP FORWARD:  



• perlperf ANOTHER STEP SIDEWAYS:  












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51.3.1 ONE STEP SIDEWAYS



Firstly, you need to establish a baseline time for the existing code, which
timing needs to be reliable and repeatable.  You’ll probably want to use the
Benchmark or Devel::NYTProf modules, or something similar, for this step,
or perhaps the Unix system time utility, whichever is appropriate.  See the
base of this document for a longer list of benchmarking and profiling modules,
and recommended further reading.









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51.3.2 ONE STEP FORWARD



Next, having examined the program for hot spots, (places where the code
seems to run slowly), change the code with the intention of making it run
faster.  Using version control software, like subversion, will ensure no
changes are irreversible.  It’s too easy to fiddle here and fiddle there -
don’t change too much at any one time or you might not discover which piece of
code really was the slow bit.









Previous: perlperf ONE STEP FORWARD, Up: perlperf OVERVIEW   [Contents]





51.3.3 ANOTHER STEP SIDEWAYS



It’s not enough to say: "that will make it run faster", you have to check it.
Rerun the code under control of the benchmarking or profiling modules, from the
first step above, and check that the new code executed the same task in
less time.  Save your work and repeat...









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51.4 GENERAL GUIDELINES



The critical thing when considering performance is to remember there is no such
thing as a Golden Bullet, which is why there are no rules, only guidelines.



It is clear that inline code is going to be faster than subroutine or method
calls, because there is less overhead, but this approach has the disadvantage
of being less maintainable and comes at the cost of greater memory usage -
there is no such thing as a free lunch.  If you are searching for an element in
a list, it can be more efficient to store the data in a hash structure, and
then simply look to see whether the key is defined, rather than to loop through
the entire array using grep() for instance.  substr() may be (a lot) faster
than grep() but not as flexible, so you have another trade-off to access.  Your
code may contain a line which takes 0.01 of a second to execute which if you
call it 1,000 times, quite likely in a program parsing even medium sized files
for instance, you already have a 10 second delay, in just one single code
location, and if you call that line 100,000 times, your entire program will
slow down to an unbearable crawl.



Using a subroutine as part of your sort is a powerful way to get exactly what
you want, but will usually be slower than the built-in alphabetic cmp and
numeric <=> sort operators.  It is possible to make multiple
passes over your data, building indices to make the upcoming sort more
efficient, and to use what is known as the OM (Orcish Maneuver) to cache the
sort keys in advance.  The cache lookup, while a good idea, can itself be a
source of slowdown by enforcing a double pass over the data - once to setup the
cache, and once to sort the data.  Using pack() to extract the required sort
key into a consistent string can be an efficient way to build a single string
to compare, instead of using multiple sort keys, which makes it possible to use
the standard, written in c and fast, perl sort() function on the output,
and is the basis of the GRT (Guttman Rossler Transform).  Some string
combinations can slow the GRT down, by just being too plain complex for it’s
own good.



For applications using database backends, the standard DBIx namespace has
tries to help with keeping things nippy, not least because it tries to not
query the database until the latest possible moment, but always read the docs
which come with your choice of libraries.  Among the many issues facing
developers dealing with databases should remain aware of is to always use
SQL placeholders and to consider pre-fetching data sets when this might
prove advantageous.  Splitting up a large file by assigning multiple processes
to parsing a single file, using say POE, threads or fork can also be a
useful way of optimizing your usage of the available CPU resources, though
this technique is fraught with concurrency issues and demands high attention to
detail.



Every case has a specific application and one or more exceptions, and there is
no replacement for running a few tests and finding out which method works best
for your particular environment, this is why writing optimal code is not an
exact science, and why we love using Perl so much - TMTOWTDI.









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51.5 BENCHMARKS



Here are a few examples to demonstrate usage of Perl’s benchmarking tools.





• perlperf Assigning and Dereferencing Variables.:  



• perlperf Search and replace or tr:  












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51.5.1 Assigning and Dereferencing Variables.



I’m sure most of us have seen code which looks like, (or worse than), this:



    if ( $obj->{_ref}->{_myscore} >= $obj->{_ref}->{_yourscore} ) {
        ...



This sort of code can be a real eyesore to read, as well as being very
sensitive to typos, and it’s much clearer to dereference the variable
explicitly.  We’re side-stepping the issue of working with object-oriented
programming techniques to encapsulate variable access via methods, only
accessible through an object.  Here we’re just discussing the technical
implementation of choice, and whether this has an effect on performance.  We
can see whether this dereferencing operation, has any overhead by putting
comparative code in a file and running a Benchmark test.



# dereference



    #!/usr/bin/perl

    use strict;
    use warnings;

    use Benchmark;

    my $ref = {
            'ref'   => {
                _myscore    => '100 + 1',
                _yourscore  => '102 - 1',
            },
    };

    timethese(1000000, {
            'direct'       => sub {
                my $x = $ref->{ref}->{_myscore} . $ref->{ref}->{_yourscore} ;
            },
            'dereference'  => sub {
                my $ref  = $ref->{ref};
                my $myscore = $ref->{_myscore};
                my $yourscore = $ref->{_yourscore};
                my $x = $myscore . $yourscore;
            },
    });



It’s essential to run any timing measurements a sufficient number of times so
the numbers settle on a numerical average, otherwise each run will naturally
fluctuate due to variations in the environment, to reduce the effect of
contention for CPU resources and network bandwidth for instance.  Running
the above code for one million iterations, we can take a look at the report
output by the Benchmark module, to see which approach is the most effective.



    $> perl dereference

    Benchmark: timing 1000000 iterations of dereference, direct...
    dereference:  2 wallclock secs ( 1.59 usr +  0.00 sys =  1.59 CPU) @ 628930.82/s (n=1000000)
        direct:  1 wallclock secs ( 1.20 usr +  0.00 sys =  1.20 CPU) @ 833333.33/s (n=1000000)



The difference is clear to see and the dereferencing approach is slower.  While
it managed to execute an average of 628,930 times a second during our test, the
direct approach managed to run an additional 204,403 times, unfortunately.
Unfortunately, because there are many examples of code written using the
multiple layer direct variable access, and it’s usually horrible.  It is,
however, minusculy faster.  The question remains whether the minute gain is
actually worth the eyestrain, or the loss of maintainability.









Previous: perlperf Assigning and Dereferencing Variables., Up: perlperf BENCHMARKS   [Contents]





51.5.2 Search and replace or tr



If we have a string which needs to be modified, while a regex will almost
always be much more flexible, tr, an oft underused tool, can still be a
useful.  One scenario might be replace all vowels with another character.  The
regex solution might look like this:



    $str =~ s/[aeiou]/x/g



The tr alternative might look like this:



    $str =~ tr/aeiou/xxxxx/



We can put that into a test file which we can run to check which approach is
the fastest, using a global $STR variable to assign to the my $str
variable so as to avoid perl trying to optimize any of the work away by
noticing it’s assigned only the once.



# regex-transliterate



    #!/usr/bin/perl

    use strict;
    use warnings;

    use Benchmark;

    my $STR = "$$-this and that";

    timethese( 1000000, {
            'sr'  => sub { my $str = $STR; $str =~ s/[aeiou]/x/g; return $str; },
            'tr'  => sub { my $str = $STR; $str =~ tr/aeiou/xxxxx/; return $str; },
    });



Running the code gives us our results:



    $> perl regex-transliterate

    Benchmark: timing 1000000 iterations of sr, tr...
            sr:  2 wallclock secs ( 1.19 usr +  0.00 sys =  1.19 CPU) @ 840336.13/s (n=1000000)
            tr:  0 wallclock secs ( 0.49 usr +  0.00 sys =  0.49 CPU) @ 2040816.33/s (n=1000000)



The tr version is a clear winner.  One solution is flexible, the other is
fast - and it’s appropriately the programmer’s choice which to use.



Check the Benchmark docs for further useful techniques.









Next: , Previous: perlperf BENCHMARKS, Up: perlperf   [Contents]





51.6 PROFILING TOOLS



A slightly larger piece of code will provide something on which a profiler can
produce more extensive reporting statistics.  This example uses the simplistic
wordmatch program which parses a given input file and spews out a short
report on the contents.



# wordmatch



    #!/usr/bin/perl

    use strict;
    use warnings;

    =head1 NAME

    filewords - word analysis of input file

    =head1 SYNOPSIS

        filewords -f inputfilename [-d]

    =head1 DESCRIPTION

    This program parses the given filename, specified with C<-f>, and displays a
    simple analysis of the words found therein.  Use the C<-d> switch to enable
    debugging messages.

    =cut

    use FileHandle;
    use Getopt::Long;

    my $debug   =  0;
    my $file    = '';

    my $result = GetOptions (
        'debug'         => \$debug,
        'file=s'        => \$file,
    );
    die("invalid args") unless $result;

    unless ( -f $file ) {
        die("Usage: $0 -f filename [-d]");
    }
    my $FH = FileHandle->new("< $file") or die("unable to open file($file): $!");

    my $i_LINES = 0;
    my $i_WORDS = 0;
    my %count   = ();

    my @lines = <$FH>;
    foreach my $line ( @lines ) {
        $i_LINES++;
        $line =~ s/\n//;
        my @words = split(/ +/, $line);
        my $i_words = scalar(@words);
        $i_WORDS = $i_WORDS + $i_words;
        debug("line: $i_LINES supplying $i_words words: @words");
        my $i_word = 0;
        foreach my $word ( @words ) {
            $i_word++;
            $count{$i_LINES}{spec} += matches($i_word, $word, '[^a-zA-Z0-9]');
            $count{$i_LINES}{only} += matches($i_word, $word, '^[^a-zA-Z0-9]+$');
            $count{$i_LINES}{cons} += matches($i_word, $word, '^[(?i:bcdfghjklmnpqrstvwxyz)]+$');
            $count{$i_LINES}{vows} += matches($i_word, $word, '^[(?i:aeiou)]+$');
            $count{$i_LINES}{caps} += matches($i_word, $word, '^[(A-Z)]+$');
        }
    }

    print report( %count );

    sub matches {
        my $i_wd  = shift;
        my $word  = shift;
        my $regex = shift;
        my $has = 0;

        if ( $word =~ /($regex)/ ) {
            $has++ if $1;
        }

        debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/");

        return $has;
    }

    sub report {
        my %report = @_;
        my %rep;

        foreach my $line ( keys %report ) {
            foreach my $key ( keys %{ $report{$line} } ) {
                $rep{$key} += $report{$line}{$key};
            }
        }

        my $report = qq|
    $0 report for $file:
    lines in file: $i_LINES
    words in file: $i_WORDS
    words with special (non-word) characters: $i_spec
    words with only special (non-word) characters: $i_only
    words with only consonants: $i_cons
    words with only capital letters: $i_caps
    words with only vowels: $i_vows
    |;

        return $report;
    }

    sub debug {
        my $message = shift;

        if ( $debug ) {
            print STDERR "DBG: $message\n";
        }
    }

    exit 0;





• perlperf Devel::DProf:  



• perlperf Devel::Profiler:  



• perlperf Devel::SmallProf:  



• perlperf Devel::FastProf:  



• perlperf Devel::NYTProf:  












Next: , Up: perlperf PROFILING TOOLS   [Contents]





51.6.1 Devel::DProf



This venerable module has been the de-facto standard for Perl code profiling
for more than a decade, but has been replaced by a number of other modules
which have brought us back to the 21st century.  Although you’re recommended to
evaluate your tool from the several mentioned here and from the CPAN list at
the base of this document, (and currently (Devel-NYTProf) seems to be the
weapon of choice - see below), we’ll take a quick look at the output from
(Devel-DProf) first, to set a baseline for Perl profiling tools.  Run the
above program under the control of Devel::DProf by using the -d switch on
the command-line.



    $> perl -d:DProf wordmatch -f perl5db.pl

    <...multiple lines snipped...>

    wordmatch report for perl5db.pl:
    lines in file: 9428
    words in file: 50243
    words with special (non-word) characters: 20480
    words with only special (non-word) characters: 7790
    words with only consonants: 4801
    words with only capital letters: 1316
    words with only vowels: 1701



Devel::DProf produces a special file, called tmon.out by default, and
this file is read by the dprofpp program, which is already installed as part
of the Devel::DProf distribution.  If you call dprofpp with no options,
it will read the tmon.out file in the current directory and produce a human
readable statistics report of the run of your program.  Note that this may take
a little time.



    $> dprofpp

    Total Elapsed Time = 2.951677 Seconds
      User+System Time = 2.871677 Seconds
    Exclusive Times
    %Time ExclSec CumulS #Calls sec/call Csec/c  Name
     102.   2.945  3.003 251215   0.0000 0.0000  main::matches
     2.40   0.069  0.069 260643   0.0000 0.0000  main::debug
     1.74   0.050  0.050      1   0.0500 0.0500  main::report
     1.04   0.030  0.049      4   0.0075 0.0123  main::BEGIN
     0.35   0.010  0.010      3   0.0033 0.0033  Exporter::as_heavy
     0.35   0.010  0.010      7   0.0014 0.0014  IO::File::BEGIN
     0.00       - -0.000      1        -      -  Getopt::Long::FindOption
     0.00       - -0.000      1        -      -  Symbol::BEGIN
     0.00       - -0.000      1        -      -  Fcntl::BEGIN
     0.00       - -0.000      1        -      -  Fcntl::bootstrap
     0.00       - -0.000      1        -      -  warnings::BEGIN
     0.00       - -0.000      1        -      -  IO::bootstrap
     0.00       - -0.000      1        -      -  Getopt::Long::ConfigDefaults
     0.00       - -0.000      1        -      -  Getopt::Long::Configure
     0.00       - -0.000      1        -      -  Symbol::gensym



dprofpp will produce some quite detailed reporting on the activity of the
wordmatch program.  The wallclock, user and system, times are at the top of
the analysis, and after this are the main columns defining which define the
report.  Check the dprofpp docs for details of the many options it supports.



See also Apache::DProf which hooks Devel::DProf into mod_perl.









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51.6.2 Devel::Profiler



Let’s take a look at the same program using a different profiler:
Devel::Profiler, a drop-in Perl-only replacement for Devel::DProf.  The
usage is very slightly different in that instead of using the special -d:
flag, you pull Devel::Profiler in directly as a module using -M.



    $> perl -MDevel::Profiler wordmatch -f perl5db.pl

    <...multiple lines snipped...>

    wordmatch report for perl5db.pl:
    lines in file: 9428
    words in file: 50243
    words with special (non-word) characters: 20480
    words with only special (non-word) characters: 7790
    words with only consonants: 4801
    words with only capital letters: 1316
    words with only vowels: 1701



Devel::Profiler generates a tmon.out file which is compatible with the
dprofpp program, thus saving the construction of a dedicated statistics
reader program.  dprofpp usage is therefore identical to the above example.



    $> dprofpp

    Total Elapsed Time =   20.984 Seconds
      User+System Time =   19.981 Seconds
    Exclusive Times
    %Time ExclSec CumulS #Calls sec/call Csec/c  Name
     49.0   9.792 14.509 251215   0.0000 0.0001  main::matches
     24.4   4.887  4.887 260643   0.0000 0.0000  main::debug
     0.25   0.049  0.049      1   0.0490 0.0490  main::report
     0.00   0.000  0.000      1   0.0000 0.0000  Getopt::Long::GetOptions
     0.00   0.000  0.000      2   0.0000 0.0000  Getopt::Long::ParseOptionSpec
     0.00   0.000  0.000      1   0.0000 0.0000  Getopt::Long::FindOption
     0.00   0.000  0.000      1   0.0000 0.0000  IO::File::new
     0.00   0.000  0.000      1   0.0000 0.0000  IO::Handle::new
     0.00   0.000  0.000      1   0.0000 0.0000  Symbol::gensym
     0.00   0.000  0.000      1   0.0000 0.0000  IO::File::open



Interestingly we get slightly different results, which is mostly because the
algorithm which generates the report is different, even though the output file
format was allegedly identical.  The elapsed, user and system times are clearly
showing the time it took for Devel::Profiler to execute its own run, but
the column listings feel more accurate somehow than the ones we had earlier
from Devel::DProf.  The 102% figure has disappeared, for example.  This is
where we have to use the tools at our disposal, and recognise their pros and
cons, before using them.  Interestingly, the numbers of calls for each
subroutine are identical in the two reports, it’s the percentages which differ.
As the author of Devel::Proviler writes:



    ...running HTML::Template's test suite under Devel::DProf shows output()
    taking NO time but Devel::Profiler shows around 10% of the time is in output().
    I don't know which to trust but my gut tells me something is wrong with
    Devel::DProf.  HTML::Template::output() is a big routine that's called for
    every test. Either way, something needs fixing.



YMMV.



See also Devel::Apache::Profiler which hooks Devel::Profiler into mod_perl.









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51.6.3 Devel::SmallProf



The Devel::SmallProf profiler examines the runtime of your Perl program and
produces a line-by-line listing to show how many times each line was called,
and how long each line took to execute.  It is called by supplying the familiar
-d flag to Perl at runtime.



    $> perl -d:SmallProf wordmatch -f perl5db.pl

    <...multiple lines snipped...>

    wordmatch report for perl5db.pl:
    lines in file: 9428
    words in file: 50243
    words with special (non-word) characters: 20480
    words with only special (non-word) characters: 7790
    words with only consonants: 4801
    words with only capital letters: 1316
    words with only vowels: 1701



Devel::SmallProf writes it’s output into a file called smallprof.out, by
default.  The format of the file looks like this:



    <num> <time> <ctime> <line>:<text>



When the program has terminated, the output may be examined and sorted using
any standard text filtering utilities.  Something like the following may be
sufficient:



    $> cat smallprof.out | grep \d*: | sort -k3 | tac | head -n20

    251215   1.65674   7.68000    75: if ( $word =~ /($regex)/ ) {
    251215   0.03264   4.40000    79: debug("word: $i_wd ".($has ? 'matches' :
    251215   0.02693   4.10000    81: return $has;
    260643   0.02841   4.07000   128: if ( $debug ) {
    260643   0.02601   4.04000   126: my $message = shift;
    251215   0.02641   3.91000    73: my $has = 0;
    251215   0.03311   3.71000    70: my $i_wd  = shift;
    251215   0.02699   3.69000    72: my $regex = shift;
    251215   0.02766   3.68000    71: my $word  = shift;
     50243   0.59726   1.00000    59:  $count{$i_LINES}{cons} =
     50243   0.48175   0.92000    61:  $count{$i_LINES}{spec} =
     50243   0.00644   0.89000    56:  my $i_cons = matches($i_word, $word,
     50243   0.48837   0.88000    63:  $count{$i_LINES}{caps} =
     50243   0.00516   0.88000    58:  my $i_caps = matches($i_word, $word, '^[(A-
     50243   0.00631   0.81000    54:  my $i_spec = matches($i_word, $word, '[^a-
     50243   0.00496   0.80000    57:  my $i_vows = matches($i_word, $word,
     50243   0.00688   0.80000    53:  $i_word++;
     50243   0.48469   0.79000    62:  $count{$i_LINES}{only} =
     50243   0.48928   0.77000    60:  $count{$i_LINES}{vows} =
     50243   0.00683   0.75000    55:  my $i_only = matches($i_word, $word, '^[^a-



You can immediately see a slightly different focus to the subroutine profiling
modules, and we start to see exactly which line of code is taking the most
time.  That regex line is looking a bit suspicious, for example.  Remember that
these tools are supposed to be used together, there is no single best way to
profile your code, you need to use the best tools for the job.



See also Apache::SmallProf which hooks Devel::SmallProf into mod_perl.









Next: , Previous: perlperf Devel::SmallProf, Up: perlperf PROFILING TOOLS   [Contents]





51.6.4 Devel::FastProf



Devel::FastProf is another Perl line profiler.  This was written with a view
to getting a faster line profiler, than is possible with for example
Devel::SmallProf, because it’s written in C.  To use Devel::FastProf,
supply the -d argument to Perl:



    $> perl -d:FastProf wordmatch -f perl5db.pl

    <...multiple lines snipped...>

    wordmatch report for perl5db.pl:
    lines in file: 9428
    words in file: 50243
    words with special (non-word) characters: 20480
    words with only special (non-word) characters: 7790
    words with only consonants: 4801
    words with only capital letters: 1316
    words with only vowels: 1701



Devel::FastProf writes statistics to the file fastprof.out in the current
directory.  The output file, which can be specified, can be interpreted by using
the fprofpp command-line program.



    $> fprofpp | head -n20

    # fprofpp output format is:
    # filename:line time count: source
    wordmatch:75 3.93338 251215: if ( $word =~ /($regex)/ ) {
    wordmatch:79 1.77774 251215: debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/");
    wordmatch:81 1.47604 251215: return $has;
    wordmatch:126 1.43441 260643: my $message = shift;
    wordmatch:128 1.42156 260643: if ( $debug ) {
    wordmatch:70 1.36824 251215: my $i_wd  = shift;
    wordmatch:71 1.36739 251215: my $word  = shift;
    wordmatch:72 1.35939 251215: my $regex = shift;



Straightaway we can see that the number of times each line has been called is
identical to the Devel::SmallProf output, and the sequence is only very
slightly different based on the ordering of the amount of time each line took
to execute, if ( $debug ) {  and my $message = shift;, for example.  The
differences in the actual times recorded might be in the algorithm used
internally, or it could be due to system resource limitations or contention.



See also the (DBIx-Profile) which will profile database queries running
under the DBIx::* namespace.









Previous: perlperf Devel::FastProf, Up: perlperf PROFILING TOOLS   [Contents]





51.6.5 Devel::NYTProf



Devel::NYTProf is the next generation of Perl code profiler, fixing many
shortcomings in other tools and implementing many cool features.  First of all it
can be used as either a line profiler, a block or a subroutine
profiler, all at once.  It can also use sub-microsecond (100ns) resolution on
systems which provide clock_gettime().  It can be started and stopped even
by the program being profiled.  It’s a one-line entry to profile mod_perl
applications.  It’s written in c and is probably the fastest profiler
available for Perl.  The list of coolness just goes on.  Enough of that, let’s
see how to it works - just use the familiar -d switch to plug it in and run
the code.



    $> perl -d:NYTProf wordmatch -f perl5db.pl

    wordmatch report for perl5db.pl:
    lines in file: 9427
    words in file: 50243
    words with special (non-word) characters: 20480
    words with only special (non-word) characters: 7790
    words with only consonants: 4801
    words with only capital letters: 1316
    words with only vowels: 1701



NYTProf will generate a report database into the file nytprof.out by
default.  Human readable reports can be generated from here by using the
supplied nytprofhtml (HTML output) and nytprofcsv (CSV output) programs.
We’ve used the Unix system html2text utility to convert the
nytprof/index.html file for convenience here.



    $> html2text nytprof/index.html

    Performance Profile Index
    For wordmatch
      Run on Fri Sep 26 13:46:39 2008
    Reported on Fri Sep 26 13:47:23 2008

             Top 15 Subroutines -- ordered by exclusive time
    |Calls |P |F |Inclusive|Exclusive|Subroutine                          |
    |      |  |  |Time     |Time     |                                    |
    |251215|5 |1 |13.09263 |10.47692 |main::              |matches        |
    |260642|2 |1 |2.71199  |2.71199  |main::              |debug          |
    |1     |1 |1 |0.21404  |0.21404  |main::              |report         |
    |2     |2 |2 |0.00511  |0.00511  |XSLoader::          |load (xsub)    |
    |14    |14|7 |0.00304  |0.00298  |Exporter::          |import         |
    |3     |1 |1 |0.00265  |0.00254  |Exporter::          |as_heavy       |
    |10    |10|4 |0.00140  |0.00140  |vars::              |import         |
    |13    |13|1 |0.00129  |0.00109  |constant::          |import         |
    |1     |1 |1 |0.00360  |0.00096  |FileHandle::        |import         |
    |3     |3 |3 |0.00086  |0.00074  |warnings::register::|import         |
    |9     |3 |1 |0.00036  |0.00036  |strict::            |bits           |
    |13    |13|13|0.00032  |0.00029  |strict::            |import         |
    |2     |2 |2 |0.00020  |0.00020  |warnings::          |import         |
    |2     |1 |1 |0.00020  |0.00020  |Getopt::Long::      |ParseOptionSpec|
    |7     |7 |6 |0.00043  |0.00020  |strict::            |unimport       |

    For more information see the full list of 189 subroutines.



The first part of the report already shows the critical information regarding
which subroutines are using the most time.  The next gives some statistics
about the source files profiled.



            Source Code Files -- ordered by exclusive time then name
    |Stmts  |Exclusive|Avg.   |Reports                     |Source File         |
    |       |Time     |       |                            |                    |
    |2699761|15.66654 |6e-06  |line   .    block   .    sub|wordmatch           |
    |35     |0.02187  |0.00062|line   .    block   .    sub|IO/Handle.pm        |
    |274    |0.01525  |0.00006|line   .    block   .    sub|Getopt/Long.pm      |
    |20     |0.00585  |0.00029|line   .    block   .    sub|Fcntl.pm            |
    |128    |0.00340  |0.00003|line   .    block   .    sub|Exporter/Heavy.pm   |
    |42     |0.00332  |0.00008|line   .    block   .    sub|IO/File.pm          |
    |261    |0.00308  |0.00001|line   .    block   .    sub|Exporter.pm         |
    |323    |0.00248  |8e-06  |line   .    block   .    sub|constant.pm         |
    |12     |0.00246  |0.00021|line   .    block   .    sub|File/Spec/Unix.pm   |
    |191    |0.00240  |0.00001|line   .    block   .    sub|vars.pm             |
    |77     |0.00201  |0.00003|line   .    block   .    sub|FileHandle.pm       |
    |12     |0.00198  |0.00016|line   .    block   .    sub|Carp.pm             |
    |14     |0.00175  |0.00013|line   .    block   .    sub|Symbol.pm           |
    |15     |0.00130  |0.00009|line   .    block   .    sub|IO.pm               |
    |22     |0.00120  |0.00005|line   .    block   .    sub|IO/Seekable.pm      |
    |198    |0.00085  |4e-06  |line   .    block   .    sub|warnings/register.pm|
    |114    |0.00080  |7e-06  |line   .    block   .    sub|strict.pm           |
    |47     |0.00068  |0.00001|line   .    block   .    sub|warnings.pm         |
    |27     |0.00054  |0.00002|line   .    block   .    sub|overload.pm         |
    |9      |0.00047  |0.00005|line   .    block   .    sub|SelectSaver.pm      |
    |13     |0.00045  |0.00003|line   .    block   .    sub|File/Spec.pm        |
    |2701595|15.73869 |       |Total                       |
    |128647 |0.74946  |       |Average                     |
    |       |0.00201  |0.00003|Median                      |
    |       |0.00121  |0.00003|Deviation                   |

    Report produced by the NYTProf 2.03 Perl profiler, developed by Tim Bunce and
    Adam Kaplan.



At this point, if you’re using the html report, you can click through the
various links to bore down into each subroutine and each line of code.  Because
we’re using the text reporting here, and there’s a whole directory full of
reports built for each source file, we’ll just display a part of the
corresponding wordmatch-line.html file, sufficient to give an idea of the
sort of output you can expect from this cool tool.



    $> html2text nytprof/wordmatch-line.html

    Performance Profile -- -block view-.-line view-.-sub view-
    For wordmatch
    Run on Fri Sep 26 13:46:39 2008
    Reported on Fri Sep 26 13:47:22 2008

    File wordmatch

     Subroutines -- ordered by exclusive time
    |Calls |P|F|Inclusive|Exclusive|Subroutine    |
    |      | | |Time     |Time     |              |
    |251215|5|1|13.09263 |10.47692 |main::|matches|
    |260642|2|1|2.71199  |2.71199  |main::|debug  |
    |1     |1|1|0.21404  |0.21404  |main::|report |
    |0     |0|0|0        |0        |main::|BEGIN  |


    |Line|Stmts.|Exclusive|Avg.   |Code                                           |
    |    |      |Time     |       |                                               |
    |1   |      |         |       |#!/usr/bin/perl                                |
    |2   |      |         |       |                                               |
    |    |      |         |       |use strict;                                    |
    |3   |3     |0.00086  |0.00029|# spent 0.00003s making 1 calls to strict::    |
    |    |      |         |       |import                                         |
    |    |      |         |       |use warnings;                                  |
    |4   |3     |0.01563  |0.00521|# spent 0.00012s making 1 calls to warnings::  |
    |    |      |         |       |import                                         |
    |5   |      |         |       |                                               |
    |6   |      |         |       |=head1 NAME                                    |
    |7   |      |         |       |                                               |
    |8   |      |         |       |filewords - word analysis of input file        |
    <...snip...>
    |62  |1     |0.00445  |0.00445|print report( %count );                        |
    |    |      |         |       |# spent 0.21404s making 1 calls to main::report|
    |63  |      |         |       |                                               |
    |    |      |         |       |# spent 23.56955s (10.47692+2.61571) within    |
    |    |      |         |       |main::matches which was called 251215 times,   |
    |    |      |         |       |avg 0.00005s/call: # 50243 times               |
    |    |      |         |       |(2.12134+0.51939s) at line 57 of wordmatch, avg|
    |    |      |         |       |0.00005s/call # 50243 times (2.17735+0.54550s) |
    |64  |      |         |       |at line 56 of wordmatch, avg 0.00005s/call #   |
    |    |      |         |       |50243 times (2.10992+0.51797s) at line 58 of   |
    |    |      |         |       |wordmatch, avg 0.00005s/call # 50243 times     |
    |    |      |         |       |(2.12696+0.51598s) at line 55 of wordmatch, avg|
    |    |      |         |       |0.00005s/call # 50243 times (1.94134+0.51687s) |
    |    |      |         |       |at line 54 of wordmatch, avg 0.00005s/call     |
    |    |      |         |       |sub matches {                                  |
    <...snip...>
    |102 |      |         |       |                                               |
    |    |      |         |       |# spent 2.71199s within main::debug which was  |
    |    |      |         |       |called 260642 times, avg 0.00001s/call: #      |
    |    |      |         |       |251215 times (2.61571+0s) by main::matches at  |
    |103 |      |         |       |line 74 of wordmatch, avg 0.00001s/call # 9427 |
    |    |      |         |       |times (0.09628+0s) at line 50 of wordmatch, avg|
    |    |      |         |       |0.00001s/call                                  |
    |    |      |         |       |sub debug {                                    |
    |104 |260642|0.58496  |2e-06  |my $message = shift;                           |
    |105 |      |         |       |                                               |
    |106 |260642|1.09917  |4e-06  |if ( $debug ) {                                |
    |107 |      |         |       |print STDERR "DBG: $message\n";                |
    |108 |      |         |       |}                                              |
    |109 |      |         |       |}                                              |
    |110 |      |         |       |                                               |
    |111 |1     |0.01501  |0.01501|exit 0;                                        |
    |112 |      |         |       |                                               |



Oodles of very useful information in there - this seems to be the way forward.



See also Devel::NYTProf::Apache which hooks Devel::NYTProf into mod_perl.









Next: , Previous: perlperf PROFILING TOOLS, Up: perlperf   [Contents]





51.7 SORTING



Perl modules are not the only tools a performance analyst has at their
disposal, system tools like time should not be overlooked as the next
example shows, where we take a quick look at sorting.  Many books, theses and
articles, have been written about efficient sorting algorithms, and this is not
the place to repeat such work, there’s several good sorting modules which
deserve taking a look at too: Sort::Maker, Sort::Key spring to mind.
However, it’s still possible to make some observations on certain Perl specific
interpretations on issues relating to sorting data sets and give an example or
two with regard to how sorting large data volumes can effect performance.
Firstly, an often overlooked point when sorting large amounts of data, one can
attempt to reduce the data set to be dealt with and in many cases grep() can
be quite useful as a simple filter:



    @data = sort grep { /$filter/ } @incoming



A command such as this can vastly reduce the volume of material to actually
sort through in the first place, and should not be too lightly disregarded
purely on the basis of its simplicity.  The KISS principle is too often
overlooked - the next example uses the simple system time utility to
demonstrate.  Let’s take a look at an actual example of sorting the contents of
a large file, an apache logfile would do.  This one has over a quarter of a
million lines, is 50M in size, and a snippet of it looks like this:



# logfile



    188.209-65-87.adsl-dyn.isp.belgacom.be - - [08/Feb/2007:12:57:16 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    188.209-65-87.adsl-dyn.isp.belgacom.be - - [08/Feb/2007:12:57:16 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    151.56.71.198 - - [08/Feb/2007:12:57:41 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1"
    151.56.71.198 - - [08/Feb/2007:12:57:42 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1"
    151.56.71.198 - - [08/Feb/2007:12:57:43 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1"
    217.113.68.60 - - [08/Feb/2007:13:02:15 +0000] "GET / HTTP/1.1" 304 - "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    217.113.68.60 - - [08/Feb/2007:13:02:16 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)"
    debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)"
    debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)"
    195.24.196.99 - - [08/Feb/2007:13:26:48 +0000] "GET / HTTP/1.0" 200 3309 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9"
    195.24.196.99 - - [08/Feb/2007:13:26:58 +0000] "GET /data/css HTTP/1.0" 404 206 "http://www.rfi.net/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9"
    195.24.196.99 - - [08/Feb/2007:13:26:59 +0000] "GET /favicon.ico HTTP/1.0" 404 209 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9"
    crawl1.cosmixcorp.com - - [08/Feb/2007:13:27:57 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "voyager/1.0"
    crawl1.cosmixcorp.com - - [08/Feb/2007:13:28:25 +0000] "GET /links.html HTTP/1.0" 200 3413 "-" "voyager/1.0"
    fhm226.internetdsl.tpnet.pl - - [08/Feb/2007:13:37:32 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    fhm226.internetdsl.tpnet.pl - - [08/Feb/2007:13:37:34 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
    80.247.140.134 - - [08/Feb/2007:13:57:35 +0000] "GET / HTTP/1.1" 200 3309 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)"
    80.247.140.134 - - [08/Feb/2007:13:57:37 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)"
    pop.compuscan.co.za - - [08/Feb/2007:14:10:43 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net"
    livebot-207-46-98-57.search.live.com - - [08/Feb/2007:14:12:04 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "msnbot/1.0 (+http://search.msn.com/msnbot.htm)"
    livebot-207-46-98-57.search.live.com - - [08/Feb/2007:14:12:04 +0000] "GET /html/oracle.html HTTP/1.0" 404 214 "-" "msnbot/1.0 (+http://search.msn.com/msnbot.htm)"
    dslb-088-064-005-154.pools.arcor-ip.net - - [08/Feb/2007:14:12:15 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net"
    196.201.92.41 - - [08/Feb/2007:14:15:01 +0000] "GET / HTTP/1.1" 200 3309 "-" "MOT-L7/08.B7.DCR MIB/2.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1"



The specific task here is to sort the 286,525 lines of this file by Response
Code, Query, Browser, Referring Url, and lastly Date.  One solution might be to
use the following code, which iterates over the files given on the
command-line.



# sort-apache-log



    #!/usr/bin/perl -n

    use strict;
    use warnings;

    my @data;

    LINE:
    while ( <> ) {
        my $line = $_;
        if (
            $line =~ m/^(
                ([\w\.\-]+)             # client
                \s*-\s*-\s*\[
                ([^]]+)                 # date
                \]\s*"\w+\s*
                (\S+)                   # query
                [^"]+"\s*
                (\d+)                   # status
                \s+\S+\s+"[^"]*"\s+"
                ([^"]*)                 # browser
                "
                .*
            )$/x
        ) {
            my @chunks = split(/ +/, $line);
            my $ip      = $1;
            my $date    = $2;
            my $query   = $3;
            my $status  = $4;
            my $browser = $5;

            push(@data, [$ip, $date, $query, $status, $browser, $line]);
        }
    }

    my @sorted = sort {
        $a->[3] cmp $b->[3]
                ||
        $a->[2] cmp $b->[2]
                ||
        $a->[0] cmp $b->[0]
                ||
        $a->[1] cmp $b->[1]
                ||
        $a->[4] cmp $b->[4]
    } @data;

    foreach my $data ( @sorted ) {
        print $data->[5];
    }

    exit 0;



When running this program, redirect STDOUT so it is possible to check the
output is correct from following test runs and use the system time utility
to check the overall runtime.



    $> time ./sort-apache-log logfile > out-sort

    real    0m17.371s
    user    0m15.757s
    sys     0m0.592s



The program took just over 17 wallclock seconds to run.  Note the different
values time outputs, it’s important to always use the same one, and to not
confuse what each one means.





Elapsed Real Time




The overall, or wallclock, time between when time was called, and when it
terminates.  The elapsed time includes both user and system times, and time
spent waiting for other users and processes on the system.  Inevitably, this is
the most approximate of the measurements given.





User CPU Time




The user time is the amount of time the entire process spent on behalf of the
user on this system executing this program.





System CPU Time




The system time is the amount of time the kernel itself spent executing
routines, or system calls, on behalf of this process user.








Running this same process as a Schwarzian Transform it is possible to
eliminate the input and output arrays for storing all the data, and work on the
input directly as it arrives too.  Otherwise, the code looks fairly similar:



# sort-apache-log-schwarzian



    #!/usr/bin/perl -n

    use strict;
    use warnings;

    print

        map $_->[0] =>

        sort {
            $a->[4] cmp $b->[4]
                    ||
            $a->[3] cmp $b->[3]
                    ||
            $a->[1] cmp $b->[1]
                    ||
            $a->[2] cmp $b->[2]
                    ||
            $a->[5] cmp $b->[5]
        }
        map  [ $_, m/^(
            ([\w\.\-]+)             # client
            \s*-\s*-\s*\[
            ([^]]+)                 # date
            \]\s*"\w+\s*
            (\S+)                   # query
            [^"]+"\s*
            (\d+)                   # status
            \s+\S+\s+"[^"]*"\s+"
            ([^"]*)                 # browser
            "
            .*
        )$/xo ]

        => <>;

    exit 0;



Run the new code against the same logfile, as above, to check the new time.



    $> time ./sort-apache-log-schwarzian logfile > out-schwarz

    real    0m9.664s
    user    0m8.873s
    sys     0m0.704s



The time has been cut in half, which is a respectable speed improvement by any
standard.  Naturally, it is important to check the output is consistent with
the first program run, this is where the Unix system cksum utility comes in.



    $> cksum out-sort out-schwarz
    3044173777 52029194 out-sort
    3044173777 52029194 out-schwarz



BTW. Beware too of pressure from managers who see you speed a program up by 50%
of the runtime once, only to get a request one month later to do the same again
(true story) - you’ll just have to point out your only human, even if you are a
Perl programmer, and you’ll see what you can do...









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51.8 LOGGING



An essential part of any good development process is appropriate error handling
with appropriately informative messages, however there exists a school of
thought which suggests that log files should be chatty, as if the chain of
unbroken output somehow ensures the survival of the program.  If speed is in
any way an issue, this approach is wrong.



A common sight is code which looks something like this:



    logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) )



The problem is that this code will always be parsed and executed, even when the
debug level set in the logging configuration file is zero.  Once the debug()
subroutine has been entered, and the internal $debug variable confirmed to
be zero, for example, the message which has been sent in will be discarded and
the program will continue.  In the example given though, the \%INC hash will
already have been dumped, and the message string constructed, all of which work
could be bypassed by a debug variable at the statement level, like this:



    logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) if $DEBUG;



This effect can be demonstrated by setting up a test script with both forms,
including a debug() subroutine to emulate typical logger() functionality.



# ifdebug



    #!/usr/bin/perl

    use strict;
    use warnings;

    use Benchmark;
    use Data::Dumper;
    my $DEBUG = 0;

    sub debug {
        my $msg = shift;

        if ( $DEBUG ) {
            print "DEBUG: $msg\n";
        }
    };

    timethese(100000, {
            'debug'       => sub {
                debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) )
            },
            'ifdebug'  => sub {
                debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if $DEBUG
            },
    });



Let’s see what Benchmark makes of this:



    $> perl ifdebug
    Benchmark: timing 100000 iterations of constant, sub...
       ifdebug:  0 wallclock secs ( 0.01 usr +  0.00 sys =  0.01 CPU) @ 10000000.00/s (n=100000)
                (warning: too few iterations for a reliable count)
         debug: 14 wallclock secs (13.18 usr +  0.04 sys = 13.22 CPU) @ 7564.30/s (n=100000)



In the one case the code, which does exactly the same thing as far as
outputting any debugging information is concerned, in other words nothing,
takes 14 seconds, and in the other case the code takes one hundredth of a
second.  Looks fairly definitive.  Use a $DEBUG variable BEFORE you call the
subroutine, rather than relying on the smart functionality inside it.





• perlperf Logging if DEBUG (constant):  












Up: perlperf LOGGING   [Contents]





51.8.1 Logging if DEBUG (constant)



It’s possible to take the previous idea a little further, by using a compile
time DEBUG constant.



# ifdebug-constant



    #!/usr/bin/perl

    use strict;
    use warnings;

    use Benchmark;
    use Data::Dumper;
    use constant
        DEBUG => 0
    ;

    sub debug {
        if ( DEBUG ) {
            my $msg = shift;
            print "DEBUG: $msg\n";
        }
    };

    timethese(100000, {
            'debug'       => sub {
                debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) )
            },
            'constant'  => sub {
                debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if DEBUG
            },
    });



Running this program produces the following output:



    $> perl ifdebug-constant
    Benchmark: timing 100000 iterations of constant, sub...
      constant:  0 wallclock secs (-0.00 usr +  0.00 sys = -0.00 CPU) @ -7205759403792793600000.00/s (n=100000)
                (warning: too few iterations for a reliable count)
           sub: 14 wallclock secs (13.09 usr +  0.00 sys = 13.09 CPU) @ 7639.42/s (n=100000)



The DEBUG constant wipes the floor with even the $debug variable,
clocking in at minus zero seconds, and generates a "warning: too few iterations
for a reliable count" message into the bargain.  To see what is really going
on, and why we had too few iterations when we thought we asked for 100000, we
can use the very useful B::Deparse to inspect the new code:



    $> perl -MO=Deparse ifdebug-constant

    use Benchmark;
    use Data::Dumper;
    use constant ('DEBUG', 0);
    sub debug {
        use warnings;
        use strict 'refs';
        0;
    }
    use warnings;
    use strict 'refs';
    timethese(100000, {'sub', sub {
        debug "A $0 logging message via process-id: $$" . Dumper(\%INC);
    }
    , 'constant', sub {
        0;
    }
    });
    ifdebug-constant syntax OK



The output shows the constant() subroutine we’re testing being replaced with
the value of the DEBUG constant: zero.  The line to be tested has been
completely optimized away, and you can’t get much more efficient than that.









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51.9 POSTSCRIPT



This document has provided several way to go about identifying hot-spots, and
checking whether any modifications have improved the runtime of the code.



As a final thought, remember that it’s not (at the time of writing) possible to
produce a useful program which will run in zero or negative time and this basic
principle can be written as: useful programs are slow by their very
definition.  It is of course possible to write a nearly instantaneous program,
but it’s not going to do very much, here’s a very efficient one:



    $> perl -e 0



Optimizing that any further is a job for p5p.









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51.10 SEE ALSO



Further reading can be found using the modules and links below.





• perlperf PERLDOCS:  



• perlperf MAN PAGES:  



• perlperf MODULES:  



• perlperf URLS:  












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51.10.1 PERLDOCS



For example: perldoc -f sort.



(perlfaq4).



perlfork NAME, (perlfunc), perlretut NAME, perlthrtut NAME.



(threads).









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51.10.2 MAN PAGES



time.









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51.10.3 MODULES



It’s not possible to individually showcase all the performance related code for
Perl here, naturally, but here’s a short list of modules from the CPAN which
deserve further attention.



    Apache::DProf
    Apache::SmallProf
    Benchmark
    DBIx::Profile
    Devel::AutoProfiler
    Devel::DProf
    Devel::DProfLB
    Devel::FastProf
    Devel::GraphVizProf
    Devel::NYTProf
    Devel::NYTProf::Apache
    Devel::Profiler
    Devel::Profile
    Devel::Profit
    Devel::SmallProf
    Devel::WxProf
    POE::Devel::Profiler
    Sort::Key
    Sort::Maker









Previous: perlperf MODULES, Up: perlperf SEE ALSO   [Contents]





51.10.4 URLS



Very useful online reference material:



    http://www.ccl4.org/~nick/P/Fast_Enough/

    http://www-128.ibm.com/developerworks/library/l-optperl.html

    http://perlbuzz.com/2007/11/bind-output-variables-in-dbi-for-speed-and-safety.html

    http://en.wikipedia.org/wiki/Performance_analysis

    http://apache.perl.org/docs/1.0/guide/performance.html

    http://perlgolf.sourceforge.net/

    http://www.sysarch.com/Perl/sort_paper.html









Previous: perlperf SEE ALSO, Up: perlperf   [Contents]





51.11 AUTHOR



Richard Foley <richard.foley@rfi.net> Copyright (c) 2008









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52 NAME POD plain old documentation





• NAME POD plain old documentation NAME:  



• NAME POD plain old documentation DESCRIPTION:  



• NAME POD plain old documentation SEE ALSO:  



• NAME POD plain old documentation AUTHOR:  












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52.1 NAME





perlpod - the Plain Old Documentation format









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52.2 DESCRIPTION



Pod is a simple-to-use markup language used for writing documentation
for Perl, Perl programs, and Perl modules.



Translators are available for converting Pod to various formats
like plain text, HTML, man pages, and more.



Pod markup consists of three basic kinds of paragraphs:
ordinary,
verbatim, and 
command.





• NAME POD plain old documentation Ordinary Paragraph:  



• NAME POD plain old documentation Verbatim Paragraph:  



• NAME POD plain old documentation Command Paragraph:  



• NAME POD plain old documentation Formatting Codes:  



• NAME POD plain old documentation The Intent:  



• NAME POD plain old documentation Embedding Pods in Perl Modules:  



• NAME POD plain old documentation Hints for Writing Pod:  












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52.2.1 Ordinary Paragraph




Most paragraphs in your documentation will be ordinary blocks
of text, like this one.  You can simply type in your text without
any markup whatsoever, and with just a blank line before and
after.  When it gets formatted, it will undergo minimal formatting, 
like being rewrapped, probably put into a proportionally spaced
font, and maybe even justified.



You can use formatting codes in ordinary paragraphs, for bold,
italic, code-style, (perlfaq)hyperlinks, and more.  Such
codes are explained in the "Formatting Codes"
section, below.









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52.2.2 Verbatim Paragraph





Verbatim paragraphs are usually used for presenting a codeblock or
other text which does not require any special parsing or formatting,
and which shouldn’t be wrapped.



A verbatim paragraph is distinguished by having its first character
be a space or a tab.  (And commonly, all its lines begin with spaces
and/or tabs.)  It should be reproduced exactly, with tabs assumed to
be on 8-column boundaries.  There are no special formatting codes,
so you can’t italicize or anything like that.  A \ means \, and
nothing else.









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52.2.3 Command Paragraph




A command paragraph is used for special treatment of whole chunks
of text, usually as headings or parts of lists.



All command paragraphs (which are typically only one line long) start
with "=", followed by an identifier, followed by arbitrary text that
the command can use however it pleases.  Currently recognized commands
are



    =pod
    =head1 Heading Text
    =head2 Heading Text
    =head3 Heading Text
    =head4 Heading Text
    =over indentlevel
    =item stuff
    =back
    =begin format
    =end format
    =for format text...
    =encoding type
    =cut



To explain them each in detail:





=head1 Heading Text














=head2 Heading Text






=head3 Heading Text






=head4 Heading Text




Head1 through head4 produce headings, head1 being the highest
level.  The text in the rest of this paragraph is the content of the
heading.  For example:



  =head2 Object Attributes



The text "Object Attributes" comprises the heading there.  (Note that
head3 and head4 are recent additions, not supported in older Pod
translators.)  The text in these heading commands can use
formatting codes, as seen here:



  =head2 Possible Values for C<$/>



Such commands are explained in the
"Formatting Codes" section, below.





=over indentlevel












=item stuff...






=back




Item, over, and back require a little more explanation:  "=over" starts
a region specifically for the generation of a list using "=item"
commands, or for indenting (groups of) normal paragraphs.  At the end
of your list, use "=back" to end it.  The indentlevel option to
"=over" indicates how far over to indent, generally in ems (where
one em is the width of an "M" in the document’s base font) or roughly
comparable units; if there is no indentlevel option, it defaults
to four.  (And some formatters may just ignore whatever indentlevel
you provide.)  In the stuff in =item stuff..., you may
use formatting codes, as seen here:



  =item Using C<$|> to Control Buffering



Such commands are explained in the
"Formatting Codes" section, below.



Note also that there are some basic rules to using "=over" ...
"=back" regions:







=cut






To end a Pod block, use a blank line,
then a line beginning with "=cut", and a blank
line after it.  This lets Perl (and the Pod formatter) know that
this is where Perl code is resuming.  (The blank line before the "=cut"
is not technically necessary, but many older Pod processors require it.)





=pod






The "=pod" command by itself doesn’t do much of anything, but it
signals to Perl (and Pod formatters) that a Pod block starts here.  A
Pod block starts with any command paragraph, so a "=pod" command is
usually used just when you want to start a Pod block with an ordinary
paragraph or a verbatim paragraph.  For example:



  =item stuff()

  This function does stuff.

  =cut

  sub stuff {
    ...
  }

  =pod

  Remember to check its return value, as in:

    stuff() || die "Couldn't do stuff!";

  =cut





=begin formatname












=end formatname






=for formatname text...




For, begin, and end will let you have regions of text/code/data that
are not generally interpreted as normal Pod text, but are passed
directly to particular formatters, or are otherwise special.  A
formatter that can use that format will use the region, otherwise it
will be completely ignored.



A command "=begin formatname", some paragraphs, and a
command "=end formatname", mean that the text/data in between
is meant for formatters that understand the special format
called formatname.  For example,



  =begin html

  <hr> <img src="thang.png">
  <p> This is a raw HTML paragraph </p>

  =end html



The command "=for formatname text..."
specifies that the remainder of just this paragraph (starting
right after formatname) is in that special format.  



  =for html <hr> <img src="thang.png">
  <p> This is a raw HTML paragraph </p>



This means the same thing as the above "=begin html" ... "=end html"
region.



That is, with "=for", you can have only one paragraph’s worth
of text (i.e., the text in "=foo targetname text..."), but with
"=begin targetname" ... "=end targetname", you can have any amount
of stuff in between.  (Note that there still must be a blank line
after the "=begin" command and a blank line before the "=end"
command.



Here are some examples of how to use these:



  =begin html

  <br>Figure 1.<br><IMG SRC="figure1.png"><br>

  =end html

  =begin text

    ---------------
    |  foo        |
    |        bar  |
    ---------------

  ^^^^ Figure 1. ^^^^

  =end text



Some format names that formatters currently are known to accept
include "roff", "man", "latex", "tex", "text", and "html".  (Some
formatters will treat some of these as synonyms.)



A format name of "comment" is common for just making notes (presumably
to yourself) that won’t appear in any formatted version of the Pod
document:



  =for comment
  Make sure that all the available options are documented!



Some formatnames will require a leading colon (as in
"=for :formatname", or
"=begin :formatname" ... "=end :formatname"),
to signal that the text is not raw data, but instead is Pod text
(i.e., possibly containing formatting codes) that’s just not for
normal formatting (e.g., may not be a normal-use paragraph, but might
be for formatting as a footnote).





=encoding encodingname






This command is used for declaring the encoding of a document.  Most
users won’t need this; but if your encoding isn’t US-ASCII or Latin-1,
then put a =encoding encodingname command early in the document so
that pod formatters will know how to decode the document.  For
encodingname, use a name recognized by the (Encode-Supported)
module.  Examples:



  =encoding utf8

  =encoding koi8-r
  
  =encoding ShiftJIS
  
  =encoding big5








=encoding affects the whole document, and must occur only once.



And don’t forget, when using any other command, that the command lasts up
until the end of its paragraph, not its line.  So in the
examples below, you can see that every command needs the blank
line after it, to end its paragraph.



Some examples of lists include:



  =over

  =item *

  First item

  =item *

  Second item

  =back

  =over

  =item Foo()

  Description of Foo function

  =item Bar()

  Description of Bar function

  =back









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52.2.4 Formatting Codes







In ordinary paragraphs and in some command paragraphs, various
formatting codes (a.k.a. "interior sequences") can be used:





I<text> – italic text








Used for emphasis ("be I<careful!>") and parameters
("redo I<LABEL>")





B<text> – bold text








Used for switches ("perl's B<-n> switch"), programs
("some systems provide a B<chfn> for that"),
emphasis ("be B<careful!>"), and so on
("and that feature is known as B<autovivification>").





C<code> – code text








Renders code in a typewriter font, or gives some other indication that
this represents program text ("C<gmtime($^T)>") or some other
form of computerese ("C<drwxr-xr-x>").





L<name> – a hyperlink








There are various syntaxes, listed below.  In the syntaxes given,
text, name, and section cannot contain the characters
’/’ and ’|’; and any ’<’ or ’>’ should be matched.





A section is started by the named heading or item.  For
example, L<perlvar/$.> or L<perlvar/"$."> both
link to the section started by "=item $." in perlvar.  And
L<perlsyn/For Loops> or L<perlsyn/"For Loops">
both link to the section started by "=head2 For Loops"
in perlsyn.



To control what text is used for display, you
use "L<text|...>", as in:





Or you can link to a web page:







E<escape> – a character escape








Very similar to HTML/XML &foo; "entity references":







F<filename> – used for filenames








Typically displayed in italics.  Example: "F<.cshrc>"





S<text> – text contains non-breaking spaces








This means that the words in text should not be broken
across lines.  Example: S<$x ? $y : $z>.





X<topic name> – an index entry








This is ignored by most formatters, but some may use it for building
indexes.  It always renders as empty-string.
Example: X<absolutizing relative URLs>





Z<> – a null (zero-effect) formatting code








This is rarely used.  It’s one way to get around using an
E<...> code sometimes.  For example, instead of
"NE<lt>3" (for "N<3") you could write
"NZ<><3" (the "Z<>" breaks up the "N" and
the "<" so they can’t be considered
the part of a (fictitious) "N<...>" code.








Most of the time, you will need only a single set of angle brackets to
delimit the beginning and end of formatting codes.  However,
sometimes you will want to put a real right angle bracket (a
greater-than sign, ’>’) inside of a formatting code.  This is particularly
common when using a formatting code to provide a different font-type for a
snippet of code.  As with all things in Perl, there is more than
one way to do it.  One way is to simply escape the closing bracket
using an E code:



    C<$a E<lt>=E<gt> $b>



This will produce: "$a <=> $b"




A more readable, and perhaps more "plain" way is to use an alternate
set of delimiters that doesn’t require a single ">" to be escaped.
Doubled angle brackets ("<<" and ">>") may be used if and only if there is
whitespace right after the opening delimiter and whitespace right
before the closing delimiter!  For example, the following will
do the trick:




    C<< $a <=> $b >>




In fact, you can use as many repeated angle-brackets as you like so
long as you have the same number of them in the opening and closing
delimiters, and make sure that whitespace immediately follows the last
’<’ of the opening delimiter, and immediately precedes the first ’>’
of the closing delimiter.  (The whitespace is ignored.)  So the
following will also work:




    C<<< $a <=> $b >>>
    C<<<<  $a <=> $b     >>>>



And they all mean exactly the same as this:



    C<$a E<lt>=E<gt> $b>



The multiple-bracket form does not affect the interpretation of the contents of
the formatting code, only how it must end.  That means that the examples above
are also exactly the same as this:



    C<< $a E<lt>=E<gt> $b >>



As a further example, this means that if you wanted to put these bits of
code in C (code) style:



    open(X, ">>thing.dat") || die $!
    $foo->bar();



you could do it like so:



    C<<< open(X, ">>thing.dat") || die $! >>>
    C<< $foo->bar(); >>



which is presumably easier to read than the old way:



    C<open(X, "E<gt>E<gt>thing.dat") || die $!>
    C<$foo-E<gt>bar();>



This is currently supported by pod2text (Pod::Text), pod2man (Pod::Man),
and any other pod2xxx or Pod::Xxxx translators that use
Pod::Parser 1.093 or later, or Pod::Tree 1.02 or later.









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52.2.5 The Intent




The intent is simplicity of use, not power of expression.  Paragraphs
look like paragraphs (block format), so that they stand out
visually, and so that I could run them through fmt easily to reformat
them (that’s F7 in my version of vi, or Esc Q in my version of
emacs).  I wanted the translator to always leave the ' and ` and
" quotes alone, in verbatim mode, so I could slurp in a
working program, shift it over four spaces, and have it print out, er,
verbatim.  And presumably in a monospace font.



The Pod format is not necessarily sufficient for writing a book.  Pod
is just meant to be an idiot-proof common source for nroff, HTML,
TeX, and other markup languages, as used for online
documentation.  Translators exist for pod2text, pod2html,
pod2man (that’s for nroff(1) and troff(1)), pod2latex, and
pod2fm.  Various others are available in CPAN.









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52.2.6 Embedding Pods in Perl Modules




You can embed Pod documentation in your Perl modules and scripts.
Start your documentation with an empty line, a "=head1" command at the
beginning, and end it with a "=cut" command and an empty line.  Perl
will ignore the Pod text.  See any of the supplied library modules for
examples.  If you’re going to put your Pod at the end of the file, and
you’re using an __END__ or __DATA__ cut mark, make sure to put an
empty line there before the first Pod command.



  __END__

  =head1 NAME

  Time::Local - efficiently compute time from local and GMT time



Without that empty line before the "=head1", many translators wouldn’t
have recognized the "=head1" as starting a Pod block.









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52.2.7 Hints for Writing Pod











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52.3 SEE ALSO



perlpodspec NAME, (perlsyn)PODs: Embedded Documentation,
perlnewmod NAME, (perldoc), (pod2html), (pod2man), (podchecker).









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52.4 AUTHOR



Larry Wall, Sean M. Burke









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53 perlpodspec





• perlpodspec NAME:  



• perlpodspec DESCRIPTION:  



• perlpodspec Pod Definitions:  



• perlpodspec Pod Commands:  



• perlpodspec Pod Formatting Codes:  



• perlpodspec Notes on Implementing Pod Processors:  



• perlpodspec About L<...> Codes:  



• perlpodspec About =over...=back Regions:  



• perlpodspec About Data Paragraphs and "=begin/=end" Regions:  



• perlpodspec SEE ALSO:  



• perlpodspec AUTHOR:  












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53.1 NAME



perlpodspec - Plain Old Documentation: format specification and notes









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53.2 DESCRIPTION



This document is detailed notes on the Pod markup language.  Most
people will only have to read (perlpod)perlpod to know how to write
in Pod, but this document may answer some incidental questions to do
with parsing and rendering Pod.



In this document, "must" / "must not", "should" /
"should not", and "may" have their conventional (cf. RFC 2119)
meanings: "X must do Y" means that if X doesn’t do Y, it’s against
this specification, and should really be fixed.  "X should do Y"
means that it’s recommended, but X may fail to do Y, if there’s a
good reason.  "X may do Y" is merely a note that X can do Y at
will (although it is up to the reader to detect any connotation of
"and I think it would be nice if X did Y" versus "it wouldn’t
really bother me if X did Y").



Notably, when I say "the parser should do Y", the
parser may fail to do Y, if the calling application explicitly
requests that the parser not do Y.  I often phrase this as
"the parser should, by default, do Y."  This doesn’t require
the parser to provide an option for turning off whatever
feature Y is (like expanding tabs in verbatim paragraphs), although
it implicates that such an option may be provided.









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53.3 Pod Definitions



Pod is embedded in files, typically Perl source files, although you
can write a file that’s nothing but Pod.



A line in a file consists of zero or more non-newline characters,
terminated by either a newline or the end of the file.



A newline sequence is usually a platform-dependent concept, but
Pod parsers should understand it to mean any of CR (ASCII 13), LF
(ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in
addition to any other system-specific meaning.  The first CR/CRLF/LF
sequence in the file may be used as the basis for identifying the
newline sequence for parsing the rest of the file.



A blank line is a line consisting entirely of zero or more spaces
(ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file.
A non-blank line is a line containing one or more characters other
than space or tab (and terminated by a newline or end-of-file).



(Note: Many older Pod parsers did not accept a line consisting of
spaces/tabs and then a newline as a blank line. The only lines they
considered blank were lines consisting of no characters at all,
terminated by a newline.)



Whitespace is used in this document as a blanket term for spaces,
tabs, and newline sequences.  (By itself, this term usually refers
to literal whitespace.  That is, sequences of whitespace characters
in Pod source, as opposed to "E<32>", which is a formatting
code that denotes a whitespace character.)



A Pod parser is a module meant for parsing Pod (regardless of
whether this involves calling callbacks or building a parse tree or
directly formatting it).  A Pod formatter (or Pod translator)
is a module or program that converts Pod to some other format (HTML,
plaintext, TeX, PostScript, RTF).  A Pod processor might be a
formatter or translator, or might be a program that does something
else with the Pod (like counting words, scanning for index points,
etc.).



Pod content is contained in Pod blocks.  A Pod block starts with a
line that matches <m/\A=[a-zA-Z]/>, and continues up to the next line
that matches m/\A=cut/ or up to the end of the file if there is
no m/\A=cut/ line.



Within a Pod block, there are Pod paragraphs.  A Pod paragraph
consists of non-blank lines of text, separated by one or more blank
lines.



For purposes of Pod processing, there are four types of paragraphs in
a Pod block:





For example: consider the following paragraphs:



  # <- that's the 0th column

  =head1 Foo

  Stuff

    $foo->bar

  =cut



Here, "=head1 Foo" and "=cut" are command paragraphs because the first
line of each matches m/\A=[a-zA-Z]/.  "[space][space]$foo->bar"
is a verbatim paragraph, because its first line starts with a literal
whitespace character (and there’s no "=begin"..."=end" region around).



The "=begin identifier" ... "=end identifier" commands stop
paragraphs that they surround from being parsed as ordinary or verbatim
paragraphs, if identifier doesn’t begin with a colon.  This
is discussed in detail in the section
About Data Paragraphs and "=begin/=end" Regions.









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53.4 Pod Commands



This section is intended to supplement and clarify the discussion in
(perlpod)Command Paragraph.  These are the currently recognized
Pod commands:





"=head1", "=head2", "=head3", "=head4"




This command indicates that the text in the remainder of the paragraph
is a heading.  That text may contain formatting codes.  Examples:



  =head1 Object Attributes

  =head3 What B<Not> to Do!





"=pod"




This command indicates that this paragraph begins a Pod block.  (If we
are already in the middle of a Pod block, this command has no effect at
all.)  If there is any text in this command paragraph after "=pod",
it must be ignored.  Examples:



  =pod

  This is a plain Pod paragraph.

  =pod This text is ignored.





"=cut"




This command indicates that this line is the end of this previously
started Pod block.  If there is any text after "=cut" on the line, it must be
ignored.  Examples:



  =cut

  =cut The documentation ends here.

  =cut
  # This is the first line of program text.
  sub foo { # This is the second.



It is an error to try to start a Pod block with a "=cut" command.  In
that case, the Pod processor must halt parsing of the input file, and
must by default emit a warning.





"=over"




This command indicates that this is the start of a list/indent
region.  If there is any text following the "=over", it must consist
of only a nonzero positive numeral.  The semantics of this numeral is
explained in the About =over...=back Regions section, further
below.  Formatting codes are not expanded.  Examples:



  =over 3

  =over 3.5

  =over





"=item"




This command indicates that an item in a list begins here.  Formatting
codes are processed.  The semantics of the (optional) text in the
remainder of this paragraph are
explained in the About =over...=back Regions section, further
below.  Examples:



  =item

  =item *

  =item      *    

  =item 14

  =item   3.

  =item C<< $thing->stuff(I<dodad>) >>

  =item For transporting us beyond seas to be tried for pretended
  offenses

  =item He is at this time transporting large armies of foreign
  mercenaries to complete the works of death, desolation and
  tyranny, already begun with circumstances of cruelty and perfidy
  scarcely paralleled in the most barbarous ages, and totally
  unworthy the head of a civilized nation.





"=back"




This command indicates that this is the end of the region begun
by the most recent "=over" command.  It permits no text after the
"=back" command.





"=begin formatname"






"=begin formatname parameter"




This marks the following paragraphs (until the matching "=end
formatname") as being for some special kind of processing.  Unless
"formatname" begins with a colon, the contained non-command
paragraphs are data paragraphs.  But if "formatname" does begin
with a colon, then non-command paragraphs are ordinary paragraphs
or data paragraphs.  This is discussed in detail in the section
About Data Paragraphs and "=begin/=end" Regions.



It is advised that formatnames match the regexp
m/\A:?[-a-zA-Z0-9_]+\z/.  Everything following whitespace after the
formatname is a parameter that may be used by the formatter when dealing
with this region.  This parameter must not be repeated in the "=end"
paragraph.  Implementors should anticipate future expansion in the
semantics and syntax of the first parameter to "=begin"/"=end"/"=for".





"=end formatname"




This marks the end of the region opened by the matching
"=begin formatname" region.  If "formatname" is not the formatname
of the most recent open "=begin formatname" region, then this
is an error, and must generate an error message.  This
is discussed in detail in the section
About Data Paragraphs and "=begin/=end" Regions.





"=for formatname text..."




This is synonymous with:



     =begin formatname

     text...

     =end formatname



That is, it creates a region consisting of a single paragraph; that
paragraph is to be treated as a normal paragraph if "formatname"
begins with a ":"; if "formatname" doesn’t begin with a colon,
then "text..." will constitute a data paragraph.  There is no way
to use "=for formatname text..." to express "text..." as a verbatim
paragraph.





"=encoding encodingname"




This command, which should occur early in the document (at least
before any non-US-ASCII data!), declares that this document is
encoded in the encoding encodingname, which must be
an encoding name that (Encode) recognizes.  (Encode’s list
of supported encodings, in (Encode-Supported), is useful here.)
If the Pod parser cannot decode the declared encoding, it 
should emit a warning and may abort parsing the document
altogether.



A document having more than one "=encoding" line should be
considered an error.  Pod processors may silently tolerate this if
the not-first "=encoding" lines are just duplicates of the
first one (e.g., if there’s a "=encoding utf8" line, and later on
another "=encoding utf8" line).  But Pod processors should complain if
there are contradictory "=encoding" lines in the same document
(e.g., if there is a "=encoding utf8" early in the document and
"=encoding big5" later).  Pod processors that recognize BOMs
may also complain if they see an "=encoding" line
that contradicts the BOM (e.g., if a document with a UTF-16LE
BOM has an "=encoding shiftjis" line).








If a Pod processor sees any command other than the ones listed
above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish",
or "=w123"), that processor must by default treat this as an
error.  It must not process the paragraph beginning with that
command, must by default warn of this as an error, and may
abort the parse.  A Pod parser may allow a way for particular
applications to add to the above list of known commands, and to
stipulate, for each additional command, whether formatting
codes should be processed.



Future versions of this specification may add additional
commands.









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53.5 Pod Formatting Codes



(Note that in previous drafts of this document and of perlpod,
formatting codes were referred to as "interior sequences", and
this term may still be found in the documentation for Pod parsers,
and in error messages from Pod processors.)



There are two syntaxes for formatting codes:





In parsing Pod, a notably tricky part is the correct parsing of
(potentially nested!) formatting codes.  Implementors should
consult the code in the parse_text routine in Pod::Parser as an
example of a correct implementation.





I<text> – italic text




See the brief discussion in (perlpod)Formatting Codes.





B<text> – bold text




See the brief discussion in (perlpod)Formatting Codes.





C<code> – code text




See the brief discussion in (perlpod)Formatting Codes.





F<filename> – style for filenames




See the brief discussion in (perlpod)Formatting Codes.





X<topic name> – an index entry




See the brief discussion in (perlpod)Formatting Codes.



This code is unusual in that most formatters completely discard
this code and its content.  Other formatters will render it with
invisible codes that can be used in building an index of
the current document.





Z<> – a null (zero-effect) formatting code




Discussed briefly in (perlpod)Formatting Codes.



This code is unusual is that it should have no content.  That is,
a processor may complain if it sees Z<potatoes>.  Whether
or not it complains, the potatoes text should ignored.





L<name> – a hyperlink




The complicated syntaxes of this code are discussed at length in
(perlpod)Formatting Codes, and implementation details are
discussed below, in About L<...> Codes.  Parsing the
contents of L<content> is tricky.  Notably, the content has to be
checked for whether it looks like a URL, or whether it has to be split
on literal "|" and/or "/" (in the right order!), and so on,
before E<...> codes are resolved.





E<escape> – a character escape




See (perlpod)Formatting Codes, and several points in
Notes on Implementing Pod Processors.





S<text> – text contains non-breaking spaces




This formatting code is syntactically simple, but semantically
complex.  What it means is that each space in the printable
content of this code signifies a non-breaking space.



Consider:



    C<$x ? $y    :  $z>

    S<C<$x ? $y     :  $z>>



Both signify the monospace (c[ode] style) text consisting of
"$x", one space, "?", one space, ":", one space, "$z".  The
difference is that in the latter, with the S code, those spaces
are not "normal" spaces, but instead are non-breaking spaces.








If a Pod processor sees any formatting code other than the ones
listed above (as in "N<...>", or "Q<...>", etc.), that
processor must by default treat this as an error.
A Pod parser may allow a way for particular
applications to add to the above list of known formatting codes;
a Pod parser might even allow a way to stipulate, for each additional
command, whether it requires some form of special processing, as
L<...> does.



Future versions of this specification may add additional
formatting codes.



Historical note:  A few older Pod processors would not see a ">" as
closing a "C<" code, if the ">" was immediately preceded by
a "-".  This was so that this:



    C<$foo->bar>



would parse as equivalent to this:



    C<$foo-E<gt>bar>



instead of as equivalent to a "C" formatting code containing 
only "$foo-", and then a "bar>" outside the "C" formatting code.  This
problem has since been solved by the addition of syntaxes like this:



    C<< $foo->bar >>



Compliant parsers must not treat "->" as special.



Formatting codes absolutely cannot span paragraphs.  If a code is
opened in one paragraph, and no closing code is found by the end of
that paragraph, the Pod parser must close that formatting code,
and should complain (as in "Unterminated I code in the paragraph
starting at line 123: ’Time objects are not...’").  So these
two paragraphs:



  I<I told you not to do this!

  Don't make me say it again!>



...must not be parsed as two paragraphs in italics (with the I
code starting in one paragraph and starting in another.)  Instead,
the first paragraph should generate a warning, but that aside, the
above code must parse as if it were:



  I<I told you not to do this!>

  Don't make me say it again!E<gt>



(In SGMLish jargon, all Pod commands are like block-level
elements, whereas all Pod formatting codes are like inline-level
elements.)









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53.6 Notes on Implementing Pod Processors



The following is a long section of miscellaneous requirements
and suggestions to do with Pod processing.











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53.7 About L<...> Codes



As you can tell from a glance at (perlpod)perlpod, the L<...>
code is the most complex of the Pod formatting codes.  The points below
will hopefully clarify what it means and how processors should deal
with it.











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53.8 About =over...=back Regions



"=over"..."=back" regions are used for various kinds of list-like
structures.  (I use the term "region" here simply as a collective
term for everything from the "=over" to the matching "=back".)











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53.9 About Data Paragraphs and "=begin/=end" Regions



Data paragraphs are typically used for inlining non-Pod data that is
to be used (typically passed through) when rendering the document to
a specific format:



  =begin rtf

  \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}

  =end rtf



The exact same effect could, incidentally, be achieved with a single
"=for" paragraph:



  =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}



(Although that is not formally a data paragraph, it has the same
meaning as one, and Pod parsers may parse it as one.)



Another example of a data paragraph:



  =begin html

  I like <em>PIE</em>!

  <hr>Especially pecan pie!

  =end html



If these were ordinary paragraphs, the Pod parser would try to
expand the "E</em>" (in the first paragraph) as a formatting
code, just like "E<lt>" or "E<eacute>".  But since this
is in a "=begin identifier"..."=end identifier" region and
the identifier "html" doesn’t begin have a ":" prefix, the contents
of this region are stored as data paragraphs, instead of being
processed as ordinary paragraphs (or if they began with a spaces
and/or tabs, as verbatim paragraphs).



As a further example: At time of writing, no "biblio" identifier is
supported, but suppose some processor were written to recognize it as
a way of (say) denoting a bibliographic reference (necessarily
containing formatting codes in ordinary paragraphs).  The fact that
"biblio" paragraphs were meant for ordinary processing would be
indicated by prefacing each "biblio" identifier with a colon:



  =begin :biblio

  Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
  Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

  =end :biblio



This would signal to the parser that paragraphs in this begin...end
region are subject to normal handling as ordinary/verbatim paragraphs
(while still tagged as meant only for processors that understand the
"biblio" identifier).  The same effect could be had with:



  =for :biblio
  Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
  Programs.>  Prentice-Hall, Englewood Cliffs, NJ.



The ":" on these identifiers means simply "process this stuff
normally, even though the result will be for some special target".
I suggest that parser APIs report "biblio" as the target identifier,
but also report that it had a ":" prefix.  (And similarly, with the
above "html", report "html" as the target identifier, and note the
lack of a ":" prefix.)



Note that a "=begin identifier"..."=end identifier" region where
identifier begins with a colon, can contain commands.  For example:



  =begin :biblio

  Wirth's classic is available in several editions, including:

  =for comment
   hm, check abebooks.com for how much used copies cost.

  =over

  =item

  Wirth, Niklaus.  1975.  I<Algorithmen und Datenstrukturen.>
  Teubner, Stuttgart.  [Yes, it's in German.]

  =item

  Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
  Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

  =back

  =end :biblio



Note, however, a "=begin identifier"..."=end identifier"
region where identifier does not begin with a colon, should not
directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back",
nor "=item".  For example, this may be considered invalid:



  =begin somedata

  This is a data paragraph.

  =head1 Don't do this!

  This is a data paragraph too.

  =end somedata



A Pod processor may signal that the above (specifically the "=head1"
paragraph) is an error.  Note, however, that the following should
not be treated as an error:



  =begin somedata

  This is a data paragraph.

  =cut

  # Yup, this isn't Pod anymore.
  sub excl { (rand() > .5) ? "hoo!" : "hah!" }

  =pod

  This is a data paragraph too.

  =end somedata



And this too is valid:



  =begin someformat

  This is a data paragraph.

    And this is a data paragraph.

  =begin someotherformat

  This is a data paragraph too.

    And this is a data paragraph too.

  =begin :yetanotherformat

  =head2 This is a command paragraph!

  This is an ordinary paragraph!

    And this is a verbatim paragraph!

  =end :yetanotherformat

  =end someotherformat

  Another data paragraph!

  =end someformat



The contents of the above "=begin :yetanotherformat" ...
"=end :yetanotherformat" region aren’t data paragraphs, because
the immediately containing region’s identifier (":yetanotherformat")
begins with a colon.  In practice, most regions that contain
data paragraphs will contain only data paragraphs; however, 
the above nesting is syntactically valid as Pod, even if it is
rare.  However, the handlers for some formats, like "html",
will accept only data paragraphs, not nested regions; and they may
complain if they see (targeted for them) nested regions, or commands,
other than "=end", "=pod", and "=cut".



Also consider this valid structure:



  =begin :biblio

  Wirth's classic is available in several editions, including:

  =over

  =item

  Wirth, Niklaus.  1975.  I<Algorithmen und Datenstrukturen.>
  Teubner, Stuttgart.  [Yes, it's in German.]

  =item

  Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
  Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

  =back

  Buy buy buy!

  =begin html

  <img src='wirth_spokesmodeling_book.png'>

  <hr>

  =end html

  Now now now!

  =end :biblio



There, the "=begin html"..."=end html" region is nested inside
the larger "=begin :biblio"..."=end :biblio" region.  Note that the
content of the "=begin html"..."=end html" region is data
paragraph(s), because the immediately containing region’s identifier
("html") doesn’t begin with a colon.



Pod parsers, when processing a series of data paragraphs one
after another (within a single region), should consider them to
be one large data paragraph that happens to contain blank lines.  So
the content of the above "=begin html"..."=end html" may be stored
as two data paragraphs (one consisting of
"<img src=’wirth_spokesmodeling_book.png’>\n"
and another consisting of "<hr>\n"), but should be stored as
a single data paragraph (consisting of 
"<img src=’wirth_spokesmodeling_book.png’>\n\n<hr>\n").



Pod processors should tolerate empty
"=begin something"..."=end something" regions,
empty "=begin :something"..."=end :something" regions, and
contentless "=for something" and "=for :something"
paragraphs.  I.e., these should be tolerated:



  =for html

  =begin html

  =end html

  =begin :biblio

  =end :biblio



Incidentally, note that there’s no easy way to express a data
paragraph starting with something that looks like a command.  Consider:



  =begin stuff

  =shazbot

  =end stuff



There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data
paragraph "=shazbot\n".  However, you can express a data paragraph consisting
of "=shazbot\n" using this code:



  =for stuff =shazbot



The situation where this is necessary, is presumably quite rare.



Note that =end commands must match the currently open =begin command.  That
is, they must properly nest.  For example, this is valid:



  =begin outer

  X

  =begin inner

  Y

  =end inner

  Z

  =end outer



while this is invalid:



  =begin outer

  X

  =begin inner

  Y

  =end outer

  Z

  =end inner



This latter is improper because when the "=end outer" command is seen, the
currently open region has the formatname "inner", not "outer".  (It just
happens that "outer" is the format name of a higher-up region.)  This is
an error.  Processors must by default report this as an error, and may halt
processing the document containing that error.  A corollary of this is that
regions cannot "overlap". That is, the latter block above does not represent
a region called "outer" which contains X and Y, overlapping a region called
"inner" which contains Y and Z.  But because it is invalid (as all
apparently overlapping regions would be), it doesn’t represent that, or
anything at all.



Similarly, this is invalid:



  =begin thing

  =end hting



This is an error because the region is opened by "thing", and the "=end"
tries to close "hting" [sic].



This is also invalid:



  =begin thing

  =end



This is invalid because every "=end" command must have a formatname
parameter.









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53.10 SEE ALSO



(perlpod), (perlsyn)PODs: Embedded Documentation,
(podchecker)









Previous: perlpodspec SEE ALSO, Up: perlpodspec   [Contents]





53.11 AUTHOR



Sean M. Burke









Next: , Previous: perlpodspec, Up: Top   [Contents]





54 perlpodstyle





• perlpodstyle NAME:  



• perlpodstyle DESCRIPTION:  



• perlpodstyle SEE ALSO 1:  



• perlpodstyle AUTHOR 1:  



• perlpodstyle COPYRIGHT AND LICENSE 1:  












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54.1 NAME



perlpodstyle - Perl POD style guide









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54.2 DESCRIPTION



These are general guidelines for how to write POD documentation for Perl
scripts and modules, based on general guidelines for writing good Unix man
pages.  All of these guidelines are, of course, optional, but following
them will make your documentation more consistent with other documentation
on the system.



Here are some simple guidelines for markup; see (perlpod) for details.





bold (B<>)




NOTE: Use extremely rarely.  Do not use bold for emphasis; that’s
what italics are for.  Restrict bold for notices like NOTE: and
WARNING:.  However, program arguments and options–but not their
names!–are written in bold (using B<>) to distinguish the -f
command-line option from the -f filetest operator.





italic (I<>)




Use italic to emphasize text, like this.  Function names are
traditionally written in italics; if you write a function as function(),
Pod::Man will take care of this for you.  Names of programs, including the
name of the program being documented, are conventionally written in italics
(using I<>) wherever they occur in normal roman text.





code (C<>)




Literal code should be in C<>.  However metasyntactic placeholders
should furthermore be nested in "italics" (actually, oblique) like
C<I<>>.  That way
C<accept(I<NEWSOCKET>, <GENERICSOCKET>)>
renders as accept(NEWSOCKET, GENERICSOCKET).





files (F<>)




Filenames, whether absolute or relative, are specified with the F<>
markup. This will render as italics, but has other semantic connotations.








References to other man pages should be in the form "manpage(section)" or
"L<manpage(section)>", and Pod::Man will automatically format
those appropriately.  Both will render as manpage(section).  The second
form, with L<>, is used to request that a POD formatter make a link
to the man page if possible.  As an exception, one normally omits the
section when referring to module documentation because not all systems
place it in section 3, although that is the default.  You may use
L<Module::Name> for module references instead, but this is
optional because the translators are supposed to recognize module
references in pod, just as they do variable references like $foo and such.



References to other programs or functions are normally in the form of man
page references so that cross-referencing tools can provide the user with
links and the like.  It’s possible to overdo this, though, so be careful not
to clutter your documentation with too much markup.  References to other
programs that are not given as man page references should be enclosed in
italics via I<>.



Major headers should be set out using a =head1 directive, and are
historically written in the rather startling ALL UPPER CASE format; this is
not mandatory, but it’s strongly recommended so that sections have
consistent naming across different software packages.  The translators are
supposed to translate all caps into small caps.  Minor headers may be
included using =head2, and are typically in mixed case.



The standard sections of a manual page are:





NAME




Mandatory section; should be a comma-separated list of programs or
functions documented by this POD page, such as:



    foo, bar - programs to do something



Manual page indexers are often extremely picky about the format of this
section, so don’t put anything in it except this line.  Every program or
function documented by this POD page should be listed, separated by a
comma and a space.  For a Perl module, just give the module name.  A
single dash, and only a single dash, should separate the list of programs
or functions from the description.  Do not use any markup such as
C<> or I<> anywhere in this line.  Functions should not be
qualified with () or the like.  The description should ideally fit on a
single line, even if a man program replaces the dash with a few tabs.





SYNOPSIS




A short usage summary for programs and functions.  This section is
mandatory for section 3 pages.  For Perl module documentation, it’s
usually convenient to have the contents of this section be a verbatim
block showing some (brief) examples of typical ways the module is used.





DESCRIPTION




Extended description and discussion of the program or functions, or the
body of the documentation for man pages that document something else.  If
particularly long, it’s a good idea to break this up into subsections
=head2 directives like:



    =head2 Normal Usage

    =head2 Advanced Features

    =head2 Writing Configuration Files



or whatever is appropriate for your documentation.



For a module, this is generally where the documentation of the interfaces
provided by the module goes, usually in the form of a list with an
=item for each interface.  Depending on how many interfaces there are,
you may want to put that documentation in separate METHODS, FUNCTIONS,
CLASS METHODS, or INSTANCE METHODS sections instead and save the
DESCRIPTION section for an overview.





OPTIONS




Detailed description of each of the command-line options taken by the
program.  This should be separate from the description for the use of
parsers like (Pod-Usage).  This is normally presented as a list, with
each option as a separate =item.  The specific option string should be
enclosed in B<>.  Any values that the option takes should be
enclosed in I<>.  For example, the section for the option
–section=manext would be introduced with:



    =item B<--section>=I<manext>



Synonymous options (like both the short and long forms) are separated by a
comma and a space on the same =item line, or optionally listed as their
own item with a reference to the canonical name.  For example, since
–section can also be written as -s, the above would be:



    =item B<-s> I<manext>, B<--section>=I<manext>



Writing the short option first is recommended because it’s easier to read.
The long option is long enough to draw the eye to it anyway and the short
option can otherwise get lost in visual noise.





RETURN VALUE




What the program or function returns, if successful.  This section can be
omitted for programs whose precise exit codes aren’t important, provided
they return 0 on success and non-zero on failure as is standard.  It
should always be present for functions.  For modules, it may be useful to
summarize return values from the module interface here, or it may be more
useful to discuss return values separately in the documentation of each
function or method the module provides.





ERRORS




Exceptions, error return codes, exit statuses, and errno settings.
Typically used for function or module documentation; program documentation
uses DIAGNOSTICS instead.  The general rule of thumb is that errors
printed to STDOUT or STDERR and intended for the end user are
documented in DIAGNOSTICS while errors passed internal to the calling
program and intended for other programmers are documented in ERRORS.  When
documenting a function that sets errno, a full list of the possible errno
values should be given here.





DIAGNOSTICS




All possible messages the program can print out and what they mean.  You
may wish to follow the same documentation style as the Perl documentation;
see perldiag(1) for more details (and look at the POD source as well).



If applicable, please include details on what the user should do to
correct the error; documenting an error as indicating "the input buffer is
too small" without telling the user how to increase the size of the input
buffer (or at least telling them that it isn’t possible) aren’t very
useful.





EXAMPLES




Give some example uses of the program or function.  Don’t skimp; users
often find this the most useful part of the documentation.  The examples
are generally given as verbatim paragraphs.



Don’t just present an example without explaining what it does.  Adding a
short paragraph saying what the example will do can increase the value of
the example immensely.





ENVIRONMENT




Environment variables that the program cares about, normally presented as
a list using =over, =item, and =back.  For example:



    =over 6

    =item HOME

    Used to determine the user's home directory.  F<.foorc> in this
    directory is read for configuration details, if it exists.

    =back



Since environment variables are normally in all uppercase, no additional
special formatting is generally needed; they’re glaring enough as it is.





FILES




All files used by the program or function, normally presented as a list,
and what it uses them for.  File names should be enclosed in F<>.
It’s particularly important to document files that will be potentially
modified.





CAVEATS




Things to take special care with, sometimes called WARNINGS.





BUGS




Things that are broken or just don’t work quite right.





RESTRICTIONS




Bugs you don’t plan to fix.  :-)





NOTES




Miscellaneous commentary.





AUTHOR




Who wrote it (use AUTHORS for multiple people).  It’s a good idea to
include your current email address (or some email address to which bug
reports should be sent) or some other contact information so that users
have a way of contacting you.  Remember that program documentation tends
to roam the wild for far longer than you expect and pick a contact method
that’s likely to last.





HISTORY




Programs derived from other sources sometimes have this.  Some people keep
a modification log here, but that usually gets long and is normally better
maintained in a separate file.





COPYRIGHT AND LICENSE




For copyright



    Copyright YEAR(s) YOUR NAME(s)



(No, (C) is not needed.  No, "all rights reserved" is not needed.)



For licensing the easiest way is to use the same licensing as Perl itself:



    This library is free software; you may redistribute it and/or modify
    it under the same terms as Perl itself.



This makes it easy for people to use your module with Perl.  Note that
this licensing example is neither an endorsement or a requirement, you are
of course free to choose any licensing.





SEE ALSO




Other man pages to check out, like man(1), man(7), makewhatis(8), or
catman(8).  Normally a simple list of man pages separated by commas, or a
paragraph giving the name of a reference work.  Man page references, if
they use the standard name(section) form, don’t have to be enclosed in
L<> (although it’s recommended), but other things in this section
probably should be when appropriate.



If the package has a mailing list, include a URL or subscription
instructions here.



If the package has a web site, include a URL here.








Documentation of object-oriented libraries or modules may want to use
CONSTRUCTORS and METHODS sections, or CLASS METHODS and INSTANCE METHODS
sections, for detailed documentation of the parts of the library and save
the DESCRIPTION section for an overview.  Large modules with a function
interface may want to use FUNCTIONS for similar reasons.  Some people use
OVERVIEW to summarize the description if it’s quite long.



Section ordering varies, although NAME must always be the first section
(you’ll break some man page systems otherwise), and NAME, SYNOPSIS,
DESCRIPTION, and OPTIONS generally always occur first and in that order if
present.  In general, SEE ALSO, AUTHOR, and similar material should be
left for last.  Some systems also move WARNINGS and NOTES to last.  The
order given above should be reasonable for most purposes.



Some systems use CONFORMING TO to note conformance to relevant standards
and MT-LEVEL to note safeness for use in threaded programs or signal
handlers.  These headings are primarily useful when documenting parts of a
C library.



Finally, as a general note, try not to use an excessive amount of markup.
As documented here and in (Pod-Man), you can safely leave Perl variables,
module names, function names, man page references, and the like unadorned
by markup, and the POD translators will figure it all out for you.  This
makes it much easier to later edit the documentation.  Note that many
existing translators will do the wrong thing with email addresses when
wrapped in L<>, so don’t do that.



You can check whether your documentation looks right by running



    % pod2text -o something.pod | less



If you have groff installed, you can get an even better look this way:



    % pod2man something.pod | groff -Tps -mandoc > something.ps



Now view the resulting Postscript file to see whether everything checks out.









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54.3 SEE ALSO



For additional information that may be more accurate for your specific
system, see either man(5) or man(7) depending on your system manual
section numbering conventions.



This documentation is maintained as part of the podlators distribution.
The current version is always available from its web site at
<http://www.eyrie.org/~eagle/software/podlators/>.









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54.4 AUTHOR



Russ Allbery <rra@stanford.edu>, with large portions of this documentation
taken from the documentation of the original pod2man implementation by
Larry Wall and Tom Christiansen.









Previous: perlpodstyle AUTHOR 1, Up: perlpodstyle   [Contents]





54.5 COPYRIGHT AND LICENSE



Copyright 1999, 2000, 2001, 2004, 2006, 2008, 2010 Russ Allbery
<rra@stanford.edu>.



This documentation is free software; you may redistribute it and/or modify
it under the same terms as Perl itself.









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55 perlpolicy





• perlpolicy NAME:  



• perlpolicy DESCRIPTION:  



• perlpolicy GOVERNANCE:  



• perlpolicy MAINTENANCE AND SUPPORT:  



• perlpolicy BACKWARD COMPATIBILITY AND DEPRECATION:  



• perlpolicy MAINTENANCE BRANCHES:  



• perlpolicy CONTRIBUTED MODULES:  



• perlpolicy DOCUMENTATION:  



• perlpolicy CREDITS:  












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55.1 NAME



perlpolicy - Various and sundry policies and commitments related to the Perl core









Next: , Previous: perlpolicy NAME, Up: perlpolicy   [Contents]





55.2 DESCRIPTION



This document is the master document which records all written
policies about how the Perl 5 Porters collectively develop and maintain
the Perl core.









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55.3 GOVERNANCE





• perlpolicy Perl 5 Porters:  












Up: perlpolicy GOVERNANCE   [Contents]





55.3.1 Perl 5 Porters



Subscribers to perl5-porters (the porters themselves) come in several flavours.
Some are quiet curious lurkers, who rarely pitch in and instead watch
the ongoing development to ensure they’re forewarned of new changes or
features in Perl.  Some are representatives of vendors, who are there
to make sure that Perl continues to compile and work on their
platforms.  Some patch any reported bug that they know how to fix,
some are actively patching their pet area (threads, Win32, the regexp
-engine), while others seem to do nothing but complain.  In other
words, it’s your usual mix of technical people.



Over this group of porters presides Larry Wall.  He has the final word
in what does and does not change in any of the Perl programming languages.
These days, Larry spends most of his time on Perl 6, while Perl 5 is
shepherded by a "pumpking", a porter responsible for deciding what
goes into each release and ensuring that releases happen on a regular
basis.



Larry sees Perl development along the lines of the US government:
there’s the Legislature (the porters), the Executive branch (the
-pumpking), and the Supreme Court (Larry).  The legislature can
discuss and submit patches to the executive branch all they like, but
the executive branch is free to veto them.  Rarely, the Supreme Court
will side with the executive branch over the legislature, or the
legislature over the executive branch.  Mostly, however, the
legislature and the executive branch are supposed to get along and
work out their differences without impeachment or court cases.



You might sometimes see reference to Rule 1 and Rule 2.  Larry’s power
as Supreme Court is expressed in The Rules:



    

  1.  Larry is always by definition right about how Perl should behave.
This means he has final veto power on the core functionality.


  2.  Larry is allowed to change his mind about any matter at a later date,
regardless of whether he previously invoked Rule 1.





Got that?  Larry is always right, even when he was wrong.  It’s rare
to see either Rule exercised, but they are often alluded to.









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55.4 MAINTENANCE AND SUPPORT



Perl 5 is developed by a community, not a corporate entity. Every change
contributed to the Perl core is the result of a donation. Typically, these
donations are contributions of code or time by individual members of our
community. On occasion, these donations come in the form of corporate
or organizational sponsorship of a particular individual or project.



As a volunteer organization, the commitments we make are heavily dependent
on the goodwill and hard work of individuals who have no obligation to
contribute to Perl.



That being said, we value Perl’s stability and security and have long
had an unwritten covenant with the broader Perl community to support
and maintain releases of Perl.



This document codifies the support and maintenance commitments that
the Perl community should expect from Perl’s developers:











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55.5 BACKWARD COMPATIBILITY AND DEPRECATION



Our community has a long-held belief that backward-compatibility is a
virtue, even when the functionality in question is a design flaw.



We would all love to unmake some mistakes we’ve made over the past
decades.  Living with every design error we’ve ever made can lead
to painful stagnation.  Unwinding our mistakes is very, very
difficult.  Doing so without actively harming our users is
nearly impossible.



Lately, ignoring or actively opposing compatibility with earlier versions
of Perl has come into vogue.  Sometimes, a change is proposed which
wants to usurp syntax which previously had another meaning.  Sometimes,
a change wants to improve previously-crazy semantics.



Down this road lies madness.



Requiring end-user programmers to change just a few language constructs,
even language constructs which no well-educated developer would ever
intentionally use is tantamount to saying "you should not upgrade to
a new release of Perl unless you have 100% test coverage and can do a
full manual audit of your codebase."  If we were to have tools capable of
reliably upgrading Perl source code from one version of Perl to another,
this concern could be significantly mitigated.



We want to ensure that Perl continues to grow and flourish in the coming
years and decades, but not at the expense of our user community.



Existing syntax and semantics should only be marked for destruction in
very limited circumstances.  If a given language feature’s continued
inclusion in the language will cause significant harm to the language
or prevent us from making needed changes to the runtime, then it may
be considered for deprecation.



Any language change which breaks backward-compatibility should be able to
be enabled or disabled lexically.  Unless code at a given scope declares
that it wants the new behavior, that new behavior should be disabled.
Which backward-incompatible changes are controlled implicitly by a
’use v5.x.y’ is a decision which should be made by the pumpking in
consultation with the community.



When a backward-incompatible change can’t be toggled lexically, the decision
to change the language must be considered very, very carefully.  If it’s
possible to move the old syntax or semantics out of the core language
and into XS-land, that XS module should be enabled by default unless
the user declares that they want a newer revision of Perl.



Historically, we’ve held ourselves to a far higher standard than
backward-compatibility – bugward-compatibility.  Any accident of
implementation or unintentional side-effect of running some bit of code
has been considered to be a feature of the language to be defended with
the same zeal as any other feature or functionality.  No matter how
frustrating these unintentional features may be to us as we continue
to improve Perl, these unintentional features often deserve our
protection.  It is very important that existing software written in
Perl continue to work correctly.  If end-user developers have adopted a
bug as a feature, we need to treat it as such.



New syntax and semantics which don’t break existing language constructs
and syntax have a much lower bar.  They merely need to prove themselves
to be useful, elegant, well designed, and well tested.





• perlpolicy Terminology:  












Up: perlpolicy BACKWARD COMPATIBILITY AND DEPRECATION   [Contents]





55.5.1 Terminology



To make sure we’re talking about the same thing when we discuss the removal
of features or functionality from the Perl core, we have specific definitions
for a few words and phrases.





experimental




If something in the Perl core is marked as experimental, we may change
its behaviour, deprecate or remove it without notice. While we’ll always
do our best to smooth the transition path for users of experimental
features, you should contact the perl5-porters mailinglist if you find
an experimental feature useful and want to help shape its future.





deprecated




If something in the Perl core is marked as deprecated, we may remove it
from the core in the next stable release series, though we may not. As of
Perl 5.12, deprecated features and modules warn the user as they’re used.
If you use a deprecated feature and believe that its removal from the Perl
core would be a mistake, please contact the perl5-porters mailinglist and
plead your case.  We don’t deprecate things without a good reason, but
sometimes there’s a counterargument we haven’t considered.  Historically,
we did not distinguish between "deprecated" and "discouraged" features.





discouraged




From time to time, we may mark language constructs and features which we
consider to have been mistakes as discouraged.  Discouraged features
aren’t candidates for removal in the next major release series, but
we may later deprecate them if they’re found to stand in the way of a
significant improvement to the Perl core.





removed




Once a feature, construct or module has been marked as deprecated for a
stable release cycle, we may remove it from the Perl core.  Unsurprisingly,
we say we’ve removed these things.














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55.6 MAINTENANCE BRANCHES







• perlpolicy Getting changes into a maint branch:  












Up: perlpolicy MAINTENANCE BRANCHES   [Contents]





55.6.1 Getting changes into a maint branch



Historically, only the pumpking cherry-picked changes from bleadperl
into maintperl.  This has...scaling problems.  At the same time,
maintenance branches of stable versions of Perl need to be treated with
great care. To that end, we’re going to try out a new process for
maint-5.12.



Any committer may cherry-pick any commit from blead to maint-5.12 if
they send mail to perl5-porters announcing their intent to cherry-pick
a specific commit along with a rationale for doing so and at least two 
other committers respond to the list giving their assent. (This policy
applies to current and former pumpkings, as well as other committers.)









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55.7 CONTRIBUTED MODULES





• perlpolicy A Social Contract about Artistic Control:  












Up: perlpolicy CONTRIBUTED MODULES   [Contents]





55.7.1 A Social Contract about Artistic Control



What follows is a statement about artistic control, defined as the ability
of authors of packages to guide the future of their code and maintain
control over their work.  It is a recognition that authors should have
control over their work, and that it is a responsibility of the rest of
the Perl community to ensure that they retain this control.  It is an
attempt to document the standards to which we, as Perl developers, intend
to hold ourselves.  It is an attempt to write down rough guidelines about
the respect we owe each other as Perl developers.



This statement is not a legal contract.  This statement is not a legal
document in any way, shape, or form.  Perl is distributed under the GNU
Public License and under the Artistic License; those are the precise legal
terms.  This statement isn’t about the law or licenses.  It’s about
community, mutual respect, trust, and good-faith cooperation.



We recognize that the Perl core, defined as the software distributed with
the heart of Perl itself, is a joint project on the part of all of us.
From time to time, a script, module, or set of modules (hereafter referred
to simply as a "module") will prove so widely useful and/or so integral to
the correct functioning of Perl itself that it should be distributed with
the Perl core.  This should never be done without the author’s explicit
consent, and a clear recognition on all parts that this means the module
is being distributed under the same terms as Perl itself.  A module author
should realize that inclusion of a module into the Perl core will
necessarily mean some loss of control over it, since changes may
occasionally have to be made on short notice or for consistency with the
rest of Perl.



Once a module has been included in the Perl core, however, everyone
involved in maintaining Perl should be aware that the module is still the
property of the original author unless the original author explicitly
gives up their ownership of it.  In particular:





In other words, the author of a module should be considered to have final
say on modifications to their module whenever possible (bearing in mind
that it’s expected that everyone involved will work together and arrive at
reasonable compromises when there are disagreements).



As a last resort, however:



If the author’s vision of the future of their module is sufficiently
different from the vision of the pumpkin holder and perl5-porters as a
whole so as to cause serious problems for Perl, the pumpkin holder may
choose to formally fork the version of the module in the Perl core from the
one maintained by the author.  This should not be done lightly and
should always if at all possible be done only after direct input
from Larry.  If this is done, it must then be made explicit in the
module as distributed with the Perl core that it is a forked version and
that while it is based on the original author’s work, it is no longer
maintained by them.  This must be noted in both the documentation and
in the comments in the source of the module.



Again, this should be a last resort only.  Ideally, this should never
happen, and every possible effort at cooperation and compromise should be
made before doing this.  If it does prove necessary to fork a module for
the overall health of Perl, proper credit must be given to the original
author in perpetuity and the decision should be constantly re-evaluated to
see if a remerging of the two branches is possible down the road.



In all dealings with contributed modules, everyone maintaining Perl should
keep in mind that the code belongs to the original author, that they may
not be on perl5-porters at any given time, and that a patch is not
official unless it has been integrated into the author’s copy of the
module.  To aid with this, and with points #1, #2, and #3 above, contact
information for the authors of all contributed modules should be kept with
the Perl distribution.



Finally, the Perl community as a whole recognizes that respect for
ownership of code, respect for artistic control, proper credit, and active
effort to prevent unintentional code skew or communication gaps is vital
to the health of the community and Perl itself.  Members of a community
should not normally have to resort to rules and laws to deal with each
other, and this document, although it contains rules so as to be clear, is
about an attitude and general approach.  The first step in any dispute
should be open communication, respect for opposing views, and an attempt
at a compromise.  In nearly every circumstance nothing more will be
necessary, and certainly no more drastic measure should be used until
every avenue of communication and discussion has failed.









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55.8 DOCUMENTATION



Perl’s documentation is an important resource for our users. It’s
incredibly important for Perl’s documentation to be reasonably coherent
and to accurately reflect the current implementation.



Just as P5P collectively maintains the codebase, we collectively
maintain the documentation.  Writing a particular bit of documentation
doesn’t give an author control of the future of that documentation.
At the same time, just as source code changes should match the style
of their surrounding blocks, so should documentation changes.



Examples in documentation should be illustrative of the concept
they’re explaining.  Sometimes, the best way to show how a
language feature works is with a small program the reader can
run without modification.  More often, examples will consist
of a snippet of code containing only the "important" bits.
The definition of "important" varies from snippet to snippet.
Sometimes it’s important to declare use strict and use warnings,
initialize all variables and fully catch every error condition.
More often than not, though, those things obscure the lesson
the example was intended to teach.



As Perl is developed by a global team of volunteers, our
documentation often contains spellings which look funny
to somebody.  Choice of American/British/Other spellings
is left as an exercise for the author of each bit of
documentation.  When patching documentation, try to emulate
the documentation around you, rather than changing the existing
prose.



In general, documentation should describe what Perl does "now" rather
than what it used to do.  It’s perfectly reasonable to include notes
in documentation about how behaviour has changed from previous releases,
but, with very few exceptions, documentation isn’t "dual-life" –
it doesn’t need to fully describe how all old versions used to work.









Previous: perlpolicy DOCUMENTATION, Up: perlpolicy   [Contents]





55.9 CREDITS



"Social Contract about Contributed Modules" originally by Russ Allbery <rra@stanford.edu> and the perl5-porters.









Next: , Previous: perlpolicy, Up: Top   [Contents]





56 perlport





• perlport NAME:  



• perlport DESCRIPTION:  



• perlport ISSUES:  



• perlport CPAN Testers:  



• perlport PLATFORMS:  



• perlport FUNCTION IMPLEMENTATIONS:  



• perlport Supported Platforms:  



• perlport EOL Platforms (Perl 5.14):  



• perlport Supported Platforms (Perl 5.8):  



• perlport SEE ALSO:  



• perlport AUTHORS / CONTRIBUTORS:  












Next: , Up: perlport   [Contents]





56.1 NAME



perlport - Writing portable Perl









Next: , Previous: perlport NAME, Up: perlport   [Contents]





56.2 DESCRIPTION



Perl runs on numerous operating systems.  While most of them share
much in common, they also have their own unique features.



This document is meant to help you to find out what constitutes portable
Perl code.  That way once you make a decision to write portably,
you know where the lines are drawn, and you can stay within them.



There is a tradeoff between taking full advantage of one particular
type of computer and taking advantage of a full range of them.
Naturally, as you broaden your range and become more diverse, the
common factors drop, and you are left with an increasingly smaller
area of common ground in which you can operate to accomplish a
particular task.  Thus, when you begin attacking a problem, it is
important to consider under which part of the tradeoff curve you
want to operate.  Specifically, you must decide whether it is
important that the task that you are coding have the full generality
of being portable, or whether to just get the job done right now.
This is the hardest choice to be made.  The rest is easy, because
Perl provides many choices, whichever way you want to approach your
problem.



Looking at it another way, writing portable code is usually about
willfully limiting your available choices.  Naturally, it takes
discipline and sacrifice to do that.  The product of portability
and convenience may be a constant.  You have been warned.



Be aware of two important points:





Not all Perl programs have to be portable




There is no reason you should not use Perl as a language to glue Unix
tools together, or to prototype a Macintosh application, or to manage the
Windows registry.  If it makes no sense to aim for portability for one
reason or another in a given program, then don’t bother.





Nearly all of Perl already is portable




Don’t be fooled into thinking that it is hard to create portable Perl
code.  It isn’t.  Perl tries its level-best to bridge the gaps between
what’s available on different platforms, and all the means available to
use those features.  Thus almost all Perl code runs on any machine
without modification.  But there are some significant issues in
writing portable code, and this document is entirely about those issues.








Here’s the general rule: When you approach a task commonly done
using a whole range of platforms, think about writing portable
code.  That way, you don’t sacrifice much by way of the implementation
choices you can avail yourself of, and at the same time you can give
your users lots of platform choices.  On the other hand, when you have to
take advantage of some unique feature of a particular platform, as is
often the case with systems programming (whether for Unix, Windows,
VMS, etc.), consider writing platform-specific code.



When the code will run on only two or three operating systems, you
may need to consider only the differences of those particular systems.
The important thing is to decide where the code will run and to be
deliberate in your decision.



The material below is separated into three main sections: main issues of
portability (ISSUES), platform-specific issues (PLATFORMS), and
built-in perl functions that behave differently on various ports
(FUNCTION IMPLEMENTATIONS).



This information should not be considered complete; it includes possibly
transient information about idiosyncrasies of some of the ports, almost
all of which are in a state of constant evolution.  Thus, this material
should be considered a perpetual work in progress
(<IMG SRC="yellow_sign.gif" ALT="Under Construction">).









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56.3 ISSUES





• perlport Newlines:  



• perlport Numbers endianness and Width:  



• perlport Files and Filesystems:  



• perlport System Interaction:  



• perlport Command names versus file pathnames:  



• perlport Networking:  



• perlport Interprocess Communication (IPC):  



• perlport External Subroutines (XS):  



• perlport Standard Modules:  



• perlport Time and Date:  



• perlport Character sets and character encoding:  



• perlport Internationalisation:  



• perlport System Resources:  



• perlport Security:  



• perlport Style:  












Next: , Up: perlport ISSUES   [Contents]





56.3.1 Newlines



In most operating systems, lines in files are terminated by newlines.
Just what is used as a newline may vary from OS to OS.  Unix
traditionally uses \012, one type of DOSish I/O uses \015\012,
and Mac OS uses \015.



Perl uses \n to represent the "logical" newline, where what is
logical may depend on the platform in use.  In MacPerl, \n always
means \015.  In DOSish perls, \n usually means \012, but when
accessing a file in "text" mode, perl uses the :crlf layer that
translates it to (or from) \015\012, depending on whether you’re
reading or writing. Unix does the same thing on ttys in canonical
mode.  \015\012 is commonly referred to as CRLF.



To trim trailing newlines from text lines use chomp().  With default 
settings that function looks for a trailing \n character and thus 
trims in a portable way.



When dealing with binary files (or text files in binary mode) be sure
to explicitly set $/ to the appropriate value for your file format
before using chomp().



Because of the "text" mode translation, DOSish perls have limitations
in using seek and tell on a file accessed in "text" mode.
Stick to seek-ing to locations you got from tell (and no
others), and you are usually free to use seek and tell even
in "text" mode.  Using seek or tell or other file operations
may be non-portable.  If you use binmode on a file, however, you
can usually seek and tell with arbitrary values in safety.



A common misconception in socket programming is that \n eq \012
everywhere.  When using protocols such as common Internet protocols,
\012 and \015 are called for specifically, and the values of
the logical \n and \r (carriage return) are not reliable.



    print SOCKET "Hi there, client!\r\n";      # WRONG
    print SOCKET "Hi there, client!\015\012";  # RIGHT



However, using \015\012 (or \cM\cJ, or \x0D\x0A) can be tedious
and unsightly, as well as confusing to those maintaining the code.  As
such, the Socket module supplies the Right Thing for those who want it.



    use Socket qw(:DEFAULT :crlf);
    print SOCKET "Hi there, client!$CRLF"      # RIGHT



When reading from a socket, remember that the default input record
separator $/ is \n, but robust socket code will recognize as
either \012 or \015\012 as end of line:



    while (<SOCKET>) {
        # ...
    }



Because both CRLF and LF end in LF, the input record separator can
be set to LF and any CR stripped later.  Better to write:



    use Socket qw(:DEFAULT :crlf);
    local($/) = LF;      # not needed if $/ is already \012

    while (<SOCKET>) {
        s/$CR?$LF/\n/;   # not sure if socket uses LF or CRLF, OK
    #   s/\015?\012/\n/; # same thing
    }



This example is preferred over the previous one–even for Unix
platforms–because now any \015’s (\cM’s) are stripped out
(and there was much rejoicing).



Similarly, functions that return text data–such as a function that
fetches a web page–should sometimes translate newlines before
returning the data, if they’ve not yet been translated to the local
newline representation.  A single line of code will often suffice:



    $data =~ s/\015?\012/\n/g;
    return $data;



Some of this may be confusing.  Here’s a handy reference to the ASCII CR
and LF characters.  You can print it out and stick it in your wallet.



    LF  eq  \012  eq  \x0A  eq  \cJ  eq  chr(10)  eq  ASCII 10
    CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  ASCII 13

             | Unix | DOS  | Mac  |
        ---------------------------
        \n   |  LF  |  LF  |  CR  |
        \r   |  CR  |  CR  |  LF  |
        \n * |  LF  | CRLF |  CR  |
        \r * |  CR  |  CR  |  LF  |
        ---------------------------
        * text-mode STDIO



The Unix column assumes that you are not accessing a serial line
(like a tty) in canonical mode.  If you are, then CR on input becomes
"\n", and "\n" on output becomes CRLF.



These are just the most common definitions of \n and \r in Perl.
There may well be others.  For example, on an EBCDIC implementation
such as z/OS (OS/390) or OS/400 (using the ILE, the PASE is ASCII-based)
the above material is similar to "Unix" but the code numbers change:



    LF  eq  \025  eq  \x15  eq  \cU  eq  chr(21)  eq  CP-1047 21
    LF  eq  \045  eq  \x25  eq           chr(37)  eq  CP-0037 37
    CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  CP-1047 13
    CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  CP-0037 13

             | z/OS | OS/400 |
        ----------------------
        \n   |  LF  |  LF    |
        \r   |  CR  |  CR    |
        \n * |  LF  |  LF    |
        \r * |  CR  |  CR    |
        ----------------------
        * text-mode STDIO









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56.3.2 Numbers endianness and Width



Different CPUs store integers and floating point numbers in different
orders (called endianness) and widths (32-bit and 64-bit being the
most common today).  This affects your programs when they attempt to transfer
numbers in binary format from one CPU architecture to another,
usually either "live" via network connection, or by storing the
numbers to secondary storage such as a disk file or tape.



Conflicting storage orders make utter mess out of the numbers.  If a
little-endian host (Intel, VAX) stores 0x12345678 (305419896 in
decimal), a big-endian host (Motorola, Sparc, PA) reads it as
0x78563412 (2018915346 in decimal).  Alpha and MIPS can be either:
Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses
them in big-endian mode.  To avoid this problem in network (socket)
connections use the pack and unpack formats n and N, the
"network" orders.  These are guaranteed to be portable.



As of perl 5.9.2, you can also use the > and < modifiers
to force big- or little-endian byte-order.  This is useful if you want
to store signed integers or 64-bit integers, for example.



You can explore the endianness of your platform by unpacking a
data structure packed in native format such as:



    print unpack("h*", pack("s2", 1, 2)), "\n";
    # '10002000' on e.g. Intel x86 or Alpha 21064 in little-endian mode
    # '00100020' on e.g. Motorola 68040



If you need to distinguish between endian architectures you could use
either of the variables set like so:



    $is_big_endian   = unpack("h*", pack("s", 1)) =~ /01/;
    $is_little_endian = unpack("h*", pack("s", 1)) =~ /^1/;



Differing widths can cause truncation even between platforms of equal
endianness.  The platform of shorter width loses the upper parts of the
number.  There is no good solution for this problem except to avoid
transferring or storing raw binary numbers.



One can circumnavigate both these problems in two ways.  Either
transfer and store numbers always in text format, instead of raw
binary, or else consider using modules like Data::Dumper (included in
the standard distribution as of Perl 5.005) and Storable (included as
of perl 5.8).  Keeping all data as text significantly simplifies matters.



The v-strings are portable only up to v2147483647 (0x7FFFFFFF), that’s
how far EBCDIC, or more precisely UTF-EBCDIC will go.









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56.3.3 Files and Filesystems



Most platforms these days structure files in a hierarchical fashion.
So, it is reasonably safe to assume that all platforms support the
notion of a "path" to uniquely identify a file on the system.  How
that path is really written, though, differs considerably.



Although similar, file path specifications differ between Unix,
Windows, Mac OS, OS/2, VMS, VOS, RISC OS, and probably others.
Unix, for example, is one of the few OSes that has the elegant idea
of a single root directory.



DOS, OS/2, VMS, VOS, and Windows can work similarly to Unix with /
as path separator, or in their own idiosyncratic ways (such as having
several root directories and various "unrooted" device files such NIL:
and LPT:).



Mac OS 9 and earlier used : as a path separator instead of /.



The filesystem may support neither hard links (link) nor
symbolic links (symlink, readlink, lstat).



The filesystem may support neither access timestamp nor change
timestamp (meaning that about the only portable timestamp is the
modification timestamp), or one second granularity of any timestamps
(e.g. the FAT filesystem limits the time granularity to two seconds).



The "inode change timestamp" (the -C filetest) may really be the
"creation timestamp" (which it is not in Unix).



VOS perl can emulate Unix filenames with / as path separator.  The
native pathname characters greater-than, less-than, number-sign, and
percent-sign are always accepted.



RISC OS perl can emulate Unix filenames with / as path
separator, or go native and use . for path separator and : to
signal filesystems and disk names.



Don’t assume Unix filesystem access semantics: that read, write,
and execute are all the permissions there are, and even if they exist,
that their semantics (for example what do r, w, and x mean on
a directory) are the Unix ones.  The various Unix/POSIX compatibility
layers usually try to make interfaces like chmod() work, but sometimes
there simply is no good mapping.



If all this is intimidating, have no (well, maybe only a little)
fear.  There are modules that can help.  The File::Spec modules
provide methods to do the Right Thing on whatever platform happens
to be running the program.



    use File::Spec::Functions;
    chdir(updir());        # go up one directory
    my $file = catfile(curdir(), 'temp', 'file.txt');
    # on Unix and Win32, './temp/file.txt'
    # on Mac OS Classic, ':temp:file.txt'
    # on VMS, '[.temp]file.txt'



File::Spec is available in the standard distribution as of version
5.004_05.  File::Spec::Functions is only in File::Spec 0.7 and later,
and some versions of perl come with version 0.6.  If File::Spec
is not updated to 0.7 or later, you must use the object-oriented
interface from File::Spec (or upgrade File::Spec).



In general, production code should not have file paths hardcoded.
Making them user-supplied or read from a configuration file is
better, keeping in mind that file path syntax varies on different
machines.



This is especially noticeable in scripts like Makefiles and test suites,
which often assume / as a path separator for subdirectories.



Also of use is File::Basename from the standard distribution, which
splits a pathname into pieces (base filename, full path to directory,
and file suffix).



Even when on a single platform (if you can call Unix a single platform),
remember not to count on the existence or the contents of particular
system-specific files or directories, like /etc/passwd,
/etc/sendmail.conf, /etc/resolv.conf, or even /tmp/.  For
example, /etc/passwd may exist but not contain the encrypted
passwords, because the system is using some form of enhanced security.
Or it may not contain all the accounts, because the system is using NIS. 
If code does need to rely on such a file, include a description of the
file and its format in the code’s documentation, then make it easy for
the user to override the default location of the file.



Don’t assume a text file will end with a newline.  They should,
but people forget.



Do not have two files or directories of the same name with different
case, like test.pl and Test.pl, as many platforms have
case-insensitive (or at least case-forgiving) filenames.  Also, try
not to have non-word characters (except for .) in the names, and
keep them to the 8.3 convention, for maximum portability, onerous a
burden though this may appear.



Likewise, when using the AutoSplit module, try to keep your functions to
8.3 naming and case-insensitive conventions; or, at the least,
make it so the resulting files have a unique (case-insensitively)
first 8 characters.



Whitespace in filenames is tolerated on most systems, but not all,
and even on systems where it might be tolerated, some utilities
might become confused by such whitespace.



Many systems (DOS, VMS ODS-2) cannot have more than one . in their
filenames.



Don’t assume > won’t be the first character of a filename.
Always use < explicitly to open a file for reading, or even
better, use the three-arg version of open, unless you want the user to
be able to specify a pipe open.



    open my $fh, '<', $existing_file) or die $!;



If filenames might use strange characters, it is safest to open it
with sysopen instead of open.  open is magic and can
translate characters like >, <, and |, which may
be the wrong thing to do.  (Sometimes, though, it’s the right thing.)
Three-arg open can also help protect against this translation in cases
where it is undesirable.



Don’t use : as a part of a filename since many systems use that for
their own semantics (Mac OS Classic for separating pathname components,
many networking schemes and utilities for separating the nodename and
the pathname, and so on).  For the same reasons, avoid @, ; and
|.



Don’t assume that in pathnames you can collapse two leading slashes
// into one: some networking and clustering filesystems have special
semantics for that.  Let the operating system to sort it out.



The portable filename characters as defined by ANSI C are



 a b c d e f g h i j k l m n o p q r t u v w x y z
 A B C D E F G H I J K L M N O P Q R T U V W X Y Z
 0 1 2 3 4 5 6 7 8 9
 . _ -



and the "-" shouldn’t be the first character.  If you want to be
hypercorrect, stay case-insensitive and within the 8.3 naming
convention (all the files and directories have to be unique within one
directory if their names are lowercased and truncated to eight
characters before the ., if any, and to three characters after the
., if any).  (And do not use .s in directory names.)









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56.3.4 System Interaction



Not all platforms provide a command line.  These are usually platforms
that rely primarily on a Graphical User Interface (GUI) for user
interaction.  A program requiring a command line interface might
not work everywhere.  This is probably for the user of the program
to deal with, so don’t stay up late worrying about it.



Some platforms can’t delete or rename files held open by the system,
this limitation may also apply to changing filesystem metainformation
like file permissions or owners.  Remember to close files when you
are done with them.  Don’t unlink or rename an open file.  Don’t
tie or open a file already tied or opened; untie or close
it first.



Don’t open the same file more than once at a time for writing, as some
operating systems put mandatory locks on such files.



Don’t assume that write/modify permission on a directory gives the
right to add or delete files/directories in that directory.  That is
filesystem specific: in some filesystems you need write/modify
permission also (or even just) in the file/directory itself.  In some
filesystems (AFS, DFS) the permission to add/delete directory entries
is a completely separate permission.



Don’t assume that a single unlink completely gets rid of the file:
some filesystems (most notably the ones in VMS) have versioned
filesystems, and unlink() removes only the most recent one (it doesn’t
remove all the versions because by default the native tools on those
platforms remove just the most recent version, too).  The portable
idiom to remove all the versions of a file is



    1 while unlink "file";



This will terminate if the file is undeleteable for some reason
(protected, not there, and so on).



Don’t count on a specific environment variable existing in %ENV.
Don’t count on %ENV entries being case-sensitive, or even
case-preserving.  Don’t try to clear %ENV by saying %ENV = ();, or,
if you really have to, make it conditional on $^O ne 'VMS' since in
VMS the %ENV table is much more than a per-process key-value string
table.



On VMS, some entries in the %ENV hash are dynamically created when
their key is used on a read if they did not previously exist.  The
values for $ENV{HOME}, $ENV{TERM}, $ENV{HOME}, and $ENV{USER},
are known to be dynamically generated.  The specific names that are
dynamically generated may vary with the version of the C library on VMS,
and more may exist than is documented.



On VMS by default, changes to the %ENV hash are persistent after the process
exits.  This can cause unintended issues.



Don’t count on signals or %SIG for anything.



Don’t count on filename globbing.  Use opendir, readdir, and
closedir instead.



Don’t count on per-program environment variables, or per-program current
directories.



Don’t count on specific values of $!, neither numeric nor
especially the strings values. Users may switch their locales causing
error messages to be translated into their languages.  If you can
trust a POSIXish environment, you can portably use the symbols defined
by the Errno module, like ENOENT.  And don’t trust on the values of $!
at all except immediately after a failed system call.









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56.3.5 Command names versus file pathnames



Don’t assume that the name used to invoke a command or program with
system or exec can also be used to test for the existence of the
file that holds the executable code for that command or program.
First, many systems have "internal" commands that are built-in to the
shell or OS and while these commands can be invoked, there is no
corresponding file.  Second, some operating systems (e.g., Cygwin,
DJGPP, OS/2, and VOS) have required suffixes for executable files;
these suffixes are generally permitted on the command name but are not
required.  Thus, a command like "perl" might exist in a file named
"perl", "perl.exe", or "perl.pm", depending on the operating system.
The variable "_exe" in the Config module holds the executable suffix,
if any.  Third, the VMS port carefully sets up $^X and
$Config{perlpath} so that no further processing is required.  This is
just as well, because the matching regular expression used below would
then have to deal with a possible trailing version number in the VMS
file name.



To convert $^X to a file pathname, taking account of the requirements
of the various operating system possibilities, say:



 use Config;
 my $thisperl = $^X;
 if ($^O ne 'VMS')
    {$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}



To convert $Config{perlpath} to a file pathname, say:



 use Config;
 my $thisperl = $Config{perlpath};
 if ($^O ne 'VMS')
    {$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}









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56.3.6 Networking



Don’t assume that you can reach the public Internet.



Don’t assume that there is only one way to get through firewalls
to the public Internet.



Don’t assume that you can reach outside world through any other port
than 80, or some web proxy.  ftp is blocked by many firewalls.



Don’t assume that you can send email by connecting to the local SMTP port.



Don’t assume that you can reach yourself or any node by the name
’localhost’.  The same goes for ’127.0.0.1’.  You will have to try both.



Don’t assume that the host has only one network card, or that it
can’t bind to many virtual IP addresses.



Don’t assume a particular network device name.



Don’t assume a particular set of ioctl()s will work.



Don’t assume that you can ping hosts and get replies.



Don’t assume that any particular port (service) will respond.



Don’t assume that Sys::Hostname (or any other API or command) returns
either a fully qualified hostname or a non-qualified hostname: it all
depends on how the system had been configured.  Also remember that for
things such as DHCP and NAT, the hostname you get back might not be
very useful.



All the above "don’t":s may look daunting, and they are, but the key
is to degrade gracefully if one cannot reach the particular network
service one wants.  Croaking or hanging do not look very professional.









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56.3.7 Interprocess Communication (IPC)



In general, don’t directly access the system in code meant to be
portable.  That means, no system, exec, fork, pipe,
``, qx//, open with a |, nor any of the other things
that makes being a perl hacker worth being.



Commands that launch external processes are generally supported on
most platforms (though many of them do not support any type of
forking).  The problem with using them arises from what you invoke
them on.  External tools are often named differently on different
platforms, may not be available in the same location, might accept
different arguments, can behave differently, and often present their
results in a platform-dependent way.  Thus, you should seldom depend
on them to produce consistent results. (Then again, if you’re calling 
netstat -a, you probably don’t expect it to run on both Unix and CP/M.)



One especially common bit of Perl code is opening a pipe to sendmail:



    open(MAIL, '|/usr/lib/sendmail -t') 
        or die "cannot fork sendmail: $!";



This is fine for systems programming when sendmail is known to be
available.  But it is not fine for many non-Unix systems, and even
some Unix systems that may not have sendmail installed.  If a portable
solution is needed, see the various distributions on CPAN that deal
with it.  Mail::Mailer and Mail::Send in the MailTools distribution are
commonly used, and provide several mailing methods, including mail,
sendmail, and direct SMTP (via Net::SMTP) if a mail transfer agent is
not available.  Mail::Sendmail is a standalone module that provides
simple, platform-independent mailing.



The Unix System V IPC (msg*(), sem*(), shm*()) is not available
even on all Unix platforms.



Do not use either the bare result of pack("N", 10, 20, 30, 40) or
bare v-strings (such as v10.20.30.40) to represent IPv4 addresses:
both forms just pack the four bytes into network order.  That this
would be equal to the C language in_addr struct (which is what the
socket code internally uses) is not guaranteed.  To be portable use
the routines of the Socket extension, such as inet_aton(),
inet_ntoa(), and sockaddr_in().



The rule of thumb for portable code is: Do it all in portable Perl, or
use a module (that may internally implement it with platform-specific
code, but expose a common interface).









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56.3.8 External Subroutines (XS)



XS code can usually be made to work with any platform, but dependent
libraries, header files, etc., might not be readily available or
portable, or the XS code itself might be platform-specific, just as Perl
code might be.  If the libraries and headers are portable, then it is
normally reasonable to make sure the XS code is portable, too.



A different type of portability issue arises when writing XS code:
availability of a C compiler on the end-user’s system.  C brings
with it its own portability issues, and writing XS code will expose
you to some of those.  Writing purely in Perl is an easier way to
achieve portability.









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56.3.9 Standard Modules



In general, the standard modules work across platforms.  Notable
exceptions are the CPAN module (which currently makes connections to external
programs that may not be available), platform-specific modules (like
ExtUtils::MM_VMS), and DBM modules.



There is no one DBM module available on all platforms.
SDBM_File and the others are generally available on all Unix and DOSish
ports, but not in MacPerl, where only NBDM_File and DB_File are
available.



The good news is that at least some DBM module should be available, and
AnyDBM_File will use whichever module it can find.  Of course, then
the code needs to be fairly strict, dropping to the greatest common
factor (e.g., not exceeding 1K for each record), so that it will
work with any DBM module.  See (AnyDBM_File) for more details.









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56.3.10 Time and Date



The system’s notion of time of day and calendar date is controlled in
widely different ways.  Don’t assume the timezone is stored in $ENV{TZ},
and even if it is, don’t assume that you can control the timezone through
that variable.  Don’t assume anything about the three-letter timezone
abbreviations (for example that MST would be the Mountain Standard Time,
it’s been known to stand for Moscow Standard Time).  If you need to
use timezones, express them in some unambiguous format like the
exact number of minutes offset from UTC, or the POSIX timezone
format.



Don’t assume that the epoch starts at 00:00:00, January 1, 1970,
because that is OS- and implementation-specific.  It is better to
store a date in an unambiguous representation.  The ISO 8601 standard
defines YYYY-MM-DD as the date format, or YYYY-MM-DDTHH:MM:SS
(that’s a literal "T" separating the date from the time).
Please do use the ISO 8601 instead of making us guess what
date 02/03/04 might be.  ISO 8601 even sorts nicely as-is.
A text representation (like "1987-12-18") can be easily converted
into an OS-specific value using a module like Date::Parse.
An array of values, such as those returned by localtime, can be
converted to an OS-specific representation using Time::Local.



When calculating specific times, such as for tests in time or date modules,
it may be appropriate to calculate an offset for the epoch.



    require Time::Local;
    my $offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);



The value for $offset in Unix will be 0, but in Mac OS Classic
will be some large number.  $offset can then be added to a Unix time
value to get what should be the proper value on any system.









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56.3.11 Character sets and character encoding



Assume very little about character sets.



Assume nothing about numerical values (ord, chr) of characters.
Do not use explicit code point ranges (like \xHH-\xHH); use for
example symbolic character classes like [:print:].



Do not assume that the alphabetic characters are encoded contiguously
(in the numeric sense).  There may be gaps.



Do not assume anything about the ordering of the characters.
The lowercase letters may come before or after the uppercase letters;
the lowercase and uppercase may be interlaced so that both "a" and "A"
come before "b"; the accented and other international characters may
be interlaced so that ä comes before "b".









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56.3.12 Internationalisation



If you may assume POSIX (a rather large assumption), you may read
more about the POSIX locale system from perllocale NAME.  The locale
system at least attempts to make things a little bit more portable,
or at least more convenient and native-friendly for non-English
users.  The system affects character sets and encoding, and date
and time formatting–amongst other things.



If you really want to be international, you should consider Unicode.
See perluniintro NAME and perlunicode NAME for more information.



If you want to use non-ASCII bytes (outside the bytes 0x00..0x7f) in
the "source code" of your code, to be portable you have to be explicit
about what bytes they are.  Someone might for example be using your
code under a UTF-8 locale, in which case random native bytes might be
illegal ("Malformed UTF-8 ...")  This means that for example embedding
ISO 8859-1 bytes beyond 0x7f into your strings might cause trouble
later.  If the bytes are native 8-bit bytes, you can use the bytes
pragma.  If the bytes are in a string (regular expression being a
curious string), you can often also use the \xHH notation instead
of embedding the bytes as-is.  (If you want to write your code in UTF-8,
you can use the utf8.) The bytes and utf8 pragmata are
available since Perl 5.6.0.









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56.3.13 System Resources



If your code is destined for systems with severely constrained (or
missing!) virtual memory systems then you want to be especially mindful
of avoiding wasteful constructs such as:



    my @lines = <$very_large_file>;            # bad

    while (<$fh>) {$file .= $_}                # sometimes bad
    my $file = join('', <$fh>);                # better



The last two constructs may appear unintuitive to most people.  The
first repeatedly grows a string, whereas the second allocates a
large chunk of memory in one go.  On some systems, the second is
more efficient that the first.









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56.3.14 Security



Most multi-user platforms provide basic levels of security, usually
implemented at the filesystem level.  Some, however, unfortunately do
not.  Thus the notion of user id, or "home" directory,
or even the state of being logged-in, may be unrecognizable on many
platforms.  If you write programs that are security-conscious, it
is usually best to know what type of system you will be running
under so that you can write code explicitly for that platform (or
class of platforms).



Don’t assume the Unix filesystem access semantics: the operating
system or the filesystem may be using some ACL systems, which are
richer languages than the usual rwx.  Even if the rwx exist,
their semantics might be different.



(From security viewpoint testing for permissions before attempting to
do something is silly anyway: if one tries this, there is potential
for race conditions. Someone or something might change the
permissions between the permissions check and the actual operation.
Just try the operation.)



Don’t assume the Unix user and group semantics: especially, don’t
expect the $< and $> (or the $( and $)) to work
for switching identities (or memberships).



Don’t assume set-uid and set-gid semantics. (And even if you do,
think twice: set-uid and set-gid are a known can of security worms.)









Previous: perlport Security, Up: perlport ISSUES   [Contents]





56.3.15 Style



For those times when it is necessary to have platform-specific code,
consider keeping the platform-specific code in one place, making porting
to other platforms easier.  Use the Config module and the special
variable $^O to differentiate platforms, as described in
PLATFORMS.



Be careful in the tests you supply with your module or programs.
Module code may be fully portable, but its tests might not be.  This
often happens when tests spawn off other processes or call external
programs to aid in the testing, or when (as noted above) the tests
assume certain things about the filesystem and paths.  Be careful not
to depend on a specific output style for errors, such as when checking
$! after a failed system call.  Using $! for anything else than
displaying it as output is doubtful (though see the Errno module for
testing reasonably portably for error value). Some platforms expect
a certain output format, and Perl on those platforms may have been
adjusted accordingly.  Most specifically, don’t anchor a regex when
testing an error value.









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56.4 CPAN Testers



Modules uploaded to CPAN are tested by a variety of volunteers on
different platforms.  These CPAN testers are notified by mail of each
new upload, and reply to the list with PASS, FAIL, NA (not applicable to
this platform), or UNKNOWN (unknown), along with any relevant notations.



The purpose of the testing is twofold: one, to help developers fix any
problems in their code that crop up because of lack of testing on other
platforms; two, to provide users with information about whether
a given module works on a given platform.



Also see: 











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56.5 PLATFORMS



As of version 5.002, Perl is built with a $^O variable that
indicates the operating system it was built on.  This was implemented
to help speed up code that would otherwise have to use Config
and use the value of $Config{osname}.  Of course, to get more
detailed information about the system, looking into %Config is
certainly recommended.



%Config cannot always be trusted, however, because it was built
at compile time.  If perl was built in one place, then transferred
elsewhere, some values may be wrong.  The values may even have been
edited after the fact.





• perlport Unix:  



• perlport DOS and Derivatives:  



• perlport VMS:  



• perlport VOS:  



• perlport EBCDIC Platforms:  



• perlport Acorn RISC OS:  



• perlport Other perls:  












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56.5.1 Unix



Perl works on a bewildering variety of Unix and Unix-like platforms (see
e.g. most of the files in the hints/ directory in the source code kit).
On most of these systems, the value of $^O (hence $Config{'osname'},
too) is determined either by lowercasing and stripping punctuation from the
first field of the string returned by typing uname -a (or a similar command)
at the shell prompt or by testing the file system for the presence of
uniquely named files such as a kernel or header file.  Here, for example,
are a few of the more popular Unix flavors:



    uname         $^O        $Config{'archname'}
    --------------------------------------------
    AIX           aix        aix
    BSD/OS        bsdos      i386-bsdos
    Darwin        darwin     darwin
    dgux          dgux       AViiON-dgux
    DYNIX/ptx     dynixptx   i386-dynixptx
    FreeBSD       freebsd    freebsd-i386    
    Haiku         haiku      BePC-haiku
    Linux         linux      arm-linux
    Linux         linux      i386-linux
    Linux         linux      i586-linux
    Linux         linux      ppc-linux
    HP-UX         hpux       PA-RISC1.1
    IRIX          irix       irix
    Mac OS X      darwin     darwin
    NeXT 3        next       next-fat
    NeXT 4        next       OPENSTEP-Mach
    openbsd       openbsd    i386-openbsd
    OSF1          dec_osf    alpha-dec_osf
    reliantunix-n svr4       RM400-svr4
    SCO_SV        sco_sv     i386-sco_sv
    SINIX-N       svr4       RM400-svr4
    sn4609        unicos     CRAY_C90-unicos
    sn6521        unicosmk   t3e-unicosmk
    sn9617        unicos     CRAY_J90-unicos
    SunOS         solaris    sun4-solaris
    SunOS         solaris    i86pc-solaris
    SunOS4        sunos      sun4-sunos



Because the value of $Config{archname} may depend on the
hardware architecture, it can vary more than the value of $^O.









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56.5.2 DOS and Derivatives



Perl has long been ported to Intel-style microcomputers running under
systems like PC-DOS, MS-DOS, OS/2, and most Windows platforms you can
bring yourself to mention (except for Windows CE, if you count that).
Users familiar with COMMAND.COM or CMD.EXE style shells should
be aware that each of these file specifications may have subtle
differences:



    my $filespec0 = "c:/foo/bar/file.txt";
    my $filespec1 = "c:\\foo\\bar\\file.txt";
    my $filespec2 = 'c:\foo\bar\file.txt';
    my $filespec3 = 'c:\\foo\\bar\\file.txt';



System calls accept either / or \ as the path separator.
However, many command-line utilities of DOS vintage treat / as
the option prefix, so may get confused by filenames containing /.
Aside from calling any external programs, / will work just fine,
and probably better, as it is more consistent with popular usage,
and avoids the problem of remembering what to backwhack and what
not to.



The DOS FAT filesystem can accommodate only "8.3" style filenames.  Under
the "case-insensitive, but case-preserving" HPFS (OS/2) and NTFS (NT)
filesystems you may have to be careful about case returned with functions
like readdir or used with functions like open or opendir.



DOS also treats several filenames as special, such as AUX, PRN,
NUL, CON, COM1, LPT1, LPT2, etc.  Unfortunately, sometimes these
filenames won’t even work if you include an explicit directory
prefix.  It is best to avoid such filenames, if you want your code
to be portable to DOS and its derivatives.  It’s hard to know what
these all are, unfortunately.



Users of these operating systems may also wish to make use of
scripts such as pl2bat.bat or pl2cmd to
put wrappers around your scripts.



Newline (\n) is translated as \015\012 by STDIO when reading from
and writing to files (see Newlines).  binmode(FILEHANDLE)
will keep \n translated as \012 for that filehandle.  Since it is a
no-op on other systems, binmode should be used for cross-platform code
that deals with binary data.  That’s assuming you realize in advance
that your data is in binary.  General-purpose programs should
often assume nothing about their data.



The $^O variable and the $Config{archname} values for various
DOSish perls are as follows:



     OS            $^O      $Config{archname}   ID    Version
     --------------------------------------------------------
     MS-DOS        dos        ?                 
     PC-DOS        dos        ?                 
     OS/2          os2        ?
     Windows 3.1   ?          ?                 0      3 01
     Windows 95    MSWin32    MSWin32-x86       1      4 00
     Windows 98    MSWin32    MSWin32-x86       1      4 10
     Windows ME    MSWin32    MSWin32-x86       1      ?
     Windows NT    MSWin32    MSWin32-x86       2      4 xx
     Windows NT    MSWin32    MSWin32-ALPHA     2      4 xx
     Windows NT    MSWin32    MSWin32-ppc       2      4 xx
     Windows 2000  MSWin32    MSWin32-x86       2      5 00
     Windows XP    MSWin32    MSWin32-x86       2      5 01
     Windows 2003  MSWin32    MSWin32-x86       2      5 02
     Windows Vista MSWin32    MSWin32-x86       2      6 00
     Windows 7     MSWin32    MSWin32-x86       2      6 01
     Windows 7     MSWin32    MSWin32-x64       2      6 01
     Windows 2008  MSWin32    MSWin32-x86       2      6 01
     Windows 2008  MSWin32    MSWin32-x64       2      6 01
     Windows CE    MSWin32    ?                 3           
     Cygwin        cygwin     cygwin



The various MSWin32 Perl’s can distinguish the OS they are running on
via the value of the fifth element of the list returned from 
Win32::GetOSVersion().  For example:



    if ($^O eq 'MSWin32') {
        my @os_version_info = Win32::GetOSVersion();
        print +('3.1','95','NT')[$os_version_info[4]],"\n";
    }



There are also Win32::IsWinNT() and Win32::IsWin95(), try perldoc Win32,
and as of libwin32 0.19 (not part of the core Perl distribution)
Win32::GetOSName().  The very portable POSIX::uname() will work too:



    c:\> perl -MPOSIX -we "print join '|', uname"
    Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86



Also see:











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56.5.3 VMS



Perl on VMS is discussed in perlvms NAME in the perl distribution.



The official name of VMS as of this writing is OpenVMS.



Perl on VMS can accept either VMS- or Unix-style file
specifications as in either of the following:



    $ perl -ne "print if /perl_setup/i" SYS$LOGIN:LOGIN.COM
    $ perl -ne "print if /perl_setup/i" /sys$login/login.com



but not a mixture of both as in:



    $ perl -ne "print if /perl_setup/i" sys$login:/login.com
    Can't open sys$login:/login.com: file specification syntax error



Interacting with Perl from the Digital Command Language (DCL) shell
often requires a different set of quotation marks than Unix shells do.
For example:



    $ perl -e "print ""Hello, world.\n"""
    Hello, world.



There are several ways to wrap your perl scripts in DCL .COM files, if
you are so inclined.  For example:



    $ write sys$output "Hello from DCL!"
    $ if p1 .eqs. ""
    $ then perl -x 'f$environment("PROCEDURE")
    $ else perl -x - 'p1 'p2 'p3 'p4 'p5 'p6 'p7 'p8
    $ deck/dollars="__END__"
    #!/usr/bin/perl

    print "Hello from Perl!\n";

    __END__
    $ endif



Do take care with $ ASSIGN/nolog/user SYS$COMMAND: SYS$INPUT if your
perl-in-DCL script expects to do things like $read = <STDIN>;.



The VMS operating system has two filesystems, known as ODS-2 and ODS-5.



For ODS-2, filenames are in the format "name.extension;version".  The
maximum length for filenames is 39 characters, and the maximum length for
extensions is also 39 characters.  Version is a number from 1 to
32767.  Valid characters are /[A-Z0-9$_-]/.



The ODS-2 filesystem is case-insensitive and does not preserve case.
Perl simulates this by converting all filenames to lowercase internally.



For ODS-5, filenames may have almost any character in them and can include
Unicode characters.  Characters that could be misinterpreted by the DCL
shell or file parsing utilities need to be prefixed with the ^
character, or replaced with hexadecimal characters prefixed with the
^ character.  Such prefixing is only needed with the pathnames are
in VMS format in applications.  Programs that can accept the Unix format
of pathnames do not need the escape characters.  The maximum length for
filenames is 255 characters.  The ODS-5 file system can handle both
a case preserved and a case sensitive mode.



ODS-5 is only available on the OpenVMS for 64 bit platforms.



Support for the extended file specifications is being done as optional
settings to preserve backward compatibility with Perl scripts that
assume the previous VMS limitations.



In general routines on VMS that get a Unix format file specification
should return it in a Unix format, and when they get a VMS format
specification they should return a VMS format unless they are documented
to do a conversion.



For routines that generate return a file specification, VMS allows setting
if the C library which Perl is built on if it will be returned in VMS
format or in Unix format.



With the ODS-2 file system, there is not much difference in syntax of
filenames without paths for VMS or Unix.  With the extended character
set available with ODS-5 there can be a significant difference.



Because of this, existing Perl scripts written for VMS were sometimes
treating VMS and Unix filenames interchangeably.  Without the extended
character set enabled, this behavior will mostly be maintained for
backwards compatibility.



When extended characters are enabled with ODS-5, the handling of
Unix formatted file specifications is to that of a Unix system.



VMS file specifications without extensions have a trailing dot.  An
equivalent Unix file specification should not show the trailing dot.



The result of all of this, is that for VMS, for portable scripts, you
can not depend on Perl to present the filenames in lowercase, to be
case sensitive, and that the filenames could be returned in either
Unix or VMS format.



And if a routine returns a file specification, unless it is intended to
convert it, it should return it in the same format as it found it.



readdir by default has traditionally returned lowercased filenames.
When the ODS-5 support is enabled, it will return the exact case of the
filename on the disk.



Files without extensions have a trailing period on them, so doing a
readdir in the default mode with a file named A.;5 will
return a. when VMS is (though that file could be opened with
open(FH, 'A')).



With support for extended file specifications and if opendir was
given a Unix format directory, a file named A.;5 will return a
and optionally in the exact case on the disk.  When opendir is given
a VMS format directory, then readdir should return a., and
again with the optionally the exact case.



RMS had an eight level limit on directory depths from any rooted logical
(allowing 16 levels overall) prior to VMS 7.2, and even with versions of
VMS on VAX up through 7.3.  Hence PERL_ROOT:[LIB.2.3.4.5.6.7.8] is a
valid directory specification but PERL_ROOT:[LIB.2.3.4.5.6.7.8.9] is
not.  Makefile.PL authors might have to take this into account, but at
least they can refer to the former as /PERL_ROOT/lib/2/3/4/5/6/7/8/.



Pumpkings and module integrators can easily see whether files with too many
directory levels have snuck into the core by running the following in the
top-level source directory:



 $ perl -ne "$_=~s/\s+.*//; print if scalar(split /\//) > 8;" < MANIFEST



The VMS::Filespec module, which gets installed as part of the build
process on VMS, is a pure Perl module that can easily be installed on
non-VMS platforms and can be helpful for conversions to and from RMS
native formats.  It is also now the only way that you should check to
see if VMS is in a case sensitive mode.



What \n represents depends on the type of file opened.  It usually
represents \012 but it could also be \015, \012, \015\012, 
\000, \040, or nothing depending on the file organization and 
record format.  The VMS::Stdio module provides access to the 
special fopen() requirements of files with unusual attributes on VMS.



TCP/IP stacks are optional on VMS, so socket routines might not be
implemented.  UDP sockets may not be supported.



The TCP/IP library support for all current versions of VMS is dynamically
loaded if present, so even if the routines are configured, they may
return a status indicating that they are not implemented.



The value of $^O on OpenVMS is "VMS".  To determine the architecture
that you are running on without resorting to loading all of %Config
you can examine the content of the @INC array like so:



    if (grep(/VMS_AXP/, @INC)) {
        print "I'm on Alpha!\n";

    } elsif (grep(/VMS_VAX/, @INC)) {
        print "I'm on VAX!\n";

    } elsif (grep(/VMS_IA64/, @INC)) {
        print "I'm on IA64!\n";

    } else {
        print "I'm not so sure about where $^O is...\n";
    }



In general, the significant differences should only be if Perl is running
on VMS_VAX or one of the 64 bit OpenVMS platforms.



On VMS, perl determines the UTC offset from the SYS$TIMEZONE_DIFFERENTIAL
logical name.  Although the VMS epoch began at 17-NOV-1858 00:00:00.00,
calls to localtime are adjusted to count offsets from
01-JAN-1970 00:00:00.00, just like Unix.



Also see:











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56.5.4 VOS



Perl on VOS (also known as OpenVOS) is discussed in README.vos
in the perl distribution (installed as (perlvos)).  Perl on VOS
can accept either VOS- or Unix-style file specifications as in
either of the following:



    $ perl -ne "print if /perl_setup/i" >system>notices
    $ perl -ne "print if /perl_setup/i" /system/notices



or even a mixture of both as in:



    $ perl -ne "print if /perl_setup/i" >system/notices



Even though VOS allows the slash character to appear in object
names, because the VOS port of Perl interprets it as a pathname
delimiting character, VOS files, directories, or links whose
names contain a slash character cannot be processed.  Such files
must be renamed before they can be processed by Perl.



Older releases of VOS (prior to OpenVOS Release 17.0) limit file
names to 32 or fewer characters, prohibit file names from
starting with a - character, and prohibit file names from
containing any character matching tr/ !#%&'()*;<=>?//.



Newer releases of VOS (OpenVOS Release 17.0 or later) support a
feature known as extended names.  On these releases, file names
can contain up to 255 characters, are prohibited from starting
with a - character, and the set of prohibited characters is
reduced to any character matching tr/#%*<>?//.  There are
restrictions involving spaces and apostrophes:  these characters
must not begin or end a name, nor can they immediately precede or
follow a period.  Additionally, a space must not immediately
precede another space or hyphen.  Specifically, the following
character combinations are prohibited:  space-space,
space-hyphen, period-space, space-period, period-apostrophe,
apostrophe-period, leading or trailing space, and leading or
trailing apostrophe.  Although an extended file name is limited
to 255 characters, a path name is still limited to 256
characters.



The value of $^O on VOS is "VOS".  To determine the
architecture that you are running on without resorting to loading
all of %Config you can examine the content of the @INC array
like so:



    if ($^O =~ /VOS/) {
        print "I'm on a Stratus box!\n";
    } else {
        print "I'm not on a Stratus box!\n";
        die;
    }



Also see:











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56.5.5 EBCDIC Platforms



Recent versions of Perl have been ported to platforms such as OS/400 on
AS/400 minicomputers as well as OS/390, VM/ESA, and BS2000 for S/390
Mainframes.  Such computers use EBCDIC character sets internally (usually
Character Code Set ID 0037 for OS/400 and either 1047 or POSIX-BC for S/390
systems).  On the mainframe perl currently works under the "Unix system
services for OS/390" (formerly known as OpenEdition), VM/ESA OpenEdition, or
the BS200 POSIX-BC system (BS2000 is supported in perl 5.6 and greater).
See (perlos390) for details.  Note that for OS/400 there is also a port of
Perl 5.8.1/5.9.0 or later to the PASE which is ASCII-based (as opposed to
ILE which is EBCDIC-based), see (perlos400). 



As of R2.5 of USS for OS/390 and Version 2.3 of VM/ESA these Unix
sub-systems do not support the #! shebang trick for script invocation.
Hence, on OS/390 and VM/ESA perl scripts can be executed with a header
similar to the following simple script:



    : # use perl
        eval 'exec /usr/local/bin/perl -S $0 ${1+"$@"}'
            if 0;
    #!/usr/local/bin/perl     # just a comment really

    print "Hello from perl!\n";



OS/390 will support the #! shebang trick in release 2.8 and beyond.
Calls to system and backticks can use POSIX shell syntax on all
S/390 systems.



On the AS/400, if PERL5 is in your library list, you may need
to wrap your perl scripts in a CL procedure to invoke them like so:



    BEGIN
      CALL PGM(PERL5/PERL) PARM('/QOpenSys/hello.pl')
    ENDPGM



This will invoke the perl script hello.pl in the root of the
QOpenSys file system.  On the AS/400 calls to system or backticks
must use CL syntax.



On these platforms, bear in mind that the EBCDIC character set may have
an effect on what happens with some perl functions (such as chr,
pack, print, printf, ord, sort, sprintf, unpack), as
well as bit-fiddling with ASCII constants using operators like ^, &
and |, not to mention dealing with socket interfaces to ASCII computers
(see Newlines).



Fortunately, most web servers for the mainframe will correctly
translate the \n in the following statement to its ASCII equivalent
(\r is the same under both Unix and OS/390 & VM/ESA):



    print "Content-type: text/html\r\n\r\n";



The values of $^O on some of these platforms includes:



    uname         $^O        $Config{'archname'}
    --------------------------------------------
    OS/390        os390      os390
    OS400         os400      os400
    POSIX-BC      posix-bc   BS2000-posix-bc
    VM/ESA        vmesa      vmesa



Some simple tricks for determining if you are running on an EBCDIC
platform could include any of the following (perhaps all):



    if ("\t" eq "\005")   { print "EBCDIC may be spoken here!\n"; }

    if (ord('A') == 193) { print "EBCDIC may be spoken here!\n"; }

    if (chr(169) eq 'z') { print "EBCDIC may be spoken here!\n"; }



One thing you may not want to rely on is the EBCDIC encoding
of punctuation characters since these may differ from code page to code
page (and once your module or script is rumoured to work with EBCDIC,
folks will want it to work with all EBCDIC character sets).



Also see:











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56.5.6 Acorn RISC OS



Because Acorns use ASCII with newlines (\n) in text files as \012 like
Unix, and because Unix filename emulation is turned on by default, 
most simple scripts will probably work "out of the box".  The native
filesystem is modular, and individual filesystems are free to be
case-sensitive or insensitive, and are usually case-preserving.  Some
native filesystems have name length limits, which file and directory
names are silently truncated to fit.  Scripts should be aware that the
standard filesystem currently has a name length limit of 10
characters, with up to 77 items in a directory, but other filesystems
may not impose such limitations.



Native filenames are of the form



    Filesystem#Special_Field::DiskName.$.Directory.Directory.File



where



    Special_Field is not usually present, but may contain . and $ .
    Filesystem =~ m|[A-Za-z0-9_]|
    DsicName   =~ m|[A-Za-z0-9_/]|
    $ represents the root directory
    . is the path separator
    @ is the current directory (per filesystem but machine global)
    ^ is the parent directory
    Directory and File =~ m|[^\0- "\.\$\%\&:\@\\^\|\177]+|



The default filename translation is roughly tr|/.|./|;



Note that "ADFS::HardDisk.$.File" ne 'ADFS::HardDisk.$.File' and that
the second stage of $ interpolation in regular expressions will fall
foul of the $. if scripts are not careful.



Logical paths specified by system variables containing comma-separated
search lists are also allowed; hence System:Modules is a valid
filename, and the filesystem will prefix Modules with each section of
System$Path until a name is made that points to an object on disk.
Writing to a new file System:Modules would be allowed only if
System$Path contains a single item list.  The filesystem will also
expand system variables in filenames if enclosed in angle brackets, so
<System$Dir>.Modules would look for the file
$ENV{'System$Dir'} . 'Modules'.  The obvious implication of this is
that fully qualified filenames can start with <> and should
be protected when open is used for input.



Because . was in use as a directory separator and filenames could not
be assumed to be unique after 10 characters, Acorn implemented the C
compiler to strip the trailing .c .h .s and .o suffix from
filenames specified in source code and store the respective files in
subdirectories named after the suffix.  Hence files are translated:



    foo.h           h.foo
    C:foo.h         C:h.foo        (logical path variable)
    sys/os.h        sys.h.os       (C compiler groks Unix-speak)
    10charname.c    c.10charname
    10charname.o    o.10charname
    11charname_.c   c.11charname   (assuming filesystem truncates at 10)



The Unix emulation library’s translation of filenames to native assumes
that this sort of translation is required, and it allows a user-defined list
of known suffixes that it will transpose in this fashion.  This may
seem transparent, but consider that with these rules foo/bar/baz.h
and foo/bar/h/baz both map to foo.bar.h.baz, and that readdir and
glob cannot and do not attempt to emulate the reverse mapping.  Other
.’s in filenames are translated to /.



As implied above, the environment accessed through %ENV is global, and
the convention is that program specific environment variables are of the
form Program$Name.  Each filesystem maintains a current directory,
and the current filesystem’s current directory is the global current
directory.  Consequently, sociable programs don’t change the current
directory but rely on full pathnames, and programs (and Makefiles) cannot
assume that they can spawn a child process which can change the current
directory without affecting its parent (and everyone else for that
matter).



Because native operating system filehandles are global and are currently 
allocated down from 255, with 0 being a reserved value, the Unix emulation
library emulates Unix filehandles.  Consequently, you can’t rely on
passing STDIN, STDOUT, or STDERR to your children.



The desire of users to express filenames of the form
<Foo$Dir>.Bar on the command line unquoted causes problems,
too: `` command output capture has to perform a guessing game.  It
assumes that a string <[^<>]+\$[^<>]> is a
reference to an environment variable, whereas anything else involving
< or > is redirection, and generally manages to be 99%
right.  Of course, the problem remains that scripts cannot rely on any
Unix tools being available, or that any tools found have Unix-like command
line arguments.



Extensions and XS are, in theory, buildable by anyone using free
tools.  In practice, many don’t, as users of the Acorn platform are
used to binary distributions.  MakeMaker does run, but no available
make currently copes with MakeMaker’s makefiles; even if and when
this should be fixed, the lack of a Unix-like shell will cause
problems with makefile rules, especially lines of the form cd
sdbm && make all, and anything using quoting.



"RISC OS" is the proper name for the operating system, but the value
in $^O is "riscos" (because we don’t like shouting).









Previous: perlport Acorn RISC OS, Up: perlport PLATFORMS   [Contents]





56.5.7 Other perls



Perl has been ported to many platforms that do not fit into any of
the categories listed above.  Some, such as AmigaOS, BeOS, HP MPE/iX,
QNX, Plan 9, and VOS, have been well-integrated into the standard
Perl source code kit.  You may need to see the ports/ directory
on CPAN for information, and possibly binaries, for the likes of:
aos, Atari ST, lynxos, riscos, Novell Netware, Tandem Guardian,
etc.  (Yes, we know that some of these OSes may fall under the
Unix category, but we are not a standards body.)



Some approximate operating system names and their $^O values
in the "OTHER" category include:



    OS            $^O        $Config{'archname'}
    ------------------------------------------
    Amiga DOS     amigaos    m68k-amigos
    BeOS          beos
    MPE/iX        mpeix      PA-RISC1.1



See also:











Next: , Previous: perlport PLATFORMS, Up: perlport   [Contents]





56.6 FUNCTION IMPLEMENTATIONS



Listed below are functions that are either completely unimplemented
or else have been implemented differently on various platforms.
Following each description will be, in parentheses, a list of
platforms that the description applies to.



The list may well be incomplete, or even wrong in some places.  When
in doubt, consult the platform-specific README files in the Perl
source distribution, and any other documentation resources accompanying
a given port.



Be aware, moreover, that even among Unix-ish systems there are variations.



For many functions, you can also query %Config, exported by
default from the Config module.  For example, to check whether the
platform has the lstat call, check $Config{d_lstat}.  See
(Config) for a full description of available variables.





• perlport Alphabetical Listing of Perl Functions:  












Up: perlport FUNCTION IMPLEMENTATIONS   [Contents]





56.6.1 Alphabetical Listing of Perl Functions





-X




-w only inspects the read-only file attribute (FILE_ATTRIBUTE_READONLY),
which determines whether the directory can be deleted, not whether it can
be written to. Directories always have read and write access unless denied
by discretionary access control lists (DACLs).  (Win32)



-r, -w, -x, and -o tell whether the file is accessible,
which may not reflect UIC-based file protections.  (VMS)



-s by name on an open file will return the space reserved on disk,
rather than the current extent.  -s on an open filehandle returns the
current size.  (RISC OS)



-R, -W, -X, -O are indistinguishable from -r, -w,
-x, -o. (Win32, VMS, RISC OS)



-g, -k, -l, -u, -A are not particularly meaningful.
(Win32, VMS, RISC OS)



-p is not particularly meaningful. (VMS, RISC OS)



-d is true if passed a device spec without an explicit directory.
(VMS)



-x (or -X) determine if a file ends in one of the executable
suffixes.  -S is meaningless.  (Win32)



-x (or -X) determine if a file has an executable file type.
(RISC OS)





alarm




Emulated using timers that must be explicitly polled whenever Perl
wants to dispatch "safe signals" and therefore cannot interrupt
blocking system calls.  (Win32)





atan2




Due to issues with various CPUs, math libraries, compilers, and standards,
results for atan2() may vary depending on any combination of the above.
Perl attempts to conform to the Open Group/IEEE standards for the results
returned from atan2(), but cannot force the issue if the system Perl is
run on does not allow it.  (Tru64, HP-UX 10.20) 



The current version of the standards for atan2() is available at 
http://www.opengroup.org/onlinepubs/009695399/functions/atan2.html.





binmode




Meaningless.  (RISC OS)



Reopens file and restores pointer; if function fails, underlying
filehandle may be closed, or pointer may be in a different position.
(VMS)



The value returned by tell may be affected after the call, and
the filehandle may be flushed. (Win32)





chmod




Only good for changing "owner" read-write access, "group", and "other"
bits are meaningless. (Win32)



Only good for changing "owner" and "other" read-write access. (RISC OS)



Access permissions are mapped onto VOS access-control list changes. (VOS)



The actual permissions set depend on the value of the CYGWIN
in the SYSTEM environment settings.  (Cygwin)





chown




Not implemented. (Win32, Plan 9, RISC OS)



Does nothing, but won’t fail. (Win32)



A little funky, because VOS’s notion of ownership is a little funky (VOS).





chroot




Not implemented. (Win32, VMS, Plan 9, RISC OS, VOS, VM/ESA)





crypt




May not be available if library or source was not provided when building
perl. (Win32)





dbmclose




Not implemented. (VMS, Plan 9, VOS)





dbmopen




Not implemented. (VMS, Plan 9, VOS)





dump




Not useful. (RISC OS)



Not supported. (Cygwin, Win32)



Invokes VMS debugger. (VMS)





exec




Implemented via Spawn. (VM/ESA)



Does not automatically flush output handles on some platforms.
(SunOS, Solaris, HP-UX)



Not supported. (Symbian OS)





exit




Emulates Unix exit() (which considers exit 1 to indicate an error) by
mapping the 1 to SS$_ABORT (44).  This behavior may be overridden
with the pragma use vmsish 'exit'.  As with the CRTL’s exit()
function, exit 0 is also mapped to an exit status of SS$_NORMAL
(1); this mapping cannot be overridden.  Any other argument to exit()
is used directly as Perl’s exit status.  On VMS, unless the future
POSIX_EXIT mode is enabled, the exit code should always be a valid
VMS exit code and not a generic number.  When the POSIX_EXIT mode is
enabled, a generic number will be encoded in a method compatible with
the C library _POSIX_EXIT macro so that it can be decoded by other
programs, particularly ones written in C, like the GNV package.  (VMS)



exit() resets file pointers, which is a problem when called 
from a child process (created by fork()) in BEGIN.  
A workaround is to use POSIX::_exit.  (Solaris)



    exit unless $Config{archname} =~ /\bsolaris\b/;
    require POSIX and POSIX::_exit(0);





fcntl




Not implemented. (Win32)



Some functions available based on the version of VMS. (VMS)





flock




Not implemented (VMS, RISC OS, VOS).





fork




Not implemented. (AmigaOS, RISC OS, VM/ESA, VMS)



Emulated using multiple interpreters.  See perlfork NAME.  (Win32)



Does not automatically flush output handles on some platforms.
(SunOS, Solaris, HP-UX)





getlogin




Not implemented. (RISC OS)





getpgrp




Not implemented. (Win32, VMS, RISC OS)





getppid




Not implemented. (Win32, RISC OS)





getpriority




Not implemented. (Win32, VMS, RISC OS, VOS, VM/ESA)





getpwnam




Not implemented. (Win32)



Not useful. (RISC OS)





getgrnam




Not implemented. (Win32, VMS, RISC OS)





getnetbyname




Not implemented. (Win32, Plan 9)





getpwuid




Not implemented. (Win32)



Not useful. (RISC OS)





getgrgid




Not implemented. (Win32, VMS, RISC OS)





getnetbyaddr




Not implemented. (Win32, Plan 9)





getprotobynumber






getservbyport






getpwent




Not implemented. (Win32, VM/ESA)





getgrent




Not implemented. (Win32, VMS, VM/ESA)





gethostbyname




gethostbyname('localhost') does not work everywhere: you may have
to use gethostbyname('127.0.0.1'). (Irix 5)





gethostent




Not implemented. (Win32)





getnetent




Not implemented. (Win32, Plan 9)





getprotoent




Not implemented. (Win32, Plan 9)





getservent




Not implemented. (Win32, Plan 9)





sethostent




Not implemented. (Win32, Plan 9, RISC OS)





setnetent




Not implemented. (Win32, Plan 9, RISC OS)





setprotoent




Not implemented. (Win32, Plan 9, RISC OS)





setservent




Not implemented. (Plan 9, Win32, RISC OS)





endpwent




Not implemented. (MPE/iX, VM/ESA, Win32)





endgrent




Not implemented. (MPE/iX, RISC OS, VM/ESA, VMS, Win32)





endhostent




Not implemented. (Win32)





endnetent




Not implemented. (Win32, Plan 9)





endprotoent




Not implemented. (Win32, Plan 9)





endservent




Not implemented. (Plan 9, Win32)





getsockopt SOCKET,LEVEL,OPTNAME




Not implemented. (Plan 9)





glob




This operator is implemented via the File::Glob extension on most
platforms.  See (File-Glob) for portability information.





gmtime




In theory, gmtime() is reliable from -2**63 to 2**63-1.  However,
because work arounds in the implementation use floating point numbers,
it will become inaccurate as the time gets larger.  This is a bug and
will be fixed in the future.



On VOS, time values are 32-bit quantities.





ioctl FILEHANDLE,FUNCTION,SCALAR




Not implemented. (VMS)



Available only for socket handles, and it does what the ioctlsocket() call
in the Winsock API does. (Win32)



Available only for socket handles. (RISC OS)





kill




Not implemented, hence not useful for taint checking. (RISC OS)



kill() doesn’t have the semantics of raise(), i.e. it doesn’t send
a signal to the identified process like it does on Unix platforms.
Instead kill($sig, $pid) terminates the process identified by $pid,
and makes it exit immediately with exit status $sig.  As in Unix, if
$sig is 0 and the specified process exists, it returns true without
actually terminating it. (Win32)



kill(-9, $pid) will terminate the process specified by $pid and
recursively all child processes owned by it.  This is different from
the Unix semantics, where the signal will be delivered to all
processes in the same process group as the process specified by
$pid. (Win32)



Is not supported for process identification number of 0 or negative
numbers. (VMS)





link




Not implemented. (MPE/iX, RISC OS, VOS)



Link count not updated because hard links are not quite that hard
(They are sort of half-way between hard and soft links). (AmigaOS)



Hard links are implemented on Win32 under NTFS only. They are
natively supported on Windows 2000 and later.  On Windows NT they
are implemented using the Windows POSIX subsystem support and the
Perl process will need Administrator or Backup Operator privileges
to create hard links.



Available on 64 bit OpenVMS 8.2 and later.  (VMS)





localtime




localtime() has the same range as gmtime, but because time zone
rules change its accuracy for historical and future times may degrade
but usually by no more than an hour.





lstat




Not implemented. (RISC OS)



Return values (especially for device and inode) may be bogus. (Win32)





msgctl






msgget






msgsnd






msgrcv




Not implemented. (Win32, VMS, Plan 9, RISC OS, VOS)





open




open to |- and -| are unsupported. (Win32, RISC OS)



Opening a process does not automatically flush output handles on some
platforms.  (SunOS, Solaris, HP-UX)





readlink




Not implemented. (Win32, VMS, RISC OS)





rename




Can’t move directories between directories on different logical volumes. (Win32)





rewinddir




Will not cause readdir() to re-read the directory stream.  The entries
already read before the rewinddir() call will just be returned again
from a cache buffer. (Win32)





select




Only implemented on sockets. (Win32, VMS)



Only reliable on sockets. (RISC OS)



Note that the select FILEHANDLE form is generally portable.





semctl






semget






semop




Not implemented. (Win32, VMS, RISC OS)





setgrent




Not implemented. (MPE/iX, VMS, Win32, RISC OS)





setpgrp




Not implemented. (Win32, VMS, RISC OS, VOS)





setpriority




Not implemented. (Win32, VMS, RISC OS, VOS)





setpwent




Not implemented. (MPE/iX, Win32, RISC OS)





setsockopt




Not implemented. (Plan 9)





shmctl






shmget






shmread






shmwrite




Not implemented. (Win32, VMS, RISC OS, VOS)





sockatmark




A relatively recent addition to socket functions, may not
be implemented even in Unix platforms.





socketpair




Not implemented. (RISC OS, VM/ESA)



Available on OpenVOS Release 17.0 or later. (VOS)



Available on 64 bit OpenVMS 8.2 and later.  (VMS)





stat




Platforms that do not have rdev, blksize, or blocks will return these
as ”, so numeric comparison or manipulation of these fields may cause
’not numeric’ warnings.



ctime not supported on UFS (Mac OS X).



ctime is creation time instead of inode change time  (Win32).



device and inode are not meaningful.  (Win32)



device and inode are not necessarily reliable.  (VMS)



mtime, atime and ctime all return the last modification time.  Device and
inode are not necessarily reliable.  (RISC OS)



dev, rdev, blksize, and blocks are not available.  inode is not
meaningful and will differ between stat calls on the same file.  (os2)



some versions of cygwin when doing a stat("foo") and if not finding it
may then attempt to stat("foo.exe") (Cygwin)



On Win32 stat() needs to open the file to determine the link count
and update attributes that may have been changed through hard links.
Setting ${^WIN32_SLOPPY_STAT} to a true value speeds up stat() by
not performing this operation. (Win32)





symlink




Not implemented. (Win32, RISC OS)



Implemented on 64 bit VMS 8.3.  VMS requires the symbolic link to be in Unix
syntax if it is intended to resolve to a valid path.





syscall




Not implemented. (Win32, VMS, RISC OS, VOS, VM/ESA)





sysopen




The traditional "0", "1", and "2" MODEs are implemented with different
numeric values on some systems.  The flags exported by Fcntl
(O_RDONLY, O_WRONLY, O_RDWR) should work everywhere though.  (Mac OS, OS/390, VM/ESA)





system




As an optimization, may not call the command shell specified in
$ENV{PERL5SHELL}.  system(1, @args) spawns an external
process and immediately returns its process designator, without
waiting for it to terminate.  Return value may be used subsequently
in wait or waitpid.  Failure to spawn() a subprocess is indicated
by setting $? to "255 << 8".  $? is set in a way compatible with
Unix (i.e. the exitstatus of the subprocess is obtained by "$? >> 8",
as described in the documentation).  (Win32)



There is no shell to process metacharacters, and the native standard is
to pass a command line terminated by "\n" "\r" or "\0" to the spawned
program.  Redirection such as > foo is performed (if at all) by
the run time library of the spawned program.  system list will call
the Unix emulation library’s exec emulation, which attempts to provide
emulation of the stdin, stdout, stderr in force in the parent, providing
the child program uses a compatible version of the emulation library.
scalar will call the native command line direct and no such emulation
of a child Unix program will exists.  Mileage will vary.  (RISC OS)



Does not automatically flush output handles on some platforms.
(SunOS, Solaris, HP-UX)



The return value is POSIX-like (shifted up by 8 bits), which only allows
room for a made-up value derived from the severity bits of the native
32-bit condition code (unless overridden by use vmsish 'status'). 
If the native condition code is one that has a POSIX value encoded, the
POSIX value will be decoded to extract the expected exit value.
For more details see perlvms $?. (VMS)





times




"cumulative" times will be bogus.  On anything other than Windows NT
or Windows 2000, "system" time will be bogus, and "user" time is
actually the time returned by the clock() function in the C runtime
library. (Win32)



Not useful. (RISC OS)





truncate




Not implemented. (Older versions of VMS)



Truncation to same-or-shorter lengths only. (VOS)



If a FILEHANDLE is supplied, it must be writable and opened in append
mode (i.e., use open(FH, '>>filename')
or sysopen(FH,...,O_APPEND|O_RDWR).  If a filename is supplied, it
should not be held open elsewhere. (Win32)





umask




Returns undef where unavailable, as of version 5.005.



umask works but the correct permissions are set only when the file
is finally closed. (AmigaOS)





utime




Only the modification time is updated. (BeOS, VMS, RISC OS)



May not behave as expected.  Behavior depends on the C runtime
library’s implementation of utime(), and the filesystem being
used.  The FAT filesystem typically does not support an "access
time" field, and it may limit timestamps to a granularity of
two seconds. (Win32)





wait






waitpid




Can only be applied to process handles returned for processes spawned
using system(1, ...) or pseudo processes created with fork(). (Win32)



Not useful. (RISC OS)














Next: , Previous: perlport FUNCTION IMPLEMENTATIONS, Up: perlport   [Contents]





56.7 Supported Platforms



The following platforms are known to build Perl 5.12 (as of April 2010,
its release date) from the standard source code distribution available
at http://www.cpan.org/src





Linux (x86, ARM, IA64)






HP-UX






AIX






Win32






Windows 2000






Windows XP






Windows Server 2003






Windows Vista






Windows Server 2008






Windows 7











Cygwin






Solaris (x86, SPARC)






OpenVMS






Alpha (7.2 and later)






I64 (8.2 and later)











Symbian






NetBSD






FreeBSD






Debian GNU/kFreeBSD






Haiku






Irix (6.5. What else?)






OpenBSD






Dragonfly BSD






QNX Neutrino RTOS (6.5.0)






MirOS BSD




Caveats:





time_t issues that may or may not be fixed











Symbian (Series 60 v3, 3.2 and 5 - what else?)






Stratus VOS / OpenVOS






AIX















Next: , Previous: perlport Supported Platforms, Up: perlport   [Contents]





56.8 EOL Platforms (Perl 5.14)



The following platforms were supported by a previous version of
Perl but have been officially removed from Perl’s source code
as of 5.12:





Atari MiNT






Apollo Domain/OS






Apple Mac OS 8/9






Tenon Machten









The following platforms were supported up to 5.10.  They may still
have worked in 5.12, but supporting code has been removed for 5.14:





Windows 95






Windows 98






Windows ME






Windows NT4















Next: , Previous: perlport EOL Platforms (Perl 5.14), Up: perlport   [Contents]





56.9 Supported Platforms (Perl 5.8)



As of July 2002 (the Perl release 5.8.0), the following platforms were
able to build Perl from the standard source code distribution
available at http://www.cpan.org/src/



        AIX
        BeOS
        BSD/OS          (BSDi)
        Cygwin
        DG/UX
        DOS DJGPP       1)
        DYNIX/ptx
        EPOC R5
        FreeBSD
        HI-UXMPP        (Hitachi) (5.8.0 worked but we didn't know it)
        HP-UX
        IRIX
        Linux
        Mac OS Classic
        Mac OS X        (Darwin)
        MPE/iX
        NetBSD
        NetWare
        NonStop-UX
        ReliantUNIX     (formerly SINIX)
        OpenBSD
        OpenVMS         (formerly VMS)
        Open UNIX       (Unixware) (since Perl 5.8.1/5.9.0)
        OS/2
        OS/400          (using the PASE) (since Perl 5.8.1/5.9.0)
        PowerUX
        POSIX-BC        (formerly BS2000)
        QNX
        Solaris
        SunOS 4
        SUPER-UX        (NEC)
        Tru64 UNIX      (formerly DEC OSF/1, Digital UNIX)
        UNICOS
        UNICOS/mk
        UTS
        VOS
        Win95/98/ME/2K/XP 2)
        WinCE
        z/OS            (formerly OS/390)
        VM/ESA

        1) in DOS mode either the DOS or OS/2 ports can be used
        2) compilers: Borland, MinGW (GCC), VC6



The following platforms worked with the previous releases (5.6 and
5.7), but we did not manage either to fix or to test these in time
for the 5.8.0 release.  There is a very good chance that many of these
will work fine with the 5.8.0.



        BSD/OS
        DomainOS
        Hurd
        LynxOS
        MachTen
        PowerMAX
        SCO SV
        SVR4
        Unixware
        Windows 3.1



Known to be broken for 5.8.0 (but 5.6.1 and 5.7.2 can be used):



        AmigaOS



The following platforms have been known to build Perl from source in
the past (5.005_03 and earlier), but we haven’t been able to verify
their status for the current release, either because the
hardware/software platforms are rare or because we don’t have an
active champion on these platforms–or both.  They used to work,
though, so go ahead and try compiling them, and let perlbug@perl.org
of any trouble.



        3b1
        A/UX
        ConvexOS
        CX/UX
        DC/OSx
        DDE SMES
        DOS EMX
        Dynix
        EP/IX
        ESIX
        FPS
        GENIX
        Greenhills
        ISC
        MachTen 68k
        MPC
        NEWS-OS
        NextSTEP
        OpenSTEP
        Opus
        Plan 9
        RISC/os
        SCO ODT/OSR
        Stellar
        SVR2
        TI1500
        TitanOS
        Ultrix
        Unisys Dynix



The following platforms have their own source code distributions and
binaries available via http://www.cpan.org/ports/



                                Perl release

        OS/400 (ILE)            5.005_02
        Tandem Guardian         5.004



The following platforms have only binaries available via
http://www.cpan.org/ports/index.html :



                                Perl release

        Acorn RISCOS            5.005_02
        AOS                     5.002
        LynxOS                  5.004_02



Although we do suggest that you always build your own Perl from
the source code, both for maximal configurability and for security,
in case you are in a hurry you can check
http://www.cpan.org/ports/index.html for binary distributions.









Next: , Previous: perlport Supported Platforms (Perl 5.8), Up: perlport   [Contents]





56.10 SEE ALSO



(perlaix), (perlamiga), (perlbeos), (perlbs2000),
(perlce), (perlcygwin), (perldgux), (perldos), (perlepoc),
perlebcdic NAME, (perlfreebsd), (perlhurd), (perlhpux), (perlirix),
(perlmacos), (perlmacosx), (perlmpeix),
(perlnetware), (perlos2), (perlos390), (perlos400),
(perlplan9), (perlqnx), (perlsolaris), (perltru64),
perlunicode NAME, (perlvmesa), perlvms NAME, (perlvos),
(perlwin32), and (Win32).









Previous: perlport SEE ALSO, Up: perlport   [Contents]





56.11 AUTHORS / CONTRIBUTORS



Abigail <abigail@foad.org>,
Charles Bailey <bailey@newman.upenn.edu>,
Graham Barr <gbarr@pobox.com>,
Tom Christiansen <tchrist@perl.com>,
Nicholas Clark <nick@ccl4.org>,
Thomas Dorner <Thomas.Dorner@start.de>,
Andy Dougherty <doughera@lafayette.edu>,
Dominic Dunlop <domo@computer.org>,
Neale Ferguson <neale@vma.tabnsw.com.au>,
David J. Fiander <davidf@mks.com>,
Paul Green <Paul.Green@stratus.com>,
M.J.T. Guy <mjtg@cam.ac.uk>,
Jarkko Hietaniemi <jhi@iki.fi>,
Luther Huffman <lutherh@stratcom.com>,
Nick Ing-Simmons <nick@ing-simmons.net>,
Andreas J. König <a.koenig@mind.de>,
Markus Laker <mlaker@contax.co.uk>,
Andrew M. Langmead <aml@world.std.com>,
Larry Moore <ljmoore@freespace.net>,
Paul Moore <Paul.Moore@uk.origin-it.com>,
Chris Nandor <pudge@pobox.com>,
Matthias Neeracher <neeracher@mac.com>,
Philip Newton <pne@cpan.org>,
Gary Ng <71564.1743@CompuServe.COM>,
Tom Phoenix <rootbeer@teleport.com>,
André Pirard <A.Pirard@ulg.ac.be>,
Peter Prymmer <pvhp@forte.com>,
Hugo van der Sanden <hv@crypt0.demon.co.uk>,
Gurusamy Sarathy <gsar@activestate.com>,
Paul J. Schinder <schinder@pobox.com>,
Michael G Schwern <schwern@pobox.com>,
Dan Sugalski <dan@sidhe.org>,
Nathan Torkington <gnat@frii.com>,
John Malmberg <wb8tyw@qsl.net>









Next: , Previous: perlport, Up: Top   [Contents]





57 perlpragma





• perlpragma NAME:  



• perlpragma DESCRIPTION:  



• perlpragma A basic example:  



• perlpragma Key naming:  



• perlpragma Implementation details:  












Next: , Up: perlpragma   [Contents]





57.1 NAME



perlpragma - how to write a user pragma









Next: , Previous: perlpragma NAME, Up: perlpragma   [Contents]





57.2 DESCRIPTION



A pragma is a module which influences some aspect of the compile time or run
time behaviour of Perl, such as strict or warnings. With Perl 5.10 you
are no longer limited to the built in pragmata; you can now create user
pragmata that modify the behaviour of user functions within a lexical scope.









Next: , Previous: perlpragma DESCRIPTION, Up: perlpragma   [Contents]





57.3 A basic example



For example, say you need to create a class implementing overloaded
mathematical operators, and would like to provide your own pragma that
functions much like use integer; You’d like this code



    use MyMaths;
    
    my $l = MyMaths->new(1.2);
    my $r = MyMaths->new(3.4);
    
    print "A: ", $l + $r, "\n";
    
    use myint;
    print "B: ", $l + $r, "\n";
    
    {
        no myint;
        print "C: ", $l + $r, "\n";
    }
    
    print "D: ", $l + $r, "\n";
    
    no myint;
    print "E: ", $l + $r, "\n";



to give the output



    A: 4.6
    B: 4
    C: 4.6
    D: 4
    E: 4.6



i.e., where use myint; is in effect, addition operations are forced
to integer, whereas by default they are not, with the default behaviour being
restored via no myint;



The minimal implementation of the package MyMaths would be something like
this:



    package MyMaths;
    use warnings;
    use strict;
    use myint();
    use overload '+' => sub {
        my ($l, $r) = @_;
        # Pass 1 to check up one call level from here
        if (myint::in_effect(1)) {
            int($$l) + int($$r);
        } else {
            $$l + $$r;
        }
    };
    
    sub new {
        my ($class, $value) = @_;
        bless \$value, $class;
    }
    
    1;



Note how we load the user pragma myint with an empty list () to
prevent its import being called.



The interaction with the Perl compilation happens inside package myint:



    package myint;
    
    use strict;
    use warnings;
    
    sub import {
        $^H{"myint/in_effect"} = 1;
    }
    
    sub unimport {
        $^H{"myint/in_effect"} = 0;
    }
    
    sub in_effect {
        my $level = shift // 0;
        my $hinthash = (caller($level))[10];
        return $hinthash->{"myint/in_effect"};
    }
    
    1;



As pragmata are implemented as modules, like any other module, use myint;
becomes



    BEGIN {
        require myint;
        myint->import();
    }



and no myint; is



    BEGIN {
        require myint;
        myint->unimport();
    }



Hence the import and unimport routines are called at compile time
for the user’s code.



User pragmata store their state by writing to the magical hash %^H,
hence these two routines manipulate it. The state information in %^H is
stored in the optree, and can be retrieved read-only at runtime with caller(),
at index 10 of the list of returned results. In the example pragma, retrieval
is encapsulated into the routine in_effect(), which takes as parameter
the number of call frames to go up to find the value of the pragma in the
user’s script. This uses caller() to determine the value of
$^H{"myint/in_effect"} when each line of the user’s script was called, and
therefore provide the correct semantics in the subroutine implementing the
overloaded addition.









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57.4 Key naming



There is only a single %^H, but arbitrarily many modules that want
to use its scoping semantics.  To avoid stepping on each other’s toes,
they need to be sure to use different keys in the hash.  It is therefore
conventional for a module to use only keys that begin with the module’s
name (the name of its main package) and a "/" character.  After this
module-identifying prefix, the rest of the key is entirely up to the
module: it may include any characters whatsoever.  For example, a module
Foo::Bar should use keys such as Foo::Bar/baz and Foo::Bar/$%/_!.
Modules following this convention all play nicely with each other.



The Perl core uses a handful of keys in %^H which do not follow this
convention, because they predate it.  Keys that follow the convention
won’t conflict with the core’s historical keys.









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57.5 Implementation details



The optree is shared between threads.  This means there is a possibility that
the optree will outlive the particular thread (and therefore the interpreter
instance) that created it, so true Perl scalars cannot be stored in the
optree.  Instead a compact form is used, which can only store values that are
integers (signed and unsigned), strings or undef - references and
floating point values are stringified.  If you need to store multiple values
or complex structures, you should serialise them, for example with pack.
The deletion of a hash key from %^H is recorded, and as ever can be
distinguished from the existence of a key with value undef with
exists.



Don’t attempt to store references to data structures as integers which
are retrieved via caller and converted back, as this will not be threadsafe.
Accesses would be to the structure without locking (which is not safe for
Perl’s scalars), and either the structure has to leak, or it has to be
freed when its creating thread terminates, which may be before the optree
referencing it is deleted, if other threads outlive it.









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58 NAME regular expression regex regexp





• NAME regular expression regex regexp NAME:  



• NAME regular expression regex regexp DESCRIPTION:  



• NAME regular expression regex regexp BUGS:  



• NAME regular expression regex regexp SEE ALSO:  












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58.1 NAME






perlre - Perl regular expressions









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58.2 DESCRIPTION



This page describes the syntax of regular expressions in Perl.



If you haven’t used regular expressions before, a quick-start
introduction is available in perlrequick NAME, and a longer tutorial
introduction is available in perlretut NAME.



For reference on how regular expressions are used in matching
operations, plus various examples of the same, see discussions of
m//, s///, qr// and ?? in (perlop)Regexp Quote-Like
Operators.





• NAME regular expression regex regexp Modifiers:  



• NAME regular expression regex regexp Regular Expressions:  



• NAME regular expression regex regexp Quoting metacharacters:  



• NAME regular expression regex regexp Extended Patterns:  



• NAME regular expression regex regexp Special Backtracking Control Verbs:  



• NAME regular expression regex regexp Backtracking:  



• NAME regular expression regex regexp Version 8 Regular Expressions:  



• NAME regular expression regex regexp Warning on \1 Instead of $1:  



• NAME regular expression regex regexp Repeated Patterns Matching a Zero-length Substring:  



• NAME regular expression regex regexp Combining RE Pieces:  



• NAME regular expression regex regexp Creating Custom RE Engines:  



• NAME regular expression regex regexp PCRE/Python Support:  












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58.2.1 Modifiers



Matching operations can have various modifiers.  Modifiers
that relate to the interpretation of the regular expression inside
are listed below.  Modifiers that alter the way a regular expression
is used by Perl are detailed in (perlop)Regexp Quote-Like Operators and
(perlop)Gory details of parsing quoted constructs.





m








Treat string as multiple lines.  That is, change "^" and "$" from matching
the start or end of the string to matching the start or end of any
line anywhere within the string.





s








Treat string as single line.  That is, change "." to match any character
whatsoever, even a newline, which normally it would not match.



Used together, as /ms, they let the "." match any character whatsoever,
while still allowing "^" and "$" to match, respectively, just after
and just before newlines within the string.





i








Do case-insensitive pattern matching.



If locale matching rules are in effect, the case map is taken from the
current
locale for code points less than 255, and from Unicode rules for larger
code points.  However, matches that would cross the Unicode
rules/non-Unicode rules boundary (ords 255/256) will not succeed.  See
perllocale NAME.



There are a number of Unicode characters that match multiple characters
under /i.  For example, LATIN SMALL LIGATURE FI
should match the sequence fi.  Perl is not
currently able to do this when the multiple characters are in the pattern and
are split between groupings, or when one or more are quantified.  Thus



 "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i;          # Matches
 "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i;    # Doesn't match!
 "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i;         # Doesn't match!

 # The below doesn't match, and it isn't clear what $1 and $2 would
 # be even if it did!!
 "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i;      # Doesn't match!



Perl doesn’t match multiple characters in an inverted bracketed
character class, which otherwise could be highly confusing.  See
(perlrecharclass)Negation.



Another bug involves character classes that match both a sequence of
multiple characters, and an initial sub-string of that sequence.  For
example,



 /[s\xDF]/i



should match both a single and a double "s", since \xDF (on ASCII
platforms) matches "ss".  However, this bug
([perl #89774])
causes it to only match a single "s", even if the final larger match
fails, and matching the double "ss" would have succeeded.



Also, Perl matching doesn’t fully conform to the current Unicode /i
recommendations, which ask that the matching be made upon the NFD
(Normalization Form Decomposed) of the text.  However, Unicode is
in the process of reconsidering and revising their recommendations.





x





Extend your pattern’s legibility by permitting whitespace and comments.
Details in /x





p







Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and
${^POSTMATCH} are available for use after matching.





g and c






Global matching, and keep the Current position after failed matching.
Unlike i, m, s and x, these two flags affect the way the regex is used
rather than the regex itself. See
perlretut Using regular expressions in Perl for further explanation
of the g and c modifiers.





a, d, l and u








These modifiers, all new in 5.14, affect which character-set semantics
(Unicode, etc.) are used, as described below in
Character set modifiers.








Regular expression modifiers are usually written in documentation
as e.g., "the /x modifier", even though the delimiter
in question might not really be a slash.  The modifiers /imsxadlup
may also be embedded within the regular expression itself using
the (?...) construct, see Extended Patterns below.





• NAME regular expression regex regexp /x:  



• NAME regular expression regex regexp Character set modifiers:  



• NAME regular expression regex regexp /l:  



• NAME regular expression regex regexp /u:  



• NAME regular expression regex regexp /d:  



• NAME regular expression regex regexp /a (and /aa):  



• NAME regular expression regex regexp Which character set modifier is in effect?:  



• NAME regular expression regex regexp Character set modifier behavior prior to Perl 5.14:  












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58.2.1.1 /x




/x tells
the regular expression parser to ignore most whitespace that is neither
backslashed nor within a character class.  You can use this to break up
your regular expression into (slightly) more readable parts.  The #
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code.  This also means that if you want real
whitespace or # characters in the pattern (outside a character
class, where they are unaffected by /x), then you’ll either have to
escape them (using backslashes or \Q...\E) or encode them using octal,
hex, or \N{} escapes.  Taken together, these features go a long way towards
making Perl’s regular expressions more readable.  Note that you have to
be careful not to include the pattern delimiter in the comment–perl has
no way of knowing you did not intend to close the pattern early.  See
the C-comment deletion code in (perlop).  Also note that anything inside
a \Q...\E stays unaffected by /x.  And note that /x doesn’t affect
space interpretation within a single multi-character construct.  For
example in \x{...}, regardless of the /x modifier, there can be no
spaces.  Same for a quantifier such as {3} or
{5,}.  Similarly, (?:...) can’t have a space between the ? and :,
but can between the ( and ?.  Within any delimiters for such a
construct, allowed spaces are not affected by /x, and depend on the
construct.  For example, \x{...} can’t have spaces because hexadecimal
numbers don’t have spaces in them.  But, Unicode properties can have spaces, so
in \p{...} there can be spaces that follow the Unicode rules, for which see
(perluniprops)Properties accessible through \p{} and \P{}.










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58.2.1.2 Character set modifiers



/d, /u, /a, and /l, available starting in 5.14, are called
the character set modifiers; they affect the character set semantics
used for the regular expression.



The /d, /u, and /l modifiers are not likely to be of much use
to you, and so you need not worry about them very much.  They exist for
Perl’s internal use, so that complex regular expression data structures
can be automatically serialized and later exactly reconstituted,
including all their nuances.  But, since Perl can’t keep a secret, and
there may be rare instances where they are useful, they are documented
here.



The /a modifier, on the other hand, may be useful.  Its purpose is to
allow code that is to work mostly on ASCII data to not have to concern
itself with Unicode.



Briefly, /l sets the character set to that of whatever Locale is in
effect at the time of the execution of the pattern match.



/u sets the character set to Unicode.



/a also sets the character set to Unicode, BUT adds several
restrictions for ASCII-safe matching.



/d is the old, problematic, pre-5.14 Default character set
behavior.  Its only use is to force that old behavior.



At any given time, exactly one of these modifiers is in effect.  Their
existence allows Perl to keep the originally compiled behavior of a
regular expression, regardless of what rules are in effect when it is
actually executed.  And if it is interpolated into a larger regex, the
original’s rules continue to apply to it, and only it.



The /l and /u modifiers are automatically selected for
regular expressions compiled within the scope of various pragmas,
and we recommend that in general, you use those pragmas instead of
specifying these modifiers explicitly.  For one thing, the modifiers
affect only pattern matching, and do not extend to even any replacement
done, whereas using the pragmas give consistent results for all
appropriate operations within their scopes.  For example,



 s/foo/\Ubar/il



will match "foo" using the locale’s rules for case-insensitive matching,
but the /l does not affect how the \U operates.  Most likely you
want both of them to use locale rules.  To do this, instead compile the
regular expression within the scope of use locale.  This both
implicitly adds the /l and applies locale rules to the \U.   The
lesson is to use locale and not /l explicitly.



Similarly, it would be better to use use feature 'unicode_strings'
instead of,



 s/foo/\Lbar/iu



to get Unicode rules, as the \L in the former (but not necessarily
the latter) would also use Unicode rules.



More detail on each of the modifiers follows.  Most likely you don’t
need to know this detail for /l, /u, and /d, and can skip ahead
to /a.









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58.2.1.3 /l



means to use the current locale’s rules (see perllocale NAME) when pattern
matching.  For example, \w will match the "word" characters of that
locale, and "/i" case-insensitive matching will match according to
the locale’s case folding rules.  The locale used will be the one in
effect at the time of execution of the pattern match.  This may not be
the same as the compilation-time locale, and can differ from one match
to another if there is an intervening call of the
setlocale() function.



Perl only supports single-byte locales.  This means that code points
above 255 are treated as Unicode no matter what locale is in effect.
Under Unicode rules, there are a few case-insensitive matches that cross
the 255/256 boundary.  These are disallowed under /l.  For example,
0xFF (on ASCII platforms) does not caselessly match the character at
0x178, LATIN CAPITAL LETTER Y WITH DIAERESIS, because 0xFF may not be
LATIN SMALL LETTER Y WITH DIAERESIS in the current locale, and Perl
has no way of knowing if that character even exists in the locale, much
less what code point it is.




This modifier may be specified to be the default by use locale, but
see Which character set modifier is in effect?.










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58.2.1.4 /u



means to use Unicode rules when pattern matching.  On ASCII platforms,
this means that the code points between 128 and 255 take on their
Latin-1 (ISO-8859-1) meanings (which are the same as Unicode’s).
(Otherwise Perl considers their meanings to be undefined.)  Thus,
under this modifier, the ASCII platform effectively becomes a Unicode
platform; and hence, for example, \w will match any of the more than
100_000 word characters in Unicode.



Unlike most locales, which are specific to a language and country pair,
Unicode classifies all the characters that are letters somewhere in
the world as
\w.  For example, your locale might not think that LATIN SMALL
LETTER ETH is a letter (unless you happen to speak Icelandic), but
Unicode does.  Similarly, all the characters that are decimal digits
somewhere in the world will match \d; this is hundreds, not 10,
possible matches.  And some of those digits look like some of the 10
ASCII digits, but mean a different number, so a human could easily think
a number is a different quantity than it really is.  For example,
BENGALI DIGIT FOUR (U+09EA) looks very much like an
ASCII DIGIT EIGHT (U+0038).  And, \d+, may match strings of digits
that are a mixture from different writing systems, creating a security
issue.  (Unicode-UCD)num() can be used to sort
this out.  Or the /a modifier can be used to force \d to match
just the ASCII 0 through 9.



Also, under this modifier, case-insensitive matching works on the full
set of Unicode
characters.  The KELVIN SIGN, for example matches the letters "k" and
"K"; and LATIN SMALL LIGATURE FF matches the sequence "ff", which,
if you’re not prepared, might make it look like a hexadecimal constant,
presenting another potential security issue.  See
http://unicode.org/reports/tr36 for a detailed discussion of Unicode
security issues.



On the EBCDIC platforms that Perl handles, the native character set is
equivalent to Latin-1.  Thus this modifier changes behavior only when
the "/i" modifier is also specified, and it turns out it affects only
two characters, giving them full Unicode semantics: the MICRO SIGN
will match the Greek capital and small letters MU, otherwise not; and
the LATIN CAPITAL LETTER SHARP S will match any of SS, Ss,
sS, and ss, otherwise not.




This modifier may be specified to be the default by use feature
'unicode_strings, use locale ':not_characters', or
(perlfunc)use 5.012 (or higher),
but see Which character set modifier is in effect?.










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58.2.1.5 /d



This modifier means to use the "Default" native rules of the platform
except when there is cause to use Unicode rules instead, as follows:



    

  1.  the target string is encoded in UTF-8; or


  2.  the pattern is encoded in UTF-8; or


  3.  the pattern explicitly mentions a code point that is above 255 (say by
\x{100}); or


  4.  the pattern uses a Unicode name (\N{...});  or


  5.  the pattern uses a Unicode property (\p{...})





Another mnemonic for this modifier is "Depends", as the rules actually
used depend on various things, and as a result you can get unexpected
results.  See perlunicode The "Unicode Bug".  The Unicode Bug has
become rather infamous, leading to yet another (printable) name for this
modifier, "Dodgy".



On ASCII platforms, the native rules are ASCII, and on EBCDIC platforms
(at least the ones that Perl handles), they are Latin-1.



Here are some examples of how that works on an ASCII platform:



 $str =  "\xDF";      # $str is not in UTF-8 format.
 $str =~ /^\w/;       # No match, as $str isn't in UTF-8 format.
 $str .= "\x{0e0b}";  # Now $str is in UTF-8 format.
 $str =~ /^\w/;       # Match! $str is now in UTF-8 format.
 chop $str;
 $str =~ /^\w/;       # Still a match! $str remains in UTF-8 format.



This modifier is automatically selected by default when none of the
others are, so yet another name for it is "Default".



Because of the unexpected behaviors associated with this modifier, you
probably should only use it to maintain weird backward compatibilities.









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58.2.1.6 /a (and /aa)



This modifier stands for ASCII-restrict (or ASCII-safe).  This modifier,
unlike the others, may be doubled-up to increase its effect.



When it appears singly, it causes the sequences \d, \s, \w, and
the Posix character classes to match only in the ASCII range.  They thus
revert to their pre-5.6, pre-Unicode meanings.  Under /a,  \d
always means precisely the digits "0" to "9"; \s means the five
characters [ \f\n\r\t]; \w means the 63 characters
[A-Za-z0-9_]; and likewise, all the Posix classes such as
[[:print:]] match only the appropriate ASCII-range characters.



This modifier is useful for people who only incidentally use Unicode,
and who do not wish to be burdened with its complexities and security
concerns.



With /a, one can write \d with confidence that it will only match
ASCII characters, and should the need arise to match beyond ASCII, you
can instead use \p{Digit} (or \p{Word} for \w).  There are
similar \p{...} constructs that can match beyond ASCII both white
space (see (perlrecharclass)Whitespace), and Posix classes (see
(perlrecharclass)POSIX Character Classes).  Thus, this modifier
doesn’t mean you can’t use Unicode, it means that to get Unicode
matching you must explicitly use a construct (\p{}, \P{}) that
signals Unicode.



As you would expect, this modifier causes, for example, \D to mean
the same thing as [^0-9]; in fact, all non-ASCII characters match
\D, \S, and \W.  \b still means to match at the boundary
between \w and \W, using the /a definitions of them (similarly
for \B).



Otherwise, /a behaves like the /u modifier, in that
case-insensitive matching uses Unicode semantics; for example, "k" will
match the Unicode \N{KELVIN SIGN} under /i matching, and code
points in the Latin1 range, above ASCII will have Unicode rules when it
comes to case-insensitive matching.



To forbid ASCII/non-ASCII matches (like "k" with \N{KELVIN SIGN}),
specify the "a" twice, for example /aai or /aia.  (The first
occurrence of "a" restricts the \d, etc., and the second occurrence
adds the /i restrictions.)  But, note that code points outside the
ASCII range will use Unicode rules for /i matching, so the modifier
doesn’t really restrict things to just ASCII; it just forbids the
intermixing of ASCII and non-ASCII.



To summarize, this modifier provides protection for applications that
don’t wish to be exposed to all of Unicode.  Specifying it twice
gives added protection.





This modifier may be specified to be the default by use re '/a'
or use re '/aa'.  If you do so, you may actually have occasion to use
the /u modifier explictly if there are a few regular expressions
where you do want full Unicode rules (but even here, it’s best if
everything were under feature "unicode_strings", along with the
use re '/aa').  Also see Which character set modifier is in
effect?.










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58.2.1.7 Which character set modifier is in effect?



Which of these modifiers is in effect at any given point in a regular
expression depends on a fairly complex set of interactions.  These have
been designed so that in general you don’t have to worry about it, but
this section gives the gory details.  As
explained below in Extended Patterns it is possible to explicitly
specify modifiers that apply only to portions of a regular expression.
The innermost always has priority over any outer ones, and one applying
to the whole expression has priority over any of the default settings that are
described in the remainder of this section.



The (re)use re '/foo' pragma can be used to set
default modifiers (including these) for regular expressions compiled
within its scope.  This pragma has precedence over the other pragmas
listed below that also change the defaults.



Otherwise, use locale sets the default modifier to /l;
and (feature)use feature 'unicode_strings, or
(perlfunc)use 5.012 (or higher) set the default to
/u when not in the same scope as either use locale
or (bytes)use bytes.
(use locale ':not_characters' also
sets the default to /u, overriding any plain use locale.)
Unlike the mechanisms mentioned above, these
affect operations besides regular expressions pattern matching, and so
give more consistent results with other operators, including using
\U, \l, etc. in substitution replacements.



If none of the above apply, for backwards compatibility reasons, the
/d modifier is the one in effect by default.  As this can lead to
unexpected results, it is best to specify which other rule set should be
used.









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58.2.1.8 Character set modifier behavior prior to Perl 5.14



Prior to 5.14, there were no explicit modifiers, but /l was implied
for regexes compiled within the scope of use locale, and /d was
implied otherwise.  However, interpolating a regex into a larger regex
would ignore the original compilation in favor of whatever was in effect
at the time of the second compilation.  There were a number of
inconsistencies (bugs) with the /d modifier, where Unicode rules
would be used when inappropriate, and vice versa.  \p{} did not imply
Unicode rules, and neither did all occurrences of \N{}, until 5.12.









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58.2.2 Regular Expressions





• NAME regular expression regex regexp Metacharacters:  



• NAME regular expression regex regexp Quantifiers:  



• NAME regular expression regex regexp Escape sequences:  



• NAME regular expression regex regexp Character Classes and other Special Escapes:  



• NAME regular expression regex regexp Assertions:  



• NAME regular expression regex regexp Capture groups:  












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58.2.2.1 Metacharacters



The patterns used in Perl pattern matching evolved from those supplied in
the Version 8 regex routines.  (The routines are derived
(distantly) from Henry Spencer’s freely redistributable reimplementation
of the V8 routines.)  See Version 8 Regular Expressions for
details.













In particular the following metacharacters have their standard egrep-ish
meanings:




    \        Quote the next metacharacter
    ^        Match the beginning of the line
    .        Match any character (except newline)
    $        Match the end of the line (or before newline at the end)
    |        Alternation
    ()       Grouping
    []       Bracketed Character class






By default, the "^" character is guaranteed to match only the
beginning of the string, the "$" character only the end (or before the
newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line.  Embedded newlines
will not be matched by "^" or "$".  You may, however, wish to treat a
string as a multi-line buffer, such that the "^" will match after any
newline within the string (except if the newline is the last character in
the string), and "$" will match before any newline.  At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator.  (Older programs did this by setting $*,
but this option was removed in perl 5.9.)






To simplify multi-line substitutions, the "." character never matches a
newline unless you use the /s modifier, which in effect tells Perl to pretend
the string is a single line–even if it isn’t.










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58.2.2.2 Quantifiers











The following standard quantifiers are recognized:




    *           Match 0 or more times
    +           Match 1 or more times
    ?           Match 1 or 0 times
    {n}         Match exactly n times
    {n,}        Match at least n times
    {n,m}       Match at least n but not more than m times



(If a curly bracket occurs in any other context and does not form part of
a backslashed sequence like \x{...}, it is treated
as a regular character.  In particular, the lower quantifier bound
is not optional.  However, in Perl v5.18, it is planned to issue a
deprecation warning for all such occurrences, and in Perl v5.20 to
require literal uses of a curly bracket to be escaped, say by preceding
them with a backslash or enclosing them within square brackets, ("\{"
or "[{]").  This change will allow for future syntax extensions (like
making the lower bound of a quantifier optional), and better error
checking of quantifiers.  Now, a typo in a quantifier silently causes
it to be treated as the literal characters.  For example,



    /o{4,3}/



looks like a quantifier that matches 0 times, since 4 is greater than 3,
but it really means to match the sequence of six characters
"o { 4 , 3 }".)



The "*" quantifier is equivalent to {0,}, the "+"
quantifier to {1,}, and the "?" quantifier to {0,1}.  n and m are limited
to non-negative integral values less than a preset limit defined when perl is built.
This is usually 32766 on the most common platforms.  The actual limit can
be seen in the error message generated by code such as this:



    $_ **= $_ , / {$_} / for 2 .. 42;













By default, a quantified subpattern is "greedy", that is, it will match as
many times as possible (given a particular starting location) while still
allowing the rest of the pattern to match.  If you want it to match the
minimum number of times possible, follow the quantifier with a "?".  Note
that the meanings don’t change, just the "greediness":




    *?        Match 0 or more times, not greedily
    +?        Match 1 or more times, not greedily
    ??        Match 0 or 1 time, not greedily
    {n}?      Match exactly n times, not greedily (redundant)
    {n,}?     Match at least n times, not greedily
    {n,m}?    Match at least n but not more than m times, not greedily



By default, when a quantified subpattern does not allow the rest of the
overall pattern to match, Perl will backtrack. However, this behaviour is
sometimes undesirable. Thus Perl provides the "possessive" quantifier form
as well.



 *+     Match 0 or more times and give nothing back
 ++     Match 1 or more times and give nothing back
 ?+     Match 0 or 1 time and give nothing back
 {n}+   Match exactly n times and give nothing back (redundant)
 {n,}+  Match at least n times and give nothing back
 {n,m}+ Match at least n but not more than m times and give nothing back



For instance,



   'aaaa' =~ /a++a/



will never match, as the a++ will gobble up all the a’s in the
string and won’t leave any for the remaining part of the pattern. This
feature can be extremely useful to give perl hints about where it
shouldn’t backtrack. For instance, the typical "match a double-quoted
string" problem can be most efficiently performed when written as:



   /"(?:[^"\\]++|\\.)*+"/



as we know that if the final quote does not match, backtracking will not
help. See the independent subexpression
(?>pattern) for more details;
possessive quantifiers are just syntactic sugar for that construct. For
instance the above example could also be written as follows:



   /"(?>(?:(?>[^"\\]+)|\\.)*)"/









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58.2.2.3 Escape sequences



Because patterns are processed as double-quoted strings, the following
also work:



 \t          tab                   (HT, TAB)
 \n          newline               (LF, NL)
 \r          return                (CR)
 \f          form feed             (FF)
 \a          alarm (bell)          (BEL)
 \e          escape (think troff)  (ESC)
 \cK         control char          (example: VT)
 \x{}, \x00  character whose ordinal is the given hexadecimal number
 \N{name}    named Unicode character or character sequence
 \N{U+263D}  Unicode character     (example: FIRST QUARTER MOON)
 \o{}, \000  character whose ordinal is the given octal number
 \l          lowercase next char (think vi)
 \u          uppercase next char (think vi)
 \L          lowercase till \E (think vi)
 \U          uppercase till \E (think vi)
 \Q          quote (disable) pattern metacharacters till \E
 \E          end either case modification or quoted section, think vi



Details are in (perlop)Quote and Quote-like Operators.









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58.2.2.4 Character Classes and other Special Escapes







In addition, Perl defines the following:




 Sequence   Note    Description
  [...]     [1]  Match a character according to the rules of the
                   bracketed character class defined by the "...".
                   Example: [a-z] matches "a" or "b" or "c" ... or "z"
  [[:...:]] [2]  Match a character according to the rules of the POSIX
                   character class "..." within the outer bracketed
                   character class.  Example: [[:upper:]] matches any
                   uppercase character.
  \w        [3]  Match a "word" character (alphanumeric plus "_", plus
                   other connector punctuation chars plus Unicode
                   marks)
  \W        [3]  Match a non-"word" character
  \s        [3]  Match a whitespace character
  \S        [3]  Match a non-whitespace character
  \d        [3]  Match a decimal digit character
  \D        [3]  Match a non-digit character
  \pP       [3]  Match P, named property.  Use \p{Prop} for longer names
  \PP       [3]  Match non-P
  \X        [4]  Match Unicode "eXtended grapheme cluster"
  \C             Match a single C-language char (octet) even if that is
                   part of a larger UTF-8 character.  Thus it breaks up
                   characters into their UTF-8 bytes, so you may end up
                   with malformed pieces of UTF-8.  Unsupported in
                   lookbehind.
  \1        [5]  Backreference to a specific capture group or buffer.
                   '1' may actually be any positive integer.
  \g1       [5]  Backreference to a specific or previous group,
  \g{-1}    [5]  The number may be negative indicating a relative
                   previous group and may optionally be wrapped in
                   curly brackets for safer parsing.
  \g{name}  [5]  Named backreference
  \k<name>  [5]  Named backreference
  \K        [6]  Keep the stuff left of the \K, don't include it in $&
  \N        [7]  Any character but \n (experimental).  Not affected by
                   /s modifier
  \v        [3]  Vertical whitespace
  \V        [3]  Not vertical whitespace
  \h        [3]  Horizontal whitespace
  \H        [3]  Not horizontal whitespace
  \R        [4]  Linebreak





[1]




See (perlrecharclass)Bracketed Character Classes for details.





[2]




See (perlrecharclass)POSIX Character Classes for details.





[3]




See (perlrecharclass)Backslash sequences for details.





[4]




See perlrebackslash Misc for details.





[5]




See Capture groups below for details.





[6]




See Extended Patterns below for details.





[7]




Note that \N has two meanings.  When of the form \N{NAME}, it matches the
character or character sequence whose name is NAME; and similarly
when of the form \N{U+hex}, it matches the character whose Unicode
code point is hex.  Otherwise it matches any character but \n.














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58.2.2.5 Assertions














Perl defines the following zero-width assertions:




    \b  Match a word boundary
    \B  Match except at a word boundary
    \A  Match only at beginning of string
    \Z  Match only at end of string, or before newline at the end
    \z  Match only at end of string
    \G  Match only at pos() (e.g. at the end-of-match position
        of prior m//g)








A word boundary (\b) is a spot between two characters
that has a \w on one side of it and a \W on the other side
of it (in either order), counting the imaginary characters off the
beginning and end of the string as matching a \W.  (Within
character classes \b represents backspace rather than a word
boundary, just as it normally does in any double-quoted string.)
The \A and \Z are just like "^" and "$", except that they
won’t match multiple times when the /m modifier is used, while
"^" and "$" will match at every internal line boundary.  To match
the actual end of the string and not ignore an optional trailing
newline, use \z.





The \G assertion can be used to chain global matches (using
m//g), as described in (perlop)Regexp Quote-Like Operators.
It is also useful when writing lex-like scanners, when you have
several patterns that you want to match against consequent substrings
of your string; see the previous reference.  The actual location
where \G will match can also be influenced by using pos() as
an lvalue: see (perlfunc)pos. Note that the rule for zero-length
matches (see Repeated Patterns Matching a Zero-length Substring)
is modified somewhat, in that contents to the left of \G are
not counted when determining the length of the match. Thus the following
will not match forever:




     my $string = 'ABC';
     pos($string) = 1;
     while ($string =~ /(.\G)/g) {
         print $1;
     }



It will print ’A’ and then terminate, as it considers the match to
be zero-width, and thus will not match at the same position twice in a
row.



It is worth noting that \G improperly used can result in an infinite
loop. Take care when using patterns that include \G in an alternation.









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58.2.2.6 Capture groups























The bracketing construct ( ... ) creates capture groups (also referred to as
capture buffers). To refer to the current contents of a group later on, within
the same pattern, use \g1 (or \g{1}) for the first, \g2 (or \g{2})
for the second, and so on.
This is called a backreference.
There is no limit to the number of captured substrings that you may use.
Groups are numbered with the leftmost open parenthesis being number 1, etc.  If
a group did not match, the associated backreference won’t match either. (This
can happen if the group is optional, or in a different branch of an
alternation.)
You can omit the "g", and write "\1", etc, but there are some issues with
this form, described below.



You can also refer to capture groups relatively, by using a negative number, so
that \g-1 and \g{-1} both refer to the immediately preceding capture
group, and \g-2 and \g{-2} both refer to the group before it.  For
example:



        /
         (Y)            # group 1
         (              # group 2
            (X)         # group 3
            \g{-1}      # backref to group 3
            \g{-3}      # backref to group 1
         )
        /x



would match the same as /(Y) ( (X) \g3 \g1 )/x.  This allows you to
interpolate regexes into larger regexes and not have to worry about the
capture groups being renumbered.



You can dispense with numbers altogether and create named capture groups.
The notation is (?<name>...) to declare and \g{name} to
reference.  (To be compatible with .Net regular expressions, \g{name} may
also be written as \k{name}, \k<name> or \k'name'.)
name must not begin with a number, nor contain hyphens.
When different groups within the same pattern have the same name, any reference
to that name assumes the leftmost defined group.  Named groups count in
absolute and relative numbering, and so can also be referred to by those
numbers.
(It’s possible to do things with named capture groups that would otherwise
require (??{}).)



Capture group contents are dynamically scoped and available to you outside the
pattern until the end of the enclosing block or until the next successful
match, whichever comes first.  (See (perlsyn)Compound Statements.)
You can refer to them by absolute number (using "$1" instead of "\g1",
etc); or by name via the %+ hash, using "$+{name}".



Braces are required in referring to named capture groups, but are optional for
absolute or relative numbered ones.  Braces are safer when creating a regex by
concatenating smaller strings.  For example if you have qr/$a$b/, and $a
contained "\g1", and $b contained "37", you would get /\g137/ which
is probably not what you intended.



The \g and \k notations were introduced in Perl 5.10.0.  Prior to that
there were no named nor relative numbered capture groups.  Absolute numbered
groups were referred to using \1,
\2, etc., and this notation is still
accepted (and likely always will be).  But it leads to some ambiguities if
there are more than 9 capture groups, as \10 could mean either the tenth
capture group, or the character whose ordinal in octal is 010 (a backspace in
ASCII).  Perl resolves this ambiguity by interpreting \10 as a backreference
only if at least 10 left parentheses have opened before it.  Likewise \11 is
a backreference only if at least 11 left parentheses have opened before it.
And so on.  \1 through \9 are always interpreted as backreferences.
There are several examples below that illustrate these perils.  You can avoid
the ambiguity by always using \g{} or \g if you mean capturing groups;
and for octal constants always using \o{}, or for \077 and below, using 3
digits padded with leading zeros, since a leading zero implies an octal
constant.



The \digit notation also works in certain circumstances outside
the pattern.  See Warning on \1 Instead of $1 below for details.



Examples:



    s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

    /(.)\g1/                        # find first doubled char
         and print "'$1' is the first doubled character\n";

    /(?<char>.)\k<char>/            # ... a different way
         and print "'$+{char}' is the first doubled character\n";

    /(?'char'.)\g1/                 # ... mix and match
         and print "'$1' is the first doubled character\n";

    if (/Time: (..):(..):(..)/) {   # parse out values
        $hours = $1;
        $minutes = $2;
        $seconds = $3;
    }

    /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/   # \g10 is a backreference
    /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/    # \10 is octal
    /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/  # \10 is a backreference
    /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal

    $a = '(.)\1';        # Creates problems when concatenated.
    $b = '(.)\g{1}';     # Avoids the problems.
    "aa" =~ /${a}/;      # True
    "aa" =~ /${b}/;      # True
    "aa0" =~ /${a}0/;    # False!
    "aa0" =~ /${b}0/;    # True
    "aa\x08" =~ /${a}0/;  # True!
    "aa\x08" =~ /${b}0/;  # False








Several special variables also refer back to portions of the previous
match.  $+ returns whatever the last bracket match matched.
$& returns the entire matched string.  (At one point $0 did
also, but now it returns the name of the program.)  $` returns
everything before the matched string.  $' returns everything
after the matched string. And $^N contains whatever was matched by
the most-recently closed group (submatch). $^N can be used in
extended patterns (see below), for example to assign a submatch to a
variable.


















These special variables, like the %+ hash and the numbered match variables
($1, $2, $3, etc.) are dynamically scoped
until the end of the enclosing block or until the next successful
match, whichever comes first.  (See (perlsyn)Compound Statements.)




NOTE: Failed matches in Perl do not reset the match variables,
which makes it easier to write code that tests for a series of more
specific cases and remembers the best match.






WARNING: Once Perl sees that you need one of $&, $`, or
$' anywhere in the program, it has to provide them for every
pattern match.  This may substantially slow your program.  Perl
uses the same mechanism to produce $1, $2, etc, so you also pay a
price for each pattern that contains capturing parentheses.  (To
avoid this cost while retaining the grouping behaviour, use the
extended regular expression (?: ... ) instead.)  But if you never
use $&, $` or $', then patterns without capturing
parentheses will not be penalized.  So avoid $&, $', and $`
if you can, but if you can’t (and some algorithms really appreciate
them), once you’ve used them once, use them at will, because you’ve
already paid the price.  As of 5.005, $& is not so costly as the
other two.






As a workaround for this problem, Perl 5.10.0 introduces ${^PREMATCH},
${^MATCH} and ${^POSTMATCH}, which are equivalent to $`, $&
and $', except that they are only guaranteed to be defined after a
successful match that was executed with the /p (preserve) modifier.
The use of these variables incurs no global performance penalty, unlike
their punctuation char equivalents, however at the trade-off that you
have to tell perl when you want to use them.










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58.2.3 Quoting metacharacters



Backslashed metacharacters in Perl are alphanumeric, such as \b,
\w, \n.  Unlike some other regular expression languages, there
are no backslashed symbols that aren’t alphanumeric.  So anything
that looks like \\, \(, \), \<, \>, \{, or \} is always
interpreted as a literal character, not a metacharacter.  This was
once used in a common idiom to disable or quote the special meanings
of regular expression metacharacters in a string that you want to
use for a pattern. Simply quote all non-"word" characters:



    $pattern =~ s/(\W)/\\$1/g;



(If use locale is set, then this depends on the current locale.)
Today it is more common to use the quotemeta() function or the \Q
metaquoting escape sequence to disable all metacharacters’ special
meanings like this:



    /$unquoted\Q$quoted\E$unquoted/



Beware that if you put literal backslashes (those not inside
interpolated variables) between \Q and \E, double-quotish
backslash interpolation may lead to confusing results.  If you
need to use literal backslashes within \Q...\E,
consult (perlop)Gory details of parsing quoted constructs.



quotemeta() and \Q are fully described in (perlfunc)quotemeta.









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58.2.4 Extended Patterns



Perl also defines a consistent extension syntax for features not
found in standard tools like awk and
lex.  The syntax for most of these is a
pair of parentheses with a question mark as the first thing within
the parentheses.  The character after the question mark indicates
the extension.



The stability of these extensions varies widely.  Some have been
part of the core language for many years.  Others are experimental
and may change without warning or be completely removed.  Check
the documentation on an individual feature to verify its current
status.



A question mark was chosen for this and for the minimal-matching
construct because 1) question marks are rare in older regular
expressions, and 2) whenever you see one, you should stop and
"question" exactly what is going on.  That’s psychology....





(?#text)





A comment.  The text is ignored.  If the /x modifier enables
whitespace formatting, a simple # will suffice.  Note that Perl closes
the comment as soon as it sees a ), so there is no way to put a literal
) in the comment.





(?adlupimsx-imsx)






(?^alupimsx)






One or more embedded pattern-match modifiers, to be turned on (or
turned off, if preceded by -) for the remainder of the pattern or
the remainder of the enclosing pattern group (if any).



This is particularly useful for dynamic patterns, such as those read in from a
configuration file, taken from an argument, or specified in a table
somewhere.  Consider the case where some patterns want to be
case-sensitive and some do not:  The case-insensitive ones merely need to
include (?i) at the front of the pattern.  For example:



    $pattern = "foobar";
    if ( /$pattern/i ) { }

    # more flexible:

    $pattern = "(?i)foobar";
    if ( /$pattern/ ) { }



These modifiers are restored at the end of the enclosing group. For example,



    ( (?i) blah ) \s+ \g1



will match blah in any case, some spaces, and an exact (including the case!)
repetition of the previous word, assuming the /x modifier, and no /i
modifier outside this group.



These modifiers do not carry over into named subpatterns called in the
enclosing group. In other words, a pattern such as ((?i)(?&NAME)) does not
change the case-sensitivity of the "NAME" pattern.



Any of these modifiers can be set to apply globally to all regular
expressions compiled within the scope of a use re.  See
(re)'/flags' mode.



Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately
after the "?" is a shorthand equivalent to d-imsx.  Flags (except
"d") may follow the caret to override it.
But a minus sign is not legal with it.



Note that the a, d, l, p, and u modifiers are special in
that they can only be enabled, not disabled, and the a, d, l, and
u modifiers are mutually exclusive: specifying one de-specifies the
others, and a maximum of one (or two a’s) may appear in the
construct.  Thus, for
example, (?-p) will warn when compiled under use warnings;
(?-d:...) and (?dl:...) are fatal errors.



Note also that the p modifier is special in that its presence
anywhere in a pattern has a global effect.





(?:pattern)







(?adluimsx-imsx:pattern)






(?^aluimsx:pattern)





This is for clustering, not capturing; it groups subexpressions like
"()", but doesn’t make backreferences as "()" does.  So



    @fields = split(/\b(?:a|b|c)\b/)



is like



    @fields = split(/\b(a|b|c)\b/)



but doesn’t spit out extra fields.  It’s also cheaper not to capture
characters if you don’t need to.



Any letters between ? and : act as flags modifiers as with
(?adluimsx-imsx).  For example,



    /(?s-i:more.*than).*million/i



is equivalent to the more verbose



    /(?:(?s-i)more.*than).*million/i



Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately
after the "?" is a shorthand equivalent to d-imsx.  Any positive
flags (except "d") may follow the caret, so



    (?^x:foo)



is equivalent to



    (?x-ims:foo)



The caret tells Perl that this cluster doesn’t inherit the flags of any
surrounding pattern, but uses the system defaults (d-imsx),
modified by any flags specified.



The caret allows for simpler stringification of compiled regular
expressions.  These look like



    (?^:pattern)



with any non-default flags appearing between the caret and the colon.
A test that looks at such stringification thus doesn’t need to have the
system default flags hard-coded in it, just the caret.  If new flags are
added to Perl, the meaning of the caret’s expansion will change to include
the default for those flags, so the test will still work, unchanged.



Specifying a negative flag after the caret is an error, as the flag is
redundant.



Mnemonic for (?^...):  A fresh beginning since the usual use of a caret is
to match at the beginning.





(?|pattern)






This is the "branch reset" pattern, which has the special property
that the capture groups are numbered from the same starting point
in each alternation branch. It is available starting from perl 5.10.0.



Capture groups are numbered from left to right, but inside this
construct the numbering is restarted for each branch.



The numbering within each branch will be as normal, and any groups
following this construct will be numbered as though the construct
contained only one branch, that being the one with the most capture
groups in it.



This construct is useful when you want to capture one of a
number of alternative matches.



Consider the following pattern.  The numbers underneath show in
which group the captured content will be stored.



    # before  ---------------branch-reset----------- after        
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
    # 1            2         2  3        2     3     4  



Be careful when using the branch reset pattern in combination with 
named captures. Named captures are implemented as being aliases to 
numbered groups holding the captures, and that interferes with the
implementation of the branch reset pattern. If you are using named
captures in a branch reset pattern, it’s best to use the same names,
in the same order, in each of the alternations:



   /(?|  (?<a> x ) (?<b> y )
      |  (?<a> z ) (?<b> w )) /x



Not doing so may lead to surprises:



  "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
  say $+ {a};   # Prints '12'
  say $+ {b};   # *Also* prints '12'.



The problem here is that both the group named a and the group
named b are aliases for the group belonging to $1.





Look-Around Assertions








Look-around assertions are zero-width patterns which match a specific
pattern without including it in $&. Positive assertions match when
their subpattern matches, negative assertions match when their subpattern
fails. Look-behind matches text up to the current match position,
look-ahead matches text following the current match position.





(?=pattern)







A zero-width positive look-ahead assertion.  For example, /\w+(?=\t)/
matches a word followed by a tab, without including the tab in $&.





(?!pattern)







A zero-width negative look-ahead assertion.  For example /foo(?!bar)/
matches any occurrence of "foo" that isn’t followed by "bar".  Note
however that look-ahead and look-behind are NOT the same thing.  You cannot
use this for look-behind.



If you are looking for a "bar" that isn’t preceded by a "foo", /(?!foo)bar/
will not do what you want.  That’s because the (?!foo) is just saying that
the next thing cannot be "foo"–and it’s not, it’s a "bar", so "foobar" will
match.  Use look-behind instead (see below).





(?<=pattern) \K








A zero-width positive look-behind assertion.  For example, /(?<=\t)\w+/
matches a word that follows a tab, without including the tab in $&.
Works only for fixed-width look-behind.



There is a special form of this construct, called \K, which causes the
regex engine to "keep" everything it had matched prior to the \K and
not include it in $&. This effectively provides variable-length
look-behind. The use of \K inside of another look-around assertion
is allowed, but the behaviour is currently not well defined.



For various reasons \K may be significantly more efficient than the
equivalent (?<=...) construct, and it is especially useful in
situations where you want to efficiently remove something following
something else in a string. For instance



  s/(foo)bar/$1/g;



can be rewritten as the much more efficient



  s/foo\Kbar//g;





(?<!pattern)







A zero-width negative look-behind assertion.  For example /(?<!bar)foo/
matches any occurrence of "foo" that does not follow "bar".  Works
only for fixed-width look-behind.










(?'NAME'pattern)






(?<NAME>pattern)








A named capture group. Identical in every respect to normal capturing
parentheses () but for the additional fact that the group
can be referred to by name in various regular expression
constructs (like \g{NAME}) and can be accessed by name
after a successful match via %+ or %-. See perlvar NAME
for more details on the %+ and %- hashes.



If multiple distinct capture groups have the same name then the
$+{NAME} will refer to the leftmost defined group in the match.



The forms (?'NAME'pattern) and (?<NAME>pattern) are equivalent.



NOTE: While the notation of this construct is the same as the similar
function in .NET regexes, the behavior is not. In Perl the groups are
numbered sequentially regardless of being named or not. Thus in the
pattern



  /(x)(?<foo>y)(z)/



$+{foo} will be the same as $2, and $3 will contain ’z’ instead of
the opposite which is what a .NET regex hacker might expect.



Currently NAME is restricted to simple identifiers only.
In other words, it must match /^[_A-Za-z][_A-Za-z0-9]*\z/ or
its Unicode extension (see (utf8)),
though it isn’t extended by the locale (see perllocale NAME).



NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines, the pattern (?P<NAME>pattern)
may be used instead of (?<NAME>pattern); however this form does not
support the use of single quotes as a delimiter for the name.





\k<NAME>






\k'NAME'




Named backreference. Similar to numeric backreferences, except that
the group is designated by name and not number. If multiple groups
have the same name then it refers to the leftmost defined group in
the current match.



It is an error to refer to a name not defined by a (?<NAME>)
earlier in the pattern.



Both forms are equivalent.



NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines, the pattern (?P=NAME)
may be used instead of \k<NAME>.





(?{ code })








WARNING: This extended regular expression feature is considered
experimental, and may be changed without notice. Code executed that
has side effects may not perform identically from version to version
due to the effect of future optimisations in the regex engine.



This zero-width assertion evaluates any embedded Perl code.  It
always succeeds, and its code is not interpolated.  Currently,
the rules to determine where the code ends are somewhat convoluted.



This feature can be used together with the special variable $^N to
capture the results of submatches in variables without having to keep
track of the number of nested parentheses. For example:



  $_ = "The brown fox jumps over the lazy dog";
  /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
  print "color = $color, animal = $animal\n";



Inside the (?{...}) block, $_ refers to the string the regular
expression is matching against. You can also use pos() to know what is
the current position of matching within this string.



The code is properly scoped in the following sense: If the assertion
is backtracked (compare Backtracking), all changes introduced after
localization are undone, so that



  $_ = 'a' x 8;
  m<
     (?{ $cnt = 0 })               # Initialize $cnt.
     (
       a
       (?{
           local $cnt = $cnt + 1;  # Update $cnt,
                                   # backtracking-safe.
       })
     )*
     aaaa
     (?{ $res = $cnt })            # On success copy to
                                   # non-localized location.
   >x;



will set $res = 4.  Note that after the match, $cnt returns to the globally
introduced value, because the scopes that restrict local operators
are unwound.



This assertion may be used as a (?(condition)yes-pattern|no-pattern)
switch.  If not used in this way, the result of evaluation of
code is put into the special variable $^R.  This happens
immediately, so $^R can be used from other (?{ code }) assertions
inside the same regular expression.



The assignment to $^R above is properly localized, so the old
value of $^R is restored if the assertion is backtracked; compare
Backtracking.



For reasons of security, this construct is forbidden if the regular
expression involves run-time interpolation of variables, unless the
perilous use re 'eval' pragma has been used (see (re)), or the
variables contain results of the qr// operator (see
(perlop)qr/STRING/msixpodual).



This restriction is due to the wide-spread and remarkably convenient
custom of using run-time determined strings as patterns.  For example:



    $re = <>;
    chomp $re;
    $string =~ /$re/;



Before Perl knew how to execute interpolated code within a pattern,
this operation was completely safe from a security point of view,
although it could raise an exception from an illegal pattern.  If
you turn on the use re 'eval', though, it is no longer secure,
so you should only do so if you are also using taint checking.
Better yet, use the carefully constrained evaluation within a Safe
compartment.  See perlsec NAME for details about both these mechanisms.



WARNING: Use of lexical (my) variables in these blocks is
broken. The result is unpredictable and will make perl unstable. The
workaround is to use global (our) variables.



WARNING: In perl 5.12.x and earlier, the regex engine
was not re-entrant, so interpolated code could not
safely invoke the regex engine either directly with
m// or s///), or indirectly with functions such as
split. Invoking the regex engine in these blocks would make perl
unstable.





(??{ code })








WARNING: This extended regular expression feature is considered
experimental, and may be changed without notice. Code executed that
has side effects may not perform identically from version to version
due to the effect of future optimisations in the regex engine.



This is a "postponed" regular subexpression.  The code is evaluated
at run time, at the moment this subexpression may match.  The result
of evaluation is considered a regular expression and matched as
if it were inserted instead of this construct.  Note that this means
that the contents of capture groups defined inside an eval’ed pattern
are not available outside of the pattern, and vice versa, there is no
way for the inner pattern returned from the code block to refer to a
capture group defined outside.  (The code block itself can use $1, etc.,
to refer to the enclosing pattern’s capture groups.)  Thus,



    ('a' x 100)=~/(??{'(.)' x 100})/



will match, it will not set $1.



The code is not interpolated.  As before, the rules to determine
where the code ends are currently somewhat convoluted.



The following pattern matches a parenthesized group:



 $re = qr{
            \(
            (?:
               (?> [^()]+ )  # Non-parens without backtracking
             |
               (??{ $re })   # Group with matching parens
            )*
            \)
         }x;



See also (?PARNO) for a different, more efficient way to accomplish
the same task.



For reasons of security, this construct is forbidden if the regular
expression involves run-time interpolation of variables, unless the
perilous use re 'eval' pragma has been used (see (re)), or the
variables contain results of the qr// operator (see
(perlop)qr/STRING/msixpodual).



In perl 5.12.x and earlier, because the regex engine was not re-entrant,
delayed code could not safely invoke the regex engine either directly with
m// or s///), or indirectly with functions such as split.



Recursing deeper than 50 times without consuming any input string will
result in a fatal error.  The maximum depth is compiled into perl, so
changing it requires a custom build.





(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)
















Similar to (??{ code }) except it does not involve compiling any code,
instead it treats the contents of a capture group as an independent
pattern that must match at the current position.  Capture groups
contained by the pattern will have the value as determined by the
outermost recursion.



PARNO is a sequence of digits (not starting with 0) whose value reflects
the paren-number of the capture group to recurse to. (?R) recurses to
the beginning of the whole pattern. (?0) is an alternate syntax for
(?R). If PARNO is preceded by a plus or minus sign then it is assumed
to be relative, with negative numbers indicating preceding capture groups
and positive ones following. Thus (?-1) refers to the most recently
declared group, and (?+1) indicates the next group to be declared.
Note that the counting for relative recursion differs from that of
relative backreferences, in that with recursion unclosed groups are
included.



The following pattern matches a function foo() which may contain
balanced parentheses as the argument.



  $re = qr{ (                   # paren group 1 (full function)
              foo
              (                 # paren group 2 (parens)
                \(
                  (             # paren group 3 (contents of parens)
                  (?:
                   (?> [^()]+ ) # Non-parens without backtracking
                  |
                   (?2)         # Recurse to start of paren group 2
                  )*
                  )
                \)
              )
            )
          }x;



If the pattern was used as follows



    'foo(bar(baz)+baz(bop))'=~/$re/
        and print "\$1 = $1\n",
                  "\$2 = $2\n",
                  "\$3 = $3\n";



the output produced should be the following:



    $1 = foo(bar(baz)+baz(bop))
    $2 = (bar(baz)+baz(bop))
    $3 = bar(baz)+baz(bop)



If there is no corresponding capture group defined, then it is a
fatal error.  Recursing deeper than 50 times without consuming any input
string will also result in a fatal error.  The maximum depth is compiled
into perl, so changing it requires a custom build.



The following shows how using negative indexing can make it
easier to embed recursive patterns inside of a qr// construct
for later use:



    my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
    if (/foo $parens \s+ + \s+ bar $parens/x) {
       # do something here...
    }



Note that this pattern does not behave the same way as the equivalent
PCRE or Python construct of the same form. In Perl you can backtrack into
a recursed group, in PCRE and Python the recursed into group is treated
as atomic. Also, modifiers are resolved at compile time, so constructs
like (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern will
be processed.





(?&NAME)





Recurse to a named subpattern. Identical to (?PARNO) except that the
parenthesis to recurse to is determined by name. If multiple parentheses have
the same name, then it recurses to the leftmost.



It is an error to refer to a name that is not declared somewhere in the
pattern.



NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines the pattern (?P>NAME)
may be used instead of (?&NAME).





(?(condition)yes-pattern|no-pattern)







(?(condition)yes-pattern)




Conditional expression. Matches yes-pattern if condition yields
a true value, matches no-pattern otherwise. A missing pattern always
matches.



(condition) should be either an integer in
parentheses (which is valid if the corresponding pair of parentheses
matched), a look-ahead/look-behind/evaluate zero-width assertion, a
name in angle brackets or single quotes (which is valid if a group
with the given name matched), or the special symbol (R) (true when
evaluated inside of recursion or eval). Additionally the R may be
followed by a number, (which will be true when evaluated when recursing
inside of the appropriate group), or by &NAME, in which case it will
be true only when evaluated during recursion in the named group.



Here’s a summary of the possible predicates:





(1) (2) ...




Checks if the numbered capturing group has matched something.





(<NAME>) (’NAME’)




Checks if a group with the given name has matched something.





(?=...) (?!...) (?<=...) (?<!...)




Checks whether the pattern matches (or does not match, for the ’!’
variants).





(?{ CODE })




Treats the return value of the code block as the condition.





(R)




Checks if the expression has been evaluated inside of recursion.





(R1) (R2) ...




Checks if the expression has been evaluated while executing directly
inside of the n-th capture group. This check is the regex equivalent of



  if ((caller(0))[3] eq 'subname') { ... }



In other words, it does not check the full recursion stack.





(R&NAME)




Similar to (R1), this predicate checks to see if we’re executing
directly inside of the leftmost group with a given name (this is the same
logic used by (?&NAME) to disambiguate). It does not check the full
stack, but only the name of the innermost active recursion.





(DEFINE)




In this case, the yes-pattern is never directly executed, and no
no-pattern is allowed. Similar in spirit to (?{0}) but more efficient.
See below for details.








For example:



    m{ ( \( )?
       [^()]+
       (?(1) \) )
     }x



matches a chunk of non-parentheses, possibly included in parentheses
themselves.



A special form is the (DEFINE) predicate, which never executes its
yes-pattern directly, and does not allow a no-pattern. This allows one to
define subpatterns which will be executed only by the recursion mechanism.
This way, you can define a set of regular expression rules that can be
bundled into any pattern you choose.



It is recommended that for this usage you put the DEFINE block at the
end of the pattern, and that you name any subpatterns defined within it.



Also, it’s worth noting that patterns defined this way probably will
not be as efficient, as the optimiser is not very clever about
handling them.



An example of how this might be used is as follows:



  /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
   (?(DEFINE)
     (?<NAME_PAT>....)
     (?<ADRESS_PAT>....)
   )/x



Note that capture groups matched inside of recursion are not accessible
after the recursion returns, so the extra layer of capturing groups is
necessary. Thus $+{NAME_PAT} would not be defined even though
$+{NAME} would be.



Finally, keep in mind that subpatterns created inside a DEFINE block
count towards the absolute and relative number of captures, so this:



    my @captures = "a" =~ /(.)                  # First capture
                           (?(DEFINE)
                               (?<EXAMPLE> 1 )  # Second capture
                           )/x;
    say scalar @captures;



Will output 2, not 1. This is particularly important if you intend to
compile the definitions with the qr// operator, and later
interpolate them in another pattern.





(?>pattern)








An "independent" subexpression, one which matches the substring
that a standalone pattern would match if anchored at the given
position, and it matches nothing other than this substring.  This
construct is useful for optimizations of what would otherwise be
"eternal" matches, because it will not backtrack (see Backtracking).
It may also be useful in places where the "grab all you can, and do not
give anything back" semantic is desirable.



For example: ^(?>a*)ab will never match, since (?>a*)
(anchored at the beginning of string, as above) will match all
characters a at the beginning of string, leaving no a for
ab to match.  In contrast, a*ab will match the same as a+b,
since the match of the subgroup a* is influenced by the following
group ab (see Backtracking).  In particular, a* inside
a*ab will match fewer characters than a standalone a*, since
this makes the tail match.



(?>pattern) does not disable backtracking altogether once it has
matched. It is still possible to backtrack past the construct, but not
into it. So ((?>a*)|(?>b*))ar will still match "bar".



An effect similar to (?>pattern) may be achieved by writing
(?=(pattern))\g{-1}.  This matches the same substring as a standalone
a+, and the following \g{-1} eats the matched string; it therefore
makes a zero-length assertion into an analogue of (?>...).
(The difference between these two constructs is that the second one
uses a capturing group, thus shifting ordinals of backreferences
in the rest of a regular expression.)



Consider this pattern:



    m{ \(
          (
            [^()]+           # x+
          |
            \( [^()]* \)
          )+
       \)
     }x



That will efficiently match a nonempty group with matching parentheses
two levels deep or less.  However, if there is no such group, it
will take virtually forever on a long string.  That’s because there
are so many different ways to split a long string into several
substrings.  This is what (.+)+ is doing, and (.+)+ is similar
to a subpattern of the above pattern.  Consider how the pattern
above detects no-match on ((()aaaaaaaaaaaaaaaaaa in several
seconds, but that each extra letter doubles this time.  This
exponential performance will make it appear that your program has
hung.  However, a tiny change to this pattern



    m{ \(
          (
            (?> [^()]+ )        # change x+ above to (?> x+ )
          |
            \( [^()]* \)
          )+
       \)
     }x



which uses (?>...) matches exactly when the one above does (verifying
this yourself would be a productive exercise), but finishes in a fourth
the time when used on a similar string with 1000000 as.  Be aware,
however, that, when this construct is followed by a
quantifier, it currently triggers a warning message under
the use warnings pragma or -w switch saying it
"matches null string many times in regex".



On simple groups, such as the pattern (?> [^()]+ ), a comparable
effect may be achieved by negative look-ahead, as in [^()]+ (?! [^()] ).
This was only 4 times slower on a string with 1000000 as.



The "grab all you can, and do not give anything back" semantic is desirable
in many situations where on the first sight a simple ()* looks like
the correct solution.  Suppose we parse text with comments being delimited
by # followed by some optional (horizontal) whitespace.  Contrary to
its appearance, #[ \t]* is not the correct subexpression to match
the comment delimiter, because it may "give up" some whitespace if
the remainder of the pattern can be made to match that way.  The correct
answer is either one of these:



    (?>#[ \t]*)
    #[ \t]*(?![ \t])



For example, to grab non-empty comments into $1, one should use either
one of these:



    / (?> \# [ \t]* ) (        .+ ) /x;
    /     \# [ \t]*   ( [^ \t] .* ) /x;



Which one you pick depends on which of these expressions better reflects
the above specification of comments.



In some literature this construct is called "atomic matching" or
"possessive matching".



Possessive quantifiers are equivalent to putting the item they are applied
to inside of one of these constructs. The following equivalences apply:



    Quantifier Form     Bracketing Form
    ---------------     ---------------
    PAT*+               (?>PAT*)
    PAT++               (?>PAT+)
    PAT?+               (?>PAT?)
    PAT{min,max}+       (?>PAT{min,max})














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58.2.5 Special Backtracking Control Verbs



WARNING: These patterns are experimental and subject to change or
removal in a future version of Perl. Their usage in production code should
be noted to avoid problems during upgrades.



These special patterns are generally of the form (*VERB:ARG). Unless
otherwise stated the ARG argument is optional; in some cases, it is
forbidden.



Any pattern containing a special backtracking verb that allows an argument
has the special behaviour that when executed it sets the current package’s
$REGERROR and $REGMARK variables. When doing so the following
rules apply:



On failure, the $REGERROR variable will be set to the ARG value of the
verb pattern, if the verb was involved in the failure of the match. If the
ARG part of the pattern was omitted, then $REGERROR will be set to the
name of the last (*MARK:NAME) pattern executed, or to TRUE if there was
none. Also, the $REGMARK variable will be set to FALSE.



On a successful match, the $REGERROR variable will be set to FALSE, and
the $REGMARK variable will be set to the name of the last
(*MARK:NAME) pattern executed.  See the explanation for the
(*MARK:NAME) verb below for more details.



NOTE: $REGERROR and $REGMARK are not magic variables like $1
and most other regex-related variables. They are not local to a scope, nor
readonly, but instead are volatile package variables similar to $AUTOLOAD.
Use local to localize changes to them to a specific scope if necessary.



If a pattern does not contain a special backtracking verb that allows an
argument, then $REGERROR and $REGMARK are not touched at all.





Verbs that take an argument






(*PRUNE) (*PRUNE:NAME)






This zero-width pattern prunes the backtracking tree at the current point
when backtracked into on failure. Consider the pattern A (*PRUNE) B,
where A and B are complex patterns. Until the (*PRUNE) verb is reached,
A may backtrack as necessary to match. Once it is reached, matching
continues in B, which may also backtrack as necessary; however, should B
not match, then no further backtracking will take place, and the pattern
will fail outright at the current starting position.



The following example counts all the possible matching strings in a
pattern (without actually matching any of them).



    'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
    print "Count=$count\n";



which produces:



    aaab
    aaa
    aa
    a
    aab
    aa
    a
    ab
    a
    Count=9



If we add a (*PRUNE) before the count like the following



    'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
    print "Count=$count\n";



we prevent backtracking and find the count of the longest matching string
at each matching starting point like so:



    aaab
    aab
    ab
    Count=3



Any number of (*PRUNE) assertions may be used in a pattern.



See also (?>pattern) and possessive quantifiers for other ways to
control backtracking. In some cases, the use of (*PRUNE) can be
replaced with a (?>pattern) with no functional difference; however,
(*PRUNE) can be used to handle cases that cannot be expressed using a
(?>pattern) alone.





(*SKIP) (*SKIP:NAME)





This zero-width pattern is similar to (*PRUNE), except that on
failure it also signifies that whatever text that was matched leading up
to the (*SKIP) pattern being executed cannot be part of any match
of this pattern. This effectively means that the regex engine "skips" forward
to this position on failure and tries to match again, (assuming that
there is sufficient room to match).



The name of the (*SKIP:NAME) pattern has special significance. If a
(*MARK:NAME) was encountered while matching, then it is that position
which is used as the "skip point". If no (*MARK) of that name was
encountered, then the (*SKIP) operator has no effect. When used
without a name the "skip point" is where the match point was when
executing the (*SKIP) pattern.



Compare the following to the examples in (*PRUNE); note the string
is twice as long:



 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
 print "Count=$count\n";



outputs



    aaab
    aaab
    Count=2



Once the ’aaab’ at the start of the string has matched, and the (*SKIP)
executed, the next starting point will be where the cursor was when the
(*SKIP) was executed.





(*MARK:NAME) (*:NAME)







This zero-width pattern can be used to mark the point reached in a string
when a certain part of the pattern has been successfully matched. This
mark may be given a name. A later (*SKIP) pattern will then skip
forward to that point if backtracked into on failure. Any number of
(*MARK) patterns are allowed, and the NAME portion may be duplicated.



In addition to interacting with the (*SKIP) pattern, (*MARK:NAME)
can be used to "label" a pattern branch, so that after matching, the
program can determine which branches of the pattern were involved in the
match.



When a match is successful, the $REGMARK variable will be set to the
name of the most recently executed (*MARK:NAME) that was involved
in the match.



This can be used to determine which branch of a pattern was matched
without using a separate capture group for each branch, which in turn
can result in a performance improvement, as perl cannot optimize
/(?:(x)|(y)|(z))/ as efficiently as something like
/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/.



When a match has failed, and unless another verb has been involved in
failing the match and has provided its own name to use, the $REGERROR
variable will be set to the name of the most recently executed
(*MARK:NAME).



See ‘(*SKIP)’ for more details.



As a shortcut (*MARK:NAME) can be written (*:NAME).





(*THEN) (*THEN:NAME)




This is similar to the "cut group" operator :: from Perl 6.  Like
(*PRUNE), this verb always matches, and when backtracked into on
failure, it causes the regex engine to try the next alternation in the
innermost enclosing group (capturing or otherwise) that has alternations.
The two branches of a (?(condition)yes-pattern|no-pattern) do not
count as an alternation, as far as (*THEN) is concerned.



Its name comes from the observation that this operation combined with the
alternation operator (|) can be used to create what is essentially a
pattern-based if/then/else block:



  ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )



Note that if this operator is used and NOT inside of an alternation then
it acts exactly like the (*PRUNE) operator.



  / A (*PRUNE) B /



is the same as



  / A (*THEN) B /



but



  / ( A (*THEN) B | C (*THEN) D ) /



is not the same as



  / ( A (*PRUNE) B | C (*PRUNE) D ) /



as after matching the A but failing on the B the (*THEN) verb will
backtrack and try C; but the (*PRUNE) verb will simply fail.










Verbs without an argument






(*COMMIT)





This is the Perl 6 "commit pattern" <commit> or :::. It’s a
zero-width pattern similar to (*SKIP), except that when backtracked
into on failure it causes the match to fail outright. No further attempts
to find a valid match by advancing the start pointer will occur again.
For example,



 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
 print "Count=$count\n";



outputs



    aaab
    Count=1



In other words, once the (*COMMIT) has been entered, and if the pattern
does not match, the regex engine will not try any further matching on the
rest of the string.





(*FAIL) (*F)






This pattern matches nothing and always fails. It can be used to force the
engine to backtrack. It is equivalent to (?!), but easier to read. In
fact, (?!) gets optimised into (*FAIL) internally.



It is probably useful only when combined with (?{}) or (??{}).





(*ACCEPT)





WARNING: This feature is highly experimental. It is not recommended
for production code.



This pattern matches nothing and causes the end of successful matching at
the point at which the (*ACCEPT) pattern was encountered, regardless of
whether there is actually more to match in the string. When inside of a
nested pattern, such as recursion, or in a subpattern dynamically generated
via (??{}), only the innermost pattern is ended immediately.



If the (*ACCEPT) is inside of capturing groups then the groups are
marked as ended at the point at which the (*ACCEPT) was encountered.
For instance:



  'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;



will match, and $1 will be AB and $2 will be B, $3 will not
be set. If another branch in the inner parentheses was matched, such as in the
string ’ACDE’, then the D and E would have to be matched as well.



















Next: , Previous: NAME regular expression regex regexp Special Backtracking Control Verbs, Up: NAME regular expression regex regexp DESCRIPTION   [Contents]





58.2.6 Backtracking





NOTE: This section presents an abstract approximation of regular
expression behavior.  For a more rigorous (and complicated) view of
the rules involved in selecting a match among possible alternatives,
see Combining RE Pieces.



A fundamental feature of regular expression matching involves the
notion called backtracking, which is currently used (when needed)
by all regular non-possessive expression quantifiers, namely *, *?, +,
+?, {n,m}, and {n,m}?.  Backtracking is often optimized
internally, but the general principle outlined here is valid.



For a regular expression to match, the entire regular expression must
match, not just part of it.  So if the beginning of a pattern containing a
quantifier succeeds in a way that causes later parts in the pattern to
fail, the matching engine backs up and recalculates the beginning
part–that’s why it’s called backtracking.



Here is an example of backtracking:  Let’s say you want to find the
word following "foo" in the string "Food is on the foo table.":



    $_ = "Food is on the foo table.";
    if ( /\b(foo)\s+(\w+)/i ) {
        print "$2 follows $1.\n";
    }



When the match runs, the first part of the regular expression (\b(foo))
finds a possible match right at the beginning of the string, and loads up
$1 with "Foo".  However, as soon as the matching engine sees that there’s
no whitespace following the "Foo" that it had saved in $1, it realizes its
mistake and starts over again one character after where it had the
tentative match.  This time it goes all the way until the next occurrence
of "foo". The complete regular expression matches this time, and you get
the expected output of "table follows foo."



Sometimes minimal matching can help a lot.  Imagine you’d like to match
everything between "foo" and "bar".  Initially, you write something
like this:



    $_ =  "The food is under the bar in the barn.";
    if ( /foo(.*)bar/ ) {
        print "got <$1>\n";
    }



Which perhaps unexpectedly yields:



  got <d is under the bar in the >



That’s because .* was greedy, so you get everything between the
first "foo" and the last "bar".  Here it’s more effective
to use minimal matching to make sure you get the text between a "foo"
and the first "bar" thereafter.



    if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
  got <d is under the >



Here’s another example. Let’s say you’d like to match a number at the end
of a string, and you also want to keep the preceding part of the match.
So you write this:



    $_ = "I have 2 numbers: 53147";
    if ( /(.*)(\d*)/ ) {                                # Wrong!
        print "Beginning is <$1>, number is <$2>.\n";
    }



That won’t work at all, because .* was greedy and gobbled up the
whole string. As \d* can match on an empty string the complete
regular expression matched successfully.



    Beginning is <I have 2 numbers: 53147>, number is <>.



Here are some variants, most of which don’t work:



    $_ = "I have 2 numbers: 53147";
    @pats = qw{
        (.*)(\d*)
        (.*)(\d+)
        (.*?)(\d*)
        (.*?)(\d+)
        (.*)(\d+)$
        (.*?)(\d+)$
        (.*)\b(\d+)$
        (.*\D)(\d+)$
    };

    for $pat (@pats) {
        printf "%-12s ", $pat;
        if ( /$pat/ ) {
            print "<$1> <$2>\n";
        } else {
            print "FAIL\n";
        }
    }



That will print out:



    (.*)(\d*)    <I have 2 numbers: 53147> <>
    (.*)(\d+)    <I have 2 numbers: 5314> <7>
    (.*?)(\d*)   <> <>
    (.*?)(\d+)   <I have > <2>
    (.*)(\d+)$   <I have 2 numbers: 5314> <7>
    (.*?)(\d+)$  <I have 2 numbers: > <53147>
    (.*)\b(\d+)$ <I have 2 numbers: > <53147>
    (.*\D)(\d+)$ <I have 2 numbers: > <53147>



As you see, this can be a bit tricky.  It’s important to realize that a
regular expression is merely a set of assertions that gives a definition
of success.  There may be 0, 1, or several different ways that the
definition might succeed against a particular string.  And if there are
multiple ways it might succeed, you need to understand backtracking to
know which variety of success you will achieve.



When using look-ahead assertions and negations, this can all get even
trickier.  Imagine you’d like to find a sequence of non-digits not
followed by "123".  You might try to write that as



    $_ = "ABC123";
    if ( /^\D*(?!123)/ ) {                # Wrong!
        print "Yup, no 123 in $_\n";
    }



But that isn’t going to match; at least, not the way you’re hoping.  It
claims that there is no 123 in the string.  Here’s a clearer picture of
why that pattern matches, contrary to popular expectations:



    $x = 'ABC123';
    $y = 'ABC445';

    print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
    print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;

    print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
    print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;



This prints



    2: got ABC
    3: got AB
    4: got ABC



You might have expected test 3 to fail because it seems to a more
general purpose version of test 1.  The important difference between
them is that test 3 contains a quantifier (\D*) and so can use
backtracking, whereas test 1 will not.  What’s happening is
that you’ve asked "Is it true that at the start of $x, following 0 or more
non-digits, you have something that’s not 123?"  If the pattern matcher had
let \D* expand to "ABC", this would have caused the whole pattern to
fail.



The search engine will initially match \D* with "ABC".  Then it will
try to match (?!123) with "123", which fails.  But because
a quantifier (\D*) has been used in the regular expression, the
search engine can backtrack and retry the match differently
in the hope of matching the complete regular expression.



The pattern really, really wants to succeed, so it uses the
standard pattern back-off-and-retry and lets \D* expand to just "AB" this
time.  Now there’s indeed something following "AB" that is not
"123".  It’s "C123", which suffices.



We can deal with this by using both an assertion and a negation.
We’ll say that the first part in $1 must be followed both by a digit
and by something that’s not "123".  Remember that the look-aheads
are zero-width expressions–they only look, but don’t consume any
of the string in their match.  So rewriting this way produces what
you’d expect; that is, case 5 will fail, but case 6 succeeds:



    print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
    print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;

    6: got ABC



In other words, the two zero-width assertions next to each other work as though
they’re ANDed together, just as you’d use any built-in assertions:  /^$/
matches only if you’re at the beginning of the line AND the end of the
line simultaneously.  The deeper underlying truth is that juxtaposition in
regular expressions always means AND, except when you write an explicit OR
using the vertical bar.  /ab/ means match "a" AND (then) match "b",
although the attempted matches are made at different positions because "a"
is not a zero-width assertion, but a one-width assertion.



WARNING: Particularly complicated regular expressions can take
exponential time to solve because of the immense number of possible
ways they can use backtracking to try for a match.  For example, without
internal optimizations done by the regular expression engine, this will
take a painfully long time to run:



    'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/



And if you used *’s in the internal groups instead of limiting them
to 0 through 5 matches, then it would take forever–or until you ran
out of stack space.  Moreover, these internal optimizations are not
always applicable.  For example, if you put {0,5} instead of *
on the external group, no current optimization is applicable, and the
match takes a long time to finish.



A powerful tool for optimizing such beasts is what is known as an
"independent group",
which does not backtrack (see (?>pattern)).  Note also that
zero-length look-ahead/look-behind assertions will not backtrack to make
the tail match, since they are in "logical" context: only
whether they match is considered relevant.  For an example
where side-effects of look-ahead might have influenced the
following match, see (?>pattern).









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58.2.7 Version 8 Regular Expressions






In case you’re not familiar with the "regular" Version 8 regex
routines, here are the pattern-matching rules not described above.



Any single character matches itself, unless it is a metacharacter
with a special meaning described here or above.  You can cause
characters that normally function as metacharacters to be interpreted
literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
character; "\\" matches a "\"). This escape mechanism is also required
for the character used as the pattern delimiter.



A series of characters matches that series of characters in the target
string, so the pattern blurfl would match "blurfl" in the target
string.



You can specify a character class, by enclosing a list of characters
in [], which will match any character from the list.  If the
first character after the "[" is "^", the class matches any character not
in the list.  Within a list, the "-" character specifies a
range, so that a-z represents all characters between "a" and "z",
inclusive.  If you want either "-" or "]" itself to be a member of a
class, put it at the start of the list (possibly after a "^"), or
escape it with a backslash.  "-" is also taken literally when it is
at the end of the list, just before the closing "]".  (The
following all specify the same class of three characters: [-az],
[az-], and [a\-z].  All are different from [a-z], which
specifies a class containing twenty-six characters, even on EBCDIC-based
character sets.)  Also, if you try to use the character
classes \w, \W, \s, \S, \d, or \D as endpoints of
a range, the "-" is understood literally.



Note also that the whole range idea is rather unportable between
character sets–and even within character sets they may cause results
you probably didn’t expect.  A sound principle is to use only ranges
that begin from and end at either alphabetics of equal case ([a-e],
[A-E]), or digits ([0-9]).  Anything else is unsafe.  If in doubt,
spell out the character sets in full.



Characters may be specified using a metacharacter syntax much like that
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc.  More generally, \nnn, where nnn is a string
of three octal digits, matches the character whose coded character set value
is nnn.  Similarly, \xnn, where nn are hexadecimal digits,
matches the character whose ordinal is nn. The expression \cx
matches the character control-x.  Finally, the "." metacharacter
matches any character except "\n" (unless you use /s).



You can specify a series of alternatives for a pattern using "|" to
separate them, so that fee|fie|foe will match any of "fee", "fie",
or "foe" in the target string (as would f(e|i|o)e).  The
first alternative includes everything from the last pattern delimiter
("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and
the last alternative contains everything from the last "|" to the next
closing pattern delimiter.  That’s why it’s common practice to include
alternatives in parentheses: to minimize confusion about where they
start and end.



Alternatives are tried from left to right, so the first
alternative found for which the entire expression matches, is the one that
is chosen. This means that alternatives are not necessarily greedy. For
example: when matching foo|foot against "barefoot", only the "foo"
part will match, as that is the first alternative tried, and it successfully
matches the target string. (This might not seem important, but it is
important when you are capturing matched text using parentheses.)



Also remember that "|" is interpreted as a literal within square brackets,
so if you write [fee|fie|foe] you’re really only matching [feio|].



Within a pattern, you may designate subpatterns for later reference
by enclosing them in parentheses, and you may refer back to the
nth subpattern later in the pattern using the metacharacter
\n or \gn.  Subpatterns are numbered based on the left to right order
of their opening parenthesis.  A backreference matches whatever
actually matched the subpattern in the string being examined, not
the rules for that subpattern.  Therefore, (0|0x)\d*\s\g1\d* will
match "0x1234 0x4321", but not "0x1234 01234", because subpattern
1 matched "0x", even though the rule 0|0x could potentially match
the leading 0 in the second number.









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58.2.8 Warning on \1 Instead of $1



Some people get too used to writing things like:



    $pattern =~ s/(\W)/\\\1/g;



This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid
shocking the
sed addicts, but it’s a dirty habit to get into.  That’s because in
PerlThink, the righthand side of an s/// is a double-quoted string.  \1 in
the usual double-quoted string means a control-A.  The customary Unix
meaning of \1 is kludged in for s///.  However, if you get into the habit
of doing that, you get yourself into trouble if you then add an /e
modifier.



    s/(\d+)/ \1 + 1 /eg;            # causes warning under -w



Or if you try to do



    s/(\d+)/\1000/;



You can’t disambiguate that by saying \{1}000, whereas you can fix it with
${1}000.  The operation of interpolation should not be confused
with the operation of matching a backreference.  Certainly they mean two
different things on the left side of the s///.









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58.2.9 Repeated Patterns Matching a Zero-length Substring



WARNING: Difficult material (and prose) ahead.  This section needs a rewrite.



Regular expressions provide a terse and powerful programming language.  As
with most other power tools, power comes together with the ability
to wreak havoc.



A common abuse of this power stems from the ability to make infinite
loops using regular expressions, with something as innocuous as:



    'foo' =~ m{ ( o? )* }x;



The o? matches at the beginning of 'foo', and since the position
in the string is not moved by the match, o? would match again and again
because of the * quantifier.  Another common way to create a similar cycle
is with the looping modifier //g:



    @matches = ( 'foo' =~ m{ o? }xg );



or



    print "match: <$&>\n" while 'foo' =~ m{ o? }xg;



or the loop implied by split().



However, long experience has shown that many programming tasks may
be significantly simplified by using repeated subexpressions that
may match zero-length substrings.  Here’s a simple example being:



    @chars = split //, $string;           # // is not magic in split
    ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /



Thus Perl allows such constructs, by forcefully breaking
the infinite loop.  The rules for this are different for lower-level
loops given by the greedy quantifiers *+{}, and for higher-level
ones like the /g modifier or split() operator.



The lower-level loops are interrupted (that is, the loop is
broken) when Perl detects that a repeated expression matched a
zero-length substring.   Thus



   m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;



is made equivalent to



   m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;



For example, this program



   #!perl -l
   "aaaaab" =~ /
     (?:
        a                 # non-zero
        |                 # or
       (?{print "hello"}) # print hello whenever this
                          #    branch is tried
       (?=(b))            # zero-width assertion
     )*  # any number of times
    /x;
   print $&;
   print $1;



prints



   hello
   aaaaa
   b



Notice that "hello" is only printed once, as when Perl sees that the sixth
iteration of the outermost (?:)* matches a zero-length string, it stops
the *.



The higher-level loops preserve an additional state between iterations:
whether the last match was zero-length.  To break the loop, the following
match after a zero-length match is prohibited to have a length of zero.
This prohibition interacts with backtracking (see Backtracking),
and so the second best match is chosen if the best match is of
zero length.



For example:



    $_ = 'bar';
    s/\w??/<$&>/g;



results in <><b><><a><><r><>.  At each position of the string the best
match given by non-greedy ?? is the zero-length match, and the second
best match is what is matched by \w.  Thus zero-length matches
alternate with one-character-long matches.



Similarly, for repeated m/()/g the second-best match is the match at the
position one notch further in the string.



The additional state of being matched with zero-length is associated with
the matched string, and is reset by each assignment to pos().
Zero-length matches at the end of the previous match are ignored
during split.









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58.2.10 Combining RE Pieces



Each of the elementary pieces of regular expressions which were described
before (such as ab or \Z) could match at most one substring
at the given position of the input string.  However, in a typical regular
expression these elementary pieces are combined into more complicated
patterns using combining operators ST, S|T, S* etc.
(in these examples S and T are regular subexpressions).



Such combinations can include alternatives, leading to a problem of choice:
if we match a regular expression a|ab against "abc", will it match
substring "a" or "ab"?  One way to describe which substring is
actually matched is the concept of backtracking (see Backtracking).
However, this description is too low-level and makes you think
in terms of a particular implementation.



Another description starts with notions of "better"/"worse".  All the
substrings which may be matched by the given regular expression can be
sorted from the "best" match to the "worst" match, and it is the "best"
match which is chosen.  This substitutes the question of "what is chosen?"
by the question of "which matches are better, and which are worse?".



Again, for elementary pieces there is no such question, since at most
one match at a given position is possible.  This section describes the
notion of better/worse for combining operators.  In the description
below S and T are regular subexpressions.





ST




Consider two possible matches, AB and A'B', A and A' are
substrings which can be matched by S, B and B' are substrings
which can be matched by T.



If A is a better match for S than A', AB is a better
match than A'B'.



If A and A' coincide: AB is a better match than AB' if
B is a better match for T than B'.





S|T




When S can match, it is a better match than when only T can match.



Ordering of two matches for S is the same as for S.  Similar for
two matches for T.





S{REPEAT_COUNT}




Matches as SSS...S (repeated as many times as necessary).





S{min,max}




Matches as S{max}|S{max-1}|...|S{min+1}|S{min}.





S{min,max}?




Matches as S{min}|S{min+1}|...|S{max-1}|S{max}.





S?, S*, S+




Same as S{0,1}, S{0,BIG_NUMBER}, S{1,BIG_NUMBER} respectively.





S??, S*?, S+?




Same as S{0,1}?, S{0,BIG_NUMBER}?, S{1,BIG_NUMBER}? respectively.





(?>S)




Matches the best match for S and only that.





(?=S), (?<=S)




Only the best match for S is considered.  (This is important only if
S has capturing parentheses, and backreferences are used somewhere
else in the whole regular expression.)





(?!S), (?<!S)




For this grouping operator there is no need to describe the ordering, since
only whether or not S can match is important.





(??{ EXPR }), (?PARNO)




The ordering is the same as for the regular expression which is
the result of EXPR, or the pattern contained by capture group PARNO.





(?(condition)yes-pattern|no-pattern)




Recall that which of yes-pattern or no-pattern actually matches is
already determined.  The ordering of the matches is the same as for the
chosen subexpression.








The above recipes describe the ordering of matches at a given position.
One more rule is needed to understand how a match is determined for the
whole regular expression: a match at an earlier position is always better
than a match at a later position.









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58.2.11 Creating Custom RE Engines



As of Perl 5.10.0, one can create custom regular expression engines.  This
is not for the faint of heart, as they have to plug in at the C level.  See
perlreapi NAME for more details.



As an alternative, overloaded constants (see (overload)) provide a simple
way to extend the functionality of the RE engine, by substituting one
pattern for another.



Suppose that we want to enable a new RE escape-sequence \Y| which
matches at a boundary between whitespace characters and non-whitespace
characters.  Note that (?=\S)(?<!\S)|(?!\S)(?<=\S) matches exactly
at these positions, so we want to have each \Y| in the place of the
more complicated version.  We can create a module customre to do
this:



    package customre;
    use overload;

    sub import {
      shift;
      die "No argument to customre::import allowed" if @_;
      overload::constant 'qr' => \&convert;
    }

    sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}

    # We must also take care of not escaping the legitimate \\Y|
    # sequence, hence the presence of '\\' in the conversion rules.
    my %rules = ( '\\' => '\\\\',
                  'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
    sub convert {
      my $re = shift;
      $re =~ s{
                \\ ( \\ | Y . )
              }
              { $rules{$1} or invalid($re,$1) }sgex;
      return $re;
    }



Now use customre enables the new escape in constant regular
expressions, i.e., those without any runtime variable interpolations.
As documented in (overload), this conversion will work only over
literal parts of regular expressions.  For \Y|$re\Y| the variable
part of this regular expression needs to be converted explicitly
(but only if the special meaning of \Y| should be enabled inside $re):



    use customre;
    $re = <>;
    chomp $re;
    $re = customre::convert $re;
    /\Y|$re\Y|/;









Previous: NAME regular expression regex regexp Creating Custom RE Engines, Up: NAME regular expression regex regexp DESCRIPTION   [Contents]





58.2.12 PCRE/Python Support



As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions
to the regex syntax. While Perl programmers are encouraged to use the
Perl-specific syntax, the following are also accepted:





(?P<NAME>pattern)




Define a named capture group. Equivalent to (?<NAME>pattern).





(?P=NAME)




Backreference to a named capture group. Equivalent to \g{NAME}.





(?P>NAME)




Subroutine call to a named capture group. Equivalent to (?&NAME).














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58.3 BUGS



Many regular expression constructs don’t work on EBCDIC platforms.



There are a number of issues with regard to case-insensitive matching
in Unicode rules.  See i under Modifiers above.



This document varies from difficult to understand to completely
and utterly opaque.  The wandering prose riddled with jargon is
hard to fathom in several places.



This document needs a rewrite that separates the tutorial content
from the reference content.









Previous: NAME regular expression regex regexp BUGS, Up: NAME regular expression regex regexp   [Contents]





58.4 SEE ALSO



perlrequick NAME.



perlretut NAME.



(perlop)Regexp Quote-Like Operators.



(perlop)Gory details of parsing quoted constructs.



(perlfaq6).



(perlfunc)pos.



perllocale NAME.



perlebcdic NAME.



Mastering Regular Expressions by Jeffrey Friedl, published
by O’Reilly and Associates.









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59 perlreapi





• perlreapi NAME:  



• perlreapi DESCRIPTION:  



• perlreapi Callbacks:  



• perlreapi The REGEXP structure:  



• perlreapi HISTORY:  



• perlreapi AUTHORS:  



• perlreapi LICENSE:  












Next: , Up: perlreapi   [Contents]





59.1 NAME



perlreapi - perl regular expression plugin interface









Next: , Previous: perlreapi NAME, Up: perlreapi   [Contents]





59.2 DESCRIPTION



As of Perl 5.9.5 there is a new interface for plugging and using other
regular expression engines than the default one.



Each engine is supposed to provide access to a constant structure of the
following format:



    typedef struct regexp_engine {
        REGEXP* (*comp) (pTHX_ const SV * const pattern, const U32 flags);
        I32     (*exec) (pTHX_ REGEXP * const rx, char* stringarg, char* strend,
                         char* strbeg, I32 minend, SV* screamer,
                         void* data, U32 flags);
        char*   (*intuit) (pTHX_ REGEXP * const rx, SV *sv, char *strpos,
                           char *strend, U32 flags,
                           struct re_scream_pos_data_s *data);
        SV*     (*checkstr) (pTHX_ REGEXP * const rx);
        void    (*free) (pTHX_ REGEXP * const rx);
        void    (*numbered_buff_FETCH) (pTHX_ REGEXP * const rx, const I32 paren,
                                 SV * const sv);
        void    (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
                                       SV const * const value);
        I32     (*numbered_buff_LENGTH) (pTHX_ REGEXP * const rx, const SV * const sv,
                                        const I32 paren);
        SV*     (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
                               SV * const value, U32 flags);
        SV*     (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
                                    const U32 flags);
        SV*     (*qr_package)(pTHX_ REGEXP * const rx);
    #ifdef USE_ITHREADS
        void*   (*dupe) (pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
    #endif



When a regexp is compiled, its engine field is then set to point at
the appropriate structure, so that when it needs to be used Perl can find
the right routines to do so.



In order to install a new regexp handler, $^H{regcomp} is set
to an integer which (when casted appropriately) resolves to one of these
structures. When compiling, the comp method is executed, and the
resulting regexp structure’s engine field is expected to point back at
the same structure.



The pTHX_ symbol in the definition is a macro used by perl under threading
to provide an extra argument to the routine holding a pointer back to
the interpreter that is executing the regexp. So under threading all
routines get an extra argument.









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59.3 Callbacks





• perlreapi comp:  



• perlreapi exec:  



• perlreapi intuit:  



• perlreapi checkstr:  



• perlreapi free:  



• perlreapi Numbered capture callbacks:  



• perlreapi Named capture callbacks:  



• perlreapi qr_package:  



• perlreapi dupe:  












Next: , Up: perlreapi Callbacks   [Contents]





59.3.1 comp



    REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);



Compile the pattern stored in pattern using the given flags and
return a pointer to a prepared REGEXP structure that can perform
the match. See The REGEXP structure below for an explanation of
the individual fields in the REGEXP struct.



The pattern parameter is the scalar that was used as the
pattern. previous versions of perl would pass two char* indicating
the start and end of the stringified pattern, the following snippet can
be used to get the old parameters:



    STRLEN plen;
    char*  exp = SvPV(pattern, plen);
    char* xend = exp + plen;



Since any scalar can be passed as a pattern it’s possible to implement
an engine that does something with an array ("ook" =~ [ qw/ eek
hlagh / ]) or with the non-stringified form of a compiled regular
expression ("ook" =~ qr/eek/). perl’s own engine will always
stringify everything using the snippet above but that doesn’t mean
other engines have to.



The flags parameter is a bitfield which indicates which of the
msixp flags the regex was compiled with. It also contains
additional info such as whether use locale is in effect.



The eogc flags are stripped out before being passed to the comp
routine. The regex engine does not need to know whether any of these
are set as those flags should only affect what perl does with the
pattern and its match variables, not how it gets compiled and
executed.



By the time the comp callback is called, some of these flags have
already had effect (noted below where applicable). However most of
their effect occurs after the comp callback has run in routines that
read the rx->extflags field which it populates.



In general the flags should be preserved in rx->extflags after
compilation, although the regex engine might want to add or delete
some of them to invoke or disable some special behavior in perl. The
flags along with any special behavior they cause are documented below:



The pattern modifiers:





/m - RXf_PMf_MULTILINE




If this is in rx->extflags it will be passed to
Perl_fbm_instr by pp_split which will treat the subject string
as a multi-line string.





/s - RXf_PMf_SINGLELINE






/i - RXf_PMf_FOLD






/x - RXf_PMf_EXTENDED




If present on a regex # comments will be handled differently by the
tokenizer in some cases.



TODO: Document those cases.





/p - RXf_PMf_KEEPCOPY




TODO: Document this





Character set




The character set semantics are determined by an enum that is contained
in this field.  This is still experimental and subject to change, but
the current interface returns the rules by use of the in-line function
get_regex_charset(const U32 flags).  The only currently documented
value returned from it is REGEX_LOCALE_CHARSET, which is set if
use locale is in effect. If present in rx->extflags,
split will use the locale dependent definition of whitespace
when RXf_SKIPWHITE or RXf_WHITE is in effect. ASCII whitespace
is defined as per (perlapi)isSPACE, and by the internal
macros is_utf8_space under UTF-8, and isSPACE_LC under use
locale.








Additional flags:





RXf_UTF8




Set if the pattern is (perlapi)SvUTF8(), set by Perl_pmruntime.



A regex engine may want to set or disable this flag during
compilation. The perl engine for instance may upgrade non-UTF-8
strings to UTF-8 if the pattern includes constructs such as \x{...}
that can only match Unicode values.





RXf_SPLIT




If split is invoked as split ' ' or with no arguments (which
really means split(' ', $_), see (perlfunc)split), perl will
set this flag. The regex engine can then check for it and set the
SKIPWHITE and WHITE extflags. To do this the perl engine does:



    if (flags & RXf_SPLIT && r->prelen == 1 && r->precomp[0] == ' ')
        r->extflags |= (RXf_SKIPWHITE|RXf_WHITE);








These flags can be set during compilation to enable optimizations in
the split operator.





RXf_SKIPWHITE




If the flag is present in rx->extflags split will delete
whitespace from the start of the subject string before it’s operated
on. What is considered whitespace depends on whether the subject is a
UTF-8 string and whether the RXf_PMf_LOCALE flag is set.



If RXf_WHITE is set in addition to this flag split will behave like
split " " under the perl engine.





RXf_START_ONLY




Tells the split operator to split the target string on newlines
(\n) without invoking the regex engine.



Perl’s engine sets this if the pattern is /^/ (plen == 1 && *exp
== '^'), even under /^/s, see (perlfunc)split. Of course a
different regex engine might want to use the same optimizations
with a different syntax.





RXf_WHITE




Tells the split operator to split the target string on whitespace
without invoking the regex engine. The definition of whitespace varies
depending on whether the target string is a UTF-8 string and on
whether RXf_PMf_LOCALE is set.



Perl’s engine sets this flag if the pattern is \s+.





RXf_NULL




Tells the split operator to split the target string on
characters. The definition of character varies depending on whether
the target string is a UTF-8 string.



Perl’s engine sets this flag on empty patterns, this optimization
makes split // much faster than it would otherwise be. It’s even
faster than unpack.














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59.3.2 exec



    I32 exec(pTHX_ REGEXP * const rx,
             char *stringarg, char* strend, char* strbeg,
             I32 minend, SV* screamer,
             void* data, U32 flags);



Execute a regexp.









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59.3.3 intuit



    char* intuit(pTHX_ REGEXP * const rx,
                  SV *sv, char *strpos, char *strend,
                  const U32 flags, struct re_scream_pos_data_s *data);



Find the start position where a regex match should be attempted,
or possibly whether the regex engine should not be run because the
pattern can’t match. This is called as appropriate by the core
depending on the values of the extflags member of the regexp
structure.









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59.3.4 checkstr



    SV* checkstr(pTHX_ REGEXP * const rx);



Return a SV containing a string that must appear in the pattern. Used
by split for optimising matches.









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59.3.5 free



    void free(pTHX_ REGEXP * const rx);



Called by perl when it is freeing a regexp pattern so that the engine
can release any resources pointed to by the pprivate member of the
regexp structure. This is only responsible for freeing private data;
perl will handle releasing anything else contained in the regexp structure.









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59.3.6 Numbered capture callbacks



Called to get/set the value of $`, $', $& and their named
equivalents, ${^PREMATCH}, ${^POSTMATCH} and $^{MATCH}, as well as the
numbered capture groups ($1, $2, ...).



The paren parameter will be -2 for $`, -1 for $', 0
for $&, 1 for $1 and so forth.



The names have been chosen by analogy with (Tie-Scalar) methods
names with an additional LENGTH callback for efficiency. However
named capture variables are currently not tied internally but
implemented via magic.





• perlreapi numbered_buff_FETCH:  



• perlreapi numbered_buff_STORE:  



• perlreapi numbered_buff_LENGTH:  












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59.3.6.1 numbered_buff_FETCH



    void numbered_buff_FETCH(pTHX_ REGEXP * const rx, const I32 paren,
                             SV * const sv);



Fetch a specified numbered capture. sv should be set to the scalar
to return, the scalar is passed as an argument rather than being
returned from the function because when it’s called perl already has a
scalar to store the value, creating another one would be
redundant. The scalar can be set with sv_setsv, sv_setpvn and
friends, see (perlapi).



This callback is where perl untaints its own capture variables under
taint mode (see perlsec NAME). See the Perl_reg_numbered_buff_fetch
function in regcomp.c for how to untaint capture variables if
that’s something you’d like your engine to do as well.









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59.3.6.2 numbered_buff_STORE



    void    (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
                                    SV const * const value);



Set the value of a numbered capture variable. value is the scalar
that is to be used as the new value. It’s up to the engine to make
sure this is used as the new value (or reject it).



Example:



    if ("ook" =~ /(o*)/) {
        # 'paren' will be '1' and 'value' will be 'ee'
        $1 =~ tr/o/e/;
    }



Perl’s own engine will croak on any attempt to modify the capture
variables, to do this in another engine use the following callback
(copied from Perl_reg_numbered_buff_store):



    void
    Example_reg_numbered_buff_store(pTHX_ REGEXP * const rx, const I32 paren,
                                                            SV const * const value)
    {
        PERL_UNUSED_ARG(rx);
        PERL_UNUSED_ARG(paren);
        PERL_UNUSED_ARG(value);

        if (!PL_localizing)
            Perl_croak(aTHX_ PL_no_modify);
    }



Actually perl will not always croak in a statement that looks
like it would modify a numbered capture variable. This is because the
STORE callback will not be called if perl can determine that it
doesn’t have to modify the value. This is exactly how tied variables
behave in the same situation:



    package CaptureVar;
    use base 'Tie::Scalar';

    sub TIESCALAR { bless [] }
    sub FETCH { undef }
    sub STORE { die "This doesn't get called" }

    package main;

    tie my $sv => "CaptureVar";
    $sv =~ y/a/b/;



Because $sv is undef when the y/// operator is applied to it
the transliteration won’t actually execute and the program won’t
die. This is different to how 5.8 and earlier versions behaved
since the capture variables were READONLY variables then, now they’ll
just die when assigned to in the default engine.









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59.3.6.3 numbered_buff_LENGTH



    I32 numbered_buff_LENGTH (pTHX_ REGEXP * const rx, const SV * const sv,
                              const I32 paren);



Get the length of a capture variable. There’s a special callback
for this so that perl doesn’t have to do a FETCH and run length on
the result, since the length is (in perl’s case) known from an offset
stored in rx->offs this is much more efficient:



    I32 s1  = rx->offs[paren].start;
    I32 s2  = rx->offs[paren].end;
    I32 len = t1 - s1;



This is a little bit more complex in the case of UTF-8, see what
Perl_reg_numbered_buff_length does with
(perlapi)is_utf8_string_loclen.









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59.3.7 Named capture callbacks



Called to get/set the value of %+ and %- as well as by some
utility functions in (re).



There are two callbacks, named_buff is called in all the cases the
FETCH, STORE, DELETE, CLEAR, EXISTS and SCALAR (Tie-Hash) callbacks
would be on changes to %+ and %- and named_buff_iter in the
same cases as FIRSTKEY and NEXTKEY.



The flags parameter can be used to determine which of these
operations the callbacks should respond to, the following flags are
currently defined:



Which (Tie-Hash) operation is being performed from the Perl level on
%+ or %+, if any:



    RXapif_FETCH
    RXapif_STORE
    RXapif_DELETE
    RXapif_CLEAR
    RXapif_EXISTS
    RXapif_SCALAR
    RXapif_FIRSTKEY
    RXapif_NEXTKEY



Whether %+ or %- is being operated on, if any.



    RXapif_ONE /* %+ */
    RXapif_ALL /* %- */



Whether this is being called as re::regname, re::regnames or
re::regnames_count, if any. The first two will be combined with
RXapif_ONE or RXapif_ALL.



    RXapif_REGNAME
    RXapif_REGNAMES
    RXapif_REGNAMES_COUNT



Internally %+ and %- are implemented with a real tied interface
via (Tie-Hash-NamedCapture). The methods in that package will call
back into these functions. However the usage of
(Tie-Hash-NamedCapture) for this purpose might change in future
releases. For instance this might be implemented by magic instead
(would need an extension to mgvtbl).





• perlreapi named_buff:  



• perlreapi named_buff_iter:  












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59.3.7.1 named_buff



    SV*     (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
                           SV * const value, U32 flags);









Previous: perlreapi named_buff, Up: perlreapi Named capture callbacks   [Contents]





59.3.7.2 named_buff_iter



    SV*     (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
                                const U32 flags);









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59.3.8 qr_package



    SV* qr_package(pTHX_ REGEXP * const rx);



The package the qr// magic object is blessed into (as seen by ref
qr//). It is recommended that engines change this to their package
name for identification regardless of whether they implement methods
on the object.



The package this method returns should also have the internal
Regexp package in its @ISA. qr//->isa("Regexp") should always
be true regardless of what engine is being used.



Example implementation might be:



    SV*
    Example_qr_package(pTHX_ REGEXP * const rx)
    {
        PERL_UNUSED_ARG(rx);
        return newSVpvs("re::engine::Example");
    }



Any method calls on an object created with qr// will be dispatched to the
package as a normal object.



    use re::engine::Example;
    my $re = qr//;
    $re->meth; # dispatched to re::engine::Example::meth()



To retrieve the REGEXP object from the scalar in an XS function use
the SvRX macro, see (perlapi)"REGEXP Functions" in perlapi.



    void meth(SV * rv)
    PPCODE:
        REGEXP * re = SvRX(sv);









Previous: perlreapi qr_package, Up: perlreapi Callbacks   [Contents]





59.3.9 dupe



    void* dupe(pTHX_ REGEXP * const rx, CLONE_PARAMS *param);



On threaded builds a regexp may need to be duplicated so that the pattern
can be used by multiple threads. This routine is expected to handle the
duplication of any private data pointed to by the pprivate member of
the regexp structure.  It will be called with the preconstructed new
regexp structure as an argument, the pprivate member will point at
the old private structure, and it is this routine’s responsibility to
construct a copy and return a pointer to it (which perl will then use to
overwrite the field as passed to this routine.)



This allows the engine to dupe its private data but also if necessary
modify the final structure if it really must.



On unthreaded builds this field doesn’t exist.









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59.4 The REGEXP structure



The REGEXP struct is defined in regexp.h. All regex engines must be able to
correctly build such a structure in their comp routine.



The REGEXP structure contains all the data that perl needs to be aware of
to properly work with the regular expression. It includes data about
optimisations that perl can use to determine if the regex engine should
really be used, and various other control info that is needed to properly
execute patterns in various contexts such as is the pattern anchored in
some way, or what flags were used during the compile, or whether the
program contains special constructs that perl needs to be aware of.



In addition it contains two fields that are intended for the private
use of the regex engine that compiled the pattern. These are the
intflags and pprivate members. pprivate is a void pointer to
an arbitrary structure whose use and management is the responsibility
of the compiling engine. perl will never modify either of these
values.



    typedef struct regexp {
        /* what engine created this regexp? */
        const struct regexp_engine* engine;

        /* what re is this a lightweight copy of? */
        struct regexp* mother_re;

        /* Information about the match that the perl core uses to manage things */
        U32 extflags;   /* Flags used both externally and internally */
        I32 minlen;     /* mininum possible length of string to match */
        I32 minlenret;  /* mininum possible length of $& */
        U32 gofs;       /* chars left of pos that we search from */

        /* substring data about strings that must appear
           in the final match, used for optimisations */
        struct reg_substr_data *substrs;

        U32 nparens;  /* number of capture groups */

        /* private engine specific data */
        U32 intflags;   /* Engine Specific Internal flags */
        void *pprivate; /* Data private to the regex engine which 
                           created this object. */

        /* Data about the last/current match. These are modified during matching*/
        U32 lastparen;            /* last open paren matched */
        U32 lastcloseparen;       /* last close paren matched */
        regexp_paren_pair *swap;  /* Swap copy of *offs */
        regexp_paren_pair *offs;  /* Array of offsets for (@-) and (@+) */

        char *subbeg;  /* saved or original string so \digit works forever. */
        SV_SAVED_COPY  /* If non-NULL, SV which is COW from original */
        I32 sublen;    /* Length of string pointed by subbeg */

        /* Information about the match that isn't often used */
        I32 prelen;           /* length of precomp */
        const char *precomp;  /* pre-compilation regular expression */

        char *wrapped;  /* wrapped version of the pattern */
        I32 wraplen;    /* length of wrapped */

        I32 seen_evals;   /* number of eval groups in the pattern - for security checks */
        HV *paren_names;  /* Optional hash of paren names */

        /* Refcount of this regexp */
        I32 refcnt;             /* Refcount of this regexp */
    } regexp;



The fields are discussed in more detail below:





• perlreapi engine:  



• perlreapi mother_re:  



• perlreapi extflags:  



• perlreapi minlen minlenret:  



• perlreapi gofs:  



• perlreapi substrs:  



• perlreapi nparens, lastparen, and lastcloseparen:  



• perlreapi intflags:  



• perlreapi pprivate:  



• perlreapi swap:  



• perlreapi offs:  



• perlreapi precomp prelen:  



• perlreapi paren_names:  



• perlreapi substrs 1:  



• perlreapi subbeg sublen saved_copy:  



• perlreapi wrapped wraplen:  



• perlreapi seen_evals:  



• perlreapi refcnt:  












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59.4.1 engine



This field points at a regexp_engine structure which contains pointers
to the subroutines that are to be used for performing a match. It
is the compiling routine’s responsibility to populate this field before
returning the regexp object.



Internally this is set to NULL unless a custom engine is specified in
$^H{regcomp}, perl’s own set of callbacks can be accessed in the struct
pointed to by RE_ENGINE_PTR.









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59.4.2 mother_re



TODO, see http://www.mail-archive.com/perl5-changes@perl.org/msg17328.html









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59.4.3 extflags



This will be used by perl to see what flags the regexp was compiled
with, this will normally be set to the value of the flags parameter by
the comp callback. See the comp documentation for
valid flags.









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59.4.4 minlen minlenret



The minimum string length required for the pattern to match.  This is used to
prune the search space by not bothering to match any closer to the end of a
string than would allow a match. For instance there is no point in even
starting the regex engine if the minlen is 10 but the string is only 5
characters long. There is no way that the pattern can match.



minlenret is the minimum length of the string that would be found
in $& after a match.



The difference between minlen and minlenret can be seen in the
following pattern:



    /ns(?=\d)/



where the minlen would be 3 but minlenret would only be 2 as the \d is
required to match but is not actually included in the matched content. This
distinction is particularly important as the substitution logic uses the
minlenret to tell whether it can do in-place substitution which can result in
considerable speedup.









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59.4.5 gofs



Left offset from pos() to start match at.









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59.4.6 substrs



Substring data about strings that must appear in the final match. This
is currently only used internally by perl’s engine for but might be
used in the future for all engines for optimisations.









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59.4.7 nparens, lastparen, and lastcloseparen



These fields are used to keep track of how many paren groups could be matched
in the pattern, which was the last open paren to be entered, and which was
the last close paren to be entered.









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59.4.8 intflags



The engine’s private copy of the flags the pattern was compiled with. Usually
this is the same as extflags unless the engine chose to modify one of them.









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59.4.9 pprivate



A void* pointing to an engine-defined data structure. The perl engine uses the
regexp_internal structure (see perlreguts Base Structures) but a custom
engine should use something else.









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59.4.10 swap



Unused. Left in for compatibility with perl 5.10.0.









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59.4.11 offs



A regexp_paren_pair structure which defines offsets into the string being
matched which correspond to the $& and $1, $2 etc. captures, the
regexp_paren_pair struct is defined as follows:



    typedef struct regexp_paren_pair {
        I32 start;
        I32 end;
    } regexp_paren_pair;



If ->offs[num].start or ->offs[num].end is -1 then that
capture group did not match. ->offs[0].start/end represents $& (or
${^MATCH under //p) and ->offs[paren].end matches $$paren where
$paren = 1>.









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59.4.12 precomp prelen



Used for optimisations. precomp holds a copy of the pattern that
was compiled and prelen its length. When a new pattern is to be
compiled (such as inside a loop) the internal regcomp operator
checks whether the last compiled REGEXP’s precomp and prelen
are equivalent to the new one, and if so uses the old pattern instead
of compiling a new one.



The relevant snippet from Perl_pp_regcomp:



        if (!re || !re->precomp || re->prelen != (I32)len ||
            memNE(re->precomp, t, len))
        /* Compile a new pattern */









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59.4.13 paren_names



This is a hash used internally to track named capture groups and their
offsets. The keys are the names of the buffers the values are dualvars,
with the IV slot holding the number of buffers with the given name and the
pv being an embedded array of I32.  The values may also be contained
independently in the data array in cases where named backreferences are
used.









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59.4.14 substrs



Holds information on the longest string that must occur at a fixed
offset from the start of the pattern, and the longest string that must
occur at a floating offset from the start of the pattern. Used to do
Fast-Boyer-Moore searches on the string to find out if its worth using
the regex engine at all, and if so where in the string to search.









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59.4.15 subbeg sublen saved_copy



Used during execution phase for managing search and replace patterns.









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59.4.16 wrapped wraplen



Stores the string qr// stringifies to. The perl engine for example
stores (?^:eek) in the case of qr/eek/.



When using a custom engine that doesn’t support the (?:) construct
for inline modifiers, it’s probably best to have qr// stringify to
the supplied pattern, note that this will create undesired patterns in
cases such as:



    my $x = qr/a|b/;  # "a|b"
    my $y = qr/c/i;   # "c"
    my $z = qr/$x$y/; # "a|bc"



There’s no solution for this problem other than making the custom
engine understand a construct like (?:).









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59.4.17 seen_evals



This stores the number of eval groups in the pattern. This is used for security
purposes when embedding compiled regexes into larger patterns with qr//.









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59.4.18 refcnt



The number of times the structure is referenced. When this falls to 0 the
regexp is automatically freed by a call to pregfree. This should be set to 1 in
each engine’s comp routine.









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59.5 HISTORY



Originally part of perlreguts NAME.









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59.6 AUTHORS



Originally written by Yves Orton, expanded by Ævar Arnfjörð
Bjarmason.









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59.7 LICENSE



Copyright 2006 Yves Orton and 2007 Ævar Arnfjörð Bjarmason.



This program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.









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60 perlrebackslash





• perlrebackslash NAME:  



• perlrebackslash DESCRIPTION:  












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60.1 NAME



perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes









Previous: perlrebackslash NAME, Up: perlrebackslash   [Contents]





60.2 DESCRIPTION



The top level documentation about Perl regular expressions
is found in (perlre).



This document describes all backslash and escape sequences. After
explaining the role of the backslash, it lists all the sequences that have
a special meaning in Perl regular expressions (in alphabetical order),
then describes each of them.



Most sequences are described in detail in different documents; the primary
purpose of this document is to have a quick reference guide describing all
backslash and escape sequences.





• perlrebackslash The backslash:  



• perlrebackslash All the sequences and escapes:  



• perlrebackslash Character Escapes:  



• perlrebackslash Modifiers:  



• perlrebackslash Character classes:  



• perlrebackslash Referencing:  



• perlrebackslash Assertions:  



• perlrebackslash Misc:  












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60.2.1 The backslash



In a regular expression, the backslash can perform one of two tasks:
it either takes away the special meaning of the character following it
(for instance, \| matches a vertical bar, it’s not an alternation),
or it is the start of a backslash or escape sequence.



The rules determining what it is are quite simple: if the character
following the backslash is an ASCII punctuation (non-word) character (that is,
anything that is not a letter, digit, or underscore), then the backslash just
takes away any special meaning of the character following it.



If the character following the backslash is an ASCII letter or an ASCII digit,
then the sequence may be special; if so, it’s listed below. A few letters have
not been used yet, so escaping them with a backslash doesn’t change them to be
special.  A future version of Perl may assign a special meaning to them, so if
you have warnings turned on, Perl issues a warning if you use such a
sequence.  [1].



It is however guaranteed that backslash or escape sequences never have a
punctuation character following the backslash, not now, and not in a future
version of Perl 5. So it is safe to put a backslash in front of a non-word
character.



Note that the backslash itself is special; if you want to match a backslash,
you have to escape the backslash with a backslash: /\\/ matches a single
backslash.





[1]




There is one exception. If you use an alphanumeric character as the
delimiter of your pattern (which you probably shouldn’t do for readability
reasons), you have to escape the delimiter if you want to match
it. Perl won’t warn then. See also (perlop)Gory details of parsing
quoted constructs.














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60.2.2 All the sequences and escapes



Those not usable within a bracketed character class (like [\da-z]) are marked
as Not in [].



 \000              Octal escape sequence.  See also \o{}.
 \1                Absolute backreference.  Not in [].
 \a                Alarm or bell.
 \A                Beginning of string.  Not in [].
 \b                Word/non-word boundary. (Backspace in []).
 \B                Not a word/non-word boundary.  Not in [].
 \cX               Control-X
 \C                Single octet, even under UTF-8.  Not in [].
 \d                Character class for digits.
 \D                Character class for non-digits.
 \e                Escape character.
 \E                Turn off \Q, \L and \U processing.  Not in [].
 \f                Form feed.
 \F                Foldcase till \E.  Not in [].
 \g{}, \g1         Named, absolute or relative backreference.  Not in []
 \G                Pos assertion.  Not in [].
 \h                Character class for horizontal whitespace.
 \H                Character class for non horizontal whitespace.
 \k{}, \k<>, \k''  Named backreference.  Not in [].
 \K                Keep the stuff left of \K.  Not in [].
 \l                Lowercase next character.  Not in [].
 \L                Lowercase till \E.  Not in [].
 \n                (Logical) newline character.
 \N                Any character but newline.  Experimental.  Not in [].
 \N{}              Named or numbered (Unicode) character or sequence.
 \o{}              Octal escape sequence.
 \p{}, \pP         Character with the given Unicode property.
 \P{}, \PP         Character without the given Unicode property.
 \Q                Quote (disable) pattern metacharacters till \E.  Not
                   in [].
 \r                Return character.
 \R                Generic new line.  Not in [].
 \s                Character class for whitespace.
 \S                Character class for non whitespace.
 \t                Tab character.
 \u                Titlecase next character.  Not in [].
 \U                Uppercase till \E.  Not in [].
 \v                Character class for vertical whitespace.
 \V                Character class for non vertical whitespace.
 \w                Character class for word characters.
 \W                Character class for non-word characters.
 \x{}, \x00        Hexadecimal escape sequence.
 \X                Unicode "extended grapheme cluster".  Not in [].
 \z                End of string.  Not in [].
 \Z                End of string.  Not in [].









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60.2.3 Character Escapes





• perlrebackslash Fixed characters:  



• perlrebackslash Example:  



• perlrebackslash Control characters:  



• perlrebackslash Example 1:  



• perlrebackslash Named or numbered characters and character sequences:  



• perlrebackslash Example 2:  



• perlrebackslash Octal escapes:  



• perlrebackslash Examples (assuming an ASCII platform):  



• perlrebackslash Disambiguation rules between old-style octal escapes and backreferences:  



• perlrebackslash Hexadecimal escapes:  



• perlrebackslash Examples (assuming an ASCII platform) 1:  












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60.2.3.1 Fixed characters



A handful of characters have a dedicated character escape. The following
table shows them, along with their ASCII code points (in decimal and hex),
their ASCII name, the control escape on ASCII platforms and a short
description.  (For EBCDIC platforms, see perlebcdic OPERATOR DIFFERENCES.)



 Seq.  Code Point  ASCII   Cntrl   Description.
       Dec    Hex
  \a     7     07    BEL    \cG    alarm or bell
  \b     8     08     BS    \cH    backspace [1]
  \e    27     1B    ESC    \c[    escape character
  \f    12     0C     FF    \cL    form feed
  \n    10     0A     LF    \cJ    line feed [2]
  \r    13     0D     CR    \cM    carriage return
  \t     9     09    TAB    \cI    tab





[1]




\b is the backspace character only inside a character class. Outside a
character class, \b is a word/non-word boundary.





[2]




\n matches a logical newline. Perl converts between \n and your
OS’s native newline character when reading from or writing to text files.














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60.2.3.2 Example



 $str =~ /\t/;   # Matches if $str contains a (horizontal) tab.









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60.2.3.3 Control characters



\c is used to denote a control character; the character following \c
determines the value of the construct.  For example the value of \cA is
chr(1), and the value of \cb is chr(2), etc.
The gory details are in (perlop)Regexp Quote-Like Operators.  A complete
list of what chr(1), etc. means for ASCII and EBCDIC platforms is in
perlebcdic OPERATOR DIFFERENCES.



Note that \c\ alone at the end of a regular expression (or doubled-quoted
string) is not valid.  The backslash must be followed by another character.
That is, \c\X means chr(28) . 'X' for all characters X.



To write platform-independent code, you must use \N{NAME} instead, like
\N{ESCAPE} or \N{U+001B}, see (charnames).



Mnemonic: control character.









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60.2.3.4 Example



 $str =~ /\cK/;  # Matches if $str contains a vertical tab (control-K).









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60.2.3.5 Named or numbered characters and character sequences



Unicode characters have a Unicode name and numeric code point (ordinal)
value.  Use the
\N{} construct to specify a character by either of these values.
Certain sequences of characters also have names.



To specify by name, the name of the character or character sequence goes
between the curly braces.



To specify a character by Unicode code point, use the form \N{U+code
point}, where code point is a number in hexadecimal that gives the
code point that Unicode has assigned to the desired character.  It is
customary but not required to use leading zeros to pad the number to 4
digits.  Thus \N{U+0041} means LATIN CAPITAL LETTER A, and you will
rarely see it written without the two leading zeros.  \N{U+0041} means
"A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).



It is even possible to give your own names to characters and character
sequences.  For details, see (charnames).



(There is an expanded internal form that you may see in debug output:
\N{U+code point.code point...}.
The ... means any number of these code points separated by dots.
This represents the sequence formed by the characters.  This is an internal
form only, subject to change, and you should not try to use it yourself.)



Mnemonic: Named character.



Note that a character or character sequence expressed as a named
or numbered character is considered a character without special
meaning by the regex engine, and will match "as is".









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60.2.3.6 Example



 $str =~ /\N{THAI CHARACTER SO SO}/;  # Matches the Thai SO SO character

 use charnames 'Cyrillic';            # Loads Cyrillic names.
 $str =~ /\N{ZHE}\N{KA}/;             # Match "ZHE" followed by "KA".









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60.2.3.7 Octal escapes



There are two forms of octal escapes.  Each is used to specify a character by
its code point specified in octal notation.



One form, available starting in Perl 5.14 looks like \o{...}, where the dots
represent one or more octal digits.  It can be used for any Unicode character.



It was introduced to avoid the potential problems with the other form,
available in all Perls.  That form consists of a backslash followed by three
octal digits.  One problem with this form is that it can look exactly like an
old-style backreference (see
Disambiguation rules between old-style octal escapes and backreferences
below.)  You can avoid this by making the first of the three digits always a
zero, but that makes \077 the largest code point specifiable.



In some contexts, a backslash followed by two or even one octal digits may be
interpreted as an octal escape, sometimes with a warning, and because of some
bugs, sometimes with surprising results.  Also, if you are creating a regex
out of smaller snippets concatenated together, and you use fewer than three
digits, the beginning of one snippet may be interpreted as adding digits to the
ending of the snippet before it.  See Absolute referencing for more
discussion and examples of the snippet problem.



Note that a character expressed as an octal escape is considered
a character without special meaning by the regex engine, and will match
"as is".



To summarize, the \o{} form is always safe to use, and the other form is
safe to use for code points through \077 when you use exactly three digits to
specify them.



Mnemonic: 0ctal or octal.









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60.2.3.8 Examples (assuming an ASCII platform)



 $str = "Perl";
 $str =~ /\o{120}/;  # Match, "\120" is "P".
 $str =~ /\120/;     # Same.
 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
 $str =~ /\120+/;    # Same.
 $str =~ /P\053/;    # No match, "\053" is "+" and taken literally.
 /\o{23073}/         # Black foreground, white background smiling face.
 /\o{4801234567}/    # Raises a warning, and yields chr(4)









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60.2.3.9 Disambiguation rules between old-style octal escapes and backreferences



Octal escapes of the \000 form outside of bracketed character classes
potentially clash with old-style backreferences.  (see Absolute referencing
below).  They both consist of a backslash followed by numbers.  So Perl has to
use heuristics to determine whether it is a backreference or an octal escape.
Perl uses the following rules to disambiguate:



    

  1.  If the backslash is followed by a single digit, it’s a backreference.


  2.  If the first digit following the backslash is a 0, it’s an octal escape.


  3.  If the number following the backslash is N (in decimal), and Perl already
has seen N capture groups, Perl considers this a backreference.  Otherwise,
it considers it an octal escape. If N has more than three digits, Perl
takes only the first three for the octal escape; the rest are matched as is.


     my $pat  = "(" x 999;
    $pat .= "a";
    $pat .= ")" x 999;
 /^($pat)\1000$/;   #  Matches 'aa'; there are 1000 capture groups.
 /^$pat\1000$/;     #  Matches 'a@0'; there are 999 capture groups
                    #    and \1000 is seen as \100 (a '@') and a '0'

    




You can force a backreference interpretation always by using the \g{...}
form.  You can the force an octal interpretation always by using the \o{...}
form, or for numbers up through \077 (= 63 decimal), by using three digits,
beginning with a "0".









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60.2.3.10 Hexadecimal escapes



Like octal escapes, there are two forms of hexadecimal escapes, but both start
with the same thing, \x.  This is followed by either exactly two hexadecimal
digits forming a number, or a hexadecimal number of arbitrary length surrounded
by curly braces. The hexadecimal number is the code point of the character you
want to express.



Note that a character expressed as one of these escapes is considered a
character without special meaning by the regex engine, and will match
"as is".



Mnemonic: hexadecimal.









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60.2.3.11 Examples (assuming an ASCII platform)



 $str = "Perl";
 $str =~ /\x50/;    # Match, "\x50" is "P".
 $str =~ /\x50+/;   # Match, "\x50" is "P", it is repeated at least once
 $str =~ /P\x2B/;   # No match, "\x2B" is "+" and taken literally.

 /\x{2603}\x{2602}/ # Snowman with an umbrella.
                    # The Unicode character 2603 is a snowman,
                    # the Unicode character 2602 is an umbrella.
 /\x{263B}/         # Black smiling face.
 /\x{263b}/         # Same, the hex digits A - F are case insensitive.









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60.2.4 Modifiers



A number of backslash sequences have to do with changing the character,
or characters following them. \l will lowercase the character following
it, while \u will uppercase (or, more accurately, titlecase) the
character following it. They provide functionality similar to the
functions lcfirst and ucfirst.



To uppercase or lowercase several characters, one might want to use
\L or \U, which will lowercase/uppercase all characters following
them, until either the end of the pattern or the next occurrence of
\E, whichever comes first. They provide functionality similar to what
the functions lc and uc provide.



\Q is used to quote (disable) pattern metacharacters, up to the next
\E or the end of the pattern. \Q adds a backslash to any character
that could have special meaning to Perl.  In the ASCII range, it quotes
every character that isn’t a letter, digit, or underscore.  See
(perlfunc)quotemeta for details on what gets quoted for non-ASCII
code points.  Using this ensures that any character between \Q and
\E will be matched literally, not interpreted as a metacharacter by
the regex engine.



\F can be used to casefold all characters following, up to the next \E
or the end of the pattern. It provides the functionality similar to
the fc function.



Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.





• perlrebackslash Examples:  












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60.2.4.1 Examples



 $sid     = "sid";
 $greg    = "GrEg";
 $miranda = "(Miranda)";
 $str     =~ /\u$sid/;        # Matches 'Sid'
 $str     =~ /\L$greg/;       # Matches 'greg'
 $str     =~ /\Q$miranda\E/;  # Matches '(Miranda)', as if the pattern
                              #   had been written as /\(Miranda\)/









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60.2.5 Character classes



Perl regular expressions have a large range of character classes. Some of
the character classes are written as a backslash sequence. We will briefly
discuss those here; full details of character classes can be found in
(perlrecharclass).



\w is a character class that matches any single word character
(letters, digits, Unicode marks, and connector punctuation (like the
underscore)).  \d is a character class that matches any decimal
digit, while the character class \s matches any whitespace character.
New in perl 5.10.0 are the classes \h and \v which match horizontal
and vertical whitespace characters.



The exact set of characters matched by \d, \s, and \w varies
depending on various pragma and regular expression modifiers.  It is
possible to restrict the match to the ASCII range by using the /a
regular expression modifier.  See (perlrecharclass).



The uppercase variants (\W, \D, \S, \H, and \V) are
character classes that match, respectively, any character that isn’t a
word character, digit, whitespace, horizontal whitespace, or vertical
whitespace.



Mnemonics: word, digit, space, horizontal, vertical.





• perlrebackslash Unicode classes:  












Up: perlrebackslash Character classes   [Contents]





60.2.5.1 Unicode classes



\pP (where P is a single letter) and \p{Property} are used to
match a character that matches the given Unicode property; properties
include things like "letter", or "thai character". Capitalizing the
sequence to \PP and \P{Property} make the sequence match a character
that doesn’t match the given Unicode property. For more details, see
(perlrecharclass)Backslash sequences and
perlunicode Unicode Character Properties.



Mnemonic: property.









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60.2.6 Referencing



If capturing parenthesis are used in a regular expression, we can refer
to the part of the source string that was matched, and match exactly the
same thing. There are three ways of referring to such backreference:
absolutely, relatively, and by name.





• perlrebackslash Absolute referencing:  



• perlrebackslash Examples 1:  



• perlrebackslash Relative referencing:  



• perlrebackslash Examples 2:  



• perlrebackslash Named referencing:  



• perlrebackslash Examples 3:  












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60.2.6.1 Absolute referencing



Either \gN (starting in Perl 5.10.0), or \N (old-style) where N
is a positive (unsigned) decimal number of any length is an absolute reference
to a capturing group.



N refers to the Nth set of parentheses, so \gN refers to whatever has
been matched by that set of parentheses.  Thus \g1 refers to the first
capture group in the regex.



The \gN form can be equivalently written as \g{N}
which avoids ambiguity when building a regex by concatenating shorter
strings.  Otherwise if you had a regex qr/$a$b/, and $a contained
"\g1", and $b contained "37", you would get /\g137/ which is
probably not what you intended.



In the \N form, N must not begin with a "0", and there must be at
least N capturing groups, or else N is considered an octal escape
(but something like \18 is the same as \0018; that is, the octal escape
"\001" followed by a literal digit "8").



Mnemonic: group.









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60.2.6.2 Examples



 /(\w+) \g1/;    # Finds a duplicated word, (e.g. "cat cat").
 /(\w+) \1/;     # Same thing; written old-style
 /(.)(.)\g2\g1/;  # Match a four letter palindrome (e.g. "ABBA").









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60.2.6.3 Relative referencing



\g-N (starting in Perl 5.10.0) is used for relative addressing.  (It can
be written as \g{-N.)  It refers to the Nth group before the
\g{-N}.



The big advantage of this form is that it makes it much easier to write
patterns with references that can be interpolated in larger patterns,
even if the larger pattern also contains capture groups.









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60.2.6.4 Examples



 /(A)        # Group 1
  (          # Group 2
    (B)      # Group 3
    \g{-1}   # Refers to group 3 (B)
    \g{-3}   # Refers to group 1 (A)
  )
 /x;         # Matches "ABBA".

 my $qr = qr /(.)(.)\g{-2}\g{-1}/;  # Matches 'abab', 'cdcd', etc.
 /$qr$qr/                           # Matches 'ababcdcd'.









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60.2.6.5 Named referencing



\g{name} (starting in Perl 5.10.0) can be used to back refer to a
named capture group, dispensing completely with having to think about capture
buffer positions.



To be compatible with .Net regular expressions, \g{name} may also be
written as \k{name}, \k<name> or \k'name'.



To prevent any ambiguity, name must not start with a digit nor contain a
hyphen.









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60.2.6.6 Examples



 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
 /(?<word>\w+) \k{word}/ # Same.
 /(?<word>\w+) \k<word>/ # Same.
 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
                         # Match a four letter palindrome (e.g. "ABBA")









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60.2.7 Assertions



Assertions are conditions that have to be true; they don’t actually
match parts of the substring. There are six assertions that are written as
backslash sequences.





\A




\A only matches at the beginning of the string. If the /m modifier
isn’t used, then /\A/ is equivalent to /^/. However, if the /m
modifier is used, then /^/ matches internal newlines, but the meaning
of /\A/ isn’t changed by the /m modifier. \A matches at the beginning
of the string regardless whether the /m modifier is used.





\z, \Z




\z and \Z match at the end of the string. If the /m modifier isn’t
used, then /\Z/ is equivalent to /$/; that is, it matches at the
end of the string, or one before the newline at the end of the string. If the
/m modifier is used, then /$/ matches at internal newlines, but the
meaning of /\Z/ isn’t changed by the /m modifier. \Z matches at
the end of the string (or just before a trailing newline) regardless whether
the /m modifier is used.



\z is just like \Z, except that it does not match before a trailing
newline. \z matches at the end of the string only, regardless of the
modifiers used, and not just before a newline.  It is how to anchor the
match to the true end of the string under all conditions.





\G




\G is usually used only in combination with the /g modifier. If the
/g modifier is used and the match is done in scalar context, Perl 
remembers where in the source string the last match ended, and the next time,
it will start the match from where it ended the previous time.



\G matches the point where the previous match on that string ended, 
or the beginning of that string if there was no previous match.



Mnemonic: Global.





\b, \B




\b matches at any place between a word and a non-word character; \B
matches at any place between characters where \b doesn’t match. \b
and \B assume there’s a non-word character before the beginning and after
the end of the source string; so \b will match at the beginning (or end)
of the source string if the source string begins (or ends) with a word
character. Otherwise, \B will match.   



Do not use something like \b=head\d\b and expect it to match the
beginning of a line.  It can’t, because for there to be a boundary before
the non-word "=", there must be a word character immediately previous.  
All boundary determinations look for word characters alone, not for
non-words characters nor for string ends.  It may help to understand how
<\b> and <\B> work by equating them as follows:



    \b  really means    (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
    \B  really means    (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))



Mnemonic: boundary.










• perlrebackslash Examples 4:  












Up: perlrebackslash Assertions   [Contents]





60.2.7.1 Examples



  "cat"   =~ /\Acat/;     # Match.
  "cat"   =~ /cat\Z/;     # Match.
  "cat\n" =~ /cat\Z/;     # Match.
  "cat\n" =~ /cat\z/;     # No match.

  "cat"   =~ /\bcat\b/;   # Matches.
  "cats"  =~ /\bcat\b/;   # No match.
  "cat"   =~ /\bcat\B/;   # No match.
  "cats"  =~ /\bcat\B/;   # Match.

  while ("cat dog" =~ /(\w+)/g) {
      print $1;           # Prints 'catdog'
  }
  while ("cat dog" =~ /\G(\w+)/g) {
      print $1;           # Prints 'cat'
  }









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60.2.8 Misc



Here we document the backslash sequences that don’t fall in one of the
categories above. These are:





\C




\C always matches a single octet, even if the source string is encoded
in UTF-8 format, and the character to be matched is a multi-octet character.
\C was introduced in perl 5.6.  This is very dangerous, because it violates
the logical character abstraction and can cause UTF-8 sequences to become malformed.



Mnemonic: oCtet.





\K




This appeared in perl 5.10.0. Anything matched left of \K is
not included in $&, and will not be replaced if the pattern is
used in a substitution. This lets you write s/PAT1 \K PAT2/REPL/x
instead of s/(PAT1) PAT2/${1}REPL/x or s/(?<=PAT1) PAT2/REPL/x.



Mnemonic: Keep.





\N




This is an experimental feature new to perl 5.12.0.  It matches any character
that is not a newline.  It is a short-hand for writing [^\n], and is
identical to the . metasymbol, except under the /s flag, which changes
the meaning of ., but not \N.



Note that \N{...} can mean a
named or numbered character.



Mnemonic: Complement of \n.





\R





\R matches a generic newline; that is, anything considered a
linebreak sequence by Unicode. This includes all characters matched by
\v (vertical whitespace), and the multi character sequence "\x0D\x0A"
(carriage return followed by a line feed, sometimes called the network
newline; it’s the end of line sequence used in Microsoft text files opened
in binary mode). \R is equivalent to (?>\x0D\x0A|\v).  (The
reason it doesn’t backtrack is that the sequence is considered
inseparable.  That means that



 "\x0D\x0A" =~ /^\R\x0A$/   # No match



fails, because the \R matches the entire string, and won’t backtrack
to match just the "\x0D".)  Since
\R can match a sequence of more than one character, it cannot be put
inside a bracketed character class; /[\R]/ is an error; use \v
instead.  \R was introduced in perl 5.10.0.



Note that this does not respect any locale that might be in effect; it
matches according to the platform’s native character set.



Mnemonic: none really. \R was picked because PCRE already uses \R,
and more importantly because Unicode recommends such a regular expression
metacharacter, and suggests \R as its notation.





\X





This matches a Unicode extended grapheme cluster.



\X matches quite well what normal (non-Unicode-programmer) usage
would consider a single character.  As an example, consider a G with some sort
of diacritic mark, such as an arrow.  There is no such single character in
Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
were a single character.



Mnemonic: eXtended Unicode character.










• perlrebackslash Examples 5:  












Up: perlrebackslash Misc   [Contents]





60.2.8.1 Examples



 "\x{256}" =~ /^\C\C$/;    # Match as chr (0x256) takes 2 octets in UTF-8.

 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
 $str =~ s/(.)\K\g1//g;    # Delete duplicated characters.

 "\n"   =~ /^\R$/;         # Match, \n   is a generic newline.
 "\r"   =~ /^\R$/;         # Match, \r   is a generic newline.
 "\r\n" =~ /^\R$/;         # Match, \r\n is a generic newline.

 "P\x{307}" =~ /^\X$/     # \X matches a P with a dot above.









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61 NAME character class





• NAME character class NAME:  



• NAME character class DESCRIPTION:  












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61.1 NAME




perlrecharclass - Perl Regular Expression Character Classes









Previous: NAME character class NAME, Up: NAME character class   [Contents]





61.2 DESCRIPTION



The top level documentation about Perl regular expressions
is found in (perlre).



This manual page discusses the syntax and use of character
classes in Perl regular expressions.



A character class is a way of denoting a set of characters
in such a way that one character of the set is matched.
It’s important to remember that: matching a character class
consumes exactly one character in the source string. (The source
string is the string the regular expression is matched against.)



There are three types of character classes in Perl regular
expressions: the dot, backslash sequences, and the form enclosed in square
brackets.  Keep in mind, though, that often the term "character class" is used
to mean just the bracketed form.  Certainly, most Perl documentation does that.





• NAME character class The dot:  



• NAME character class Backslash sequences:  



• NAME character class Bracketed Character Classes:  












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61.2.1 The dot



The dot (or period), . is probably the most used, and certainly
the most well-known character class. By default, a dot matches any
character, except for the newline. That default can be changed to
add matching the newline by using the single line modifier: either
for the entire regular expression with the /s modifier, or
locally with (?s).  (The experimental \N backslash sequence, described
below, matches any character except newline without regard to the
single line modifier.)



Here are some examples:



 "a"  =~  /./       # Match
 "."  =~  /./       # Match
 ""   =~  /./       # No match (dot has to match a character)
 "\n" =~  /./       # No match (dot does not match a newline)
 "\n" =~  /./s      # Match (global 'single line' modifier)
 "\n" =~  /(?s:.)/  # Match (local 'single line' modifier)
 "ab" =~  /^.$/     # No match (dot matches one character)









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61.2.2 Backslash sequences


















A backslash sequence is a sequence of characters, the first one of which is a
backslash.  Perl ascribes special meaning to many such sequences, and some of
these are character classes.  That is, they match a single character each,
provided that the character belongs to the specific set of characters defined
by the sequence.



Here’s a list of the backslash sequences that are character classes.  They
are discussed in more detail below.  (For the backslash sequences that aren’t
character classes, see perlrebackslash NAME.)



 \d             Match a decimal digit character.
 \D             Match a non-decimal-digit character.
 \w             Match a "word" character.
 \W             Match a non-"word" character.
 \s             Match a whitespace character.
 \S             Match a non-whitespace character.
 \h             Match a horizontal whitespace character.
 \H             Match a character that isn't horizontal whitespace.
 \v             Match a vertical whitespace character.
 \V             Match a character that isn't vertical whitespace.
 \N             Match a character that isn't a newline.  Experimental.
 \pP, \p{Prop}  Match a character that has the given Unicode property.
 \PP, \P{Prop}  Match a character that doesn't have the Unicode property





• NAME character class \N:  



• NAME character class Digits:  



• NAME character class Word characters:  



• NAME character class Whitespace:  



• NAME character class Unicode Properties:  



• NAME character class Examples:  












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61.2.2.1 \N



\N is new in 5.12, and is experimental.  It, like the dot, matches any
character that is not a newline. The difference is that \N is not influenced
by the single line regular expression modifier (see The dot above).  Note
that the form \N{...} may mean something completely different.  When the
{...} is a (perlre)quantifier, it means to match a non-newline
character that many times.  For example, \N{3} means to match 3
non-newlines; \N{5,} means to match 5 or more non-newlines.  But if {...}
is not a legal quantifier, it is presumed to be a named character.  See
(charnames) for those.  For example, none of \N{COLON}, \N{4F}, and
\N{F4} contain legal quantifiers, so Perl will try to find characters whose
names are respectively COLON, 4F, and F4.









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61.2.2.2 Digits



\d matches a single character considered to be a decimal digit.
If the /a regular expression modifier is in effect, it matches [0-9].
Otherwise, it
matches anything that is matched by \p{Digit}, which includes [0-9].
(An unlikely possible exception is that under locale matching rules, the
current locale might not have [0-9] matched by \d, and/or might match
other characters whose code point is less than 256.  Such a locale
definition would be in violation of the C language standard, but Perl
doesn’t currently assume anything in regard to this.)



What this means is that unless the /a modifier is in effect \d not
only matches the digits ’0’ - ’9’, but also Arabic, Devanagari, and
digits from other languages.  This may cause some confusion, and some
security issues.



Some digits that \d matches look like some of the [0-9] ones, but
have different values.  For example, BENGALI DIGIT FOUR (U+09EA) looks
very much like an ASCII DIGIT EIGHT (U+0038).  An application that
is expecting only the ASCII digits might be misled, or if the match is
\d+, the matched string might contain a mixture of digits from
different writing systems that look like they signify a number different
than they actually do.  (Unicode-UCD)num() can
be used to safely
calculate the value, returning undef if the input string contains
such a mixture.



What \p{Digit} means (and hence \d except under the /a
modifier) is \p{General_Category=Decimal_Number}, or synonymously,
\p{General_Category=Digit}.  Starting with Unicode version 4.1, this
is the same set of characters matched by \p{Numeric_Type=Decimal}.
But Unicode also has a different property with a similar name,
\p{Numeric_Type=Digit}, which matches a completely different set of
characters.  These characters are things such as CIRCLED DIGIT ONE
or subscripts, or are from writing systems that lack all ten digits.



The design intent is for \d to exactly match the set of characters
that can safely be used with "normal" big-endian positional decimal
syntax, where, for example 123 means one ’hundred’, plus two ’tens’,
plus three ’ones’.  This positional notation does not necessarily apply
to characters that match the other type of "digit",
\p{Numeric_Type=Digit}, and so \d doesn’t match them.



The Tamil digits (U+0BE6 - U+0BEF) can also legally be
used in old-style Tamil numbers in which they would appear no more than
one in a row, separated by characters that mean "times 10", "times 100",
etc.  (See http://www.unicode.org/notes/tn21.)



Any character not matched by \d is matched by \D.









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61.2.2.3 Word characters



A \w matches a single alphanumeric character (an alphabetic character, or a
decimal digit) or a connecting punctuation character, such as an
underscore ("_").  It does not match a whole word.  To match a whole
word, use \w+.  This isn’t the same thing as matching an English word, but
in the ASCII range it is the same as a string of Perl-identifier
characters.





If the /a modifier is in effect ...




\w matches the 63 characters [a-zA-Z0-9_].





otherwise ...






For code points above 255 ...




\w matches the same as \p{Word} matches in this range.  That is,
it matches Thai letters, Greek letters, etc.  This includes connector
punctuation (like the underscore) which connect two words together, or
diacritics, such as a COMBINING TILDE and the modifier letters, which
are generally used to add auxiliary markings to letters.





For code points below 256 ...






if locale rules are in effect ...




\w matches the platform’s native underscore character plus whatever
the locale considers to be alphanumeric.





if Unicode rules are in effect or if on an EBCDIC platform ...




\w matches exactly what \p{Word} matches.





otherwise ...




\w matches [a-zA-Z0-9_].


















Which rules apply are determined as described in (perlre)Which character set modifier is in effect?.



There are a number of security issues with the full Unicode list of word
characters.  See http://unicode.org/reports/tr36.



Also, for a somewhat finer-grained set of characters that are in programming
language identifiers beyond the ASCII range, you may wish to instead use the
more customized Unicode Properties, \p{ID_Start},
\p{ID_Continue}, \p{XID_Start}, and \p{XID_Continue}.  See
http://unicode.org/reports/tr31.



Any character not matched by \w is matched by \W.









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61.2.2.4 Whitespace



\s matches any single character considered whitespace.





If the /a modifier is in effect ...




\s matches the 5 characters [\t\n\f\r ]; that is, the horizontal tab,
the newline, the form feed, the carriage return, and the space.  (Note
that it doesn’t match the vertical tab, \cK on ASCII platforms.)





otherwise ...






For code points above 255 ...




\s matches exactly the code points above 255 shown with an "s" column
in the table below.





For code points below 256 ...






if locale rules are in effect ...




\s matches whatever the locale considers to be whitespace.  Note that
this is likely to include the vertical space, unlike non-locale \s
matching.





if Unicode rules are in effect or if on an EBCDIC platform ...




\s matches exactly the characters shown with an "s" column in the
table below.





otherwise ...




\s matches [\t\n\f\r ].
Note that this list doesn’t include the non-breaking space.


















Which rules apply are determined as described in (perlre)Which character set modifier is in effect?.



Any character not matched by \s is matched by \S.



\h matches any character considered horizontal whitespace;
this includes the platform’s space and tab characters and several others
listed in the table below.  \H matches any character
not considered horizontal whitespace.  They use the platform’s native
character set, and do not consider any locale that may otherwise be in
use.



\v matches any character considered vertical whitespace;
this includes the platform’s carriage return and line feed characters (newline)
plus several other characters, all listed in the table below.
\V matches any character not considered vertical whitespace.
They use the platform’s native character set, and do not consider any
locale that may otherwise be in use.



\R matches anything that can be considered a newline under Unicode
rules. It’s not a character class, as it can match a multi-character
sequence. Therefore, it cannot be used inside a bracketed character
class; use \v instead (vertical whitespace).  It uses the platform’s
native character set, and does not consider any locale that may
otherwise be in use.
Details are discussed in perlrebackslash NAME.



Note that unlike \s (and \d and \w), \h and \v always match
the same characters, without regard to other factors, such as the active
locale or whether the source string is in UTF-8 format.



One might think that \s is equivalent to [\h\v]. This is not true.
The difference is that the vertical tab ("\x0b") is not matched by
\s; it is however considered vertical whitespace.



The following table is a complete listing of characters matched by
\s, \h and \v as of Unicode 6.0.



The first column gives the Unicode code point of the character (in hex format),
the second column gives the (Unicode) name. The third column indicates
by which class(es) the character is matched (assuming no locale or EBCDIC code
page is in effect that changes the \s matching).



 0x0009        CHARACTER TABULATION   h s
 0x000a              LINE FEED (LF)    vs
 0x000b             LINE TABULATION    v
 0x000c              FORM FEED (FF)    vs
 0x000d        CARRIAGE RETURN (CR)    vs
 0x0020                       SPACE   h s
 0x0085             NEXT LINE (NEL)    vs  [1]
 0x00a0              NO-BREAK SPACE   h s  [1]
 0x1680            OGHAM SPACE MARK   h s
 0x180e   MONGOLIAN VOWEL SEPARATOR   h s
 0x2000                     EN QUAD   h s
 0x2001                     EM QUAD   h s
 0x2002                    EN SPACE   h s
 0x2003                    EM SPACE   h s
 0x2004          THREE-PER-EM SPACE   h s
 0x2005           FOUR-PER-EM SPACE   h s
 0x2006            SIX-PER-EM SPACE   h s
 0x2007                FIGURE SPACE   h s
 0x2008           PUNCTUATION SPACE   h s
 0x2009                  THIN SPACE   h s
 0x200a                  HAIR SPACE   h s
 0x2028              LINE SEPARATOR    vs
 0x2029         PARAGRAPH SEPARATOR    vs
 0x202f       NARROW NO-BREAK SPACE   h s
 0x205f   MEDIUM MATHEMATICAL SPACE   h s
 0x3000           IDEOGRAPHIC SPACE   h s





[1]




NEXT LINE and NO-BREAK SPACE may or may not match \s depending
on the rules in effect.  See
the beginning of this section.














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61.2.2.5 Unicode Properties



\pP and \p{Prop} are character classes to match characters that fit given
Unicode properties.  One letter property names can be used in the \pP form,
with the property name following the \p, otherwise, braces are required.
When using braces, there is a single form, which is just the property name
enclosed in the braces, and a compound form which looks like \p{name=value},
which means to match if the property "name" for the character has that particular
"value".
For instance, a match for a number can be written as /\pN/ or as
/\p{Number}/, or as /\p{Number=True}/.
Lowercase letters are matched by the property Lowercase_Letter which
has the short form Ll. They need the braces, so are written as /\p{Ll}/ or
/\p{Lowercase_Letter}/, or /\p{General_Category=Lowercase_Letter}/
(the underscores are optional).
/\pLl/ is valid, but means something different.
It matches a two character string: a letter (Unicode property \pL),
followed by a lowercase l.



If neither the /a modifier nor locale rules are in effect, the use of
a Unicode property will force the regular expression into using Unicode
rules.



Note that almost all properties are immune to case-insensitive matching.
That is, adding a /i regular expression modifier does not change what
they match.  There are two sets that are affected.  The first set is
Uppercase_Letter,
Lowercase_Letter,
and Titlecase_Letter,
all of which match Cased_Letter under /i matching.
The second set is
Uppercase,
Lowercase,
and Titlecase,
all of which match Cased under /i matching.
(The difference between these sets is that some things, such as Roman
numerals, come in both upper and lower case, so they are Cased, but
aren’t considered to be letters, so they aren’t Cased_Letters. They’re
actually Letter_Numbers.)
This set also includes its subsets PosixUpper and PosixLower, both
of which under /i match PosixAlpha.



For more details on Unicode properties, see perlunicode Unicode Character Properties; for a
complete list of possible properties, see
(perluniprops)Properties accessible through \p{} and \P{},
which notes all forms that have /i differences.
It is also possible to define your own properties. This is discussed in
perlunicode User-Defined Character Properties.



Unicode properties are defined (surprise!) only on Unicode code points.
A warning is raised and all matches fail on non-Unicode code points
(those above the legal Unicode maximum of 0x10FFFF).  This can be
somewhat surprising,



 chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails.
 chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Also fails!



Even though these two matches might be thought of as complements, they
are so only on Unicode code points.









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61.2.2.6 Examples



 "a"  =~  /\w/      # Match, "a" is a 'word' character.
 "7"  =~  /\w/      # Match, "7" is a 'word' character as well.
 "a"  =~  /\d/      # No match, "a" isn't a digit.
 "7"  =~  /\d/      # Match, "7" is a digit.
 " "  =~  /\s/      # Match, a space is whitespace.
 "a"  =~  /\D/      # Match, "a" is a non-digit.
 "7"  =~  /\D/      # No match, "7" is not a non-digit.
 " "  =~  /\S/      # No match, a space is not non-whitespace.

 " "  =~  /\h/      # Match, space is horizontal whitespace.
 " "  =~  /\v/      # No match, space is not vertical whitespace.
 "\r" =~  /\v/      # Match, a return is vertical whitespace.

 "a"  =~  /\pL/     # Match, "a" is a letter.
 "a"  =~  /\p{Lu}/  # No match, /\p{Lu}/ matches upper case letters.

 "\x{0e0b}" =~ /\p{Thai}/  # Match, \x{0e0b} is the character
                           # 'THAI CHARACTER SO SO', and that's in
                           # Thai Unicode class.
 "a"  =~  /\P{Lao}/ # Match, as "a" is not a Laotian character.



It is worth emphasizing that \d, \w, etc, match single characters, not
complete numbers or words. To match a number (that consists of digits),
use \d+; to match a word, use \w+.  But be aware of the security
considerations in doing so, as mentioned above.









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61.2.3 Bracketed Character Classes



The third form of character class you can use in Perl regular expressions
is the bracketed character class.  In its simplest form, it lists the characters
that may be matched, surrounded by square brackets, like this: [aeiou].
This matches one of a, e, i, o or u.  Like the other
character classes, exactly one character is matched.* To match
a longer string consisting of characters mentioned in the character
class, follow the character class with a (perlre)quantifier.  For
instance, [aeiou]+ matches one or more lowercase English vowels.



Repeating a character in a character class has no
effect; it’s considered to be in the set only once.



Examples:



 "e"  =~  /[aeiou]/        # Match, as "e" is listed in the class.
 "p"  =~  /[aeiou]/        # No match, "p" is not listed in the class.
 "ae" =~  /^[aeiou]$/      # No match, a character class only matches
                           # a single character.
 "ae" =~  /^[aeiou]+$/     # Match, due to the quantifier.

 -------



* There is an exception to a bracketed character class matching a
single character only.  When the class is to match caselessly under /i
matching rules, and a character inside the class matches a
multiple-character sequence caselessly under Unicode rules, the class
(when not inverted) will also match that sequence.  For
example, Unicode says that the letter LATIN SMALL LETTER SHARP S
should match the sequence ss under /i rules.  Thus,



 'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i             # Matches
 'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i    # Matches





• NAME character class Special Characters Inside a Bracketed Character Class:  



• NAME character class Character Ranges:  



• NAME character class Negation:  



• NAME character class Backslash Sequences:  



• NAME character class POSIX Character Classes:  



• NAME character class Negation of POSIX character classes:  



• NAME character class [= =] and [. .]:  



• NAME character class Examples 1:  












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61.2.3.1 Special Characters Inside a Bracketed Character Class



Most characters that are meta characters in regular expressions (that
is, characters that carry a special meaning like ., *, or () lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, [()] matches either an opening
parenthesis, or a closing parenthesis, and the parens inside the character
class don’t group or capture.



Characters that may carry a special meaning inside a character class are:
\, ^, -, [ and ], and are discussed below. They can be
escaped with a backslash, although this is sometimes not needed, in which
case the backslash may be omitted.



The sequence \b is special inside a bracketed character class. While
outside the character class, \b is an assertion indicating a point
that does not have either two word characters or two non-word characters
on either side, inside a bracketed character class, \b matches a
backspace character.



The sequences
\a,
\c,
\e,
\f,
\n,
\N{NAME},
\N{U+hex char},
\r,
\t,
and
\x
are also special and have the same meanings as they do outside a
bracketed character class.  (However, inside a bracketed character
class, if \N{NAME} expands to a sequence of characters, only the first
one in the sequence is used, with a warning.)



Also, a backslash followed by two or three octal digits is considered an octal
number.



A [ is not special inside a character class, unless it’s the start of a
POSIX character class (see POSIX Character Classes below). It normally does
not need escaping.



A ] is normally either the end of a POSIX character class (see
POSIX Character Classes below), or it signals the end of the bracketed
character class.  If you want to include a ] in the set of characters, you
must generally escape it.



However, if the ] is the first (or the second if the first
character is a caret) character of a bracketed character class, it
does not denote the end of the class (as you cannot have an empty class)
and is considered part of the set of characters that can be matched without
escaping.



Examples:



 "+"   =~ /[+?*]/     #  Match, "+" in a character class is not special.
 "\cH" =~ /[\b]/      #  Match, \b inside in a character class
                      #  is equivalent to a backspace.
 "]"   =~ /[][]/      #  Match, as the character class contains.
                      #  both [ and ].
 "[]"  =~ /[[]]/      #  Match, the pattern contains a character class
                      #  containing just ], and the character class is
                      #  followed by a ].









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61.2.3.2 Character Ranges



It is not uncommon to want to match a range of characters. Luckily, instead
of listing all characters in the range, one may use the hyphen (-).
If inside a bracketed character class you have two characters separated
by a hyphen, it’s treated as if all characters between the two were in
the class. For instance, [0-9] matches any ASCII digit, and [a-m]
matches any lowercase letter from the first half of the ASCII alphabet.



Note that the two characters on either side of the hyphen are not
necessarily both letters or both digits. Any character is possible,
although not advisable.  ['-?] contains a range of characters, but
most people will not know which characters that means.  Furthermore,
such ranges may lead to portability problems if the code has to run on
a platform that uses a different character set, such as EBCDIC.



If a hyphen in a character class cannot syntactically be part of a range, for
instance because it is the first or the last character of the character class,
or if it immediately follows a range, the hyphen isn’t special, and so is
considered a character to be matched literally.  If you want a hyphen in
your set of characters to be matched and its position in the class is such
that it could be considered part of a range, you must escape that hyphen
with a backslash.



Examples:



 [a-z]       #  Matches a character that is a lower case ASCII letter.
 [a-fz]      #  Matches any letter between 'a' and 'f' (inclusive) or
             #  the letter 'z'.
 [-z]        #  Matches either a hyphen ('-') or the letter 'z'.
 [a-f-m]     #  Matches any letter between 'a' and 'f' (inclusive), the
             #  hyphen ('-'), or the letter 'm'.
 ['-?]       #  Matches any of the characters  '()*+,-./0123456789:;<=>?
             #  (But not on an EBCDIC platform).









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61.2.3.3 Negation



It is also possible to instead list the characters you do not want to
match. You can do so by using a caret (^) as the first character in the
character class. For instance, [^a-z] matches any character that is not a
lowercase ASCII letter, which therefore includes more than a million
Unicode code points.  The class is said to be "negated" or "inverted".



This syntax make the caret a special character inside a bracketed character
class, but only if it is the first character of the class. So if you want
the caret as one of the characters to match, either escape the caret or
else don’t list it first.



In inverted bracketed character classes, Perl ignores the Unicode rules
that normally say that certain characters should match a sequence of
multiple characters under caseless /i matching.  Following those
rules could lead to highly confusing situations:



 "ss" =~ /^[^\xDF]+$/ui;   # Matches!



This should match any sequences of characters that aren’t \xDF nor
what \xDF matches under /i.  "s" isn’t \xDF, but Unicode
says that "ss" is what \xDF matches under /i.  So which one
"wins"? Do you fail the match because the string has ss or accept it
because it has an s followed by another s?  Perl has chosen the
latter.



Examples:



 "e"  =~  /[^aeiou]/   #  No match, the 'e' is listed.
 "x"  =~  /[^aeiou]/   #  Match, as 'x' isn't a lowercase vowel.
 "^"  =~  /[^^]/       #  No match, matches anything that isn't a caret.
 "^"  =~  /[x^]/       #  Match, caret is not special here.









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61.2.3.4 Backslash Sequences



You can put any backslash sequence character class (with the exception of
\N and \R) inside a bracketed character class, and it will act just
as if you had put all characters matched by the backslash sequence inside the
character class. For instance, [a-f\d] matches any decimal digit, or any
of the lowercase letters between ’a’ and ’f’ inclusive.



\N within a bracketed character class must be of the forms \N{name}
or \N{U+hex char}, and NOT be the form that matches non-newlines,
for the same reason that a dot . inside a bracketed character class loses
its special meaning: it matches nearly anything, which generally isn’t what you
want to happen.



Examples:



 /[\p{Thai}\d]/     # Matches a character that is either a Thai
                    # character, or a digit.
 /[^\p{Arabic}()]/  # Matches a character that is neither an Arabic
                    # character, nor a parenthesis.



Backslash sequence character classes cannot form one of the endpoints
of a range.  Thus, you can’t say:



 /[\p{Thai}-\d]/     # Wrong!









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61.2.3.5 POSIX Character Classes




















POSIX character classes have the form [:class:], where class is
name, and the [: and :] delimiters. POSIX character classes only appear
inside bracketed character classes, and are a convenient and descriptive
way of listing a group of characters.



Be careful about the syntax,



 # Correct:
 $string =~ /[[:alpha:]]/

 # Incorrect (will warn):
 $string =~ /[:alpha:]/



The latter pattern would be a character class consisting of a colon,
and the letters a, l, p and h.
POSIX character classes can be part of a larger bracketed character class.
For example,



 [01[:alpha:]%]



is valid and matches ’0’, ’1’, any alphabetic character, and the percent sign.



Perl recognizes the following POSIX character classes:



 alpha  Any alphabetical character ("[A-Za-z]").
 alnum  Any alphanumeric character. ("[A-Za-z0-9]")
 ascii  Any character in the ASCII character set.
 blank  A GNU extension, equal to a space or a horizontal tab ("\t").
 cntrl  Any control character.  See Note [2] below.
 digit  Any decimal digit ("[0-9]"), equivalent to "\d".
 graph  Any printable character, excluding a space.  See Note [3] below.
 lower  Any lowercase character ("[a-z]").
 print  Any printable character, including a space.  See Note [4] below.
 punct  Any graphical character excluding "word" characters.  Note [5].
 space  Any whitespace character. "\s" plus the vertical tab ("\cK").
 upper  Any uppercase character ("[A-Z]").
 word   A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
 xdigit Any hexadecimal digit ("[0-9a-fA-F]").



Most POSIX character classes have two Unicode-style \p property
counterparts.  (They are not official Unicode properties, but Perl extensions
derived from official Unicode properties.)  The table below shows the relation
between POSIX character classes and these counterparts.



One counterpart, in the column labelled "ASCII-range Unicode" in
the table, matches only characters in the ASCII character set.



The other counterpart, in the column labelled "Full-range Unicode", matches any
appropriate characters in the full Unicode character set.  For example,
\p{Alpha} matches not just the ASCII alphabetic characters, but any
character in the entire Unicode character set considered alphabetic.
An entry in the column labelled "backslash sequence" is a (short)
equivalent.



 [[:...:]]      ASCII-range          Full-range  backslash  Note
                 Unicode              Unicode     sequence
 -----------------------------------------------------
   alpha      \p{PosixAlpha}       \p{XPosixAlpha}
   alnum      \p{PosixAlnum}       \p{XPosixAlnum}
   ascii      \p{ASCII}
   blank      \p{PosixBlank}       \p{XPosixBlank}  \h      [1]
                                   or \p{HorizSpace}        [1]
   cntrl      \p{PosixCntrl}       \p{XPosixCntrl}          [2]
   digit      \p{PosixDigit}       \p{XPosixDigit}  \d
   graph      \p{PosixGraph}       \p{XPosixGraph}          [3]
   lower      \p{PosixLower}       \p{XPosixLower}
   print      \p{PosixPrint}       \p{XPosixPrint}          [4]
   punct      \p{PosixPunct}       \p{XPosixPunct}          [5]
              \p{PerlSpace}        \p{XPerlSpace}   \s      [6]
   space      \p{PosixSpace}       \p{XPosixSpace}          [6]
   upper      \p{PosixUpper}       \p{XPosixUpper}
   word       \p{PosixWord}        \p{XPosixWord}   \w
   xdigit     \p{PosixXDigit}      \p{XPosixXDigit}





[1]




\p{Blank} and \p{HorizSpace} are synonyms.





[2]




Control characters don’t produce output as such, but instead usually control
the terminal somehow: for example, newline and backspace are control characters.
In the ASCII range, characters whose code points are between 0 and 31 inclusive,
plus 127 (DEL) are control characters.



On EBCDIC platforms, it is likely that the code page will define [[:cntrl:]]
to be the EBCDIC equivalents of the ASCII controls, plus the controls
that in Unicode have code pointss from 128 through 159.





[3]




Any character that is graphical, that is, visible. This class consists
of all alphanumeric characters and all punctuation characters.





[4]




All printable characters, which is the set of all graphical characters
plus those whitespace characters which are not also controls.





[5]




\p{PosixPunct} and [[:punct:]] in the ASCII range match all
non-controls, non-alphanumeric, non-space characters:
[-!"#$%&'()*+,./:;<=>?@[\\\]^_`{|}~] (although if a locale is in effect,
it could alter the behavior of [[:punct:]]).



The similarly named property, \p{Punct}, matches a somewhat different
set in the ASCII range, namely
[-!"#%&'()*,./:;?@[\\\]_{}].  That is, it is missing the nine
characters [$+<=>^`|~].
This is because Unicode splits what POSIX considers to be punctuation into two
categories, Punctuation and Symbols.



\p{XPosixPunct} and (under Unicode rules) [[:punct:]], match what
\p{PosixPunct} matches in the ASCII range, plus what \p{Punct}
matches.  This is different than strictly matching according to
\p{Punct}.  Another way to say it is that
if Unicode rules are in effect, [[:punct:]] matches all characters
that Unicode considers punctuation, plus all ASCII-range characters that
Unicode considers symbols.





[6]




\p{SpacePerl} and \p{Space} differ only in that in non-locale
matching, \p{Space} additionally
matches the vertical tab, \cK.   Same for the two ASCII-only range forms.








There are various other synonyms that can be used besides the names
listed in the table.  For example, \p{PosixAlpha} can be written as
\p{Alpha}.  All are listed in
(perluniprops)Properties accessible through \p{} and \P{},
plus all characters matched by each ASCII-range property.



Both the \p counterparts always assume Unicode rules are in effect.
On ASCII platforms, this means they assume that the code points from 128
to 255 are Latin-1, and that means that using them under locale rules is
unwise unless the locale is guaranteed to be Latin-1 or UTF-8.  In contrast, the
POSIX character classes are useful under locale rules.  They are
affected by the actual rules in effect, as follows:





If the /a modifier, is in effect ...




Each of the POSIX classes matches exactly the same as their ASCII-range
counterparts.





otherwise ...






For code points above 255 ...




The POSIX class matches the same as its Full-range counterpart.





For code points below 256 ...






if locale rules are in effect ...




The POSIX class matches according to the locale, except that
word uses the platform’s native underscore character, no matter what
the locale is.





if Unicode rules are in effect or if on an EBCDIC platform ...




The POSIX class matches the same as the Full-range counterpart.





otherwise ...




The POSIX class matches the same as the ASCII range counterpart.


















Which rules apply are determined as described in
(perlre)Which character set modifier is in effect?.



It is proposed to change this behavior in a future release of Perl so that
whether or not Unicode rules are in effect would not change the
behavior:  Outside of locale or an EBCDIC code page, the POSIX classes
would behave like their ASCII-range counterparts.  If you wish to
comment on this proposal, send email to perl5-porters@perl.org.









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61.2.3.6 Negation of POSIX character classes




A Perl extension to the POSIX character class is the ability to
negate it. This is done by prefixing the class name with a caret (^).
Some examples:



     POSIX         ASCII-range     Full-range  backslash
                    Unicode         Unicode    sequence
 -----------------------------------------------------
 [[:^digit:]]   \P{PosixDigit}  \P{XPosixDigit}   \D
 [[:^space:]]   \P{PosixSpace}  \P{XPosixSpace}
                \P{PerlSpace}   \P{XPerlSpace}    \S
 [[:^word:]]    \P{PerlWord}    \P{XPosixWord}    \W



The backslash sequence can mean either ASCII- or Full-range Unicode,
depending on various factors as described in (perlre)Which character set modifier is in effect?.









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61.2.3.7 [= =] and [. .]



Perl recognizes the POSIX character classes [=class=] and
[.class.], but does not (yet?) support them.  Any attempt to use
either construct raises an exception.









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61.2.3.8 Examples



 /[[:digit:]]/            # Matches a character that is a digit.
 /[01[:lower:]]/          # Matches a character that is either a
                          # lowercase letter, or '0' or '1'.
 /[[:digit:][:^xdigit:]]/ # Matches a character that can be anything
                          # except the letters 'a' to 'f'.  This is
                          # because the main character class is composed
                          # of two POSIX character classes that are ORed
                          # together, one that matches any digit, and
                          # the other that matches anything that isn't a
                          # hex digit.  The result matches all
                          # characters except the letters 'a' to 'f' and
                          # 'A' to 'F'.









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62 NAME reference pointer data structure structure struct





• NAME reference pointer data structure structure struct NAME:  



• NAME reference pointer data structure structure struct NOTE:  



• NAME reference pointer data structure structure struct DESCRIPTION:  



• NAME reference pointer data structure structure struct WARNING:  



• NAME reference pointer data structure structure struct SEE ALSO:  












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62.1 NAME








perlref - Perl references and nested data structures









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62.2 NOTE



This is complete documentation about all aspects of references.
For a shorter, tutorial introduction to just the essential features,
see perlreftut NAME.









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62.3 DESCRIPTION



Before release 5 of Perl it was difficult to represent complex data
structures, because all references had to be symbolic–and even then
it was difficult to refer to a variable instead of a symbol table entry.
Perl now not only makes it easier to use symbolic references to variables,
but also lets you have "hard" references to any piece of data or code.
Any scalar may hold a hard reference.  Because arrays and hashes contain
scalars, you can now easily build arrays of arrays, arrays of hashes,
hashes of arrays, arrays of hashes of functions, and so on.



Hard references are smart–they keep track of reference counts for you,
automatically freeing the thing referred to when its reference count goes
to zero.  (Reference counts for values in self-referential or
cyclic data structures may not go to zero without a little help; see
Circular References for a detailed explanation.)
If that thing happens to be an object, the object is destructed.  See
(perlobj) for more about objects.  (In a sense, everything in Perl is an
object, but we usually reserve the word for references to objects that
have been officially "blessed" into a class package.)







Symbolic references are names of variables or other objects, just as a
symbolic link in a Unix filesystem contains merely the name of a file.
The *glob notation is something of a symbolic reference.  (Symbolic
references are sometimes called "soft references", but please don’t call
them that; references are confusing enough without useless synonyms.)






In contrast, hard references are more like hard links in a Unix file
system: They are used to access an underlying object without concern for
what its (other) name is.  When the word "reference" is used without an
adjective, as in the following paragraph, it is usually talking about a
hard reference.




References are easy to use in Perl.  There is just one overriding
principle: Perl does no implicit referencing or dereferencing.  When a
scalar is holding a reference, it always behaves as a simple scalar.  It
doesn’t magically start being an array or hash or subroutine; you have to
tell it explicitly to do so, by dereferencing it.



References are easy to use in Perl.  There is just one overriding
principle: in general, Perl does no implicit referencing or dereferencing.
When a scalar is holding a reference, it always behaves as a simple scalar.
It doesn’t magically start being an array or hash or subroutine; you have to
tell it explicitly to do so, by dereferencing it.



That said, be aware that Perl version 5.14 introduces an exception
to the rule, for syntactic convenience.  Experimental array and hash container
function behavior allows array and hash references to be handled by Perl as
if they had been explicitly syntactically dereferenced.  See
(perl5140delta)Syntactical Enhancements
and (perlfunc) for details.





• NAME reference pointer data structure structure struct Making References:  



• NAME reference pointer data structure structure struct Using References:  



• NAME reference pointer data structure structure struct Circular References:  



• NAME reference pointer data structure structure struct Symbolic references:  



• NAME reference pointer data structure structure struct Not-so-symbolic references:  



• NAME reference pointer data structure structure struct Pseudo-hashes: Using an array as a hash:  



• NAME reference pointer data structure structure struct Function Templates:  












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62.3.1 Making References





References can be created in several ways.





1.






By using the backslash operator on a variable, subroutine, or value.
(This works much like the & (address-of) operator in C.)  
This typically creates another reference to a variable, because
there’s already a reference to the variable in the symbol table.  But
the symbol table reference might go away, and you’ll still have the
reference that the backslash returned.  Here are some examples:



    $scalarref = \$foo;
    $arrayref  = \@ARGV;
    $hashref   = \%ENV;
    $coderef   = \&handler;
    $globref   = \*foo;



It isn’t possible to create a true reference to an IO handle (filehandle
or dirhandle) using the backslash operator.  The most you can get is a
reference to a typeglob, which is actually a complete symbol table entry.
But see the explanation of the *foo{THING} syntax below.  However,
you can still use type globs and globrefs as though they were IO handles.





2.












A reference to an anonymous array can be created using square
brackets:



    $arrayref = [1, 2, ['a', 'b', 'c']];



Here we’ve created a reference to an anonymous array of three elements
whose final element is itself a reference to another anonymous array of three
elements.  (The multidimensional syntax described later can be used to
access this.  For example, after the above, $arrayref->[2][1] would have
the value "b".)



Taking a reference to an enumerated list is not the same
as using square brackets–instead it’s the same as creating
a list of references!



    @list = (\$a, \@b, \%c);
    @list = \($a, @b, %c);      # same thing!



As a special case, \(@foo) returns a list of references to the contents
of @foo, not a reference to @foo itself.  Likewise for %foo,
except that the key references are to copies (since the keys are just
strings rather than full-fledged scalars).





3.













A reference to an anonymous hash can be created using curly
brackets:



    $hashref = {
        'Adam'  => 'Eve',
        'Clyde' => 'Bonnie',
    };



Anonymous hash and array composers like these can be intermixed freely to
produce as complicated a structure as you want.  The multidimensional
syntax described below works for these too.  The values above are
literals, but variables and expressions would work just as well, because
assignment operators in Perl (even within local() or my()) are executable
statements, not compile-time declarations.



Because curly brackets (braces) are used for several other things
including BLOCKs, you may occasionally have to disambiguate braces at the
beginning of a statement by putting a + or a return in front so
that Perl realizes the opening brace isn’t starting a BLOCK.  The economy and
mnemonic value of using curlies is deemed worth this occasional extra
hassle.



For example, if you wanted a function to make a new hash and return a
reference to it, you have these options:



    sub hashem {        { @_ } }   # silently wrong
    sub hashem {       +{ @_ } }   # ok
    sub hashem { return { @_ } }   # ok



On the other hand, if you want the other meaning, you can do this:



    sub showem {        { @_ } }   # ambiguous (currently ok, but may change)
    sub showem {       {; @_ } }   # ok
    sub showem { { return @_ } }   # ok



The leading +{ and {; always serve to disambiguate
the expression to mean either the HASH reference, or the BLOCK.





4.











A reference to an anonymous subroutine can be created by using
sub without a subname:



    $coderef = sub { print "Boink!\n" };



Note the semicolon.  Except for the code
inside not being immediately executed, a sub {} is not so much a
declaration as it is an operator, like do{} or eval{}.  (However, no
matter how many times you execute that particular line (unless you’re in an
eval("...")), $coderef will still have a reference to the same
anonymous subroutine.)



Anonymous subroutines act as closures with respect to my() variables,
that is, variables lexically visible within the current scope.  Closure
is a notion out of the Lisp world that says if you define an anonymous
function in a particular lexical context, it pretends to run in that
context even when it’s called outside the context.



In human terms, it’s a funny way of passing arguments to a subroutine when
you define it as well as when you call it.  It’s useful for setting up
little bits of code to run later, such as callbacks.  You can even
do object-oriented stuff with it, though Perl already provides a different
mechanism to do that–see (perlobj).



You might also think of closure as a way to write a subroutine
template without using eval().  Here’s a small example of how
closures work:



    sub newprint {
        my $x = shift;
        return sub { my $y = shift; print "$x, $y!\n"; };
    }
    $h = newprint("Howdy");
    $g = newprint("Greetings");

    # Time passes...

    &$h("world");
    &$g("earthlings");



This prints



    Howdy, world!
    Greetings, earthlings!



Note particularly that $x continues to refer to the value passed
into newprint() despite "my $x" having gone out of scope by the
time the anonymous subroutine runs.  That’s what a closure is all
about.



This applies only to lexical variables, by the way.  Dynamic variables
continue to work as they have always worked.  Closure is not something
that most Perl programmers need trouble themselves about to begin with.





5.






References are often returned by special subroutines called constructors.  Perl
objects are just references to a special type of object that happens to know
which package it’s associated with.  Constructors are just special subroutines
that know how to create that association.  They do so by starting with an
ordinary reference, and it remains an ordinary reference even while it’s also
being an object.  Constructors are often named new().  You can call them
indirectly:



    $objref = new Doggie( Tail => 'short', Ears => 'long' );



But that can produce ambiguous syntax in certain cases, so it’s often
better to use the direct method invocation approach:



    $objref   = Doggie->new(Tail => 'short', Ears => 'long');

    use Term::Cap;
    $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });

    use Tk;
    $main    = MainWindow->new();
    $menubar = $main->Frame(-relief              => "raised",
                            -borderwidth         => 2)





6.





References of the appropriate type can spring into existence if you
dereference them in a context that assumes they exist.  Because we haven’t
talked about dereferencing yet, we can’t show you any examples yet.





7.






A reference can be created by using a special syntax, lovingly known as
the *foo{THING} syntax.  *foo{THING} returns a reference to the THING
slot in *foo (which is the symbol table entry which holds everything
known as foo).



    $scalarref = *foo{SCALAR};
    $arrayref  = *ARGV{ARRAY};
    $hashref   = *ENV{HASH};
    $coderef   = *handler{CODE};
    $ioref     = *STDIN{IO};
    $globref   = *foo{GLOB};
    $formatref = *foo{FORMAT};



All of these are self-explanatory except for *foo{IO}.  It returns
the IO handle, used for file handles ((perlfunc)open), sockets
((perlfunc)socket and (perlfunc)socketpair), and directory
handles ((perlfunc)opendir).  For compatibility with previous
versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}, though it
is deprecated as of 5.8.0.  If deprecation warnings are in effect, it will warn
of its use.



*foo{THING} returns undef if that particular THING hasn’t been used yet,
except in the case of scalars.  *foo{SCALAR} returns a reference to an
anonymous scalar if $foo hasn’t been used yet.  This might change in a
future release.



*foo{IO} is an alternative to the *HANDLE mechanism given in
perldata Typeglobs and Filehandles for passing filehandles
into or out of subroutines, or storing into larger data structures.
Its disadvantage is that it won’t create a new filehandle for you.
Its advantage is that you have less risk of clobbering more than
you want to with a typeglob assignment.  (It still conflates file
and directory handles, though.)  However, if you assign the incoming
value to a scalar instead of a typeglob as we do in the examples
below, there’s no risk of that happening.



    splutter(*STDOUT);          # pass the whole glob
    splutter(*STDOUT{IO});      # pass both file and dir handles

    sub splutter {
        my $fh = shift;
        print $fh "her um well a hmmm\n";
    }

    $rec = get_rec(*STDIN);     # pass the whole glob
    $rec = get_rec(*STDIN{IO}); # pass both file and dir handles

    sub get_rec {
        my $fh = shift;
        return scalar <$fh>;
    }














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62.3.2 Using References






That’s it for creating references.  By now you’re probably dying to
know how to use references to get back to your long-lost data.  There
are several basic methods.



    

  1.  Anywhere you’d put an identifier (or chain of identifiers) as part
of a variable or subroutine name, you can replace the identifier with
a simple scalar variable containing a reference of the correct type:


        $bar = $$scalarref;
    push(@$arrayref, $filename);
    $$arrayref[0] = "January";
    $$hashref{"KEY"} = "VALUE";
    &$coderef(1,2,3);
    print $globref "output\n";

    

    It’s important to understand that we are specifically not dereferencing
$arrayref[0] or $hashref{"KEY"} there.  The dereference of the
scalar variable happens before it does any key lookups.  Anything more
complicated than a simple scalar variable must use methods 2 or 3 below.
However, a "simple scalar" includes an identifier that itself uses method
1 recursively.  Therefore, the following prints "howdy".

    

        $refrefref = \\\"howdy";
    print $$$$refrefref;

    

  2.  Anywhere you’d put an identifier (or chain of identifiers) as part of a
variable or subroutine name, you can replace the identifier with a
BLOCK returning a reference of the correct type.  In other words, the
previous examples could be written like this:


        $bar = ${$scalarref};
    push(@{$arrayref}, $filename);
    ${$arrayref}[0] = "January";
    ${$hashref}{"KEY"} = "VALUE";
    &{$coderef}(1,2,3);
    $globref->print("output\n");  # iff IO::Handle is loaded

    

    Admittedly, it’s a little silly to use the curlies in this case, but
the BLOCK can contain any arbitrary expression, in particular,
subscripted expressions:

    

        &{ $dispatch{$index} }(1,2,3);      # call correct routine

    

    Because of being able to omit the curlies for the simple case of $$x,
people often make the mistake of viewing the dereferencing symbols as
proper operators, and wonder about their precedence.  If they were,
though, you could use parentheses instead of braces.  That’s not the case.
Consider the difference below; case 0 is a short-hand version of case 1,
not case 2:

    

        $$hashref{"KEY"}   = "VALUE";       # CASE 0
    ${$hashref}{"KEY"} = "VALUE";       # CASE 1
    ${$hashref{"KEY"}} = "VALUE";       # CASE 2
    ${$hashref->{"KEY"}} = "VALUE";     # CASE 3

    

    Case 2 is also deceptive in that you’re accessing a variable
called %hashref, not dereferencing through $hashref to the hash
it’s presumably referencing.  That would be case 3.

    

  3.  Subroutine calls and lookups of individual array elements arise often
enough that it gets cumbersome to use method 2.  As a form of
syntactic sugar, the examples for method 2 may be written:


        $arrayref->[0] = "January";   # Array element
    $hashref->{"KEY"} = "VALUE";  # Hash element
    $coderef->(1,2,3);            # Subroutine call

    

    The left side of the arrow can be any expression returning a reference,
including a previous dereference.  Note that $array[$x] is not the
same thing as $array->[$x] here:

    

        $array[$x]->{"foo"}->[0] = "January";

    

    This is one of the cases we mentioned earlier in which references could
spring into existence when in an lvalue context.  Before this
statement, $array[$x] may have been undefined.  If so, it’s
automatically defined with a hash reference so that we can look up
{"foo"} in it.  Likewise $array[$x]->{"foo"} will automatically get
defined with an array reference so that we can look up [0] in it.
This process is called autovivification.

    

    One more thing here.  The arrow is optional between brackets
subscripts, so you can shrink the above down to

    

        $array[$x]{"foo"}[0] = "January";

    

    Which, in the degenerate case of using only ordinary arrays, gives you
multidimensional arrays just like C’s:

    

        $score[$x][$y][$z] += 42;

    

    Well, okay, not entirely like C’s arrays, actually.  C doesn’t know how
to grow its arrays on demand.  Perl does.

    

  4.  If a reference happens to be a reference to an object, then there are
probably methods to access the things referred to, and you should probably
stick to those methods unless you’re in the class package that defines the
object’s methods.  In other words, be nice, and don’t violate the object’s
encapsulation without a very good reason.  Perl does not enforce
encapsulation.  We are not totalitarians here.  We do expect some basic
civility though.






Using a string or number as a reference produces a symbolic reference,
as explained above.  Using a reference as a number produces an
integer representing its storage location in memory.  The only
useful thing to be done with this is to compare two references
numerically to see whether they refer to the same location.




    if ($ref1 == $ref2) {  # cheap numeric compare of references
        print "refs 1 and 2 refer to the same thing\n";
    }




Using a reference as a string produces both its referent’s type,
including any package blessing as described in (perlobj), as well
as the numeric address expressed in hex.  The ref() operator returns
just the type of thing the reference is pointing to, without the
address.  See (perlfunc)ref for details and examples of its use.




The bless() operator may be used to associate the object a reference
points to with a package functioning as an object class.  See (perlobj).



A typeglob may be dereferenced the same way a reference can, because
the dereference syntax always indicates the type of reference desired.
So ${*foo} and ${\$foo} both indicate the same scalar variable.



Here’s a trick for interpolating a subroutine call into a string:



    print "My sub returned @{[mysub(1,2,3)]} that time.\n";



The way it works is that when the @{...} is seen in the double-quoted
string, it’s evaluated as a block.  The block creates a reference to an
anonymous array containing the results of the call to mysub(1,2,3).  So
the whole block returns a reference to an array, which is then
dereferenced by @{...} and stuck into the double-quoted string. This
chicanery is also useful for arbitrary expressions:



    print "That yields @{[$n + 5]} widgets\n";



Similarly, an expression that returns a reference to a scalar can be
dereferenced via ${...}. Thus, the above expression may be written
as:



    print "That yields ${\($n + 5)} widgets\n";









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62.3.3 Circular References





It is possible to create a "circular reference" in Perl, which can lead
to memory leaks. A circular reference occurs when two references
contain a reference to each other, like this:



    my $foo = {};
    my $bar = { foo => $foo };
    $foo->{bar} = $bar;



You can also create a circular reference with a single variable:



    my $foo;
    $foo = \$foo;



In this case, the reference count for the variables will never reach 0,
and the references will never be garbage-collected. This can lead to
memory leaks.



Because objects in Perl are implemented as references, it’s possible to
have circular references with objects as well. Imagine a TreeNode class
where each node references its parent and child nodes. Any node with a
parent will be part of a circular reference.



You can break circular references by creating a "weak reference". A
weak reference does not increment the reference count for a variable,
which means that the object can go out of scope and be destroyed. You
can weaken a reference with the weaken function exported by the
(Scalar-Util) module.



Here’s how we can make the first example safer:



    use Scalar::Util 'weaken';

    my $foo = {};
    my $bar = { foo => $foo };
    $foo->{bar} = $bar;

    weaken $foo->{bar};



The reference from $foo to $bar has been weakened. When the
$bar variable goes out of scope, it will be garbage-collected. The
next time you look at the value of the $foo->{bar} key, it will
be undef.



This action at a distance can be confusing, so you should be careful
with your use of weaken. You should weaken the reference in the
variable that will go out of scope first. That way, the longer-lived
variable will contain the expected reference until it goes out of
scope.









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62.3.4 Symbolic references







We said that references spring into existence as necessary if they are
undefined, but we didn’t say what happens if a value used as a
reference is already defined, but isn’t a hard reference.  If you
use it as a reference, it’ll be treated as a symbolic
reference.  That is, the value of the scalar is taken to be the name
of a variable, rather than a direct link to a (possibly) anonymous
value.



People frequently expect it to work like this.  So it does.



    $name = "foo";
    $$name = 1;                 # Sets $foo
    ${$name} = 2;               # Sets $foo
    ${$name x 2} = 3;           # Sets $foofoo
    $name->[0] = 4;             # Sets $foo[0]
    @$name = ();                # Clears @foo
    &$name();                   # Calls &foo() (as in Perl 4)
    $pack = "THAT";
    ${"${pack}::$name"} = 5;    # Sets $THAT::foo without eval



This is powerful, and slightly dangerous, in that it’s possible
to intend (with the utmost sincerity) to use a hard reference, and
accidentally use a symbolic reference instead.  To protect against
that, you can say



    use strict 'refs';



and then only hard references will be allowed for the rest of the enclosing
block.  An inner block may countermand that with



    no strict 'refs';



Only package variables (globals, even if localized) are visible to
symbolic references.  Lexical variables (declared with my()) aren’t in
a symbol table, and thus are invisible to this mechanism.  For example:



    local $value = 10;
    $ref = "value";
    {
        my $value = 20;
        print $$ref;
    }



This will still print 10, not 20.  Remember that local() affects package
variables, which are all "global" to the package.









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62.3.5 Not-so-symbolic references



Since Perl verion 5.001, brackets around a symbolic reference can simply
serve to isolate an identifier or variable name from the rest of an
expression, just as they always have within a string.  For example,



    $push = "pop on ";
    print "${push}over";



has always meant to print "pop on over", even though push is
a reserved word.  In 5.001, this was generalized to work the same
without the enclosing double quotes, so that



    print ${push} . "over";



and even



    print ${ push } . "over";



will have the same effect.  (This would have been a syntax error in
Perl 5.000, though Perl 4 allowed it in the spaceless form.)  This
construct is not considered to be a symbolic reference when you’re
using strict refs:



    use strict 'refs';
    ${ bareword };      # Okay, means $bareword.
    ${ "bareword" };    # Error, symbolic reference.



Similarly, because of all the subscripting that is done using single words,
the same rule applies to any bareword that is used for subscripting a hash.
So now, instead of writing



    $array{ "aaa" }{ "bbb" }{ "ccc" }



you can write just



    $array{ aaa }{ bbb }{ ccc }



and not worry about whether the subscripts are reserved words.  In the
rare event that you do wish to do something like



    $array{ shift }



you can force interpretation as a reserved word by adding anything that
makes it more than a bareword:



    $array{ shift() }
    $array{ +shift }
    $array{ shift @_ }



The use warnings pragma or the -w switch will warn you if it
interprets a reserved word as a string.
But it will no longer warn you about using lowercase words, because the
string is effectively quoted.









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62.3.6 Pseudo-hashes: Using an array as a hash






Pseudo-hashes have been removed from Perl.  The ’fields’ pragma
remains available.









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62.3.7 Function Templates











As explained above, an anonymous function with access to the lexical
variables visible when that function was compiled, creates a closure.  It
retains access to those variables even though it doesn’t get run until
later, such as in a signal handler or a Tk callback.



Using a closure as a function template allows us to generate many functions
that act similarly.  Suppose you wanted functions named after the colors
that generated HTML font changes for the various colors:



    print "Be ", red("careful"), "with that ", green("light");



The red() and green() functions would be similar.  To create these,
we’ll assign a closure to a typeglob of the name of the function we’re
trying to build.  



    @colors = qw(red blue green yellow orange purple violet);
    for my $name (@colors) {
        no strict 'refs';       # allow symbol table manipulation
        *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
    } 



Now all those different functions appear to exist independently.  You can
call red(), RED(), blue(), BLUE(), green(), etc.  This technique saves on
both compile time and memory use, and is less error-prone as well, since
syntax checks happen at compile time.  It’s critical that any variables in
the anonymous subroutine be lexicals in order to create a proper closure.
That’s the reasons for the my on the loop iteration variable.



This is one of the only places where giving a prototype to a closure makes
much sense.  If you wanted to impose scalar context on the arguments of
these functions (probably not a wise idea for this particular example),
you could have written it this way instead:



    *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };



However, since prototype checking happens at compile time, the assignment
above happens too late to be of much use.  You could address this by
putting the whole loop of assignments within a BEGIN block, forcing it
to occur during compilation.



Access to lexicals that change over time–like those in the for loop
above, basically aliases to elements from the surrounding lexical scopes–
only works with anonymous subs, not with named subroutines. Generally
said, named subroutines do not nest properly and should only be declared
in the main package scope.



This is because named subroutines are created at compile time so their
lexical variables get assigned to the parent lexicals from the first
execution of the parent block. If a parent scope is entered a second
time, its lexicals are created again, while the nested subs still
reference the old ones.



Anonymous subroutines get to capture each time you execute the sub
operator, as they are created on the fly. If you are accustomed to using
nested subroutines in other programming languages with their own private
variables, you’ll have to work at it a bit in Perl.  The intuitive coding
of this type of thing incurs mysterious warnings about "will not stay
shared" due to the reasons explained above. 
For example, this won’t work:



    sub outer {
        my $x = $_[0] + 35;
        sub inner { return $x * 19 }   # WRONG
        return $x + inner();
    }



A work-around is the following:



    sub outer {
        my $x = $_[0] + 35;
        local *inner = sub { return $x * 19 };
        return $x + inner();
    }



Now inner() can only be called from within outer(), because of the
temporary assignments of the anonymous subroutine. But when it does,
it has normal access to the lexical variable $x from the scope of
outer() at the time outer is invoked.



This has the interesting effect of creating a function local to another
function, something not normally supported in Perl.









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62.4 WARNING





You may not (usefully) use a reference as the key to a hash.  It will be
converted into a string:



    $x{ \$a } = $a;



If you try to dereference the key, it won’t do a hard dereference, and
you won’t accomplish what you’re attempting.  You might want to do something
more like



    $r = \@a;
    $x{ $r } = $r;



And then at least you can use the values(), which will be
real refs, instead of the keys(), which won’t.



The standard Tie::RefHash module provides a convenient workaround to this.









Previous: NAME reference pointer data structure structure struct WARNING, Up: NAME reference pointer data structure structure struct   [Contents]





62.5 SEE ALSO



Besides the obvious documents, source code can be instructive.
Some pathological examples of the use of references can be found
in the t/op/ref.t regression test in the Perl source directory.



See also (perldsc) and perllol NAME for how to use references to create
complex data structures, and perlootut NAME and (perlobj)
for how to use them to create objects.









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63 perlreftut





• perlreftut NAME:  



• perlreftut DESCRIPTION:  



• perlreftut Who Needs Complicated Data Structures?:  



• perlreftut The Solution:  



• perlreftut Syntax:  



• perlreftut Solution:  



• perlreftut The Rest:  



• perlreftut Summary:  



• perlreftut Credits:  












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63.1 NAME



perlreftut - Mark’s very short tutorial about references









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63.2 DESCRIPTION



One of the most important new features in Perl 5 was the capability to
manage complicated data structures like multidimensional arrays and
nested hashes.  To enable these, Perl 5 introduced a feature called
’references’, and using references is the key to managing complicated,
structured data in Perl.  Unfortunately, there’s a lot of funny syntax
to learn, and the main manual page can be hard to follow.  The manual
is quite complete, and sometimes people find that a problem, because
it can be hard to tell what is important and what isn’t.



Fortunately, you only need to know 10% of what’s in the main page to get
90% of the benefit.  This page will show you that 10%.









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63.3 Who Needs Complicated Data Structures?



One problem that came up all the time in Perl 4 was how to represent a
hash whose values were lists.  Perl 4 had hashes, of course, but the
values had to be scalars; they couldn’t be lists.



Why would you want a hash of lists?  Let’s take a simple example: You
have a file of city and country names, like this:



        Chicago, USA
        Frankfurt, Germany
        Berlin, Germany
        Washington, USA
        Helsinki, Finland
        New York, USA



and you want to produce an output like this, with each country mentioned
once, and then an alphabetical list of the cities in that country:



        Finland: Helsinki.
        Germany: Berlin, Frankfurt.
        USA:  Chicago, New York, Washington.



The natural way to do this is to have a hash whose keys are country
names.  Associated with each country name key is a list of the cities in
that country.  Each time you read a line of input, split it into a country
and a city, look up the list of cities already known to be in that
country, and append the new city to the list.  When you’re done reading
the input, iterate over the hash as usual, sorting each list of cities
before you print it out.



If hash values can’t be lists, you lose.  In Perl 4, hash values can’t
be lists; they can only be strings.  You lose.  You’d probably have to
combine all the cities into a single string somehow, and then when
time came to write the output, you’d have to break the string into a
list, sort the list, and turn it back into a string.  This is messy
and error-prone.  And it’s frustrating, because Perl already has
perfectly good lists that would solve the problem if only you could
use them.









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63.4 The Solution



By the time Perl 5 rolled around, we were already stuck with this
design: Hash values must be scalars.  The solution to this is
references.



A reference is a scalar value that refers to an entire array or an
entire hash (or to just about anything else).  Names are one kind of
reference that you’re already familiar with.  Think of the President
of the United States: a messy, inconvenient bag of blood and bones.
But to talk about him, or to represent him in a computer program, all
you need is the easy, convenient scalar string "Barack Obama".



References in Perl are like names for arrays and hashes.  They’re
Perl’s private, internal names, so you can be sure they’re
unambiguous.  Unlike "Barack Obama", a reference only refers to one
thing, and you always know what it refers to.  If you have a reference
to an array, you can recover the entire array from it.  If you have a
reference to a hash, you can recover the entire hash.  But the
reference is still an easy, compact scalar value.



You can’t have a hash whose values are arrays; hash values can only be
scalars.  We’re stuck with that.  But a single reference can refer to
an entire array, and references are scalars, so you can have a hash of
references to arrays, and it’ll act a lot like a hash of arrays, and
it’ll be just as useful as a hash of arrays.



We’ll come back to this city-country problem later, after we’ve seen
some syntax for managing references.









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63.5 Syntax



There are just two ways to make a reference, and just two ways to use
it once you have it.





• perlreftut Making References:  



• perlreftut Using References:  



• perlreftut An Example:  



• perlreftut Arrow Rule:  












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63.5.1 Making References





• perlreftut Make Rule 1:  












Up: perlreftut Making References   [Contents]





63.5.1.1 Make Rule 1



If you put a \ in front of a variable, you get a
reference to that variable.



    $aref = \@array;         # $aref now holds a reference to @array
    $href = \%hash;          # $href now holds a reference to %hash
    $sref = \$scalar;        # $sref now holds a reference to $scalar



Once the reference is stored in a variable like $aref or $href, you
can copy it or store it just the same as any other scalar value:



    $xy = $aref;             # $xy now holds a reference to @array
    $p[3] = $href;           # $p[3] now holds a reference to %hash
    $z = $p[3];              # $z now holds a reference to %hash



These examples show how to make references to variables with names.
Sometimes you want to make an array or a hash that doesn’t have a
name.  This is analogous to the way you like to be able to use the
string "\n" or the number 80 without having to store it in a named
variable first.



Make Rule 2



[ ITEMS ] makes a new, anonymous array, and returns a reference to
that array.  { ITEMS } makes a new, anonymous hash, and returns a
reference to that hash.



    $aref = [ 1, "foo", undef, 13 ];
    # $aref now holds a reference to an array

    $href = { APR => 4, AUG => 8 };
    # $href now holds a reference to a hash



The references you get from rule 2 are the same kind of
references that you get from rule 1:



        # This:
        $aref = [ 1, 2, 3 ];

        # Does the same as this:
        @array = (1, 2, 3);
        $aref = \@array;



The first line is an abbreviation for the following two lines, except
that it doesn’t create the superfluous array variable @array.



If you write just [], you get a new, empty anonymous array.
If you write just {}, you get a new, empty anonymous hash.









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63.5.2 Using References



What can you do with a reference once you have it?  It’s a scalar
value, and we’ve seen that you can store it as a scalar and get it back
again just like any scalar.  There are just two more ways to use it:





• perlreftut Use Rule 1:  



• perlreftut Use Rule 2:  












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63.5.2.1 Use Rule 1



You can always use an array reference, in curly braces, in place of
the name of an array.  For example, @{$aref} instead of @array.



Here are some examples of that:



Arrays:



        @a              @{$aref}                An array
        reverse @a      reverse @{$aref}        Reverse the array
        $a[3]           ${$aref}[3]             An element of the array
        $a[3] = 17;     ${$aref}[3] = 17        Assigning an element



On each line are two expressions that do the same thing.  The
left-hand versions operate on the array @a.  The right-hand
versions operate on the array that is referred to by $aref.  Once
they find the array they’re operating on, both versions do the same
things to the arrays.



Using a hash reference is exactly the same:



        %h              %{$href}              A hash
        keys %h         keys %{$href}         Get the keys from the hash
        $h{'red'}       ${$href}{'red'}       An element of the hash
        $h{'red'} = 17  ${$href}{'red'} = 17  Assigning an element



Whatever you want to do with a reference, Use Rule 1 tells you how
to do it.  You just write the Perl code that you would have written
for doing the same thing to a regular array or hash, and then replace
the array or hash name with {$reference}.  "How do I loop over an
array when all I have is a reference?"  Well, to loop over an array, you
would write



        for my $element (@array) {
           ...
        }



so replace the array name, @array, with the reference:



        for my $element (@{$aref}) {
           ...
        }



"How do I print out the contents of a hash when all I have is a
reference?"  First write the code for printing out a hash:



        for my $key (keys %hash) {
          print "$key => $hash{$key}\n";
        }



And then replace the hash name with the reference:



        for my $key (keys %{$href}) {
          print "$key => ${$href}{$key}\n";
        }









Previous: perlreftut Use Rule 1, Up: perlreftut Using References   [Contents]





63.5.2.2 Use Rule 2



Use Rule 1 is all you really need, because it tells you how to do
absolutely everything you ever need to do with references.  But the
most common thing to do with an array or a hash is to extract a single
element, and the Use Rule 1 notation is cumbersome.  So there is an
abbreviation.



${$aref}[3] is too hard to read, so you can write $aref->[3]
instead.



${$href}{red} is too hard to read, so you can write
$href->{red} instead.



If $aref holds a reference to an array, then $aref->[3] is
the fourth element of the array.  Don’t confuse this with $aref[3],
which is the fourth element of a totally different array, one
deceptively named @aref.  $aref and @aref are unrelated the
same way that $item and @item are.



Similarly, $href->{'red'} is part of the hash referred to by
the scalar variable $href, perhaps even one with no name.
$href{'red'} is part of the deceptively named %href hash.  It’s
easy to forget to leave out the ->, and if you do, you’ll get
bizarre results when your program gets array and hash elements out of
totally unexpected hashes and arrays that weren’t the ones you wanted
to use.









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63.5.3 An Example



Let’s see a quick example of how all this is useful.



First, remember that [1, 2, 3] makes an anonymous array containing
(1, 2, 3), and gives you a reference to that array.



Now think about



        @a = ( [1, 2, 3],
               [4, 5, 6],
               [7, 8, 9]
             );



@a is an array with three elements, and each one is a reference to
another array.



$a[1] is one of these references.  It refers to an array, the array
containing (4, 5, 6), and because it is a reference to an array,
Use Rule 2 says that we can write $a[1]->[2] to get the
third element from that array.  $a[1]->[2] is the 6.
Similarly, $a[0]->[1] is the 2.  What we have here is like a
two-dimensional array; you can write $a[ROW]->[COLUMN] to get
or set the element in any row and any column of the array.



The notation still looks a little cumbersome, so there’s one more
abbreviation:









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63.5.4 Arrow Rule



In between two subscripts, the arrow is optional.



Instead of $a[1]->[2], we can write $a[1][2]; it means the
same thing.  Instead of $a[0]->[1] = 23, we can write
$a[0][1] = 23; it means the same thing.



Now it really looks like two-dimensional arrays!



You can see why the arrows are important.  Without them, we would have
had to write ${$a[1]}[2] instead of $a[1][2].  For
three-dimensional arrays, they let us write $x[2][3][5] instead of
the unreadable ${${$x[2]}[3]}[5].









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63.6 Solution



Here’s the answer to the problem I posed earlier, of reformatting a
file of city and country names.



    1   my %table;

    2   while (<>) {
    3    chomp;
    4     my ($city, $country) = split /, /;
    5     $table{$country} = [] unless exists $table{$country};
    6     push @{$table{$country}}, $city;
    7   }

    8   foreach $country (sort keys %table) {
    9     print "$country: ";
   10     my @cities = @{$table{$country}};
   11     print join ', ', sort @cities;
   12     print ".\n";
   13   }



The program has two pieces: Lines 2–7 read the input and build a data
structure, and lines 8-13 analyze the data and print out the report.
We’re going to have a hash, %table, whose keys are country names,
and whose values are references to arrays of city names.  The data
structure will look like this:



           %table
        +-------+---+
        |       |   |   +-----------+--------+
        |Germany| *---->| Frankfurt | Berlin |
        |       |   |   +-----------+--------+
        +-------+---+
        |       |   |   +----------+
        |Finland| *---->| Helsinki |
        |       |   |   +----------+
        +-------+---+
        |       |   |   +---------+------------+----------+
        |  USA  | *---->| Chicago | Washington | New York |
        |       |   |   +---------+------------+----------+
        +-------+---+



We’ll look at output first.  Supposing we already have this structure,
how do we print it out?



    8   foreach $country (sort keys %table) {
    9     print "$country: ";
   10     my @cities = @{$table{$country}};
   11     print join ', ', sort @cities;
   12     print ".\n";
   13   }



%table is an
ordinary hash, and we get a list of keys from it, sort the keys, and
loop over the keys as usual.  The only use of references is in line 10.
$table{$country} looks up the key $country in the hash
and gets the value, which is a reference to an array of cities in that country.
Use Rule 1 says that
we can recover the array by saying
@{$table{$country}}.  Line 10 is just like



        @cities = @array;



except that the name array has been replaced by the reference
{$table{$country}}.  The @ tells Perl to get the entire array.
Having gotten the list of cities, we sort it, join it, and print it
out as usual.



Lines 2-7 are responsible for building the structure in the first
place.  Here they are again:



    2   while (<>) {
    3    chomp;
    4     my ($city, $country) = split /, /;
    5     $table{$country} = [] unless exists $table{$country};
    6     push @{$table{$country}}, $city;
    7   }



Lines 2-4 acquire a city and country name.  Line 5 looks to see if the
country is already present as a key in the hash.  If it’s not, the
program uses the [] notation (Make Rule 2) to manufacture a new,
empty anonymous array of cities, and installs a reference to it into
the hash under the appropriate key.



Line 6 installs the city name into the appropriate array.
$table{$country} now holds a reference to the array of cities seen
in that country so far.  Line 6 is exactly like



        push @array, $city;



except that the name array has been replaced by the reference
{$table{$country}}.  The push adds a city name to the end of the
referred-to array.



There’s one fine point I skipped.  Line 5 is unnecessary, and we can
get rid of it.



    2   while (<>) {
    3    chomp;
    4     my ($city, $country) = split /, /;
    5   ####  $table{$country} = [] unless exists $table{$country};
    6     push @{$table{$country}}, $city;
    7   }



If there’s already an entry in %table for the current $country,
then nothing is different.  Line 6 will locate the value in
$table{$country}, which is a reference to an array, and push
$city into the array.  But
what does it do when
$country holds a key, say Greece, that is not yet in %table?



This is Perl, so it does the exact right thing.  It sees that you want
to push Athens onto an array that doesn’t exist, so it helpfully
makes a new, empty, anonymous array for you, installs it into
%table, and then pushes Athens onto it.  This is called
’autovivification’–bringing things to life automatically.  Perl saw
that they key wasn’t in the hash, so it created a new hash entry
automatically. Perl saw that you wanted to use the hash value as an
array, so it created a new empty array and installed a reference to it
in the hash automatically.  And as usual, Perl made the array one
element longer to hold the new city name.









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63.7 The Rest



I promised to give you 90% of the benefit with 10% of the details, and
that means I left out 90% of the details.  Now that you have an
overview of the important parts, it should be easier to read the
(perlref) manual page, which discusses 100% of the details.



Some of the highlights of (perlref):





You might prefer to go on to perllol NAME instead of (perlref); it
discusses lists of lists and multidimensional arrays in detail.  After
that, you should move on to (perldsc); it’s a Data Structure Cookbook
that shows recipes for using and printing out arrays of hashes, hashes
of arrays, and other kinds of data.









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63.8 Summary



Everyone needs compound data structures, and in Perl the way you get
them is with references.  There are four important rules for managing
references: Two for making references and two for using them.  Once
you know these rules you can do most of the important things you need
to do with references.









Previous: perlreftut Summary, Up: perlreftut   [Contents]





63.9 Credits



Author: Mark Jason Dominus, Plover Systems (mjd-perl-ref+@plover.com)



This article originally appeared in The Perl Journal
( http://www.tpj.com/ ) volume 3, #2.  Reprinted with permission.



The original title was Understand References Today.





• perlreftut Distribution Conditions:  












Up: perlreftut Credits   [Contents]





63.9.1 Distribution Conditions



Copyright 1998 The Perl Journal.



This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.



Irrespective of its distribution, all code examples in these files are
hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun or for profit
as you see fit.  A simple comment in the code giving credit would be
courteous but is not required.









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64 perlreguts





• perlreguts NAME:  



• perlreguts DESCRIPTION:  



• perlreguts OVERVIEW:  



• perlreguts Process Overview:  



• perlreguts MISCELLANEOUS:  



• perlreguts SEE ALSO:  



• perlreguts AUTHOR:  



• perlreguts LICENCE:  



• perlreguts REFERENCES:  












Next: , Up: perlreguts   [Contents]





64.1 NAME



perlreguts - Description of the Perl regular expression engine.









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64.2 DESCRIPTION



This document is an attempt to shine some light on the guts of the regex
engine and how it works. The regex engine represents a significant chunk
of the perl codebase, but is relatively poorly understood. This document
is a meagre attempt at addressing this situation. It is derived from the
author’s experience, comments in the source code, other papers on the
regex engine, feedback on the perl5-porters mail list, and no doubt other
places as well.



NOTICE! It should be clearly understood that the behavior and
structures discussed in this represents the state of the engine as the
author understood it at the time of writing. It is NOT an API
definition, it is purely an internals guide for those who want to hack
the regex engine, or understand how the regex engine works. Readers of
this document are expected to understand perl’s regex syntax and its
usage in detail. If you want to learn about the basics of Perl’s
regular expressions, see (perlre). And if you want to replace the
regex engine with your own, see perlreapi NAME.









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64.3 OVERVIEW





• perlreguts A quick note on terms:  



• perlreguts What is a regular expression engine?:  



• perlreguts Structure of a Regexp Program:  












Next: , Up: perlreguts OVERVIEW   [Contents]





64.3.1 A quick note on terms



There is some debate as to whether to say "regexp" or "regex". In this
document we will use the term "regex" unless there is a special reason
not to, in which case we will explain why.



When speaking about regexes we need to distinguish between their source
code form and their internal form. In this document we will use the term
"pattern" when we speak of their textual, source code form, and the term
"program" when we speak of their internal representation. These
correspond to the terms S-regex and B-regex that Mark Jason
Dominus employs in his paper on "Rx" ([1] in REFERENCES).









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64.3.2 What is a regular expression engine?



A regular expression engine is a program that takes a set of constraints
specified in a mini-language, and then applies those constraints to a
target string, and determines whether or not the string satisfies the
constraints. See (perlre) for a full definition of the language.



In less grandiose terms, the first part of the job is to turn a pattern into
something the computer can efficiently use to find the matching point in
the string, and the second part is performing the search itself.



To do this we need to produce a program by parsing the text. We then
need to execute the program to find the point in the string that
matches. And we need to do the whole thing efficiently.









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64.3.3 Structure of a Regexp Program





• perlreguts High Level:  



• perlreguts Regops:  



• perlreguts What regop is next?:  












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64.3.3.1 High Level



Although it is a bit confusing and some people object to the terminology, it
is worth taking a look at a comment that has
been in regexp.h for years:



This is essentially a linear encoding of a nondeterministic
finite-state machine (aka syntax charts or "railroad normal form" in
parsing technology).



The term "railroad normal form" is a bit esoteric, with "syntax
diagram/charts", or "railroad diagram/charts" being more common terms.
Nevertheless it provides a useful mental image of a regex program: each
node can be thought of as a unit of track, with a single entry and in
most cases a single exit point (there are pieces of track that fork, but
statistically not many), and the whole forms a layout with a
single entry and single exit point. The matching process can be thought
of as a car that moves along the track, with the particular route through
the system being determined by the character read at each possible
connector point. A car can fall off the track at any point but it may
only proceed as long as it matches the track.



Thus the pattern /foo(?:\w+|\d+|\s+)bar/ can be thought of as the
following chart:



                      [start]
                         |
                       <foo>
                         |
                   +-----+-----+
                   |     |     |
                 <\w+> <\d+> <\s+>
                   |     |     |
                   +-----+-----+
                         |
                       <bar>
                         |
                       [end]



The truth of the matter is that perl’s regular expressions these days are
much more complex than this kind of structure, but visualising it this way
can help when trying to get your bearings, and it matches the
current implementation pretty closely.



To be more precise, we will say that a regex program is an encoding
of a graph. Each node in the graph corresponds to part of
the original regex pattern, such as a literal string or a branch,
and has a pointer to the nodes representing the next component
to be matched. Since "node" and "opcode" already have other meanings in the
perl source, we will call the nodes in a regex program "regops".



The program is represented by an array of regnode structures, one or
more of which represent a single regop of the program. Struct
regnode is the smallest struct needed, and has a field structure which is
shared with all the other larger structures.



The "next" pointers of all regops except BRANCH implement concatenation;
a "next" pointer with a BRANCH on both ends of it is connecting two
alternatives.  [Here we have one of the subtle syntax dependencies: an
individual BRANCH (as opposed to a collection of them) is never
concatenated with anything because of operator precedence.]



The operand of some types of regop is a literal string; for others,
it is a regop leading into a sub-program.  In particular, the operand
of a BRANCH node is the first regop of the branch.



NOTE: As the railroad metaphor suggests, this is not a tree
structure:  the tail of the branch connects to the thing following the
set of BRANCHes.  It is a like a single line of railway track that
splits as it goes into a station or railway yard and rejoins as it comes
out the other side.









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64.3.3.2 Regops



The base structure of a regop is defined in regexp.h as follows:



    struct regnode {
        U8  flags;    /* Various purposes, sometimes overridden */
        U8  type;     /* Opcode value as specified by regnodes.h */
        U16 next_off; /* Offset in size regnode */
    };



Other larger regnode-like structures are defined in regcomp.h. They
are almost like subclasses in that they have the same fields as
regnode, with possibly additional fields following in
the structure, and in some cases the specific meaning (and name)
of some of base fields are overridden. The following is a more
complete description.





regnode_1






regnode_2




regnode_1 structures have the same header, followed by a single
four-byte argument; regnode_2 structures contain two two-byte
arguments instead:



    regnode_1                U32 arg1;
    regnode_2                U16 arg1;  U16 arg2;





regnode_string




regnode_string structures, used for literal strings, follow the header
with a one-byte length and then the string data. Strings are padded on
the end with zero bytes so that the total length of the node is a
multiple of four bytes:



    regnode_string           char string[1];
                             U8 str_len; /* overrides flags */





regnode_charclass




Character classes are represented by regnode_charclass structures,
which have a four-byte argument and then a 32-byte (256-bit) bitmap
indicating which characters are included in the class.



    regnode_charclass        U32 arg1;
                             char bitmap[ANYOF_BITMAP_SIZE];





regnode_charclass_class




There is also a larger form of a char class structure used to represent
POSIX char classes called regnode_charclass_class which has an
additional 4-byte (32-bit) bitmap indicating which POSIX char classes
have been included.



    regnode_charclass_class  U32 arg1;
                             char bitmap[ANYOF_BITMAP_SIZE];
                             char classflags[ANYOF_CLASSBITMAP_SIZE];








regnodes.h defines an array called regarglen[] which gives the size
of each opcode in units of size regnode (4-byte). A macro is used
to calculate the size of an EXACT node based on its str_len field.



The regops are defined in regnodes.h which is generated from
regcomp.sym by regcomp.pl. Currently the maximum possible number
of distinct regops is restricted to 256, with about a quarter already
used.



A set of macros makes accessing the fields
easier and more consistent. These include OP(), which is used to determine
the type of a regnode-like structure; NEXT_OFF(), which is the offset to
the next node (more on this later); ARG(), ARG1(), ARG2(), ARG_SET(),
and equivalents for reading and setting the arguments; and STR_LEN(),
STRING() and OPERAND() for manipulating strings and regop bearing
types.









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64.3.3.3 What regop is next?



There are three distinct concepts of "next" in the regex engine, and
it is important to keep them clear.











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64.4 Process Overview



Broadly speaking, performing a match of a string against a pattern
involves the following steps:





A. Compilation






1. Parsing for size






2. Parsing for construction






3. Peep-hole optimisation and analysis











B. Execution






4. Start position and no-match optimisations






5. Program execution














Where these steps occur in the actual execution of a perl program is
determined by whether the pattern involves interpolating any string
variables. If interpolation occurs, then compilation happens at run time. If it
does not, then compilation is performed at compile time. (The /o modifier changes this,
as does qr// to a certain extent.) The engine doesn’t really care that
much.





• perlreguts Compilation:  



• perlreguts Execution:  












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64.4.1 Compilation



This code resides primarily in regcomp.c, along with the header files
regcomp.h, regexp.h and regnodes.h.



Compilation starts with pregcomp(), which is mostly an initialisation
wrapper which farms work out to two other routines for the heavy lifting: the
first is reg(), which is the start point for parsing; the second,
study_chunk(), is responsible for optimisation.



Initialisation in pregcomp() mostly involves the creation and data-filling
of a special structure, RExC_state_t (defined in regcomp.c).
Almost all internally-used routines in regcomp.h take a pointer to one
of these structures as their first argument, with the name pRExC_state.
This structure is used to store the compilation state and contains many
fields. Likewise there are many macros which operate on this
variable: anything that looks like RExC_xxxx is a macro that operates on
this pointer/structure.





• perlreguts Parsing for size:  



• perlreguts Parsing for construction:  



• perlreguts Parse Call Graph and a Grammar:  



• perlreguts Debug Output:  



• perlreguts Peep-hole Optimisation and Analysis:  












Next: , Up: perlreguts Compilation   [Contents]





64.4.1.1 Parsing for size



In this pass the input pattern is parsed in order to calculate how much
space is needed for each regop we would need to emit. The size is also
used to determine whether long jumps will be required in the program.



This stage is controlled by the macro SIZE_ONLY being set.



The parse proceeds pretty much exactly as it does during the
construction phase, except that most routines are short-circuited to
change the size field RExC_size and not do anything else.









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64.4.1.2 Parsing for construction



Once the size of the program has been determined, the pattern is parsed
again, but this time for real. Now SIZE_ONLY will be false, and the
actual construction can occur.



reg() is the start of the parse process. It is responsible for
parsing an arbitrary chunk of pattern up to either the end of the
string, or the first closing parenthesis it encounters in the pattern.
This means it can be used to parse the top-level regex, or any section
inside of a grouping parenthesis. It also handles the "special parens"
that perl’s regexes have. For instance when parsing /x(?:foo)y/ reg()
will at one point be called to parse from the "?" symbol up to and
including the ")".



Additionally, reg() is responsible for parsing the one or more
branches from the pattern, and for "finishing them off" by correctly
setting their next pointers. In order to do the parsing, it repeatedly
calls out to regbranch(), which is responsible for handling up to the
first | symbol it sees.



regbranch() in turn calls regpiece() which
handles "things" followed by a quantifier. In order to parse the
"things", regatom() is called. This is the lowest level routine, which
parses out constant strings, character classes, and the
various special symbols like $. If regatom() encounters a "("
character it in turn calls reg().



The routine regtail() is called by both reg() and regbranch()
in order to "set the tail pointer" correctly. When executing and
we get to the end of a branch, we need to go to the node following the
grouping parens. When parsing, however, we don’t know where the end will
be until we get there, so when we do we must go back and update the
offsets as appropriate. regtail is used to make this easier.



A subtlety of the parsing process means that a regex like /foo/ is
originally parsed into an alternation with a single branch. It is only
afterwards that the optimiser converts single branch alternations into the
simpler form.









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64.4.1.3 Parse Call Graph and a Grammar



The call graph looks like this:



    reg()                        # parse a top level regex, or inside of parens
        regbranch()              # parse a single branch of an alternation
            regpiece()           # parse a pattern followed by a quantifier
                regatom()        # parse a simple pattern
                    regclass()   #   used to handle a class
                    reg()        #   used to handle a parenthesised subpattern
                    ....
            ...
            regtail()            # finish off the branch
        ...
        regtail()                # finish off the branch sequence. Tie each
                                 # branch's tail to the tail of the sequence
                                 # (NEW) In Debug mode this is
                                 # regtail_study().



A grammar form might be something like this:



    atom  : constant | class
    quant : '*' | '+' | '?' | '{min,max}'
    _branch: piece
           | piece _branch
           | nothing
    branch: _branch
          | _branch '|' branch
    group : '(' branch ')'
    _piece: atom | group
    piece : _piece
          | _piece quant









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64.4.1.4 Debug Output



In the 5.9.x development version of perl you can use re Debug => 'PARSE'
to see some trace information about the parse process. We will start with some
simple patterns and build up to more complex patterns.



So when we parse /foo/ we see something like the following table. The
left shows what is being parsed, and the number indicates where the next regop
would go. The stuff on the right is the trace output of the graph. The
names are chosen to be short to make it less dense on the screen. ’tsdy’
is a special form of regtail() which does some extra analysis.



 >foo<             1    reg
                          brnc
                            piec
                              atom
 ><                4      tsdy~ EXACT <foo> (EXACT) (1)
                              ~ attach to END (3) offset to 2



The resulting program then looks like:



   1: EXACT <foo>(3)
   3: END(0)



As you can see, even though we parsed out a branch and a piece, it was ultimately
only an atom. The final program shows us how things work. We have an EXACT regop,
followed by an END regop. The number in parens indicates where the regnext of
the node goes. The regnext of an END regop is unused, as END regops mean
we have successfully matched. The number on the left indicates the position of
the regop in the regnode array.



Now let’s try a harder pattern. We will add a quantifier, so now we have the pattern
/foo+/. We will see that regbranch() calls regpiece() twice.



 >foo+<            1    reg
                          brnc
                            piec
                              atom
 >o+<              3        piec
                              atom
 ><                6        tail~ EXACT <fo> (1)
                   7      tsdy~ EXACT <fo> (EXACT) (1)
                              ~ PLUS (END) (3)
                              ~ attach to END (6) offset to 3



And we end up with the program:



   1: EXACT <fo>(3)
   3: PLUS(6)
   4:   EXACT <o>(0)
   6: END(0)



Now we have a special case. The EXACT regop has a regnext of 0. This is
because if it matches it should try to match itself again. The PLUS regop
handles the actual failure of the EXACT regop and acts appropriately (going
to regnode 6 if the EXACT matched at least once, or failing if it didn’t).



Now for something much more complex: /x(?:foo*|b[a][rR])(foo|bar)$/



 >x(?:foo*|b...    1    reg
                          brnc
                            piec
                              atom
 >(?:foo*|b[...    3        piec
                              atom
 >?:foo*|b[a...                 reg
 >foo*|b[a][...                   brnc
                                    piec
                                      atom
 >o*|b[a][rR...    5                piec
                                      atom
 >|b[a][rR])...    8                tail~ EXACT <fo> (3)
 >b[a][rR])(...    9              brnc
                  10                piec
                                      atom
 >[a][rR])(f...   12                piec
                                      atom
 >a][rR])(fo...                         clas
 >[rR])(foo|...   14                tail~ EXACT <b> (10)
                                    piec
                                      atom
 >rR])(foo|b...                         clas
 >)(foo|bar)...   25                tail~ EXACT <a> (12)
                                  tail~ BRANCH (3)
                  26              tsdy~ BRANCH (END) (9)
                                      ~ attach to TAIL (25) offset to 16
                                  tsdy~ EXACT <fo> (EXACT) (4)
                                      ~ STAR (END) (6)
                                      ~ attach to TAIL (25) offset to 19
                                  tsdy~ EXACT <b> (EXACT) (10)
                                      ~ EXACT <a> (EXACT) (12)
                                      ~ ANYOF[Rr] (END) (14)
                                      ~ attach to TAIL (25) offset to 11
 >(foo|bar)$<               tail~ EXACT <x> (1)
                            piec
                              atom
 >foo|bar)$<                    reg
                  28              brnc
                                    piec
                                      atom
 >|bar)$<         31              tail~ OPEN1 (26)
 >bar)$<                          brnc
                  32                piec
                                      atom
 >)$<             34              tail~ BRANCH (28)
                  36              tsdy~ BRANCH (END) (31)
                                      ~ attach to CLOSE1 (34) offset to 3
                                  tsdy~ EXACT <foo> (EXACT) (29)
                                      ~ attach to CLOSE1 (34) offset to 5
                                  tsdy~ EXACT <bar> (EXACT) (32)
                                      ~ attach to CLOSE1 (34) offset to 2
 >$<                        tail~ BRANCH (3)
                                ~ BRANCH (9)
                                ~ TAIL (25)
                            piec
                              atom
 ><               37        tail~ OPEN1 (26)
                                ~ BRANCH (28)
                                ~ BRANCH (31)
                                ~ CLOSE1 (34)
                  38      tsdy~ EXACT <x> (EXACT) (1)
                              ~ BRANCH (END) (3)
                              ~ BRANCH (END) (9)
                              ~ TAIL (END) (25)
                              ~ OPEN1 (END) (26)
                              ~ BRANCH (END) (28)
                              ~ BRANCH (END) (31)
                              ~ CLOSE1 (END) (34)
                              ~ EOL (END) (36)
                              ~ attach to END (37) offset to 1



Resulting in the program



   1: EXACT <x>(3)
   3: BRANCH(9)
   4:   EXACT <fo>(6)
   6:   STAR(26)
   7:     EXACT <o>(0)
   9: BRANCH(25)
  10:   EXACT <ba>(14)
  12:   OPTIMIZED (2 nodes)
  14:   ANYOF[Rr](26)
  25: TAIL(26)
  26: OPEN1(28)
  28:   TRIE-EXACT(34)
        [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
          <foo>
          <bar>
  30:   OPTIMIZED (4 nodes)
  34: CLOSE1(36)
  36: EOL(37)
  37: END(0)



Here we can see a much more complex program, with various optimisations in
play. At regnode 10 we see an example where a character class with only
one character in it was turned into an EXACT node. We can also see where
an entire alternation was turned into a TRIE-EXACT node. As a consequence,
some of the regnodes have been marked as optimised away. We can see that
the $ symbol has been converted into an EOL regop, a special piece of
code that looks for \n or the end of the string.



The next pointer for BRANCHes is interesting in that it points at where
execution should go if the branch fails. When executing, if the engine
tries to traverse from a branch to a regnext that isn’t a branch then
the engine will know that the entire set of branches has failed.









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64.4.1.5 Peep-hole Optimisation and Analysis



The regular expression engine can be a weighty tool to wield. On long
strings and complex patterns it can end up having to do a lot of work
to find a match, and even more to decide that no match is possible.
Consider a situation like the following pattern.



   'ababababababababababab' =~ /(a|b)*z/



The (a|b)* part can match at every char in the string, and then fail
every time because there is no z in the string. So obviously we can
avoid using the regex engine unless there is a z in the string.
Likewise in a pattern like:



   /foo(\w+)bar/



In this case we know that the string must contain a foo which must be
followed by bar. We can use Fast Boyer-Moore matching as implemented
in fbm_instr() to find the location of these strings. If they don’t exist
then we don’t need to resort to the much more expensive regex engine.
Even better, if they do exist then we can use their positions to
reduce the search space that the regex engine needs to cover to determine
if the entire pattern matches.



There are various aspects of the pattern that can be used to facilitate
optimisations along these lines:





Another form of optimisation that can occur is the post-parse "peep-hole"
optimisation, where inefficient constructs are replaced by more efficient
constructs. The TAIL regops which are used during parsing to mark the end
of branches and the end of groups are examples of this. These regops are used
as place-holders during construction and "always match" so they can be
"optimised away" by making the things that point to the TAIL point to the
thing that TAIL points to, thus "skipping" the node.



Another optimisation that can occur is that of "EXACT merging" which is
where two consecutive EXACT nodes are merged into a single
regop. An even more aggressive form of this is that a branch
sequence of the form EXACT BRANCH ... EXACT can be converted into a
TRIE-EXACT regop.



All of this occurs in the routine study_chunk() which uses a special
structure scan_data_t to store the analysis that it has performed, and
does the "peep-hole" optimisations as it goes.



The code involved in study_chunk() is extremely cryptic. Be careful. :-)









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64.4.2 Execution



Execution of a regex generally involves two phases, the first being
finding the start point in the string where we should match from,
and the second being running the regop interpreter.



If we can tell that there is no valid start point then we don’t bother running
interpreter at all. Likewise, if we know from the analysis phase that we
cannot detect a short-cut to the start position, we go straight to the
interpreter.



The two entry points are re_intuit_start() and pregexec(). These routines
have a somewhat incestuous relationship with overlap between their functions,
and pregexec() may even call re_intuit_start() on its own. Nevertheless
other parts of the perl source code may call into either, or both.



Execution of the interpreter itself used to be recursive, but thanks to the
efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an
internal stack is maintained on the heap and the routine is fully
iterative. This can make it tricky as the code is quite conservative
about what state it stores, with the result that two consecutive lines in the
code can actually be running in totally different contexts due to the
simulated recursion.





• perlreguts Start position and no-match optimisations:  



• perlreguts Program execution:  












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64.4.2.1 Start position and no-match optimisations



re_intuit_start() is responsible for handling start points and no-match
optimisations as determined by the results of the analysis done by
study_chunk() (and described in Peep-hole Optimisation and Analysis).



The basic structure of this routine is to try to find the start- and/or
end-points of where the pattern could match, and to ensure that the string
is long enough to match the pattern. It tries to use more efficient
methods over less efficient methods and may involve considerable
cross-checking of constraints to find the place in the string that matches.
For instance it may try to determine that a given fixed string must be
not only present but a certain number of chars before the end of the
string, or whatever.



It calls several other routines, such as fbm_instr() which does
Fast Boyer Moore matching and find_byclass() which is responsible for
finding the start using the first mandatory regop in the program.



When the optimisation criteria have been satisfied, reg_try() is called
to perform the match.









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64.4.2.2 Program execution



pregexec() is the main entry point for running a regex. It contains
support for initialising the regex interpreter’s state, running
re_intuit_start() if needed, and running the interpreter on the string
from various start positions as needed. When it is necessary to use
the regex interpreter pregexec() calls regtry().



regtry() is the entry point into the regex interpreter. It expects
as arguments a pointer to a regmatch_info structure and a pointer to
a string.  It returns an integer 1 for success and a 0 for failure.
It is basically a set-up wrapper around regmatch().



regmatch is the main "recursive loop" of the interpreter. It is
basically a giant switch statement that implements a state machine, where
the possible states are the regops themselves, plus a number of additional
intermediate and failure states. A few of the states are implemented as
subroutines but the bulk are inline code.









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64.5 MISCELLANEOUS





• perlreguts Unicode and Localisation Support:  



• perlreguts Base Structures:  












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64.5.1 Unicode and Localisation Support



When dealing with strings containing characters that cannot be represented
using an eight-bit character set, perl uses an internal representation
that is a permissive version of Unicode’s UTF-8 encoding[2]. This uses single
bytes to represent characters from the ASCII character set, and sequences
of two or more bytes for all other characters. (See perlunitut NAME
for more information about the relationship between UTF-8 and perl’s
encoding, utf8. The difference isn’t important for this discussion.)



No matter how you look at it, Unicode support is going to be a pain in a
regex engine. Tricks that might be fine when you have 256 possible
characters often won’t scale to handle the size of the UTF-8 character
set.  Things you can take for granted with ASCII may not be true with
Unicode. For instance, in ASCII, it is safe to assume that
sizeof(char1) == sizeof(char2), but in UTF-8 it isn’t. Unicode case folding is
vastly more complex than the simple rules of ASCII, and even when not
using Unicode but only localised single byte encodings, things can get
tricky (for example, LATIN SMALL LETTER SHARP S (U+00DF, ß)
should match ’SS’ in localised case-insensitive matching).



Making things worse is that UTF-8 support was a later addition to the
regex engine (as it was to perl) and this necessarily  made things a lot
more complicated. Obviously it is easier to design a regex engine with
Unicode support in mind from the beginning than it is to retrofit it to
one that wasn’t.



Nearly all regops that involve looking at the input string have
two cases, one for UTF-8, and one not. In fact, it’s often more complex
than that, as the pattern may be UTF-8 as well.



Care must be taken when making changes to make sure that you handle
UTF-8 properly, both at compile time and at execution time, including
when the string and pattern are mismatched.



The following comment in regcomp.h gives an example of exactly how
tricky this can be:



    Two problematic code points in Unicode casefolding of EXACT nodes:

    U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
    U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS

    which casefold to

    Unicode                      UTF-8

    U+03B9 U+0308 U+0301         0xCE 0xB9 0xCC 0x88 0xCC 0x81
    U+03C5 U+0308 U+0301         0xCF 0x85 0xCC 0x88 0xCC 0x81

    This means that in case-insensitive matching (or "loose matching",
    as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
    byte length of the above casefolded versions) can match a target
    string of length two (the byte length of UTF-8 encoded U+0390 or
    U+03B0). This would rather mess up the minimum length computation.

    What we'll do is to look for the tail four bytes, and then peek
    at the preceding two bytes to see whether we need to decrease
    the minimum length by four (six minus two).

    Thanks to the design of UTF-8, there cannot be false matches:
    A sequence of valid UTF-8 bytes cannot be a subsequence of
    another valid sequence of UTF-8 bytes.









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64.5.2 Base Structures



The regexp structure described in perlreapi NAME is common to all
regex engines. Two of its fields that are intended for the private use
of the regex engine that compiled the pattern. These are the
intflags and pprivate members. The pprivate is a void pointer to
an arbitrary structure whose use and management is the responsibility
of the compiling engine. perl will never modify either of these
values. In the case of the stock engine the structure pointed to by
pprivate is called regexp_internal.



Its pprivate and intflags fields contain data
specific to each engine.



There are two structures used to store a compiled regular expression.
One, the regexp structure described in perlreapi NAME is populated by
the engine currently being. used and some of its fields read by perl to
implement things such as the stringification of qr//.



The other structure is pointed to be the regexp struct’s
pprivate and is in addition to intflags in the same struct
considered to be the property of the regex engine which compiled the
regular expression; 



The regexp structure contains all the data that perl needs to be aware of
to properly work with the regular expression. It includes data about
optimisations that perl can use to determine if the regex engine should
really be used, and various other control info that is needed to properly
execute patterns in various contexts such as is the pattern anchored in
some way, or what flags were used during the compile, or whether the
program contains special constructs that perl needs to be aware of.



In addition it contains two fields that are intended for the private use
of the regex engine that compiled the pattern. These are the intflags
and pprivate members. The pprivate is a void pointer to an arbitrary
structure whose use and management is the responsibility of the compiling
engine. perl will never modify either of these values.



As mentioned earlier, in the case of the default engines, the pprivate
will be a pointer to a regexp_internal structure which holds the compiled
program and any additional data that is private to the regex engine
implementation.





• perlreguts Perl's pprivate structure:  












Up: perlreguts Base Structures   [Contents]





64.5.2.1 Perl’s pprivate structure



The following structure is used as the pprivate struct by perl’s
regex engine. Since it is specific to perl it is only of curiosity
value to other engine implementations.



    typedef struct regexp_internal {
            regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
            U32 *offsets;           /* offset annotations 20001228 MJD 
                                       data about mapping the program to the 
                                       string*/
            regnode *regstclass;    /* Optional startclass as identified or constructed
                                       by the optimiser */
            struct reg_data *data;  /* Additional miscellaneous data used by the program.
                                       Used to make it easier to clone and free arbitrary
                                       data that the regops need. Often the ARG field of
                                       a regop is an index into this structure */
            regnode program[1];     /* Unwarranted chumminess with compiler. */
    } regexp_internal;





swap




swap formerly was an extra set of startp/endp stored in a
regexp_paren_ofs struct. This was used when the last successful match
was from the same pattern as the current pattern, so that a partial
match didn’t overwrite the previous match’s results, but it caused a
problem with re-entrant code such as trying to build the UTF-8 swashes.
Currently unused and left for backward compatibility with 5.10.0.





offsets




Offsets holds a mapping of offset in the program
to offset in the precomp string. This is only used by ActiveState’s
visual regex debugger.





regstclass




Special regop that is used by re_intuit_start() to check if a pattern
can match at a certain position. For instance if the regex engine knows
that the pattern must start with a ’Z’ then it can scan the string until
it finds one and then launch the regex engine from there. The routine
that handles this is called find_by_class(). Sometimes this field
points at a regop embedded in the program, and sometimes it points at
an independent synthetic regop that has been constructed by the optimiser.





data




This field points at a reg_data structure, which is defined as follows



    struct reg_data {
        U32 count;
        U8 *what;
        void* data[1];
    };



This structure is used for handling data structures that the regex engine
needs to handle specially during a clone or free operation on the compiled
product. Each element in the data array has a corresponding element in the
what array. During compilation regops that need special structures stored
will add an element to each array using the add_data() routine and then store
the index in the regop.





program




Compiled program. Inlined into the structure so the entire struct can be
treated as a single blob.














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64.6 SEE ALSO



perlreapi NAME



(perlre)



perlunitut NAME









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64.7 AUTHOR



by Yves Orton, 2006.



With excerpts from Perl, and contributions and suggestions from
Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus,
Stephen McCamant, and David Landgren.









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64.8 LICENCE



Same terms as Perl.









Previous: perlreguts LICENCE, Up: perlreguts   [Contents]





64.9 REFERENCES



[1] http://perl.plover.com/Rx/paper/



[2] http://www.unicode.org









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65 perlrequick





• perlrequick NAME:  



• perlrequick DESCRIPTION:  



• perlrequick The Guide:  



• perlrequick BUGS:  



• perlrequick SEE ALSO:  



• perlrequick AUTHOR AND COPYRIGHT:  












Next: , Up: perlrequick   [Contents]





65.1 NAME



perlrequick - Perl regular expressions quick start









Next: , Previous: perlrequick NAME, Up: perlrequick   [Contents]





65.2 DESCRIPTION



This page covers the very basics of understanding, creating and
using regular expressions (’regexes’) in Perl.









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65.3 The Guide





• perlrequick Simple word matching:  



• perlrequick Using character classes:  



• perlrequick Matching this or that:  



• perlrequick Grouping things and hierarchical matching:  



• perlrequick Extracting matches:  



• perlrequick Matching repetitions:  



• perlrequick More matching:  



• perlrequick Search and replace:  



• perlrequick The split operator:  












Next: , Up: perlrequick The Guide   [Contents]





65.3.1 Simple word matching



The simplest regex is simply a word, or more generally, a string of
characters.  A regex consisting of a word matches any string that
contains that word:



    "Hello World" =~ /World/;  # matches



In this statement, World is a regex and the // enclosing
/World/ tells Perl to search a string for a match.  The operator
=~ associates the string with the regex match and produces a true
value if the regex matched, or false if the regex did not match.  In
our case, World matches the second word in "Hello World", so the
expression is true.  This idea has several variations.



Expressions like this are useful in conditionals:



    print "It matches\n" if "Hello World" =~ /World/;



The sense of the match can be reversed by using !~ operator:



    print "It doesn't match\n" if "Hello World" !~ /World/;



The literal string in the regex can be replaced by a variable:



    $greeting = "World";
    print "It matches\n" if "Hello World" =~ /$greeting/;



If you’re matching against $_, the $_ =~ part can be omitted:



    $_ = "Hello World";
    print "It matches\n" if /World/;



Finally, the // default delimiters for a match can be changed to
arbitrary delimiters by putting an 'm' out front:



    "Hello World" =~ m!World!;   # matches, delimited by '!'
    "Hello World" =~ m{World};   # matches, note the matching '{}'
    "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
                                 # '/' becomes an ordinary char



Regexes must match a part of the string exactly in order for the
statement to be true:



    "Hello World" =~ /world/;  # doesn't match, case sensitive
    "Hello World" =~ /o W/;    # matches, ' ' is an ordinary char
    "Hello World" =~ /World /; # doesn't match, no ' ' at end



Perl will always match at the earliest possible point in the string:



    "Hello World" =~ /o/;       # matches 'o' in 'Hello'
    "That hat is red" =~ /hat/; # matches 'hat' in 'That'



Not all characters can be used ’as is’ in a match.  Some characters,
called metacharacters, are reserved for use in regex notation.
The metacharacters are



    {}[]()^$.|*+?\



A metacharacter can be matched by putting a backslash before it:



    "2+2=4" =~ /2+2/;    # doesn't match, + is a metacharacter
    "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
    'C:\WIN32' =~ /C:\\WIN/;                       # matches
    "/usr/bin/perl" =~ /\/usr\/bin\/perl/;  # matches



In the last regex, the forward slash '/' is also backslashed,
because it is used to delimit the regex.



Non-printable ASCII characters are represented by escape sequences.
Common examples are \t for a tab, \n for a newline, and \r
for a carriage return.  Arbitrary bytes are represented by octal
escape sequences, e.g., \033, or hexadecimal escape sequences,
e.g., \x1B:



    "1000\t2000" =~ m(0\t2)      # matches
    "cat"      =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat



Regexes are treated mostly as double-quoted strings, so variable
substitution works:



    $foo = 'house';
    'cathouse' =~ /cat$foo/;   # matches
    'housecat' =~ /${foo}cat/; # matches



With all of the regexes above, if the regex matched anywhere in the
string, it was considered a match.  To specify where it should
match, we would use the anchor metacharacters ^ and $.  The
anchor ^ means match at the beginning of the string and the anchor
$ means match at the end of the string, or before a newline at the
end of the string.  Some examples:



    "housekeeper" =~ /keeper/;         # matches
    "housekeeper" =~ /^keeper/;        # doesn't match
    "housekeeper" =~ /keeper$/;        # matches
    "housekeeper\n" =~ /keeper$/;      # matches
    "housekeeper" =~ /^housekeeper$/;  # matches









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65.3.2 Using character classes



A character class allows a set of possible characters, rather than
just a single character, to match at a particular point in a regex.
Character classes are denoted by brackets [...], with the set of
characters to be possibly matched inside.  Here are some examples:



    /cat/;            # matches 'cat'
    /[bcr]at/;        # matches 'bat', 'cat', or 'rat'
    "abc" =~ /[cab]/; # matches 'a'



In the last statement, even though 'c' is the first character in
the class, the earliest point at which the regex can match is 'a'.



    /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
                    # 'yes', 'Yes', 'YES', etc.
    /yes/i;         # also match 'yes' in a case-insensitive way



The last example shows a match with an 'i' modifier, which makes
the match case-insensitive.



Character classes also have ordinary and special characters, but the
sets of ordinary and special characters inside a character class are
different than those outside a character class.  The special
characters for a character class are -]\^$ and are matched using an
escape:



   /[\]c]def/; # matches ']def' or 'cdef'
   $x = 'bcr';
   /[$x]at/;   # matches 'bat, 'cat', or 'rat'
   /[\$x]at/;  # matches '$at' or 'xat'
   /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'



The special character '-' acts as a range operator within character
classes, so that the unwieldy [0123456789] and [abc...xyz]
become the svelte [0-9] and [a-z]:



    /item[0-9]/;  # matches 'item0' or ... or 'item9'
    /[0-9a-fA-F]/;  # matches a hexadecimal digit



If '-' is the first or last character in a character class, it is
treated as an ordinary character.



The special character ^ in the first position of a character class
denotes a negated character class, which matches any character but
those in the brackets.  Both [...] and [^...] must match a
character, or the match fails.  Then



    /[^a]at/;  # doesn't match 'aat' or 'at', but matches
               # all other 'bat', 'cat, '0at', '%at', etc.
    /[^0-9]/;  # matches a non-numeric character
    /[a^]at/;  # matches 'aat' or '^at'; here '^' is ordinary



Perl has several abbreviations for common character classes. (These
definitions are those that Perl uses in ASCII-safe mode with the /a modifier.
Otherwise they could match many more non-ASCII Unicode characters as
well.  See (perlrecharclass)Backslash sequences for details.)





The \d\s\w\D\S\W abbreviations can be used both inside and outside
of character classes.  Here are some in use:



    /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
    /[\d\s]/;         # matches any digit or whitespace character
    /\w\W\w/;         # matches a word char, followed by a
                      # non-word char, followed by a word char
    /..rt/;           # matches any two chars, followed by 'rt'
    /end\./;          # matches 'end.'
    /end[.]/;         # same thing, matches 'end.'



The word anchor  \b matches a boundary between a word
character and a non-word character \w\W or \W\w:



    $x = "Housecat catenates house and cat";
    $x =~ /\bcat/;  # matches cat in 'catenates'
    $x =~ /cat\b/;  # matches cat in 'housecat'
    $x =~ /\bcat\b/;  # matches 'cat' at end of string



In the last example, the end of the string is considered a word
boundary.









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65.3.3 Matching this or that



We can match different character strings with the alternation
metacharacter '|'.  To match dog or cat, we form the regex
dog|cat.  As before, Perl will try to match the regex at the
earliest possible point in the string.  At each character position,
Perl will first try to match the first alternative, dog.  If
dog doesn’t match, Perl will then try the next alternative, cat.
If cat doesn’t match either, then the match fails and Perl moves to
the next position in the string.  Some examples:



    "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
    "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"



Even though dog is the first alternative in the second regex,
cat is able to match earlier in the string.



    "cats"          =~ /c|ca|cat|cats/; # matches "c"
    "cats"          =~ /cats|cat|ca|c/; # matches "cats"



At a given character position, the first alternative that allows the
regex match to succeed will be the one that matches. Here, all the
alternatives match at the first string position, so the first matches.









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65.3.4 Grouping things and hierarchical matching



The grouping metacharacters () allow a part of a regex to be
treated as a single unit.  Parts of a regex are grouped by enclosing
them in parentheses.  The regex house(cat|keeper) means match
house followed by either cat or keeper.  Some more examples
are



    /(a|b)b/;    # matches 'ab' or 'bb'
    /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere

    /house(cat|)/;  # matches either 'housecat' or 'house'
    /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
                        # 'house'.  Note groups can be nested.

    "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                             # because '20\d\d' can't match









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65.3.5 Extracting matches



The grouping metacharacters () also allow the extraction of the
parts of a string that matched.  For each grouping, the part that
matched inside goes into the special variables $1, $2, etc.
They can be used just as ordinary variables:



    # extract hours, minutes, seconds
    $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
    $hours = $1;
    $minutes = $2;
    $seconds = $3;



In list context, a match /regex/ with groupings will return the
list of matched values ($1,$2,...).  So we could rewrite it as



    ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);



If the groupings in a regex are nested, $1 gets the group with the
leftmost opening parenthesis, $2 the next opening parenthesis,
etc.  For example, here is a complex regex and the matching variables
indicated below it:



    /(ab(cd|ef)((gi)|j))/;
     1  2      34



Associated with the matching variables $1, $2, ... are
the backreferences \g1, \g2, ...  Backreferences are
matching variables that can be used inside a regex:



    /(\w\w\w)\s\g1/; # find sequences like 'the the' in string



$1, $2, ... should only be used outside of a regex, and \g1,
\g2, ... only inside a regex.









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65.3.6 Matching repetitions



The quantifier metacharacters ?, *, +, and {} allow us
to determine the number of repeats of a portion of a regex we
consider to be a match.  Quantifiers are put immediately after the
character, character class, or grouping that we want to specify.  They
have the following meanings:





Here are some examples:



    /[a-z]+\s+\d*/;  # match a lowercase word, at least some space, and
                     # any number of digits
    /(\w+)\s+\g1/;    # match doubled words of arbitrary length
    $year =~ /^\d{2,4}$/;  # make sure year is at least 2 but not more
                           # than 4 digits
    $year =~ /^\d{4}$|^\d{2}$/;    # better match; throw out 3 digit dates



These quantifiers will try to match as much of the string as possible,
while still allowing the regex to match.  So we have



    $x = 'the cat in the hat';
    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 matches)



The first quantifier .* grabs as much of the string as possible
while still having the regex match. The second quantifier .* has
no string left to it, so it matches 0 times.









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65.3.7 More matching



There are a few more things you might want to know about matching
operators.
The global modifier //g allows the matching operator to match
within a string as many times as possible.  In scalar context,
successive matches against a string will have //g jump from match
to match, keeping track of position in the string as it goes along.
You can get or set the position with the pos() function.
For example,



    $x = "cat dog house"; # 3 words
    while ($x =~ /(\w+)/g) {
        print "Word is $1, ends at position ", pos $x, "\n";
    }



prints



    Word is cat, ends at position 3
    Word is dog, ends at position 7
    Word is house, ends at position 13



A failed match or changing the target string resets the position.  If
you don’t want the position reset after failure to match, add the
//c, as in /regex/gc.



In list context, //g returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regex.  So



    @words = ($x =~ /(\w+)/g);  # matches,
                                # $word[0] = 'cat'
                                # $word[1] = 'dog'
                                # $word[2] = 'house'









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65.3.8 Search and replace



Search and replace is performed using s/regex/replacement/modifiers.
The replacement is a Perl double-quoted string that replaces in the
string whatever is matched with the regex.  The operator =~ is
also used here to associate a string with s///.  If matching
against $_, the $_ =~ can be dropped.  If there is a match,
s/// returns the number of substitutions made; otherwise it returns
false.  Here are a few examples:



    $x = "Time to feed the cat!";
    $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
    $y = "'quoted words'";
    $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
                           # $y contains "quoted words"



With the s/// operator, the matched variables $1, $2, etc.
are immediately available for use in the replacement expression. With
the global modifier, s///g will search and replace all occurrences
of the regex in the string:



    $x = "I batted 4 for 4";
    $x =~ s/4/four/;   # $x contains "I batted four for 4"
    $x = "I batted 4 for 4";
    $x =~ s/4/four/g;  # $x contains "I batted four for four"



The non-destructive modifier s///r causes the result of the substitution
to be returned instead of modifying $_ (or whatever variable the
substitute was bound to with =~):



    $x = "I like dogs.";
    $y = $x =~ s/dogs/cats/r;
    print "$x $y\n"; # prints "I like dogs. I like cats."

    $x = "Cats are great.";
    print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
    # prints "Hedgehogs are great."

    @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
    # @foo is now qw(X X X 1 2 3)



The evaluation modifier s///e wraps an eval{...} around the
replacement string and the evaluated result is substituted for the
matched substring.  Some examples:



    # reverse all the words in a string
    $x = "the cat in the hat";
    $x =~ s/(\w+)/reverse $1/ge;   # $x contains "eht tac ni eht tah"

    # convert percentage to decimal
    $x = "A 39% hit rate";
    $x =~ s!(\d+)%!$1/100!e;       # $x contains "A 0.39 hit rate"



The last example shows that s/// can use other delimiters, such as
s!!! and s{}{}, and even s{}//.  If single quotes are used
s''', then the regex and replacement are treated as single-quoted
strings.









Previous: perlrequick Search and replace, Up: perlrequick The Guide   [Contents]





65.3.9 The split operator



split /regex/, string splits string into a list of substrings
and returns that list.  The regex determines the character sequence
that string is split with respect to.  For example, to split a
string into words, use



    $x = "Calvin and Hobbes";
    @word = split /\s+/, $x;  # $word[0] = 'Calvin'
                              # $word[1] = 'and'
                              # $word[2] = 'Hobbes'



To extract a comma-delimited list of numbers, use



    $x = "1.618,2.718,   3.142";
    @const = split /,\s*/, $x;  # $const[0] = '1.618'
                                # $const[1] = '2.718'
                                # $const[2] = '3.142'



If the empty regex // is used, the string is split into individual
characters.  If the regex has groupings, then the list produced contains
the matched substrings from the groupings as well:



    $x = "/usr/bin";
    @parts = split m!(/)!, $x;  # $parts[0] = ''
                                # $parts[1] = '/'
                                # $parts[2] = 'usr'
                                # $parts[3] = '/'
                                # $parts[4] = 'bin'



Since the first character of $x matched the regex, split prepended
an empty initial element to the list.









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65.4 BUGS



None.









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65.5 SEE ALSO



This is just a quick start guide.  For a more in-depth tutorial on
regexes, see perlretut NAME and for the reference page, see (perlre).









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65.6 AUTHOR AND COPYRIGHT



Copyright (c) 2000 Mark Kvale
All rights reserved.



This document may be distributed under the same terms as Perl itself.





• perlrequick Acknowledgments:  












Up: perlrequick AUTHOR AND COPYRIGHT   [Contents]





65.6.1 Acknowledgments



The author would like to thank Mark-Jason Dominus, Tom Christiansen,
Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
comments.









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66 perlreref





• perlreref NAME:  



• perlreref DESCRIPTION:  



• perlreref AUTHOR:  



• perlreref SEE ALSO:  



• perlreref THANKS:  












Next: , Up: perlreref   [Contents]





66.1 NAME



perlreref - Perl Regular Expressions Reference









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66.2 DESCRIPTION



This is a quick reference to Perl’s regular expressions.
For full information see (perlre) and (perlop), as well
as the SEE ALSO section in this document.





• perlreref OPERATORS:  



• perlreref SYNTAX:  



• perlreref ESCAPE SEQUENCES:  



• perlreref CHARACTER CLASSES:  



• perlreref ANCHORS:  



• perlreref QUANTIFIERS:  



• perlreref EXTENDED CONSTRUCTS:  



• perlreref VARIABLES:  



• perlreref FUNCTIONS:  



• perlreref TERMINOLOGY:  












Next: , Up: perlreref DESCRIPTION   [Contents]





66.2.1 OPERATORS



=~ determines to which variable the regex is applied.
In its absence, $_ is used.



    $var =~ /foo/;



!~ determines to which variable the regex is applied,
and negates the result of the match; it returns
false if the match succeeds, and true if it fails.



    $var !~ /foo/;



m/pattern/msixpogcdual searches a string for a pattern match,
applying the given options.



    m  Multiline mode - ^ and $ match internal lines
    s  match as a Single line - . matches \n
    i  case-Insensitive
    x  eXtended legibility - free whitespace and comments
    p  Preserve a copy of the matched string -
       ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
    o  compile pattern Once
    g  Global - all occurrences
    c  don't reset pos on failed matches when using /g
    a  restrict \d, \s, \w and [:posix:] to match ASCII only
    aa (two a's) also /i matches exclude ASCII/non-ASCII
    l  match according to current locale
    u  match according to Unicode rules
    d  match according to native rules unless something indicates
       Unicode



If ’pattern’ is an empty string, the last successfully matched
regex is used. Delimiters other than ’/’ may be used for both this
operator and the following ones. The leading m can be omitted
if the delimiter is ’/’.



qr/pattern/msixpodual lets you store a regex in a variable,
or pass one around. Modifiers as for m//, and are stored
within the regex.



s/pattern/replacement/msixpogcedual substitutes matches of
’pattern’ with ’replacement’. Modifiers as for m//,
with two additions:



    e  Evaluate 'replacement' as an expression
    r  Return substitution and leave the original string untouched.



’e’ may be specified multiple times. ’replacement’ is interpreted
as a double quoted string unless a single-quote (') is the delimiter.



?pattern? is like m/pattern/ but matches only once. No alternate
delimiters can be used.  Must be reset with reset().









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66.2.2 SYNTAX



 \       Escapes the character immediately following it
 .       Matches any single character except a newline (unless /s is
           used)
 ^       Matches at the beginning of the string (or line, if /m is used)
 $       Matches at the end of the string (or line, if /m is used)
 *       Matches the preceding element 0 or more times
 +       Matches the preceding element 1 or more times
 ?       Matches the preceding element 0 or 1 times
 {...}   Specifies a range of occurrences for the element preceding it
 [...]   Matches any one of the characters contained within the brackets
 (...)   Groups subexpressions for capturing to $1, $2...
 (?:...) Groups subexpressions without capturing (cluster)
 |       Matches either the subexpression preceding or following it
 \g1 or \g{1}, \g2 ...    Matches the text from the Nth group
 \1, \2, \3 ...           Matches the text from the Nth group
 \g-1 or \g{-1}, \g-2 ... Matches the text from the Nth previous group
 \g{name}     Named backreference
 \k<name>     Named backreference
 \k'name'     Named backreference
 (?P=name)    Named backreference (python syntax)









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66.2.3 ESCAPE SEQUENCES



These work as in normal strings.



   \a       Alarm (beep)
   \e       Escape
   \f       Formfeed
   \n       Newline
   \r       Carriage return
   \t       Tab
   \037     Char whose ordinal is the 3 octal digits, max \777
   \o{2307} Char whose ordinal is the octal number, unrestricted
   \x7f     Char whose ordinal is the 2 hex digits, max \xFF
   \x{263a} Char whose ordinal is the hex number, unrestricted
   \cx      Control-x
   \N{name} A named Unicode character or character sequence
   \N{U+263D} A Unicode character by hex ordinal

   \l  Lowercase next character
   \u  Titlecase next character
   \L  Lowercase until \E
   \U  Uppercase until \E
   \F  Foldcase until \E
   \Q  Disable pattern metacharacters until \E
   \E  End modification



For Titlecase, see Titlecase.



This one works differently from normal strings:



   \b  An assertion, not backspace, except in a character class









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66.2.4 CHARACTER CLASSES



   [amy]    Match 'a', 'm' or 'y'
   [f-j]    Dash specifies "range"
   [f-j-]   Dash escaped or at start or end means 'dash'
   [^f-j]   Caret indicates "match any character _except_ these"



The following sequences (except \N) work within or without a character class.
The first six are locale aware, all are Unicode aware. See perllocale NAME
and perlunicode NAME for details.



   \d      A digit
   \D      A nondigit
   \w      A word character
   \W      A non-word character
   \s      A whitespace character
   \S      A non-whitespace character
   \h      An horizontal whitespace
   \H      A non horizontal whitespace
   \N      A non newline (when not followed by '{NAME}'; experimental;
           not valid in a character class; equivalent to [^\n]; it's
           like '.' without /s modifier)
   \v      A vertical whitespace
   \V      A non vertical whitespace
   \R      A generic newline           (?>\v|\x0D\x0A)

   \C      Match a byte (with Unicode, '.' matches a character)
   \pP     Match P-named (Unicode) property
   \p{...} Match Unicode property with name longer than 1 character
   \PP     Match non-P
   \P{...} Match lack of Unicode property with name longer than 1 char
   \X      Match Unicode extended grapheme cluster



POSIX character classes and their Unicode and Perl equivalents:



            ASCII-         Full-
   POSIX    range          range    backslash
 [[:...:]]  \p{...}        \p{...}   sequence    Description

 -----------------------------------------------------------------------
 alnum   PosixAlnum       XPosixAlnum            Alpha plus Digit
 alpha   PosixAlpha       XPosixAlpha            Alphabetic characters
 ascii   ASCII                                   Any ASCII character
 blank   PosixBlank       XPosixBlank   \h       Horizontal whitespace;
                                                   full-range also
                                                   written as
                                                   \p{HorizSpace} (GNU
                                                   extension)
 cntrl   PosixCntrl       XPosixCntrl            Control characters
 digit   PosixDigit       XPosixDigit   \d       Decimal digits
 graph   PosixGraph       XPosixGraph            Alnum plus Punct
 lower   PosixLower       XPosixLower            Lowercase characters
 print   PosixPrint       XPosixPrint            Graph plus Print, but
                                                   not any Cntrls
 punct   PosixPunct       XPosixPunct            Punctuation and Symbols
                                                   in ASCII-range; just
                                                   punct outside it
 space   PosixSpace       XPosixSpace            [\s\cK]
         PerlSpace        XPerlSpace    \s       Perl's whitespace def'n
 upper   PosixUpper       XPosixUpper            Uppercase characters
 word    PosixWord        XPosixWord    \w       Alnum + Unicode marks +
                                                   connectors, like '_'
                                                   (Perl extension)
 xdigit  ASCII_Hex_Digit  XPosixDigit            Hexadecimal digit,
                                                    ASCII-range is
                                                    [0-9A-Fa-f]



Also, various synonyms like \p{Alpha} for \p{XPosixAlpha}; all listed
in (perluniprops)Properties accessible through \p{} and \P{}



Within a character class:



    POSIX      traditional   Unicode
  [:digit:]       \d        \p{Digit}
  [:^digit:]      \D        \P{Digit}









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66.2.5 ANCHORS



All are zero-width assertions.



   ^  Match string start (or line, if /m is used)
   $  Match string end (or line, if /m is used) or before newline
   \b Match word boundary (between \w and \W)
   \B Match except at word boundary (between \w and \w or \W and \W)
   \A Match string start (regardless of /m)
   \Z Match string end (before optional newline)
   \z Match absolute string end
   \G Match where previous m//g left off
   \K Keep the stuff left of the \K, don't include it in $&









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66.2.6 QUANTIFIERS



Quantifiers are greedy by default and match the longest leftmost.



   Maximal Minimal Possessive Allowed range
   ------- ------- ---------- -------------
   {n,m}   {n,m}?  {n,m}+     Must occur at least n times
                              but no more than m times
   {n,}    {n,}?   {n,}+      Must occur at least n times
   {n}     {n}?    {n}+       Must occur exactly n times
   *       *?      *+         0 or more times (same as {0,})
   +       +?      ++         1 or more times (same as {1,})
   ?       ??      ?+         0 or 1 time (same as {0,1})



The possessive forms (new in Perl 5.10) prevent backtracking: what gets
matched by a pattern with a possessive quantifier will not be backtracked
into, even if that causes the whole match to fail.



There is no quantifier {,n}. That’s interpreted as a literal string.









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66.2.7 EXTENDED CONSTRUCTS



   (?#text)          A comment
   (?:...)           Groups subexpressions without capturing (cluster)
   (?pimsx-imsx:...) Enable/disable option (as per m// modifiers)
   (?=...)           Zero-width positive lookahead assertion
   (?!...)           Zero-width negative lookahead assertion
   (?<=...)          Zero-width positive lookbehind assertion
   (?<!...)          Zero-width negative lookbehind assertion
   (?>...)           Grab what we can, prohibit backtracking
   (?|...)           Branch reset
   (?<name>...)      Named capture
   (?'name'...)      Named capture
   (?P<name>...)     Named capture (python syntax)
   (?{ code })       Embedded code, return value becomes $^R
   (??{ code })      Dynamic regex, return value used as regex
   (?N)              Recurse into subpattern number N
   (?-N), (?+N)      Recurse into Nth previous/next subpattern
   (?R), (?0)        Recurse at the beginning of the whole pattern
   (?&name)          Recurse into a named subpattern
   (?P>name)         Recurse into a named subpattern (python syntax)
   (?(cond)yes|no)
   (?(cond)yes)      Conditional expression, where "cond" can be:
                     (?=pat)   look-ahead
                     (?!pat)   negative look-ahead
                     (?<=pat)  look-behind
                     (?<!pat)  negative look-behind
                     (N)       subpattern N has matched something
                     (<name>)  named subpattern has matched something
                     ('name')  named subpattern has matched something
                     (?{code}) code condition
                     (R)       true if recursing
                     (RN)      true if recursing into Nth subpattern
                     (R&name)  true if recursing into named subpattern
                     (DEFINE)  always false, no no-pattern allowed









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66.2.8 VARIABLES



   $_    Default variable for operators to use

   $`    Everything prior to matched string
   $&    Entire matched string
   $'    Everything after to matched string

   ${^PREMATCH}   Everything prior to matched string
   ${^MATCH}      Entire matched string
   ${^POSTMATCH}  Everything after to matched string



The use of $`, $& or $' will slow down all regex use
within your program. Consult perlvar NAME for @-
to see equivalent expressions that won’t cause slow down.
See also (Devel-SawAmpersand). Starting with Perl 5.10, you
can also use the equivalent variables ${^PREMATCH}, ${^MATCH}
and ${^POSTMATCH}, but for them to be defined, you have to
specify the /p (preserve) modifier on your regular expression.



   $1, $2 ...  hold the Xth captured expr
   $+    Last parenthesized pattern match
   $^N   Holds the most recently closed capture
   $^R   Holds the result of the last (?{...}) expr
   @-    Offsets of starts of groups. $-[0] holds start of whole match
   @+    Offsets of ends of groups. $+[0] holds end of whole match
   %+    Named capture groups
   %-    Named capture groups, as array refs



Captured groups are numbered according to their opening paren.









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66.2.9 FUNCTIONS



   lc          Lowercase a string
   lcfirst     Lowercase first char of a string
   uc          Uppercase a string
   ucfirst     Titlecase first char of a string
   fc          Foldcase a string

   pos         Return or set current match position
   quotemeta   Quote metacharacters
   reset       Reset ?pattern? status
   study       Analyze string for optimizing matching

   split       Use a regex to split a string into parts



The first five of these are like the escape sequences \L, \l,
\U, \u, and \F.  For Titlecase, see Titlecase; For
Foldcase, see Foldcase.









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66.2.10 TERMINOLOGY





• perlreref Titlecase:  



• perlreref Foldcase:  












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66.2.10.1 Titlecase



Unicode concept which most often is equal to uppercase, but for
certain characters like the German "sharp s" there is a difference.









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66.2.10.2 Foldcase



Unicode form that is useful when comparing strings regardless of case,
as certain characters have compex one-to-many case mappings. Primarily a
variant of lowercase.









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66.3 AUTHOR



Iain Truskett. Updated by the Perl 5 Porters.



This document may be distributed under the same terms as Perl itself.









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66.4 SEE ALSO











Previous: perlreref SEE ALSO, Up: perlreref   [Contents]





66.5 THANKS



David P.C. Wollmann,
Richard Soderberg,
Sean M. Burke,
Tom Christiansen,
Jim Cromie,
and
Jeffrey Goff
for useful advice.









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67 perlretut





• perlretut NAME:  



• perlretut DESCRIPTION:  



• perlretut Part 1: The basics:  



• perlretut Part 2: Power tools:  



• perlretut BUGS:  



• perlretut SEE ALSO:  



• perlretut AUTHOR AND COPYRIGHT:  












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67.1 NAME



perlretut - Perl regular expressions tutorial









Next: , Previous: perlretut NAME, Up: perlretut   [Contents]





67.2 DESCRIPTION



This page provides a basic tutorial on understanding, creating and
using regular expressions in Perl.  It serves as a complement to the
reference page on regular expressions (perlre).  Regular expressions
are an integral part of the m//, s///, qr// and split
operators and so this tutorial also overlaps with
(perlop)Regexp Quote-Like Operators and (perlfunc)split.



Perl is widely renowned for excellence in text processing, and regular
expressions are one of the big factors behind this fame.  Perl regular
expressions display an efficiency and flexibility unknown in most
other computer languages.  Mastering even the basics of regular
expressions will allow you to manipulate text with surprising ease.



What is a regular expression?  A regular expression is simply a string
that describes a pattern.  Patterns are in common use these days;
examples are the patterns typed into a search engine to find web pages
and the patterns used to list files in a directory, e.g., ls *.txt
or dir *.*.  In Perl, the patterns described by regular expressions
are used to search strings, extract desired parts of strings, and to
do search and replace operations.



Regular expressions have the undeserved reputation of being abstract
and difficult to understand.  Regular expressions are constructed using
simple concepts like conditionals and loops and are no more difficult
to understand than the corresponding if conditionals and while
loops in the Perl language itself.  In fact, the main challenge in
learning regular expressions is just getting used to the terse
notation used to express these concepts.



This tutorial flattens the learning curve by discussing regular
expression concepts, along with their notation, one at a time and with
many examples.  The first part of the tutorial will progress from the
simplest word searches to the basic regular expression concepts.  If
you master the first part, you will have all the tools needed to solve
about 98% of your needs.  The second part of the tutorial is for those
comfortable with the basics and hungry for more power tools.  It
discusses the more advanced regular expression operators and
introduces the latest cutting-edge innovations.



A note: to save time, ’regular expression’ is often abbreviated as
regexp or regex.  Regexp is a more natural abbreviation than regex, but
is harder to pronounce.  The Perl pod documentation is evenly split on
regexp vs regex; in Perl, there is more than one way to abbreviate it.
We’ll use regexp in this tutorial.









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67.3 Part 1: The basics





• perlretut Simple word matching:  



• perlretut Using character classes:  



• perlretut Matching this or that:  



• perlretut Grouping things and hierarchical matching:  



• perlretut Extracting matches:  



• perlretut Backreferences:  



• perlretut Relative backreferences:  



• perlretut Named backreferences:  



• perlretut Alternative capture group numbering:  



• perlretut Position information:  



• perlretut Non-capturing groupings:  



• perlretut Matching repetitions:  



• perlretut Possessive quantifiers:  



• perlretut Building a regexp:  



• perlretut Using regular expressions in Perl:  












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67.3.1 Simple word matching



The simplest regexp is simply a word, or more generally, a string of
characters.  A regexp consisting of a word matches any string that
contains that word:



    "Hello World" =~ /World/;  # matches



What is this Perl statement all about? "Hello World" is a simple
double-quoted string.  World is the regular expression and the
// enclosing /World/ tells Perl to search a string for a match.
The operator =~ associates the string with the regexp match and
produces a true value if the regexp matched, or false if the regexp
did not match.  In our case, World matches the second word in
"Hello World", so the expression is true.  Expressions like this
are useful in conditionals:



    if ("Hello World" =~ /World/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }



There are useful variations on this theme.  The sense of the match can
be reversed by using the !~ operator:



    if ("Hello World" !~ /World/) {
        print "It doesn't match\n";
    }
    else {
        print "It matches\n";
    }



The literal string in the regexp can be replaced by a variable:



    $greeting = "World";
    if ("Hello World" =~ /$greeting/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }



If you’re matching against the special default variable $_, the
$_ =~ part can be omitted:



    $_ = "Hello World";
    if (/World/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }



And finally, the // default delimiters for a match can be changed
to arbitrary delimiters by putting an 'm' out front:



    "Hello World" =~ m!World!;   # matches, delimited by '!'
    "Hello World" =~ m{World};   # matches, note the matching '{}'
    "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
                                 # '/' becomes an ordinary char



/World/, m!World!, and m{World} all represent the
same thing.  When, e.g., the quote (") is used as a delimiter, the forward
slash '/' becomes an ordinary character and can be used in this regexp
without trouble.



Let’s consider how different regexps would match "Hello World":



    "Hello World" =~ /world/;  # doesn't match
    "Hello World" =~ /o W/;    # matches
    "Hello World" =~ /oW/;     # doesn't match
    "Hello World" =~ /World /; # doesn't match



The first regexp world doesn’t match because regexps are
case-sensitive.  The second regexp matches because the substring
'o W' occurs in the string "Hello World".  The space
character ’ ’ is treated like any other character in a regexp and is
needed to match in this case.  The lack of a space character is the
reason the third regexp 'oW' doesn’t match.  The fourth regexp
'World ' doesn’t match because there is a space at the end of the
regexp, but not at the end of the string.  The lesson here is that
regexps must match a part of the string exactly in order for the
statement to be true.



If a regexp matches in more than one place in the string, Perl will
always match at the earliest possible point in the string:



    "Hello World" =~ /o/;       # matches 'o' in 'Hello'
    "That hat is red" =~ /hat/; # matches 'hat' in 'That'



With respect to character matching, there are a few more points you
need to know about.   First of all, not all characters can be used ’as
is’ in a match.  Some characters, called metacharacters, are reserved
for use in regexp notation.  The metacharacters are



    {}[]()^$.|*+?\



The significance of each of these will be explained
in the rest of the tutorial, but for now, it is important only to know
that a metacharacter can be matched by putting a backslash before it:



    "2+2=4" =~ /2+2/;    # doesn't match, + is a metacharacter
    "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
    "The interval is [0,1)." =~ /[0,1)./     # is a syntax error!
    "The interval is [0,1)." =~ /\[0,1\)\./  # matches
    "#!/usr/bin/perl" =~ /#!\/usr\/bin\/perl/;  # matches



In the last regexp, the forward slash '/' is also backslashed,
because it is used to delimit the regexp.  This can lead to LTS
(leaning toothpick syndrome), however, and it is often more readable
to change delimiters.



    "#!/usr/bin/perl" =~ m!#\!/usr/bin/perl!;  # easier to read



The backslash character '\' is a metacharacter itself and needs to
be backslashed:



    'C:\WIN32' =~ /C:\\WIN/;   # matches



In addition to the metacharacters, there are some ASCII characters
which don’t have printable character equivalents and are instead
represented by escape sequences.  Common examples are \t for a
tab, \n for a newline, \r for a carriage return and \a for a
bell (or alert).  If your string is better thought of as a sequence of arbitrary
bytes, the octal escape sequence, e.g., \033, or hexadecimal escape
sequence, e.g., \x1B may be a more natural representation for your
bytes.  Here are some examples of escapes:



    "1000\t2000" =~ m(0\t2)   # matches
    "1000\n2000" =~ /0\n20/   # matches
    "1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"
    "cat"   =~ /\o{143}\x61\x74/ # matches in ASCII, but a weird way
                                 # to spell cat



If you’ve been around Perl a while, all this talk of escape sequences
may seem familiar.  Similar escape sequences are used in double-quoted
strings and in fact the regexps in Perl are mostly treated as
double-quoted strings.  This means that variables can be used in
regexps as well.  Just like double-quoted strings, the values of the
variables in the regexp will be substituted in before the regexp is
evaluated for matching purposes.  So we have:



    $foo = 'house';
    'housecat' =~ /$foo/;      # matches
    'cathouse' =~ /cat$foo/;   # matches
    'housecat' =~ /${foo}cat/; # matches



So far, so good.  With the knowledge above you can already perform
searches with just about any literal string regexp you can dream up.
Here is a very simple emulation of the Unix grep program:



    % cat > simple_grep
    #!/usr/bin/perl
    $regexp = shift;
    while (<>) {
        print if /$regexp/;
    }
    ^D

    % chmod +x simple_grep

    % simple_grep abba /usr/dict/words
    Babbage
    cabbage
    cabbages
    sabbath
    Sabbathize
    Sabbathizes
    sabbatical
    scabbard
    scabbards



This program is easy to understand.  #!/usr/bin/perl is the standard
way to invoke a perl program from the shell.
$regexp = shift; saves the first command line argument as the
regexp to be used, leaving the rest of the command line arguments to
be treated as files.  while (<>) loops over all the lines in
all the files.  For each line, print if /$regexp/; prints the
line if the regexp matches the line.  In this line, both print and
/$regexp/ use the default variable $_ implicitly.



With all of the regexps above, if the regexp matched anywhere in the
string, it was considered a match.  Sometimes, however, we’d like to
specify where in the string the regexp should try to match.  To do
this, we would use the anchor metacharacters ^ and $.  The
anchor ^ means match at the beginning of the string and the anchor
$ means match at the end of the string, or before a newline at the
end of the string.  Here is how they are used:



    "housekeeper" =~ /keeper/;    # matches
    "housekeeper" =~ /^keeper/;   # doesn't match
    "housekeeper" =~ /keeper$/;   # matches
    "housekeeper\n" =~ /keeper$/; # matches



The second regexp doesn’t match because ^ constrains keeper to
match only at the beginning of the string, but "housekeeper" has
keeper starting in the middle.  The third regexp does match, since the
$ constrains keeper to match only at the end of the string.



When both ^ and $ are used at the same time, the regexp has to
match both the beginning and the end of the string, i.e., the regexp
matches the whole string.  Consider



    "keeper" =~ /^keep$/;      # doesn't match
    "keeper" =~ /^keeper$/;    # matches
    ""       =~ /^$/;          # ^$ matches an empty string



The first regexp doesn’t match because the string has more to it than
keep.  Since the second regexp is exactly the string, it
matches.  Using both ^ and $ in a regexp forces the complete
string to match, so it gives you complete control over which strings
match and which don’t.  Suppose you are looking for a fellow named
bert, off in a string by himself:



    "dogbert" =~ /bert/;   # matches, but not what you want

    "dilbert" =~ /^bert/;  # doesn't match, but ..
    "bertram" =~ /^bert/;  # matches, so still not good enough

    "bertram" =~ /^bert$/; # doesn't match, good
    "dilbert" =~ /^bert$/; # doesn't match, good
    "bert"    =~ /^bert$/; # matches, perfect



Of course, in the case of a literal string, one could just as easily
use the string comparison $string eq 'bert' and it would be
more efficient.   The  ^...$ regexp really becomes useful when we
add in the more powerful regexp tools below.









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67.3.2 Using character classes



Although one can already do quite a lot with the literal string
regexps above, we’ve only scratched the surface of regular expression
technology.  In this and subsequent sections we will introduce regexp
concepts (and associated metacharacter notations) that will allow a
regexp to represent not just a single character sequence, but a whole
class of them.



One such concept is that of a character class.  A character class
allows a set of possible characters, rather than just a single
character, to match at a particular point in a regexp.  Character
classes are denoted by brackets [...], with the set of characters
to be possibly matched inside.  Here are some examples:



    /cat/;       # matches 'cat'
    /[bcr]at/;   # matches 'bat, 'cat', or 'rat'
    /item[0123456789]/;  # matches 'item0' or ... or 'item9'
    "abc" =~ /[cab]/;    # matches 'a'



In the last statement, even though 'c' is the first character in
the class, 'a' matches because the first character position in the
string is the earliest point at which the regexp can match.



    /[yY][eE][sS]/;      # match 'yes' in a case-insensitive way
                         # 'yes', 'Yes', 'YES', etc.



This regexp displays a common task: perform a case-insensitive
match.  Perl provides a way of avoiding all those brackets by simply
appending an 'i' to the end of the match.  Then /[yY][eE][sS]/;
can be rewritten as /yes/i;.  The 'i' stands for
case-insensitive and is an example of a modifier of the matching
operation.  We will meet other modifiers later in the tutorial.



We saw in the section above that there were ordinary characters, which
represented themselves, and special characters, which needed a
backslash \ to represent themselves.  The same is true in a
character class, but the sets of ordinary and special characters
inside a character class are different than those outside a character
class.  The special characters for a character class are -]\^$ (and
the pattern delimiter, whatever it is).
] is special because it denotes the end of a character class.  $ is
special because it denotes a scalar variable.  \ is special because
it is used in escape sequences, just like above.  Here is how the
special characters ]$\ are handled:



   /[\]c]def/; # matches ']def' or 'cdef'
   $x = 'bcr';
   /[$x]at/;   # matches 'bat', 'cat', or 'rat'
   /[\$x]at/;  # matches '$at' or 'xat'
   /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'



The last two are a little tricky.  In [\$x], the backslash protects
the dollar sign, so the character class has two members $ and x.
In [\\$x], the backslash is protected, so $x is treated as a
variable and substituted in double quote fashion.



The special character '-' acts as a range operator within character
classes, so that a contiguous set of characters can be written as a
range.  With ranges, the unwieldy [0123456789] and [abc...xyz]
become the svelte [0-9] and [a-z].  Some examples are



    /item[0-9]/;  # matches 'item0' or ... or 'item9'
    /[0-9bx-z]aa/;  # matches '0aa', ..., '9aa',
                    # 'baa', 'xaa', 'yaa', or 'zaa'
    /[0-9a-fA-F]/;  # matches a hexadecimal digit
    /[0-9a-zA-Z_]/; # matches a "word" character,
                    # like those in a Perl variable name



If '-' is the first or last character in a character class, it is
treated as an ordinary character; [-ab], [ab-] and [a\-b] are
all equivalent.



The special character ^ in the first position of a character class
denotes a negated character class, which matches any character but
those in the brackets.  Both [...] and [^...] must match a
character, or the match fails.  Then



    /[^a]at/;  # doesn't match 'aat' or 'at', but matches
               # all other 'bat', 'cat, '0at', '%at', etc.
    /[^0-9]/;  # matches a non-numeric character
    /[a^]at/;  # matches 'aat' or '^at'; here '^' is ordinary



Now, even [0-9] can be a bother to write multiple times, so in the
interest of saving keystrokes and making regexps more readable, Perl
has several abbreviations for common character classes, as shown below.
Since the introduction of Unicode, unless the //a modifier is in
effect, these character classes match more than just a few characters in
the ASCII range.





The //a modifier, available starting in Perl 5.14,  is used to
restrict the matches of \d, \s, and \w to just those in the ASCII range.
It is useful to keep your program from being needlessly exposed to full
Unicode (and its accompanying security considerations) when all you want
is to process English-like text.  (The "a" may be doubled, //aa, to
provide even more restrictions, preventing case-insensitive matching of
ASCII with non-ASCII characters; otherwise a Unicode "Kelvin Sign"
would caselessly match a "k" or "K".)



The \d\s\w\D\S\W abbreviations can be used both inside and outside
of character classes.  Here are some in use:



    /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
    /[\d\s]/;         # matches any digit or whitespace character
    /\w\W\w/;         # matches a word char, followed by a
                      # non-word char, followed by a word char
    /..rt/;           # matches any two chars, followed by 'rt'
    /end\./;          # matches 'end.'
    /end[.]/;         # same thing, matches 'end.'



Because a period is a metacharacter, it needs to be escaped to match
as an ordinary period. Because, for example, \d and \w are sets
of characters, it is incorrect to think of [^\d\w] as [\D\W]; in
fact [^\d\w] is the same as [^\w], which is the same as
[\W]. Think DeMorgan’s laws.



An anchor useful in basic regexps is the word anchor
\b.  This matches a boundary between a word character and a non-word
character \w\W or \W\w:



    $x = "Housecat catenates house and cat";
    $x =~ /cat/;    # matches cat in 'housecat'
    $x =~ /\bcat/;  # matches cat in 'catenates'
    $x =~ /cat\b/;  # matches cat in 'housecat'
    $x =~ /\bcat\b/;  # matches 'cat' at end of string



Note in the last example, the end of the string is considered a word
boundary.



You might wonder why '.' matches everything but "\n" - why not
every character? The reason is that often one is matching against
lines and would like to ignore the newline characters.  For instance,
while the string "\n" represents one line, we would like to think
of it as empty.  Then



    ""   =~ /^$/;    # matches
    "\n" =~ /^$/;    # matches, $ anchors before "\n"

    ""   =~ /./;      # doesn't match; it needs a char
    ""   =~ /^.$/;    # doesn't match; it needs a char
    "\n" =~ /^.$/;    # doesn't match; it needs a char other than "\n"
    "a"  =~ /^.$/;    # matches
    "a\n"  =~ /^.$/;  # matches, $ anchors before "\n"



This behavior is convenient, because we usually want to ignore
newlines when we count and match characters in a line.  Sometimes,
however, we want to keep track of newlines.  We might even want ^
and $ to anchor at the beginning and end of lines within the
string, rather than just the beginning and end of the string.  Perl
allows us to choose between ignoring and paying attention to newlines
by using the //s and //m modifiers.  //s and //m stand for
single line and multi-line and they determine whether a string is to
be treated as one continuous string, or as a set of lines.  The two
modifiers affect two aspects of how the regexp is interpreted: 1) how
the '.' character class is defined, and 2) where the anchors ^
and $ are able to match.  Here are the four possible combinations:





Here are examples of //s and //m in action:



    $x = "There once was a girl\nWho programmed in Perl\n";

    $x =~ /^Who/;   # doesn't match, "Who" not at start of string
    $x =~ /^Who/s;  # doesn't match, "Who" not at start of string
    $x =~ /^Who/m;  # matches, "Who" at start of second line
    $x =~ /^Who/sm; # matches, "Who" at start of second line

    $x =~ /girl.Who/;   # doesn't match, "." doesn't match "\n"
    $x =~ /girl.Who/s;  # matches, "." matches "\n"
    $x =~ /girl.Who/m;  # doesn't match, "." doesn't match "\n"
    $x =~ /girl.Who/sm; # matches, "." matches "\n"



Most of the time, the default behavior is what is wanted, but //s and
//m are occasionally very useful.  If //m is being used, the start
of the string can still be matched with \A and the end of the string
can still be matched with the anchors \Z (matches both the end and
the newline before, like $), and \z (matches only the end):



    $x =~ /^Who/m;   # matches, "Who" at start of second line
    $x =~ /\AWho/m;  # doesn't match, "Who" is not at start of string

    $x =~ /girl$/m;  # matches, "girl" at end of first line
    $x =~ /girl\Z/m; # doesn't match, "girl" is not at end of string

    $x =~ /Perl\Z/m; # matches, "Perl" is at newline before end
    $x =~ /Perl\z/m; # doesn't match, "Perl" is not at end of string



We now know how to create choices among classes of characters in a
regexp.  What about choices among words or character strings? Such
choices are described in the next section.









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67.3.3 Matching this or that



Sometimes we would like our regexp to be able to match different
possible words or character strings.  This is accomplished by using
the alternation metacharacter |.  To match dog or cat, we
form the regexp dog|cat.  As before, Perl will try to match the
regexp at the earliest possible point in the string.  At each
character position, Perl will first try to match the first
alternative, dog.  If dog doesn’t match, Perl will then try the
next alternative, cat.  If cat doesn’t match either, then the
match fails and Perl moves to the next position in the string.  Some
examples:



    "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
    "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"



Even though dog is the first alternative in the second regexp,
cat is able to match earlier in the string.



    "cats"          =~ /c|ca|cat|cats/; # matches "c"
    "cats"          =~ /cats|cat|ca|c/; # matches "cats"



Here, all the alternatives match at the first string position, so the
first alternative is the one that matches.  If some of the
alternatives are truncations of the others, put the longest ones first
to give them a chance to match.



    "cab" =~ /a|b|c/ # matches "c"
                     # /a|b|c/ == /[abc]/



The last example points out that character classes are like
alternations of characters.  At a given character position, the first
alternative that allows the regexp match to succeed will be the one
that matches.









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67.3.4 Grouping things and hierarchical matching



Alternation allows a regexp to choose among alternatives, but by
itself it is unsatisfying.  The reason is that each alternative is a whole
regexp, but sometime we want alternatives for just part of a
regexp.  For instance, suppose we want to search for housecats or
housekeepers.  The regexp housecat|housekeeper fits the bill, but is
inefficient because we had to type house twice.  It would be nice to
have parts of the regexp be constant, like house, and some
parts have alternatives, like cat|keeper.



The grouping metacharacters () solve this problem.  Grouping
allows parts of a regexp to be treated as a single unit.  Parts of a
regexp are grouped by enclosing them in parentheses.  Thus we could solve
the housecat|housekeeper by forming the regexp as
house(cat|keeper).  The regexp house(cat|keeper) means match
house followed by either cat or keeper.  Some more examples
are



    /(a|b)b/;    # matches 'ab' or 'bb'
    /(ac|b)b/;   # matches 'acb' or 'bb'
    /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere
    /(a|[bc])d/; # matches 'ad', 'bd', or 'cd'

    /house(cat|)/;  # matches either 'housecat' or 'house'
    /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
                        # 'house'.  Note groups can be nested.

    /(19|20|)\d\d/;  # match years 19xx, 20xx, or the Y2K problem, xx
    "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                             # because '20\d\d' can't match



Alternations behave the same way in groups as out of them: at a given
string position, the leftmost alternative that allows the regexp to
match is taken.  So in the last example at the first string position,
"20" matches the second alternative, but there is nothing left over
to match the next two digits \d\d.  So Perl moves on to the next
alternative, which is the null alternative and that works, since
"20" is two digits.



The process of trying one alternative, seeing if it matches, and
moving on to the next alternative, while going back in the string
from where the previous alternative was tried, if it doesn’t, is called
backtracking.  The term ’backtracking’ comes from the idea that
matching a regexp is like a walk in the woods.  Successfully matching
a regexp is like arriving at a destination.  There are many possible
trailheads, one for each string position, and each one is tried in
order, left to right.  From each trailhead there may be many paths,
some of which get you there, and some which are dead ends.  When you
walk along a trail and hit a dead end, you have to backtrack along the
trail to an earlier point to try another trail.  If you hit your
destination, you stop immediately and forget about trying all the
other trails.  You are persistent, and only if you have tried all the
trails from all the trailheads and not arrived at your destination, do
you declare failure.  To be concrete, here is a step-by-step analysis
of what Perl does when it tries to match the regexp



    "abcde" =~ /(abd|abc)(df|d|de)/;





0




Start with the first letter in the string ’a’.





1




Try the first alternative in the first group ’abd’.





2




Match ’a’ followed by ’b’. So far so good.





3




’d’ in the regexp doesn’t match ’c’ in the string - a dead
end.  So backtrack two characters and pick the second alternative in
the first group ’abc’.





4




Match ’a’ followed by ’b’ followed by ’c’.  We are on a roll
and have satisfied the first group. Set $1 to ’abc’.





5




Move on to the second group and pick the first alternative
’df’.





6




Match the ’d’.





7




’f’ in the regexp doesn’t match ’e’ in the string, so a dead
end.  Backtrack one character and pick the second alternative in the
second group ’d’.





8




’d’ matches. The second grouping is satisfied, so set $2 to
’d’.





9




We are at the end of the regexp, so we are done! We have
matched ’abcd’ out of the string "abcde".








There are a couple of things to note about this analysis.  First, the
third alternative in the second group ’de’ also allows a match, but we
stopped before we got to it - at a given character position, leftmost
wins.  Second, we were able to get a match at the first character
position of the string ’a’.  If there were no matches at the first
position, Perl would move to the second character position ’b’ and
attempt the match all over again.  Only when all possible paths at all
possible character positions have been exhausted does Perl give
up and declare $string =~ /(abd|abc)(df|d|de)/; to be false.



Even with all this work, regexp matching happens remarkably fast.  To
speed things up, Perl compiles the regexp into a compact sequence of
opcodes that can often fit inside a processor cache.  When the code is
executed, these opcodes can then run at full throttle and search very
quickly.









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67.3.5 Extracting matches



The grouping metacharacters () also serve another completely
different function: they allow the extraction of the parts of a string
that matched.  This is very useful to find out what matched and for
text processing in general.  For each grouping, the part that matched
inside goes into the special variables $1, $2, etc.  They can be
used just as ordinary variables:



    # extract hours, minutes, seconds
    if ($time =~ /(\d\d):(\d\d):(\d\d)/) {    # match hh:mm:ss format
        $hours = $1;
        $minutes = $2;
        $seconds = $3;
    }



Now, we know that in scalar context,
$time =~ /(\d\d):(\d\d):(\d\d)/ returns a true or false
value.  In list context, however, it returns the list of matched values
($1,$2,$3).  So we could write the code more compactly as



    # extract hours, minutes, seconds
    ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);



If the groupings in a regexp are nested, $1 gets the group with the
leftmost opening parenthesis, $2 the next opening parenthesis,
etc.  Here is a regexp with nested groups:



    /(ab(cd|ef)((gi)|j))/;
     1  2      34



If this regexp matches, $1 contains a string starting with
'ab', $2 is either set to 'cd' or 'ef', $3 equals either
'gi' or 'j', and $4 is either set to 'gi', just like $3,
or it remains undefined.



For convenience, Perl sets $+ to the string held by the highest numbered
$1, $2,... that got assigned (and, somewhat related, $^N to the
value of the $1, $2,... most-recently assigned; i.e. the $1,
$2,... associated with the rightmost closing parenthesis used in the
match).









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67.3.6 Backreferences



Closely associated with the matching variables $1, $2, ... are
the backreferences \g1, \g2,...  Backreferences are simply
matching variables that can be used inside a regexp.  This is a
really nice feature; what matches later in a regexp is made to depend on
what matched earlier in the regexp.  Suppose we wanted to look
for doubled words in a text, like ’the the’.  The following regexp finds
all 3-letter doubles with a space in between:



    /\b(\w\w\w)\s\g1\b/;



The grouping assigns a value to \g1, so that the same 3-letter sequence
is used for both parts.



A similar task is to find words consisting of two identical parts:



    % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\g1$' /usr/dict/words
    beriberi
    booboo
    coco
    mama
    murmur
    papa



The regexp has a single grouping which considers 4-letter
combinations, then 3-letter combinations, etc., and uses \g1 to look for
a repeat.  Although $1 and \g1 represent the same thing, care should be
taken to use matched variables $1, $2,... only outside a regexp
and backreferences \g1, \g2,... only inside a regexp; not doing
so may lead to surprising and unsatisfactory results.









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67.3.7 Relative backreferences



Counting the opening parentheses to get the correct number for a
backreference is error-prone as soon as there is more than one
capturing group.  A more convenient technique became available
with Perl 5.10: relative backreferences. To refer to the immediately
preceding capture group one now may write \g{-1}, the next but
last is available via \g{-2}, and so on.



Another good reason in addition to readability and maintainability
for using relative backreferences is illustrated by the following example,
where a simple pattern for matching peculiar strings is used:



    $a99a = '([a-z])(\d)\g2\g1';   # matches a11a, g22g, x33x, etc.



Now that we have this pattern stored as a handy string, we might feel
tempted to use it as a part of some other pattern:



    $line = "code=e99e";
    if ($line =~ /^(\w+)=$a99a$/){   # unexpected behavior!
        print "$1 is valid\n";
    } else {
        print "bad line: '$line'\n";
    }



But this doesn’t match, at least not the way one might expect. Only
after inserting the interpolated $a99a and looking at the resulting
full text of the regexp is it obvious that the backreferences have
backfired. The subexpression (\w+) has snatched number 1 and
demoted the groups in $a99a by one rank. This can be avoided by
using relative backreferences:



    $a99a = '([a-z])(\d)\g{-1}\g{-2}';  # safe for being interpolated









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67.3.8 Named backreferences



Perl 5.10 also introduced named capture groups and named backreferences.
To attach a name to a capturing group, you write either
(?<name>...) or (?'name'...).  The backreference may
then be written as \g{name}.  It is permissible to attach the
same name to more than one group, but then only the leftmost one of the
eponymous set can be referenced.  Outside of the pattern a named
capture group is accessible through the %+ hash.



Assuming that we have to match calendar dates which may be given in one
of the three formats yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy, we can write
three suitable patterns where we use ’d’, ’m’ and ’y’ respectively as the
names of the groups capturing the pertaining components of a date. The
matching operation combines the three patterns as alternatives:



    $fmt1 = '(?<y>\d\d\d\d)-(?<m>\d\d)-(?<d>\d\d)';
    $fmt2 = '(?<m>\d\d)/(?<d>\d\d)/(?<y>\d\d\d\d)';
    $fmt3 = '(?<d>\d\d)\.(?<m>\d\d)\.(?<y>\d\d\d\d)';
    for my $d qw( 2006-10-21 15.01.2007 10/31/2005 ){
        if ( $d =~ m{$fmt1|$fmt2|$fmt3} ){
            print "day=$+{d} month=$+{m} year=$+{y}\n";
        }
    }



If any of the alternatives matches, the hash %+ is bound to contain the
three key-value pairs.









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67.3.9 Alternative capture group numbering



Yet another capturing group numbering technique (also as from Perl 5.10)
deals with the problem of referring to groups within a set of alternatives.
Consider a pattern for matching a time of the day, civil or military style:



    if ( $time =~ /(\d\d|\d):(\d\d)|(\d\d)(\d\d)/ ){
        # process hour and minute
    }



Processing the results requires an additional if statement to determine
whether $1 and $2 or $3 and $4 contain the goodies. It would
be easier if we could use group numbers 1 and 2 in second alternative as
well, and this is exactly what the parenthesized construct (?|...),
set around an alternative achieves. Here is an extended version of the
previous pattern:



    if ( $time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/ ){
        print "hour=$1 minute=$2 zone=$3\n";
    }



Within the alternative numbering group, group numbers start at the same
position for each alternative. After the group, numbering continues
with one higher than the maximum reached across all the alternatives.









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67.3.10 Position information



In addition to what was matched, Perl (since 5.6.0) also provides the
positions of what was matched as contents of the @- and @+
arrays. $-[0] is the position of the start of the entire match and
$+[0] is the position of the end. Similarly, $-[n] is the
position of the start of the $n match and $+[n] is the position
of the end. If $n is undefined, so are $-[n] and $+[n]. Then
this code



    $x = "Mmm...donut, thought Homer";
    $x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
    foreach $expr (1..$#-) {
        print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
    }



prints



    Match 1: 'Mmm' at position (0,3)
    Match 2: 'donut' at position (6,11)



Even if there are no groupings in a regexp, it is still possible to
find out what exactly matched in a string.  If you use them, Perl
will set $` to the part of the string before the match, will set $&
to the part of the string that matched, and will set $' to the part
of the string after the match.  An example:



    $x = "the cat caught the mouse";
    $x =~ /cat/;  # $` = 'the ', $& = 'cat', $' = ' caught the mouse'
    $x =~ /the/;  # $` = '', $& = 'the', $' = ' cat caught the mouse'



In the second match, $` equals '' because the regexp matched at the
first character position in the string and stopped; it never saw the
second ’the’.  It is important to note that using $` and $'
slows down regexp matching quite a bit, while $& slows it down to a
lesser extent, because if they are used in one regexp in a program,
they are generated for all regexps in the program.  So if raw
performance is a goal of your application, they should be avoided.
If you need to extract the corresponding substrings, use @- and
@+ instead:



    $` is the same as substr( $x, 0, $-[0] )
    $& is the same as substr( $x, $-[0], $+[0]-$-[0] )
    $' is the same as substr( $x, $+[0] )



As of Perl 5.10, the ${^PREMATCH}, ${^MATCH} and ${^POSTMATCH}
variables may be used. These are only set if the /p modifier is present.
Consequently they do not penalize the rest of the program.









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67.3.11 Non-capturing groupings



A group that is required to bundle a set of alternatives may or may not be
useful as a capturing group.  If it isn’t, it just creates a superfluous
addition to the set of available capture group values, inside as well as
outside the regexp.  Non-capturing groupings, denoted by (?:regexp),
still allow the regexp to be treated as a single unit, but don’t establish
a capturing group at the same time.  Both capturing and non-capturing
groupings are allowed to co-exist in the same regexp.  Because there is
no extraction, non-capturing groupings are faster than capturing
groupings.  Non-capturing groupings are also handy for choosing exactly
which parts of a regexp are to be extracted to matching variables:



    # match a number, $1-$4 are set, but we only want $1
    /([+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?)/;

    # match a number faster , only $1 is set
    /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?)/;

    # match a number, get $1 = whole number, $2 = exponent
    /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE]([+-]?\d+))?)/;



Non-capturing groupings are also useful for removing nuisance
elements gathered from a split operation where parentheses are
required for some reason:



    $x = '12aba34ba5';
    @num = split /(a|b)+/, $x;    # @num = ('12','a','34','a','5')
    @num = split /(?:a|b)+/, $x;  # @num = ('12','34','5')









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67.3.12 Matching repetitions



The examples in the previous section display an annoying weakness.  We
were only matching 3-letter words, or chunks of words of 4 letters or
less.  We’d like to be able to match words or, more generally, strings
of any length, without writing out tedious alternatives like
\w\w\w\w|\w\w\w|\w\w|\w.



This is exactly the problem the quantifier metacharacters ?,
*, +, and {} were created for.  They allow us to delimit the
number of repeats for a portion of a regexp we consider to be a
match.  Quantifiers are put immediately after the character, character
class, or grouping that we want to specify.  They have the following
meanings:





Here are some examples:



    /[a-z]+\s+\d*/;  # match a lowercase word, at least one space, and
                     # any number of digits
    /(\w+)\s+\g1/;    # match doubled words of arbitrary length
    /y(es)?/i;       # matches 'y', 'Y', or a case-insensitive 'yes'
    $year =~ /^\d{2,4}$/;  # make sure year is at least 2 but not more
                           # than 4 digits
    $year =~ /^\d{4}$|^\d{2}$/;    # better match; throw out 3-digit dates
    $year =~ /^\d{2}(\d{2})?$/;  # same thing written differently. However,
                                 # this captures the last two digits in $1
                                 # and the other does not.

    % simple_grep '^(\w+)\g1$' /usr/dict/words   # isn't this easier?
    beriberi
    booboo
    coco
    mama
    murmur
    papa



For all of these quantifiers, Perl will try to match as much of the
string as possible, while still allowing the regexp to succeed.  Thus
with /a?.../, Perl will first try to match the regexp with the a
present; if that fails, Perl will try to match the regexp without the
a present.  For the quantifier *, we get the following:



    $x = "the cat in the hat";
    $x =~ /^(.*)(cat)(.*)$/; # matches,
                             # $1 = 'the '
                             # $2 = 'cat'
                             # $3 = ' in the hat'



Which is what we might expect, the match finds the only cat in the
string and locks onto it.  Consider, however, this regexp:



    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 characters match)



One might initially guess that Perl would find the at in cat and
stop there, but that wouldn’t give the longest possible string to the
first quantifier .*.  Instead, the first quantifier .* grabs as
much of the string as possible while still having the regexp match.  In
this example, that means having the at sequence with the final at
in the string.  The other important principle illustrated here is that,
when there are two or more elements in a regexp, the leftmost
quantifier, if there is one, gets to grab as much of the string as
possible, leaving the rest of the regexp to fight over scraps.  Thus in
our example, the first quantifier .* grabs most of the string, while
the second quantifier .* gets the empty string.   Quantifiers that
grab as much of the string as possible are called maximal match or
greedy quantifiers.



When a regexp can match a string in several different ways, we can use
the principles above to predict which way the regexp will match:





As we have seen above, Principle 0 overrides the others. The regexp
will be matched as early as possible, with the other principles
determining how the regexp matches at that earliest character
position.



Here is an example of these principles in action:



    $x = "The programming republic of Perl";
    $x =~ /^(.+)(e|r)(.*)$/;  # matches,
                              # $1 = 'The programming republic of Pe'
                              # $2 = 'r'
                              # $3 = 'l'



This regexp matches at the earliest string position, 'T'.  One
might think that e, being leftmost in the alternation, would be
matched, but r produces the longest string in the first quantifier.



    $x =~ /(m{1,2})(.*)$/;  # matches,
                            # $1 = 'mm'
                            # $2 = 'ing republic of Perl'



Here, The earliest possible match is at the first 'm' in
programming. m{1,2} is the first quantifier, so it gets to match
a maximal mm.



    $x =~ /.*(m{1,2})(.*)$/;  # matches,
                              # $1 = 'm'
                              # $2 = 'ing republic of Perl'



Here, the regexp matches at the start of the string. The first
quantifier .* grabs as much as possible, leaving just a single
'm' for the second quantifier m{1,2}.



    $x =~ /(.?)(m{1,2})(.*)$/;  # matches,
                                # $1 = 'a'
                                # $2 = 'mm'
                                # $3 = 'ing republic of Perl'



Here, .? eats its maximal one character at the earliest possible
position in the string, 'a' in programming, leaving m{1,2}
the opportunity to match both m’s. Finally,



    "aXXXb" =~ /(X*)/; # matches with $1 = ''



because it can match zero copies of 'X' at the beginning of the
string.  If you definitely want to match at least one 'X', use
X+, not X*.



Sometimes greed is not good.  At times, we would like quantifiers to
match a minimal piece of string, rather than a maximal piece.  For
this purpose, Larry Wall created the minimal match or
non-greedy quantifiers ??, *?, +?, and {}?.  These are
the usual quantifiers with a ? appended to them.  They have the
following meanings:





Let’s look at the example above, but with minimal quantifiers:



    $x = "The programming republic of Perl";
    $x =~ /^(.+?)(e|r)(.*)$/; # matches,
                              # $1 = 'Th'
                              # $2 = 'e'
                              # $3 = ' programming republic of Perl'



The minimal string that will allow both the start of the string ^
and the alternation to match is Th, with the alternation e|r
matching e.  The second quantifier .* is free to gobble up the
rest of the string.



    $x =~ /(m{1,2}?)(.*?)$/;  # matches,
                              # $1 = 'm'
                              # $2 = 'ming republic of Perl'



The first string position that this regexp can match is at the first
'm' in programming. At this position, the minimal m{1,2}?
matches just one 'm'.  Although the second quantifier .*? would
prefer to match no characters, it is constrained by the end-of-string
anchor $ to match the rest of the string.



    $x =~ /(.*?)(m{1,2}?)(.*)$/;  # matches,
                                  # $1 = 'The progra'
                                  # $2 = 'm'
                                  # $3 = 'ming republic of Perl'



In this regexp, you might expect the first minimal quantifier .*?
to match the empty string, because it is not constrained by a ^
anchor to match the beginning of the word.  Principle 0 applies here,
however.  Because it is possible for the whole regexp to match at the
start of the string, it will match at the start of the string.  Thus
the first quantifier has to match everything up to the first m.  The
second minimal quantifier matches just one m and the third
quantifier matches the rest of the string.



    $x =~ /(.??)(m{1,2})(.*)$/;  # matches,
                                 # $1 = 'a'
                                 # $2 = 'mm'
                                 # $3 = 'ing republic of Perl'



Just as in the previous regexp, the first quantifier .?? can match
earliest at position 'a', so it does.  The second quantifier is
greedy, so it matches mm, and the third matches the rest of the
string.



We can modify principle 3 above to take into account non-greedy
quantifiers:





Just like alternation, quantifiers are also susceptible to
backtracking.  Here is a step-by-step analysis of the example



    $x = "the cat in the hat";
    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 matches)





0




Start with the first letter in the string ’t’.





1




The first quantifier ’.*’ starts out by matching the whole
string ’the cat in the hat’.





2




’a’ in the regexp element ’at’ doesn’t match the end of the
string.  Backtrack one character.





3




’a’ in the regexp element ’at’ still doesn’t match the last
letter of the string ’t’, so backtrack one more character.





4




Now we can match the ’a’ and the ’t’.





5




Move on to the third element ’.*’.  Since we are at the end of
the string and ’.*’ can match 0 times, assign it the empty string.





6




We are done!








Most of the time, all this moving forward and backtracking happens
quickly and searching is fast. There are some pathological regexps,
however, whose execution time exponentially grows with the size of the
string.  A typical structure that blows up in your face is of the form



    /(a|b+)*/;



The problem is the nested indeterminate quantifiers.  There are many
different ways of partitioning a string of length n between the +
and *: one repetition with b+ of length n, two repetitions with
the first b+ length k and the second with length n-k, m repetitions
whose bits add up to length n, etc.  In fact there are an exponential
number of ways to partition a string as a function of its length.  A
regexp may get lucky and match early in the process, but if there is
no match, Perl will try every possibility before giving up.  So be
careful with nested *’s, {n,m}’s, and +’s.  The book
Mastering Regular Expressions by Jeffrey Friedl gives a wonderful
discussion of this and other efficiency issues.









Next: , Previous: perlretut Matching repetitions, Up: perlretut Part 1: The basics   [Contents]





67.3.13 Possessive quantifiers



Backtracking during the relentless search for a match may be a waste
of time, particularly when the match is bound to fail.  Consider
the simple pattern



    /^\w+\s+\w+$/; # a word, spaces, a word



Whenever this is applied to a string which doesn’t quite meet the
pattern’s expectations such as "abc  " or "abc  def ",
the regex engine will backtrack, approximately once for each character
in the string.  But we know that there is no way around taking all
of the initial word characters to match the first repetition, that all
spaces must be eaten by the middle part, and the same goes for the second
word.



With the introduction of the possessive quantifiers in Perl 5.10, we
have a way of instructing the regex engine not to backtrack, with the
usual quantifiers with a + appended to them.  This makes them greedy as
well as stingy; once they succeed they won’t give anything back to permit
another solution. They have the following meanings:





These possessive quantifiers represent a special case of a more general
concept, the independent subexpression, see below.



As an example where a possessive quantifier is suitable we consider
matching a quoted string, as it appears in several programming languages.
The backslash is used as an escape character that indicates that the
next character is to be taken literally, as another character for the
string.  Therefore, after the opening quote, we expect a (possibly
empty) sequence of alternatives: either some character except an
unescaped quote or backslash or an escaped character.



    /"(?:[^"\\]++|\\.)*+"/;









Next: , Previous: perlretut Possessive quantifiers, Up: perlretut Part 1: The basics   [Contents]





67.3.14 Building a regexp



At this point, we have all the basic regexp concepts covered, so let’s
give a more involved example of a regular expression.  We will build a
regexp that matches numbers.



The first task in building a regexp is to decide what we want to match
and what we want to exclude.  In our case, we want to match both
integers and floating point numbers and we want to reject any string
that isn’t a number.



The next task is to break the problem down into smaller problems that
are easily converted into a regexp.



The simplest case is integers.  These consist of a sequence of digits,
with an optional sign in front.  The digits we can represent with
\d+ and the sign can be matched with [+-].  Thus the integer
regexp is



    /[+-]?\d+/;  # matches integers



A floating point number potentially has a sign, an integral part, a
decimal point, a fractional part, and an exponent.  One or more of these
parts is optional, so we need to check out the different
possibilities.  Floating point numbers which are in proper form include
123., 0.345, .34, -1e6, and 25.4E-72.  As with integers, the sign out
front is completely optional and can be matched by [+-]?.  We can
see that if there is no exponent, floating point numbers must have a
decimal point, otherwise they are integers.  We might be tempted to
model these with \d*\.\d*, but this would also match just a single
decimal point, which is not a number.  So the three cases of floating
point number without exponent are



   /[+-]?\d+\./;  # 1., 321., etc.
   /[+-]?\.\d+/;  # .1, .234, etc.
   /[+-]?\d+\.\d+/;  # 1.0, 30.56, etc.



These can be combined into a single regexp with a three-way alternation:



   /[+-]?(\d+\.\d+|\d+\.|\.\d+)/;  # floating point, no exponent



In this alternation, it is important to put '\d+\.\d+' before
'\d+\.'.  If '\d+\.' were first, the regexp would happily match that
and ignore the fractional part of the number.



Now consider floating point numbers with exponents.  The key
observation here is that both integers and numbers with decimal
points are allowed in front of an exponent.  Then exponents, like the
overall sign, are independent of whether we are matching numbers with
or without decimal points, and can be ’decoupled’ from the
mantissa.  The overall form of the regexp now becomes clear:



    /^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;



The exponent is an e or E, followed by an integer.  So the
exponent regexp is



   /[eE][+-]?\d+/;  # exponent



Putting all the parts together, we get a regexp that matches numbers:



   /^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/;  # Ta da!



Long regexps like this may impress your friends, but can be hard to
decipher.  In complex situations like this, the //x modifier for a
match is invaluable.  It allows one to put nearly arbitrary whitespace
and comments into a regexp without affecting their meaning.  Using it,
we can rewrite our ’extended’ regexp in the more pleasing form



   /^
      [+-]?         # first, match an optional sign
      (             # then match integers or f.p. mantissas:
          \d+\.\d+  # mantissa of the form a.b
         |\d+\.     # mantissa of the form a.
         |\.\d+     # mantissa of the form .b
         |\d+       # integer of the form a
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;



If whitespace is mostly irrelevant, how does one include space
characters in an extended regexp? The answer is to backslash it
'\ ' or put it in a character class [ ].  The same thing
goes for pound signs: use \# or [#].  For instance, Perl allows
a space between the sign and the mantissa or integer, and we could add
this to our regexp as follows:



   /^
      [+-]?\ *      # first, match an optional sign *and space*
      (             # then match integers or f.p. mantissas:
          \d+\.\d+  # mantissa of the form a.b
         |\d+\.     # mantissa of the form a.
         |\.\d+     # mantissa of the form .b
         |\d+       # integer of the form a
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;



In this form, it is easier to see a way to simplify the
alternation.  Alternatives 1, 2, and 4 all start with \d+, so it
could be factored out:



   /^
      [+-]?\ *      # first, match an optional sign
      (             # then match integers or f.p. mantissas:
          \d+       # start out with a ...
          (
              \.\d* # mantissa of the form a.b or a.
          )?        # ? takes care of integers of the form a
         |\.\d+     # mantissa of the form .b
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;



or written in the compact form,



    /^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;



This is our final regexp.  To recap, we built a regexp by





These are also the typical steps involved in writing a computer
program.  This makes perfect sense, because regular expressions are
essentially programs written in a little computer language that specifies
patterns.









Previous: perlretut Building a regexp, Up: perlretut Part 1: The basics   [Contents]





67.3.15 Using regular expressions in Perl



The last topic of Part 1 briefly covers how regexps are used in Perl
programs.  Where do they fit into Perl syntax?



We have already introduced the matching operator in its default
/regexp/ and arbitrary delimiter m!regexp! forms.  We have used
the binding operator =~ and its negation !~ to test for string
matches.  Associated with the matching operator, we have discussed the
single line //s, multi-line //m, case-insensitive //i and
extended //x modifiers.  There are a few more things you might
want to know about matching operators.





• perlretut Prohibiting substitution:  



• perlretut Global matching:  



• perlretut Search and replace:  



• perlretut The split function:  












Next: , Up: perlretut Using regular expressions in Perl   [Contents]





67.3.15.1 Prohibiting substitution



If you change $pattern after the first substitution happens, Perl
will ignore it.  If you don’t want any substitutions at all, use the
special delimiter m'':



    @pattern = ('Seuss');
    while (<>) {
        print if m'@pattern';  # matches literal '@pattern', not 'Seuss'
    }



Similar to strings, m'' acts like apostrophes on a regexp; all other
m delimiters act like quotes.  If the regexp evaluates to the empty string,
the regexp in the last successful match is used instead.  So we have



    "dog" =~ /d/;  # 'd' matches
    "dogbert =~ //;  # this matches the 'd' regexp used before









Next: , Previous: perlretut Prohibiting substitution, Up: perlretut Using regular expressions in Perl   [Contents]





67.3.15.2 Global matching



The final two modifiers we will discuss here,
//g and //c, concern multiple matches.
The modifier //g stands for global matching and allows the
matching operator to match within a string as many times as possible.
In scalar context, successive invocations against a string will have
//g jump from match to match, keeping track of position in the
string as it goes along.  You can get or set the position with the
pos() function.



The use of //g is shown in the following example.  Suppose we have
a string that consists of words separated by spaces.  If we know how
many words there are in advance, we could extract the words using
groupings:



    $x = "cat dog house"; # 3 words
    $x =~ /^\s*(\w+)\s+(\w+)\s+(\w+)\s*$/; # matches,
                                           # $1 = 'cat'
                                           # $2 = 'dog'
                                           # $3 = 'house'



But what if we had an indeterminate number of words? This is the sort
of task //g was made for.  To extract all words, form the simple
regexp (\w+) and loop over all matches with /(\w+)/g:



    while ($x =~ /(\w+)/g) {
        print "Word is $1, ends at position ", pos $x, "\n";
    }



prints



    Word is cat, ends at position 3
    Word is dog, ends at position 7
    Word is house, ends at position 13



A failed match or changing the target string resets the position.  If
you don’t want the position reset after failure to match, add the
//c, as in /regexp/gc.  The current position in the string is
associated with the string, not the regexp.  This means that different
strings have different positions and their respective positions can be
set or read independently.



In list context, //g returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regexp.  So if
we wanted just the words, we could use



    @words = ($x =~ /(\w+)/g);  # matches,
                                # $words[0] = 'cat'
                                # $words[1] = 'dog'
                                # $words[2] = 'house'



Closely associated with the //g modifier is the \G anchor.  The
\G anchor matches at the point where the previous //g match left
off.  \G allows us to easily do context-sensitive matching:



    $metric = 1;  # use metric units
    ...
    $x = <FILE>;  # read in measurement
    $x =~ /^([+-]?\d+)\s*/g;  # get magnitude
    $weight = $1;
    if ($metric) { # error checking
        print "Units error!" unless $x =~ /\Gkg\./g;
    }
    else {
        print "Units error!" unless $x =~ /\Glbs\./g;
    }
    $x =~ /\G\s+(widget|sprocket)/g;  # continue processing



The combination of //g and \G allows us to process the string a
bit at a time and use arbitrary Perl logic to decide what to do next.
Currently, the \G anchor is only fully supported when used to anchor
to the start of the pattern.



\G is also invaluable in processing fixed-length records with
regexps.  Suppose we have a snippet of coding region DNA, encoded as
base pair letters ATCGTTGAAT... and we want to find all the stop
codons TGA.  In a coding region, codons are 3-letter sequences, so
we can think of the DNA snippet as a sequence of 3-letter records.  The
naive regexp



    # expanded, this is "ATC GTT GAA TGC AAA TGA CAT GAC"
    $dna = "ATCGTTGAATGCAAATGACATGAC";
    $dna =~ /TGA/;



doesn’t work; it may match a TGA, but there is no guarantee that
the match is aligned with codon boundaries, e.g., the substring
GTT GAA gives a match.  A better solution is



    while ($dna =~ /(\w\w\w)*?TGA/g) {  # note the minimal *?
        print "Got a TGA stop codon at position ", pos $dna, "\n";
    }



which prints



    Got a TGA stop codon at position 18
    Got a TGA stop codon at position 23



Position 18 is good, but position 23 is bogus.  What happened?



The answer is that our regexp works well until we get past the last
real match.  Then the regexp will fail to match a synchronized TGA
and start stepping ahead one character position at a time, not what we
want.  The solution is to use \G to anchor the match to the codon
alignment:



    while ($dna =~ /\G(\w\w\w)*?TGA/g) {
        print "Got a TGA stop codon at position ", pos $dna, "\n";
    }



This prints



    Got a TGA stop codon at position 18



which is the correct answer.  This example illustrates that it is
important not only to match what is desired, but to reject what is not
desired.



(There are other regexp modifiers that are available, such as
//o, but their specialized uses are beyond the
scope of this introduction.  )









Next: , Previous: perlretut Global matching, Up: perlretut Using regular expressions in Perl   [Contents]





67.3.15.3 Search and replace



Regular expressions also play a big role in search and replace
operations in Perl.  Search and replace is accomplished with the
s/// operator.  The general form is
s/regexp/replacement/modifiers, with everything we know about
regexps and modifiers applying in this case as well.  The
replacement is a Perl double-quoted string that replaces in the
string whatever is matched with the regexp.  The operator =~ is
also used here to associate a string with s///.  If matching
against $_, the $_ =~ can be dropped.  If there is a match,
s/// returns the number of substitutions made; otherwise it returns
false.  Here are a few examples:



    $x = "Time to feed the cat!";
    $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
    if ($x =~ s/^(Time.*hacker)!$/$1 now!/) {
        $more_insistent = 1;
    }
    $y = "'quoted words'";
    $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
                           # $y contains "quoted words"



In the last example, the whole string was matched, but only the part
inside the single quotes was grouped.  With the s/// operator, the
matched variables $1, $2, etc. are immediately available for use
in the replacement expression, so we use $1 to replace the quoted
string with just what was quoted.  With the global modifier, s///g
will search and replace all occurrences of the regexp in the string:



    $x = "I batted 4 for 4";
    $x =~ s/4/four/;   # doesn't do it all:
                       # $x contains "I batted four for 4"
    $x = "I batted 4 for 4";
    $x =~ s/4/four/g;  # does it all:
                       # $x contains "I batted four for four"



If you prefer ’regex’ over ’regexp’ in this tutorial, you could use
the following program to replace it:



    % cat > simple_replace
    #!/usr/bin/perl
    $regexp = shift;
    $replacement = shift;
    while (<>) {
        s/$regexp/$replacement/g;
        print;
    }
    ^D

    % simple_replace regexp regex perlretut.pod



In simple_replace we used the s///g modifier to replace all
occurrences of the regexp on each line.  (Even though the regular
expression appears in a loop, Perl is smart enough to compile it
only once.)  As with simple_grep, both the
print and the s/$regexp/$replacement/g use $_ implicitly.



If you don’t want s/// to change your original variable you can use
the non-destructive substitute modifier, s///r.  This changes the
behavior so that s///r returns the final substituted string
(instead of the number of substitutions):



    $x = "I like dogs.";
    $y = $x =~ s/dogs/cats/r;
    print "$x $y\n";



That example will print "I like dogs. I like cats". Notice the original
$x variable has not been affected. The overall
result of the substitution is instead stored in $y. If the
substitution doesn’t affect anything then the original string is
returned:



    $x = "I like dogs.";
    $y = $x =~ s/elephants/cougars/r;
    print "$x $y\n"; # prints "I like dogs. I like dogs."



One other interesting thing that the s///r flag allows is chaining
substitutions:



    $x = "Cats are great.";
    print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
    # prints "Hedgehogs are great."



A modifier available specifically to search and replace is the
s///e evaluation modifier.  s///e treats the
replacement text as Perl code, rather than a double-quoted
string.  The value that the code returns is substituted for the
matched substring.  s///e is useful if you need to do a bit of
computation in the process of replacing text.  This example counts
character frequencies in a line:



    $x = "Bill the cat";
    $x =~ s/(.)/$chars{$1}++;$1/eg;  # final $1 replaces char with itself
    print "frequency of '$_' is $chars{$_}\n"
        foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);



This prints



    frequency of ' ' is 2
    frequency of 't' is 2
    frequency of 'l' is 2
    frequency of 'B' is 1
    frequency of 'c' is 1
    frequency of 'e' is 1
    frequency of 'h' is 1
    frequency of 'i' is 1
    frequency of 'a' is 1



As with the match m// operator, s/// can use other delimiters,
such as s!!! and s{}{}, and even s{}//.  If single quotes are
used s''', then the regexp and replacement are
treated as single-quoted strings and there are no
variable substitutions.  s/// in list context
returns the same thing as in scalar context, i.e., the number of
matches.









Previous: perlretut Search and replace, Up: perlretut Using regular expressions in Perl   [Contents]





67.3.15.4 The split function



The split() function is another place where a regexp is used.
split /regexp/, string, limit separates the string operand into
a list of substrings and returns that list.  The regexp must be designed
to match whatever constitutes the separators for the desired substrings.
The limit, if present, constrains splitting into no more than limit
number of strings.  For example, to split a string into words, use



    $x = "Calvin and Hobbes";
    @words = split /\s+/, $x;  # $word[0] = 'Calvin'
                               # $word[1] = 'and'
                               # $word[2] = 'Hobbes'



If the empty regexp // is used, the regexp always matches and
the string is split into individual characters.  If the regexp has
groupings, then the resulting list contains the matched substrings from the
groupings as well.  For instance,



    $x = "/usr/bin/perl";
    @dirs = split m!/!, $x;  # $dirs[0] = ''
                             # $dirs[1] = 'usr'
                             # $dirs[2] = 'bin'
                             # $dirs[3] = 'perl'
    @parts = split m!(/)!, $x;  # $parts[0] = ''
                                # $parts[1] = '/'
                                # $parts[2] = 'usr'
                                # $parts[3] = '/'
                                # $parts[4] = 'bin'
                                # $parts[5] = '/'
                                # $parts[6] = 'perl'



Since the first character of $x matched the regexp, split prepended
an empty initial element to the list.



If you have read this far, congratulations! You now have all the basic
tools needed to use regular expressions to solve a wide range of text
processing problems.  If this is your first time through the tutorial,
why not stop here and play around with regexps a while....  Part 2
concerns the more esoteric aspects of regular expressions and those
concepts certainly aren’t needed right at the start.









Next: , Previous: perlretut Part 1: The basics, Up: perlretut   [Contents]





67.4 Part 2: Power tools



OK, you know the basics of regexps and you want to know more.  If
matching regular expressions is analogous to a walk in the woods, then
the tools discussed in Part 1 are analogous to topo maps and a
compass, basic tools we use all the time.  Most of the tools in part 2
are analogous to flare guns and satellite phones.  They aren’t used
too often on a hike, but when we are stuck, they can be invaluable.



What follows are the more advanced, less used, or sometimes esoteric
capabilities of Perl regexps.  In Part 2, we will assume you are
comfortable with the basics and concentrate on the advanced features.





• perlretut More on characters, strings, and character classes:  



• perlretut Compiling and saving regular expressions:  



• perlretut Composing regular expressions at runtime:  



• perlretut Embedding comments and modifiers in a regular expression:  



• perlretut Looking ahead and looking behind:  



• perlretut Using independent subexpressions to prevent backtracking:  



• perlretut Conditional expressions:  



• perlretut Defining named patterns:  



• perlretut Recursive patterns:  



• perlretut A bit of magic: executing Perl code in a regular expression:  



• perlretut Backtracking control verbs:  



• perlretut Pragmas and debugging:  












Next: , Up: perlretut Part 2: Power tools   [Contents]





67.4.1 More on characters, strings, and character classes



There are a number of escape sequences and character classes that we
haven’t covered yet.



There are several escape sequences that convert characters or strings
between upper and lower case, and they are also available within
patterns.  \l and \u convert the next character to lower or
upper case, respectively:



    $x = "perl";
    $string =~ /\u$x/;  # matches 'Perl' in $string
    $x = "M(rs?|s)\\."; # note the double backslash
    $string =~ /\l$x/;  # matches 'mr.', 'mrs.', and 'ms.',



A \L or \U indicates a lasting conversion of case, until
terminated by \E or thrown over by another \U or \L:



    $x = "This word is in lower case:\L SHOUT\E";
    $x =~ /shout/;       # matches
    $x = "I STILL KEYPUNCH CARDS FOR MY 360"
    $x =~ /\Ukeypunch/;  # matches punch card string



If there is no \E, case is converted until the end of the
string. The regexps \L\u$word or \u\L$word convert the first
character of $word to uppercase and the rest of the characters to
lowercase.



Control characters can be escaped with \c, so that a control-Z
character would be matched with \cZ.  The escape sequence
\Q...\E quotes, or protects most non-alphabetic characters.   For
instance,



    $x = "\QThat !^*&%~& cat!";
    $x =~ /\Q!^*&%~&\E/;  # check for rough language



It does not protect $ or @, so that variables can still be
substituted.



\Q, \L, \l, \U, \u and \E are actually part of
double-quotish syntax, and not part of regexp syntax proper.  They will
work if they appear in a regular expression embedded directly in a
program, but not when contained in a string that is interpolated in a
pattern.



With the advent of 5.6.0, Perl regexps can handle more than just the
standard ASCII character set.  Perl now supports Unicode, a standard
for representing the alphabets from virtually all of the world’s written
languages, and a host of symbols.  Perl’s text strings are Unicode strings, so
they can contain characters with a value (codepoint or character number) higher
than 255.



What does this mean for regexps? Well, regexp users don’t need to know
much about Perl’s internal representation of strings.  But they do need
to know 1) how to represent Unicode characters in a regexp and 2) that
a matching operation will treat the string to be searched as a sequence
of characters, not bytes.  The answer to 1) is that Unicode characters
greater than chr(255) are represented using the \x{hex} notation, because
\x hex (without curly braces) doesn’t go further than 255.  (Starting in Perl
5.14, if you’re an octal fan, you can also use \o{oct}.)



    /\x{263a}/;  # match a Unicode smiley face :)



NOTE: In Perl 5.6.0 it used to be that one needed to say use
utf8 to use any Unicode features.  This is no more the case: for
almost all Unicode processing, the explicit utf8 pragma is not
needed.  (The only case where it matters is if your Perl script is in
Unicode and encoded in UTF-8, then an explicit use utf8 is needed.)



Figuring out the hexadecimal sequence of a Unicode character you want
or deciphering someone else’s hexadecimal Unicode regexp is about as
much fun as programming in machine code.  So another way to specify
Unicode characters is to use the named character escape
sequence \N{name}.  name is a name for the Unicode character, as
specified in the Unicode standard.  For instance, if we wanted to
represent or match the astrological sign for the planet Mercury, we
could use



    $x = "abc\N{MERCURY}def";
    $x =~ /\N{MERCURY}/;   # matches



One can also use "short" names:



    print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";
    print "\N{greek:Sigma} is an upper-case sigma.\n";



You can also restrict names to a certain alphabet by specifying the
(charnames) pragma:



    use charnames qw(greek);
    print "\N{sigma} is Greek sigma\n";



An index of character names is available on-line from the Unicode
Consortium, http://www.unicode.org/charts/charindex.html; explanatory
material with links to other resources at
http://www.unicode.org/standard/where.



The answer to requirement 2) is, as of 5.6.0, that a regexp (mostly)
uses Unicode characters.  (The "mostly" is for messy backward
compatibility reasons, but starting in Perl 5.14, any regex compiled in
the scope of a use feature 'unicode_strings' (which is automatically
turned on within the scope of a use 5.012 or higher) will turn that
"mostly" into "always".  If you want to handle Unicode properly, you
should ensure that 'unicode_strings' is turned on.)
Internally, this is encoded to bytes using either UTF-8 or a native 8
bit encoding, depending on the history of the string, but conceptually
it is a sequence of characters, not bytes. See perlunitut NAME for a
tutorial about that.



Let us now discuss Unicode character classes.  Just as with Unicode
characters, there are named Unicode character classes represented by the
\p{name} escape sequence.  Closely associated is the \P{name}
character class, which is the negation of the \p{name} class.  For
example, to match lower and uppercase characters,



    $x = "BOB";
    $x =~ /^\p{IsUpper}/;   # matches, uppercase char class
    $x =~ /^\P{IsUpper}/;   # doesn't match, char class sans uppercase
    $x =~ /^\p{IsLower}/;   # doesn't match, lowercase char class
    $x =~ /^\P{IsLower}/;   # matches, char class sans lowercase



(The "Is" is optional.)



Here is the association between some Perl named classes and the
traditional Unicode classes:



    Perl class name  Unicode class name or regular expression

    IsAlpha          /^[LM]/
    IsAlnum          /^[LMN]/
    IsASCII          $code <= 127
    IsCntrl          /^C/
    IsBlank          $code =~ /^(0020|0009)$/ || /^Z[^lp]/
    IsDigit          Nd
    IsGraph          /^([LMNPS]|Co)/
    IsLower          Ll
    IsPrint          /^([LMNPS]|Co|Zs)/
    IsPunct          /^P/
    IsSpace          /^Z/ || ($code =~ /^(0009|000A|000B|000C|000D)$/
    IsSpacePerl      /^Z/ || ($code =~ /^(0009|000A|000C|000D|0085|2028|2029)$/
    IsUpper          /^L[ut]/
    IsWord           /^[LMN]/ || $code eq "005F"
    IsXDigit         $code =~ /^00(3[0-9]|[46][1-6])$/



You can also use the official Unicode class names with \p and
\P, like \p{L} for Unicode ’letters’, \p{Lu} for uppercase
letters, or \P{Nd} for non-digits.  If a name is just one
letter, the braces can be dropped.  For instance, \pM is the
character class of Unicode ’marks’, for example accent marks.
For the full list see perlunicode NAME.



Unicode has also been separated into various sets of characters
which you can test with \p{...} (in) and \P{...} (not in).
To test whether a character is (or is not) an element of a script
you would use the script name, for example \p{Latin}, \p{Greek},
or \P{Katakana}.



What we have described so far is the single form of the \p{...} character
classes.  There is also a compound form which you may run into.  These
look like \p{name=value} or \p{name:value} (the equals sign and colon
can be used interchangeably).  These are more general than the single form,
and in fact most of the single forms are just Perl-defined shortcuts for common
compound forms.  For example, the script examples in the previous paragraph
could be written equivalently as \p{Script=Latin}, \p{Script:Greek}, and
\P{script=katakana} (case is irrelevant between the {} braces).  You may
never have to use the compound forms, but sometimes it is necessary, and their
use can make your code easier to understand.



\X is an abbreviation for a character class that comprises
a Unicode extended grapheme cluster.  This represents a "logical character":
what appears to be a single character, but may be represented internally by more
than one.  As an example, using the Unicode full names, e.g., A + COMBINING RING is a grapheme cluster with base character A and combining character
COMBINING RING, which translates in Danish to A with the circle atop it,
as in the word Angstrom.



For the full and latest information about Unicode see the latest
Unicode standard, or the Unicode Consortium’s website http://www.unicode.org



As if all those classes weren’t enough, Perl also defines POSIX-style
character classes.  These have the form [:name:], with name the
name of the POSIX class.  The POSIX classes are alpha, alnum,
ascii, cntrl, digit, graph, lower, print, punct,
space, upper, and xdigit, and two extensions, word (a Perl
extension to match \w), and blank (a GNU extension).  The //a
modifier restricts these to matching just in the ASCII range; otherwise
they can match the same as their corresponding Perl Unicode classes:
[:upper:] is the same as \p{IsUpper}, etc.  (There are some
exceptions and gotchas with this; see (perlrecharclass) for a full
discussion.) The [:digit:], [:word:], and
[:space:] correspond to the familiar \d, \w, and \s
character classes.  To negate a POSIX class, put a ^ in front of
the name, so that, e.g., [:^digit:] corresponds to \D and, under
Unicode, \P{IsDigit}.  The Unicode and POSIX character classes can
be used just like \d, with the exception that POSIX character
classes can only be used inside of a character class:



    /\s+[abc[:digit:]xyz]\s*/;  # match a,b,c,x,y,z, or a digit
    /^=item\s[[:digit:]]/;      # match '=item',
                                # followed by a space and a digit
    /\s+[abc\p{IsDigit}xyz]\s+/;  # match a,b,c,x,y,z, or a digit
    /^=item\s\p{IsDigit}/;        # match '=item',
                                  # followed by a space and a digit



Whew! That is all the rest of the characters and character classes.









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67.4.2 Compiling and saving regular expressions



In Part 1 we mentioned that Perl compiles a regexp into a compact
sequence of opcodes.  Thus, a compiled regexp is a data structure
that can be stored once and used again and again.  The regexp quote
qr// does exactly that: qr/string/ compiles the string as a
regexp and transforms the result into a form that can be assigned to a
variable:



    $reg = qr/foo+bar?/;  # reg contains a compiled regexp



Then $reg can be used as a regexp:



    $x = "fooooba";
    $x =~ $reg;     # matches, just like /foo+bar?/
    $x =~ /$reg/;   # same thing, alternate form



$reg can also be interpolated into a larger regexp:



    $x =~ /(abc)?$reg/;  # still matches



As with the matching operator, the regexp quote can use different
delimiters, e.g., qr!!, qr{} or qr~~.  Apostrophes
as delimiters (qr'') inhibit any interpolation.



Pre-compiled regexps are useful for creating dynamic matches that
don’t need to be recompiled each time they are encountered.  Using
pre-compiled regexps, we write a grep_step program which greps
for a sequence of patterns, advancing to the next pattern as soon
as one has been satisfied.



    % cat > grep_step
    #!/usr/bin/perl
    # grep_step - match <number> regexps, one after the other
    # usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...

    $number = shift;
    $regexp[$_] = shift foreach (0..$number-1);
    @compiled = map qr/$_/, @regexp;
    while ($line = <>) {
        if ($line =~ /$compiled[0]/) {
            print $line;
            shift @compiled;
            last unless @compiled;
        }
    }
    ^D

    % grep_step 3 shift print last grep_step
    $number = shift;
            print $line;
            last unless @compiled;



Storing pre-compiled regexps in an array @compiled allows us to
simply loop through the regexps without any recompilation, thus gaining
flexibility without sacrificing speed.









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67.4.3 Composing regular expressions at runtime



Backtracking is more efficient than repeated tries with different regular
expressions.  If there are several regular expressions and a match with
any of them is acceptable, then it is possible to combine them into a set
of alternatives.  If the individual expressions are input data, this
can be done by programming a join operation.  We’ll exploit this idea in
an improved version of the simple_grep program: a program that matches
multiple patterns:



    % cat > multi_grep
    #!/usr/bin/perl
    # multi_grep - match any of <number> regexps
    # usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...

    $number = shift;
    $regexp[$_] = shift foreach (0..$number-1);
    $pattern = join '|', @regexp;

    while ($line = <>) {
        print $line if $line =~ /$pattern/;
    }
    ^D

    % multi_grep 2 shift for multi_grep
    $number = shift;
    $regexp[$_] = shift foreach (0..$number-1);



Sometimes it is advantageous to construct a pattern from the input
that is to be analyzed and use the permissible values on the left
hand side of the matching operations.  As an example for this somewhat
paradoxical situation, let’s assume that our input contains a command
verb which should match one out of a set of available command verbs,
with the additional twist that commands may be abbreviated as long as
the given string is unique. The program below demonstrates the basic
algorithm.



    % cat > keymatch
    #!/usr/bin/perl
    $kwds = 'copy compare list print';
    while( $command = <> ){
        $command =~ s/^\s+|\s+$//g;  # trim leading and trailing spaces
        if( ( @matches = $kwds =~ /\b$command\w*/g ) == 1 ){
            print "command: '@matches'\n";
        } elsif( @matches == 0 ){
            print "no such command: '$command'\n";
        } else {
            print "not unique: '$command' (could be one of: @matches)\n";
        }
    }
    ^D

    % keymatch
    li
    command: 'list'
    co
    not unique: 'co' (could be one of: copy compare)
    printer
    no such command: 'printer'



Rather than trying to match the input against the keywords, we match the
combined set of keywords against the input.  The pattern matching
operation $kwds =~ /\b($command\w*)/g does several things at the
same time. It makes sure that the given command begins where a keyword
begins (\b). It tolerates abbreviations due to the added \w*. It
tells us the number of matches (scalar @matches) and all the keywords
that were actually matched.  You could hardly ask for more.









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67.4.4 Embedding comments and modifiers in a regular expression



Starting with this section, we will be discussing Perl’s set of
extended patterns.  These are extensions to the traditional regular
expression syntax that provide powerful new tools for pattern
matching.  We have already seen extensions in the form of the minimal
matching constructs ??, *?, +?, {n,m}?, and {n,}?.  Most
of the extensions below have the form (?char...), where the
char is a character that determines the type of extension.



The first extension is an embedded comment (?#text).  This embeds a
comment into the regular expression without affecting its meaning.  The
comment should not have any closing parentheses in the text.  An
example is



    /(?# Match an integer:)[+-]?\d+/;



This style of commenting has been largely superseded by the raw,
freeform commenting that is allowed with the //x modifier.



Most modifiers, such as //i, //m, //s and //x (or any
combination thereof) can also be embedded in
a regexp using (?i), (?m), (?s), and (?x).  For instance,



    /(?i)yes/;  # match 'yes' case insensitively
    /yes/i;     # same thing
    /(?x)(          # freeform version of an integer regexp
             [+-]?  # match an optional sign
             \d+    # match a sequence of digits
         )
    /x;



Embedded modifiers can have two important advantages over the usual
modifiers.  Embedded modifiers allow a custom set of modifiers to
each regexp pattern.  This is great for matching an array of regexps
that must have different modifiers:



    $pattern[0] = '(?i)doctor';
    $pattern[1] = 'Johnson';
    ...
    while (<>) {
        foreach $patt (@pattern) {
            print if /$patt/;
        }
    }



The second advantage is that embedded modifiers (except //p, which
modifies the entire regexp) only affect the regexp
inside the group the embedded modifier is contained in.  So grouping
can be used to localize the modifier’s effects:



    /Answer: ((?i)yes)/;  # matches 'Answer: yes', 'Answer: YES', etc.



Embedded modifiers can also turn off any modifiers already present
by using, e.g., (?-i).  Modifiers can also be combined into
a single expression, e.g., (?s-i) turns on single line mode and
turns off case insensitivity.



Embedded modifiers may also be added to a non-capturing grouping.
(?i-m:regexp) is a non-capturing grouping that matches regexp
case insensitively and turns off multi-line mode.









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67.4.5 Looking ahead and looking behind



This section concerns the lookahead and lookbehind assertions.  First,
a little background.



In Perl regular expressions, most regexp elements ’eat up’ a certain
amount of string when they match.  For instance, the regexp element
[abc}] eats up one character of the string when it matches, in the
sense that Perl moves to the next character position in the string
after the match.  There are some elements, however, that don’t eat up
characters (advance the character position) if they match.  The examples
we have seen so far are the anchors.  The anchor ^ matches the
beginning of the line, but doesn’t eat any characters.  Similarly, the
word boundary anchor \b matches wherever a character matching \w
is next to a character that doesn’t, but it doesn’t eat up any
characters itself.  Anchors are examples of zero-width assertions:
zero-width, because they consume
no characters, and assertions, because they test some property of the
string.  In the context of our walk in the woods analogy to regexp
matching, most regexp elements move us along a trail, but anchors have
us stop a moment and check our surroundings.  If the local environment
checks out, we can proceed forward.  But if the local environment
doesn’t satisfy us, we must backtrack.



Checking the environment entails either looking ahead on the trail,
looking behind, or both.  ^ looks behind, to see that there are no
characters before.  $ looks ahead, to see that there are no
characters after.  \b looks both ahead and behind, to see if the
characters on either side differ in their "word-ness".



The lookahead and lookbehind assertions are generalizations of the
anchor concept.  Lookahead and lookbehind are zero-width assertions
that let us specify which characters we want to test for.  The
lookahead assertion is denoted by (?=regexp) and the lookbehind
assertion is denoted by (?<=fixed-regexp).  Some examples are



    $x = "I catch the housecat 'Tom-cat' with catnip";
    $x =~ /cat(?=\s)/;   # matches 'cat' in 'housecat'
    @catwords = ($x =~ /(?<=\s)cat\w+/g);  # matches,
                                           # $catwords[0] = 'catch'
                                           # $catwords[1] = 'catnip'
    $x =~ /\bcat\b/;  # matches 'cat' in 'Tom-cat'
    $x =~ /(?<=\s)cat(?=\s)/; # doesn't match; no isolated 'cat' in
                              # middle of $x



Note that the parentheses in (?=regexp) and (?<=regexp) are
non-capturing, since these are zero-width assertions.  Thus in the
second regexp, the substrings captured are those of the whole regexp
itself.  Lookahead (?=regexp) can match arbitrary regexps, but
lookbehind (?<=fixed-regexp) only works for regexps of fixed
width, i.e., a fixed number of characters long.  Thus
(?<=(ab|bc)) is fine, but (?<=(ab)*) is not.  The
negated versions of the lookahead and lookbehind assertions are
denoted by (?!regexp) and (?<!fixed-regexp) respectively.
They evaluate true if the regexps do not match:



    $x = "foobar";
    $x =~ /foo(?!bar)/;  # doesn't match, 'bar' follows 'foo'
    $x =~ /foo(?!baz)/;  # matches, 'baz' doesn't follow 'foo'
    $x =~ /(?<!\s)foo/;  # matches, there is no \s before 'foo'



The \C is unsupported in lookbehind, because the already
treacherous definition of \C would become even more so
when going backwards.



Here is an example where a string containing blank-separated words,
numbers and single dashes is to be split into its components.
Using /\s+/ alone won’t work, because spaces are not required between
dashes, or a word or a dash. Additional places for a split are established
by looking ahead and behind:



    $str = "one two - --6-8";
    @toks = split / \s+              # a run of spaces
                  | (?<=\S) (?=-)    # any non-space followed by '-'
                  | (?<=-)  (?=\S)   # a '-' followed by any non-space
                  /x, $str;          # @toks = qw(one two - - - 6 - 8)









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67.4.6 Using independent subexpressions to prevent backtracking



Independent subexpressions are regular expressions, in the
context of a larger regular expression, that function independently of
the larger regular expression.  That is, they consume as much or as
little of the string as they wish without regard for the ability of
the larger regexp to match.  Independent subexpressions are represented
by (?>regexp).  We can illustrate their behavior by first
considering an ordinary regexp:



    $x = "ab";
    $x =~ /a*ab/;  # matches



This obviously matches, but in the process of matching, the
subexpression a* first grabbed the a.  Doing so, however,
wouldn’t allow the whole regexp to match, so after backtracking, a*
eventually gave back the a and matched the empty string.  Here, what
a* matched was dependent on what the rest of the regexp matched.



Contrast that with an independent subexpression:



    $x =~ /(?>a*)ab/;  # doesn't match!



The independent subexpression (?>a*) doesn’t care about the rest
of the regexp, so it sees an a and grabs it.  Then the rest of the
regexp ab cannot match.  Because (?>a*) is independent, there
is no backtracking and the independent subexpression does not give
up its a.  Thus the match of the regexp as a whole fails.  A similar
behavior occurs with completely independent regexps:



    $x = "ab";
    $x =~ /a*/g;   # matches, eats an 'a'
    $x =~ /\Gab/g; # doesn't match, no 'a' available



Here //g and \G create a ’tag team’ handoff of the string from
one regexp to the other.  Regexps with an independent subexpression are
much like this, with a handoff of the string to the independent
subexpression, and a handoff of the string back to the enclosing
regexp.



The ability of an independent subexpression to prevent backtracking
can be quite useful.  Suppose we want to match a non-empty string
enclosed in parentheses up to two levels deep.  Then the following
regexp matches:



    $x = "abc(de(fg)h";  # unbalanced parentheses
    $x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x;



The regexp matches an open parenthesis, one or more copies of an
alternation, and a close parenthesis.  The alternation is two-way, with
the first alternative [^()]+ matching a substring with no
parentheses and the second alternative \([^()]*\)  matching a
substring delimited by parentheses.  The problem with this regexp is
that it is pathological: it has nested indeterminate quantifiers
of the form (a+|b)+.  We discussed in Part 1 how nested quantifiers
like this could take an exponentially long time to execute if there
was no match possible.  To prevent the exponential blowup, we need to
prevent useless backtracking at some point.  This can be done by
enclosing the inner quantifier as an independent subexpression:



    $x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x;



Here, (?>[^()]+) breaks the degeneracy of string partitioning
by gobbling up as much of the string as possible and keeping it.   Then
match failures fail much more quickly.









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67.4.7 Conditional expressions



A conditional expression is a form of if-then-else statement
that allows one to choose which patterns are to be matched, based on
some condition.  There are two types of conditional expression:
(?(condition)yes-regexp) and
(?(condition)yes-regexp|no-regexp).  (?(condition)yes-regexp) is
like an 'if () {}' statement in Perl.  If the condition is true,
the yes-regexp will be matched.  If the condition is false, the
yes-regexp will be skipped and Perl will move onto the next regexp
element.  The second form is like an 'if () {} else {}' statement
in Perl.  If the condition is true, the yes-regexp will be
matched, otherwise the no-regexp will be matched.



The condition can have several forms.  The first form is simply an
integer in parentheses (integer).  It is true if the corresponding
backreference \integer matched earlier in the regexp.  The same
thing can be done with a name associated with a capture group, written
as (<name>) or ('name').  The second form is a bare
zero-width assertion (?...), either a lookahead, a lookbehind, or a
code assertion (discussed in the next section).  The third set of forms
provides tests that return true if the expression is executed within
a recursion ((R)) or is being called from some capturing group,
referenced either by number ((R1), (R2),...) or by name
((R&name)).



The integer or name form of the condition allows us to choose,
with more flexibility, what to match based on what matched earlier in the
regexp. This searches for words of the form "$x$x" or "$x$y$y$x":



    % simple_grep '^(\w+)(\w+)?(?(2)\g2\g1|\g1)$' /usr/dict/words
    beriberi
    coco
    couscous
    deed
    ...
    toot
    toto
    tutu



The lookbehind condition allows, along with backreferences,
an earlier part of the match to influence a later part of the
match.  For instance,



    /[ATGC]+(?(?<=AA)G|C)$/;



matches a DNA sequence such that it either ends in AAG, or some
other base pair combination and C.  Note that the form is
(?(?<=AA)G|C) and not (?((?<=AA))G|C); for the
lookahead, lookbehind or code assertions, the parentheses around the
conditional are not needed.









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67.4.8 Defining named patterns



Some regular expressions use identical subpatterns in several places.
Starting with Perl 5.10, it is possible to define named subpatterns in
a section of the pattern so that they can be called up by name
anywhere in the pattern.  This syntactic pattern for this definition
group is (?(DEFINE)(?<name>pattern)...).  An insertion
of a named pattern is written as (?&name).



The example below illustrates this feature using the pattern for
floating point numbers that was presented earlier on.  The three
subpatterns that are used more than once are the optional sign, the
digit sequence for an integer and the decimal fraction.  The DEFINE
group at the end of the pattern contains their definition.  Notice
that the decimal fraction pattern is the first place where we can
reuse the integer pattern.



   /^ (?&osg)\ * ( (?&int)(?&dec)? | (?&dec) )
      (?: [eE](?&osg)(?&int) )?
    $
    (?(DEFINE)
      (?<osg>[-+]?)         # optional sign
      (?<int>\d++)          # integer
      (?<dec>\.(?&int))     # decimal fraction
    )/x









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67.4.9 Recursive patterns



This feature (introduced in Perl 5.10) significantly extends the
power of Perl’s pattern matching.  By referring to some other
capture group anywhere in the pattern with the construct
(?group-ref), the pattern within the referenced group is used
as an independent subpattern in place of the group reference itself.
Because the group reference may be contained within the group it
refers to, it is now possible to apply pattern matching to tasks that
hitherto required a recursive parser.



To illustrate this feature, we’ll design a pattern that matches if
a string contains a palindrome. (This is a word or a sentence that,
while ignoring spaces, interpunctuation and case, reads the same backwards
as forwards. We begin by observing that the empty string or a string
containing just one word character is a palindrome. Otherwise it must
have a word character up front and the same at its end, with another
palindrome in between.



    /(?: (\w) (?...Here be a palindrome...) \g{-1} | \w? )/x



Adding \W* at either end to eliminate what is to be ignored, we already
have the full pattern:



    my $pp = qr/^(\W* (?: (\w) (?1) \g{-1} | \w? ) \W*)$/ix;
    for $s ( "saippuakauppias", "A man, a plan, a canal: Panama!" ){
        print "'$s' is a palindrome\n" if $s =~ /$pp/;
    }



In (?...) both absolute and relative backreferences may be used.
The entire pattern can be reinserted with (?R) or (?0).
If you prefer to name your groups, you can use (?&name) to
recurse into that group.









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67.4.10 A bit of magic: executing Perl code in a regular expression



Normally, regexps are a part of Perl expressions.
Code evaluation expressions turn that around by allowing
arbitrary Perl code to be a part of a regexp.  A code evaluation
expression is denoted (?{code}), with code a string of Perl
statements.



Be warned that this feature is considered experimental, and may be
changed without notice.



Code expressions are zero-width assertions, and the value they return
depends on their environment.  There are two possibilities: either the
code expression is used as a conditional in a conditional expression
(?(condition)...), or it is not.  If the code expression is a
conditional, the code is evaluated and the result (i.e., the result of
the last statement) is used to determine truth or falsehood.  If the
code expression is not used as a conditional, the assertion always
evaluates true and the result is put into the special variable
$^R.  The variable $^R can then be used in code expressions later
in the regexp.  Here are some silly examples:



    $x = "abcdef";
    $x =~ /abc(?{print "Hi Mom!";})def/; # matches,
                                         # prints 'Hi Mom!'
    $x =~ /aaa(?{print "Hi Mom!";})def/; # doesn't match,
                                         # no 'Hi Mom!'



Pay careful attention to the next example:



    $x =~ /abc(?{print "Hi Mom!";})ddd/; # doesn't match,
                                         # no 'Hi Mom!'
                                         # but why not?



At first glance, you’d think that it shouldn’t print, because obviously
the ddd isn’t going to match the target string. But look at this
example:



    $x =~ /abc(?{print "Hi Mom!";})[dD]dd/; # doesn't match,
                                            # but _does_ print



Hmm. What happened here? If you’ve been following along, you know that
the above pattern should be effectively (almost) the same as the last one;
enclosing the d in a character class isn’t going to change what it
matches. So why does the first not print while the second one does?



The answer lies in the optimizations the regex engine makes. In the first
case, all the engine sees are plain old characters (aside from the
?{} construct). It’s smart enough to realize that the string ’ddd’
doesn’t occur in our target string before actually running the pattern
through. But in the second case, we’ve tricked it into thinking that our
pattern is more complicated. It takes a look, sees our
character class, and decides that it will have to actually run the
pattern to determine whether or not it matches, and in the process of
running it hits the print statement before it discovers that we don’t
have a match.



To take a closer look at how the engine does optimizations, see the
section Pragmas and debugging below.



More fun with ?{}:



    $x =~ /(?{print "Hi Mom!";})/;       # matches,
                                         # prints 'Hi Mom!'
    $x =~ /(?{$c = 1;})(?{print "$c";})/;  # matches,
                                           # prints '1'
    $x =~ /(?{$c = 1;})(?{print "$^R";})/; # matches,
                                           # prints '1'



The bit of magic mentioned in the section title occurs when the regexp
backtracks in the process of searching for a match.  If the regexp
backtracks over a code expression and if the variables used within are
localized using local, the changes in the variables produced by the
code expression are undone! Thus, if we wanted to count how many times
a character got matched inside a group, we could use, e.g.,



    $x = "aaaa";
    $count = 0;  # initialize 'a' count
    $c = "bob";  # test if $c gets clobbered
    $x =~ /(?{local $c = 0;})         # initialize count
           ( a                        # match 'a'
             (?{local $c = $c + 1;})  # increment count
           )*                         # do this any number of times,
           aa                         # but match 'aa' at the end
           (?{$count = $c;})          # copy local $c var into $count
          /x;
    print "'a' count is $count, \$c variable is '$c'\n";



This prints



    'a' count is 2, $c variable is 'bob'



If we replace the  (?{local $c = $c + 1;}) with
 (?{$c = $c + 1;}), the variable changes are not undone
during backtracking, and we get



    'a' count is 4, $c variable is 'bob'



Note that only localized variable changes are undone.  Other side
effects of code expression execution are permanent.  Thus



    $x = "aaaa";
    $x =~ /(a(?{print "Yow\n";}))*aa/;



produces



   Yow
   Yow
   Yow
   Yow



The result $^R is automatically localized, so that it will behave
properly in the presence of backtracking.



This example uses a code expression in a conditional to match a
definite article, either ’the’ in English or ’der|die|das’ in German:



    $lang = 'DE';  # use German
    ...
    $text = "das";
    print "matched\n"
        if $text =~ /(?(?{
                          $lang eq 'EN'; # is the language English?
                         })
                       the |             # if so, then match 'the'
                       (der|die|das)     # else, match 'der|die|das'
                     )
                    /xi;



Note that the syntax here is (?(?{...})yes-regexp|no-regexp), not
(?((?{...}))yes-regexp|no-regexp).  In other words, in the case of a
code expression, we don’t need the extra parentheses around the
conditional.



If you try to use code expressions with interpolating variables, Perl
may surprise you:



    $bar = 5;
    $pat = '(?{ 1 })';
    /foo(?{ $bar })bar/; # compiles ok, $bar not interpolated
    /foo(?{ 1 })$bar/;   # compile error!
    /foo${pat}bar/;      # compile error!

    $pat = qr/(?{ $foo = 1 })/;  # precompile code regexp
    /foo${pat}bar/;      # compiles ok



If a regexp has (1) code expressions and interpolating variables, or
(2) a variable that interpolates a code expression, Perl treats the
regexp as an error. If the code expression is precompiled into a
variable, however, interpolating is ok. The question is, why is this
an error?



The reason is that variable interpolation and code expressions
together pose a security risk.  The combination is dangerous because
many programmers who write search engines often take user input and
plug it directly into a regexp:



    $regexp = <>;       # read user-supplied regexp
    $chomp $regexp;     # get rid of possible newline
    $text =~ /$regexp/; # search $text for the $regexp



If the $regexp variable contains a code expression, the user could
then execute arbitrary Perl code.  For instance, some joker could
search for system('rm -rf *'); to erase your files.  In this
sense, the combination of interpolation and code expressions taints
your regexp.  So by default, using both interpolation and code
expressions in the same regexp is not allowed.  If you’re not
concerned about malicious users, it is possible to bypass this
security check by invoking use re 'eval':



    use re 'eval';       # throw caution out the door
    $bar = 5;
    $pat = '(?{ 1 })';
    /foo(?{ 1 })$bar/;   # compiles ok
    /foo${pat}bar/;      # compiles ok



Another form of code expression is the pattern code expression.
The pattern code expression is like a regular code expression, except
that the result of the code evaluation is treated as a regular
expression and matched immediately.  A simple example is



    $length = 5;
    $char = 'a';
    $x = 'aaaaabb';
    $x =~ /(??{$char x $length})/x; # matches, there are 5 of 'a'



This final example contains both ordinary and pattern code
expressions.  It detects whether a binary string 1101010010001... has a
Fibonacci spacing 0,1,1,2,3,5,...  of the 1’s:



    $x = "1101010010001000001";
    $z0 = ''; $z1 = '0';   # initial conditions
    print "It is a Fibonacci sequence\n"
        if $x =~ /^1         # match an initial '1'
                    (?:
                       ((??{ $z0 })) # match some '0'
                       1             # and then a '1'
                       (?{ $z0 = $z1; $z1 .= $^N; })
                    )+   # repeat as needed
                  $      # that is all there is
                 /x;
    printf "Largest sequence matched was %d\n", length($z1)-length($z0);



Remember that $^N is set to whatever was matched by the last
completed capture group. This prints



    It is a Fibonacci sequence
    Largest sequence matched was 5



Ha! Try that with your garden variety regexp package...



Note that the variables $z0 and $z1 are not substituted when the
regexp is compiled, as happens for ordinary variables outside a code
expression.  Rather, the code expressions are evaluated when Perl
encounters them during the search for a match.



The regexp without the //x modifier is



    /^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/



which shows that spaces are still possible in the code parts. Nevertheless,
when working with code and conditional expressions, the extended form of
regexps is almost necessary in creating and debugging regexps.









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67.4.11 Backtracking control verbs



Perl 5.10 introduced a number of control verbs intended to provide
detailed control over the backtracking process, by directly influencing
the regexp engine and by providing monitoring techniques.  As all
the features in this group are experimental and subject to change or
removal in a future version of Perl, the interested reader is
referred to (perlre)Special Backtracking Control Verbs for a
detailed description.



Below is just one example, illustrating the control verb (*FAIL),
which may be abbreviated as (*F). If this is inserted in a regexp
it will cause it to fail, just as it would at some
mismatch between the pattern and the string. Processing
of the regexp continues as it would after any "normal"
failure, so that, for instance, the next position in the string or another
alternative will be tried. As failing to match doesn’t preserve capture
groups or produce results, it may be necessary to use this in
combination with embedded code.



   %count = ();
   "supercalifragilisticexpialidocious" =~
       /([aeiou])(?{ $count{$1}++; })(*FAIL)/i;
   printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);



The pattern begins with a class matching a subset of letters.  Whenever
this matches, a statement like $count{'a'}++; is executed, incrementing
the letter’s counter. Then (*FAIL) does what it says, and
the regexp engine proceeds according to the book: as long as the end of
the string hasn’t been reached, the position is advanced before looking
for another vowel. Thus, match or no match makes no difference, and the
regexp engine proceeds until the entire string has been inspected.
(It’s remarkable that an alternative solution using something like



   $count{lc($_)}++ for split('', "supercalifragilisticexpialidocious");
   printf "%3d '%s'\n", $count2{$_}, $_ for ( qw{ a e i o u } );



is considerably slower.)









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67.4.12 Pragmas and debugging



Speaking of debugging, there are several pragmas available to control
and debug regexps in Perl.  We have already encountered one pragma in
the previous section, use re 'eval';, that allows variable
interpolation and code expressions to coexist in a regexp.  The other
pragmas are



    use re 'taint';
    $tainted = <>;
    @parts = ($tainted =~ /(\w+)\s+(\w+)/; # @parts is now tainted



The taint pragma causes any substrings from a match with a tainted
variable to be tainted as well.  This is not normally the case, as
regexps are often used to extract the safe bits from a tainted
variable.  Use taint when you are not extracting safe bits, but are
performing some other processing.  Both taint and eval pragmas
are lexically scoped, which means they are in effect only until
the end of the block enclosing the pragmas.



    use re '/m';  # or any other flags
    $multiline_string =~ /^foo/; # /m is implied



The re '/flags' pragma (introduced in Perl
5.14) turns on the given regular expression flags
until the end of the lexical scope.  See
(re)'/flags' mode for more
detail.



    use re 'debug';
    /^(.*)$/s;       # output debugging info

    use re 'debugcolor';
    /^(.*)$/s;       # output debugging info in living color



The global debug and debugcolor pragmas allow one to get
detailed debugging info about regexp compilation and
execution.  debugcolor is the same as debug, except the debugging
information is displayed in color on terminals that can display
termcap color sequences.  Here is example output:



    % perl -e 'use re "debug"; "abc" =~ /a*b+c/;'
    Compiling REx 'a*b+c'
    size 9 first at 1
       1: STAR(4)
       2:   EXACT <a>(0)
       4: PLUS(7)
       5:   EXACT <b>(0)
       7: EXACT <c>(9)
       9: END(0)
    floating 'bc' at 0..2147483647 (checking floating) minlen 2
    Guessing start of match, REx 'a*b+c' against 'abc'...
    Found floating substr 'bc' at offset 1...
    Guessed: match at offset 0
    Matching REx 'a*b+c' against 'abc'
      Setting an EVAL scope, savestack=3
       0 <> <abc>             |  1:  STAR
                               EXACT <a> can match 1 times out of 32767...
      Setting an EVAL scope, savestack=3
       1 <a> <bc>             |  4:    PLUS
                               EXACT <b> can match 1 times out of 32767...
      Setting an EVAL scope, savestack=3
       2 <ab> <c>             |  7:      EXACT <c>
       3 <abc> <>             |  9:      END
    Match successful!
    Freeing REx: 'a*b+c'



If you have gotten this far into the tutorial, you can probably guess
what the different parts of the debugging output tell you.  The first
part



    Compiling REx 'a*b+c'
    size 9 first at 1
       1: STAR(4)
       2:   EXACT <a>(0)
       4: PLUS(7)
       5:   EXACT <b>(0)
       7: EXACT <c>(9)
       9: END(0)



describes the compilation stage.  STAR(4) means that there is a
starred object, in this case 'a', and if it matches, goto line 4,
i.e., PLUS(7).  The middle lines describe some heuristics and
optimizations performed before a match:



    floating 'bc' at 0..2147483647 (checking floating) minlen 2
    Guessing start of match, REx 'a*b+c' against 'abc'...
    Found floating substr 'bc' at offset 1...
    Guessed: match at offset 0



Then the match is executed and the remaining lines describe the
process:



    Matching REx 'a*b+c' against 'abc'
      Setting an EVAL scope, savestack=3
       0 <> <abc>             |  1:  STAR
                               EXACT <a> can match 1 times out of 32767...
      Setting an EVAL scope, savestack=3
       1 <a> <bc>             |  4:    PLUS
                               EXACT <b> can match 1 times out of 32767...
      Setting an EVAL scope, savestack=3
       2 <ab> <c>             |  7:      EXACT <c>
       3 <abc> <>             |  9:      END
    Match successful!
    Freeing REx: 'a*b+c'



Each step is of the form n <x> <y>, with <x> the
part of the string matched and <y> the part not yet
matched.  The |  1:  STAR says that Perl is at line number 1
in the compilation list above.  See
perldebguts Debugging Regular Expressions for much more detail.



An alternative method of debugging regexps is to embed print
statements within the regexp.  This provides a blow-by-blow account of
the backtracking in an alternation:



    "that this" =~ m@(?{print "Start at position ", pos, "\n";})
                     t(?{print "t1\n";})
                     h(?{print "h1\n";})
                     i(?{print "i1\n";})
                     s(?{print "s1\n";})
                         |
                     t(?{print "t2\n";})
                     h(?{print "h2\n";})
                     a(?{print "a2\n";})
                     t(?{print "t2\n";})
                     (?{print "Done at position ", pos, "\n";})
                    @x;



prints



    Start at position 0
    t1
    h1
    t2
    h2
    a2
    t2
    Done at position 4









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67.5 BUGS



Code expressions, conditional expressions, and independent expressions
are experimental.  Don’t use them in production code.  Yet.









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67.6 SEE ALSO



This is just a tutorial.  For the full story on Perl regular
expressions, see the (perlre) regular expressions reference page.



For more information on the matching m// and substitution s///
operators, see (perlop)Regexp Quote-Like Operators.  For
information on the split operation, see (perlfunc)split.



For an excellent all-around resource on the care and feeding of
regular expressions, see the book Mastering Regular Expressions by
Jeffrey Friedl (published by O’Reilly, ISBN 1556592-257-3).









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67.7 AUTHOR AND COPYRIGHT



Copyright (c) 2000 Mark Kvale
All rights reserved.



This document may be distributed under the same terms as Perl itself.





• perlretut Acknowledgments:  












Up: perlretut AUTHOR AND COPYRIGHT   [Contents]





67.7.1 Acknowledgments



The inspiration for the stop codon DNA example came from the ZIP
code example in chapter 7 of Mastering Regular Expressions.



The author would like to thank Jeff Pinyan, Andrew Johnson, Peter
Haworth, Ronald J Kimball, and Joe Smith for all their helpful
comments.









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68 perlrun





• perlrun NAME:  



• perlrun SYNOPSIS:  



• perlrun DESCRIPTION:  



• perlrun ENVIRONMENT:  












Next: , Up: perlrun   [Contents]





68.1 NAME



perlrun - how to execute the Perl interpreter









Next: , Previous: perlrun NAME, Up: perlrun   [Contents]





68.2 SYNOPSIS



perl	[ -sTtuUWX ]
	[ -hv ] [ -V[:configvar] ]
	[ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
	[ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]
	[ -Idir ] [ -m[-]module ] [ -M[-]’module...’ ] [ -f ]
	[ -C [number/list] ]
	[ -S ]
	[ -x[dir] ]
	[ -i[extension] ]
	[ [-e|-E] ’command’ ] [ – ] [ programfile ] [ argument ]...









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68.3 DESCRIPTION



The normal way to run a Perl program is by making it directly
executable, or else by passing the name of the source file as an
argument on the command line.  (An interactive Perl environment
is also possible–see (perldebug) for details on how to do that.)
Upon startup, Perl looks for your program in one of the following
places:



    

  1.  Specified line by line via -e or -E switches on the command line.


  2.  Contained in the file specified by the first filename on the command line.
(Note that systems supporting the #! notation invoke interpreters this
way. See Location of Perl.)


  3.  Passed in implicitly via standard input.  This works only if there are
no filename arguments–to pass arguments to a STDIN-read program you
must explicitly specify a "-" for the program name.





With methods 2 and 3, Perl starts parsing the input file from the
beginning, unless you’ve specified a -x switch, in which case it
scans for the first line starting with #! and containing the word
"perl", and starts there instead.  This is useful for running a program
embedded in a larger message.  (In this case you would indicate the end
of the program using the __END__ token.)



The #! line is always examined for switches as the line is being
parsed.  Thus, if you’re on a machine that allows only one argument
with the #! line, or worse, doesn’t even recognize the #! line, you
still can get consistent switch behaviour regardless of how Perl was
invoked, even if -x was used to find the beginning of the program.



Because historically some operating systems silently chopped off
kernel interpretation of the #! line after 32 characters, some
switches may be passed in on the command line, and some may not;
you could even get a "-" without its letter, if you’re not careful.
You probably want to make sure that all your switches fall either
before or after that 32-character boundary.  Most switches don’t
actually care if they’re processed redundantly, but getting a "-"
instead of a complete switch could cause Perl to try to execute
standard input instead of your program.  And a partial -I switch
could also cause odd results.



Some switches do care if they are processed twice, for instance
combinations of -l and -0.  Either put all the switches after
the 32-character boundary (if applicable), or replace the use of
-0digits by BEGIN{ $/ = "\0digits"; }.



Parsing of the #! switches starts wherever "perl" is mentioned in the line.
The sequences "-*" and "- " are specifically ignored so that you could,
if you were so inclined, say



    #!/bin/sh
    #! -*-perl-*-
    eval 'exec perl -x -wS $0 ${1+"$@"}'
        if 0;



to let Perl see the -p switch.



A similar trick involves the env program, if you have it.



    #!/usr/bin/env perl



The examples above use a relative path to the perl interpreter,
getting whatever version is first in the user’s path.  If you want
a specific version of Perl, say, perl5.005_57, you should place
that directly in the #! line’s path.



If the #! line does not contain the word "perl" nor the word "indir"
the program named after the #! is executed instead of the Perl
interpreter.  This is slightly bizarre, but it helps people on machines
that don’t do #!, because they can tell a program that their SHELL is
/usr/bin/perl, and Perl will then dispatch the program to the correct
interpreter for them.



After locating your program, Perl compiles the entire program to an
internal form.  If there are any compilation errors, execution of the
program is not attempted.  (This is unlike the typical shell script,
which might run part-way through before finding a syntax error.)



If the program is syntactically correct, it is executed.  If the program
runs off the end without hitting an exit() or die() operator, an implicit
exit(0) is provided to indicate successful completion.





• perlrun #! and quoting on non-Unix systems:  



• perlrun Location of Perl:  



• perlrun Command Switches:  












Next: , Up: perlrun DESCRIPTION   [Contents]





68.3.1 #! and quoting on non-Unix systems





Unix’s #! technique can be simulated on other systems:





OS/2




Put



    extproc perl -S -your_switches



as the first line in *.cmd file (-S due to a bug in cmd.exe’s
‘extproc’ handling).





MS-DOS




Create a batch file to run your program, and codify it in
ALTERNATE_SHEBANG (see the dosish.h file in the source
distribution for more information).





Win95/NT




The Win95/NT installation, when using the ActiveState installer for Perl,
will modify the Registry to associate the .pl extension with the perl
interpreter.  If you install Perl by other means (including building from
the sources), you may have to modify the Registry yourself.  Note that
this means you can no longer tell the difference between an executable
Perl program and a Perl library file.





VMS




Put



 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
 $ exit++ + ++$status != 0 and $exit = $status = undef;



at the top of your program, where -mysw are any command line switches you
want to pass to Perl.  You can now invoke the program directly, by saying
perl program, or as a DCL procedure, by saying @program (or implicitly
via DCL$PATH by just using the name of the program).



This incantation is a bit much to remember, but Perl will display it for
you if you say perl "-V:startperl".








Command-interpreters on non-Unix systems have rather different ideas
on quoting than Unix shells.  You’ll need to learn the special
characters in your command-interpreter (*, \ and " are
common) and how to protect whitespace and these characters to run
one-liners (see -e below).



On some systems, you may have to change single-quotes to double ones,
which you must not do on Unix or Plan 9 systems.  You might also
have to change a single % to a %%.



For example:



    # Unix
    perl -e 'print "Hello world\n"'

    # MS-DOS, etc.
    perl -e "print \"Hello world\n\""

    # VMS
    perl -e "print ""Hello world\n"""



The problem is that none of this is reliable: it depends on the
command and it is entirely possible neither works.  If 4DOS were
the command shell, this would probably work better:



    perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""



CMD.EXE in Windows NT slipped a lot of standard Unix functionality in
when nobody was looking, but just try to find documentation for its
quoting rules.



There is no general solution to all of this.  It’s just a mess.









Next: , Previous: perlrun #! and quoting on non-Unix systems, Up: perlrun DESCRIPTION   [Contents]





68.3.2 Location of Perl




It may seem obvious to say, but Perl is useful only when users can
easily find it.  When possible, it’s good for both /usr/bin/perl
and /usr/local/bin/perl to be symlinks to the actual binary.  If
that can’t be done, system administrators are strongly encouraged
to put (symlinks to) perl and its accompanying utilities into a
directory typically found along a user’s PATH, or in some other
obvious and convenient place.



In this documentation, #!/usr/bin/perl on the first line of the program
will stand in for whatever method works on your system.  You are
advised to use a specific path if you care about a specific version.



    #!/usr/local/bin/perl5.00554



or if you just want to be running at least version, place a statement
like this at the top of your program:



    use 5.005_54;









Previous: perlrun Location of Perl, Up: perlrun DESCRIPTION   [Contents]





68.3.3 Command Switches





As with all standard commands, a single-character switch may be
clustered with the following switch, if any.



    #!/usr/bin/perl -spi.orig   # same as -s -p -i.orig



Switches include:





-0[octal/hexadecimal]






specifies the input record separator ($/) as an octal or
hexadecimal number.  If there are no digits, the null character is the
separator.  Other switches may precede or follow the digits.  For
example, if you have a version of find which can print filenames
terminated by the null character, you can say this:



    find . -name '*.orig' -print0 | perl -n0e unlink



The special value 00 will cause Perl to slurp files in paragraph mode.
Any value 0400 or above will cause Perl to slurp files whole, but by convention
the value 0777 is the one normally used for this purpose.



You can also specify the separator character using hexadecimal notation:
-0xHHH..., where the H are valid hexadecimal digits.  Unlike
the octal form, this one may be used to specify any Unicode character, even
those beyond 0xFF.  So if you really want a record separator of 0777,
specify it as -0x1FF.  (This means that you cannot use the -x option
with a directory name that consists of hexadecimal digits, or else Perl
will think you have specified a hex number to -0.)





-a






turns on autosplit mode when used with a -n or -p.  An implicit
split command to the @F array is done as the first thing inside the
implicit while loop produced by the -n or -p.



    perl -ane 'print pop(@F), "\n";'



is equivalent to



    while (<>) {
        @F = split(' ');
        print pop(@F), "\n";
    }



An alternate delimiter may be specified using -F.





-C [number/list]





The -C flag controls some of the Perl Unicode features.



As of 5.8.1, the -C can be followed either by a number or a list
of option letters.  The letters, their numeric values, and effects
are as follows; listing the letters is equal to summing the numbers.



    I     1   STDIN is assumed to be in UTF-8
    O     2   STDOUT will be in UTF-8
    E     4   STDERR will be in UTF-8
    S     7   I + O + E
    i     8   UTF-8 is the default PerlIO layer for input streams
    o    16   UTF-8 is the default PerlIO layer for output streams
    D    24   i + o
    A    32   the @ARGV elements are expected to be strings encoded
              in UTF-8
    L    64   normally the "IOEioA" are unconditional, the L makes
              them conditional on the locale environment variables
              (the LC_ALL, LC_TYPE, and LANG, in the order of
              decreasing precedence) -- if the variables indicate
              UTF-8, then the selected "IOEioA" are in effect
    a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
              code in debugging mode.



For example, -COE and -C6 will both turn on UTF-8-ness on both
STDOUT and STDERR.  Repeating letters is just redundant, not cumulative
nor toggling.



The io options mean that any subsequent open() (or similar I/O
operations) in the current file scope will have the :utf8 PerlIO layer
implicitly applied to them, in other words, UTF-8 is expected from any
input stream, and UTF-8 is produced to any output stream.  This is just
the default, with explicit layers in open() and with binmode() one can
manipulate streams as usual.



-C on its own (not followed by any number or option list), or the
empty string "" for the PERL_UNICODE environment variable, has the
same effect as -CSDL.  In other words, the standard I/O handles and
the default open() layer are UTF-8-fied but only if the locale
environment variables indicate a UTF-8 locale.  This behaviour follows
the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.
(See (perl581delta)UTF-8 no longer default under UTF-8 locales.)



You can use -C0 (or "0" for PERL_UNICODE) to explicitly
disable all the above Unicode features.



The read-only magic variable ${^UNICODE} reflects the numeric value
of this setting.  This variable is set during Perl startup and is
thereafter read-only.  If you want runtime effects, use the three-arg
open() (see (perlfunc)open), the two-arg binmode() (see (perlfunc)binmode),
and the open pragma (see (open)).



(In Perls earlier than 5.8.1 the -C switch was a Win32-only switch
that enabled the use of Unicode-aware "wide system call" Win32 APIs.
This feature was practically unused, however, and the command line
switch was therefore "recycled".)



Note: Since perl 5.10.1, if the -C option is used on the #! line,
it must be specified on the command line as well, since the standard streams
are already set up at this point in the execution of the perl interpreter.
You can also use binmode() to set the encoding of an I/O stream.





-c





causes Perl to check the syntax of the program and then exit without
executing it.  Actually, it will execute and BEGIN, UNITCHECK,
or CHECK blocks and any use statements: these are considered as
occurring outside the execution of your program.  INIT and END
blocks, however, will be skipped.





-d








-dt




runs the program under the Perl debugger.  See (perldebug).
If t is specified, it indicates to the debugger that threads
will be used in the code being debugged.





-d:MOD[=bar,baz]








-dt:MOD[=bar,baz]




runs the program under the control of a debugging, profiling, or tracing
module installed as Devel::MOD. E.g., -d:DProf executes the
program using the Devel::DProf profiler.  As with the -M flag, options
may be passed to the Devel::MOD package where they will be received
and interpreted by the Devel::MOD::import routine.  Again, like -M,
use --d:-MOD to call Devel::MOD::unimport instead of import.  The
comma-separated list of options must follow a = character.  If t is
specified, it indicates to the debugger that threads will be used in the
code being debugged.  See (perldebug).





-Dletters









-Dnumber




sets debugging flags.  To watch how it executes your program, use
-Dtls.  (This works only if debugging is compiled into your
Perl.)  Another nice value is -Dx, which lists your compiled
syntax tree.  And -Dr displays compiled regular expressions;
the format of the output is explained in perldebguts NAME.



As an alternative, specify a number instead of list of letters (e.g.,
-D14 is equivalent to -Dtls):



        1  p  Tokenizing and parsing (with v, displays parse stack)
        2  s  Stack snapshots (with v, displays all stacks)
        4  l  Context (loop) stack processing
        8  t  Trace execution
       16  o  Method and overloading resolution
       32  c  String/numeric conversions
       64  P  Print profiling info, source file input state
      128  m  Memory and SV allocation
      256  f  Format processing
      512  r  Regular expression parsing and execution
     1024  x  Syntax tree dump
     2048  u  Tainting checks
     4096  U  Unofficial, User hacking (reserved for private,
              unreleased use)
     8192  H  Hash dump -- usurps values()
    16384  X  Scratchpad allocation
    32768  D  Cleaning up
   131072  T  Tokenizing
   262144  R  Include reference counts of dumped variables (eg when
              using -Ds)
   524288  J  show s,t,P-debug (don't Jump over) on opcodes within
              package DB
  1048576  v  Verbose: use in conjunction with other flags
  2097152  C  Copy On Write
  4194304  A  Consistency checks on internal structures
  8388608  q  quiet - currently only suppresses the "EXECUTING"
              message
 16777216  M  trace smart match resolution
 33554432  B  dump suBroutine definitions, including special Blocks
              like BEGIN



All these flags require -DDEBUGGING when you compile the Perl
executable (but see :opd in (Devel-Peek) or (re)'debug' mode
which may change this).
See the INSTALL file in the Perl source distribution
for how to do this.  This flag is automatically set if you include -g
option when Configure asks you about optimizer/debugger flags.



If you’re just trying to get a print out of each line of Perl code
as it executes, the way that sh -x provides for shell scripts,
you can’t use Perl’s -D switch.  Instead do this



  # If you have "env" utility
  env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # Bourne shell syntax
  $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

  # csh syntax
  % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)



See (perldebug) for details and variations.





-e commandline





may be used to enter one line of program.  If -e is given, Perl
will not look for a filename in the argument list.  Multiple -e
commands may be given to build up a multi-line script.  Make sure
to use semicolons where you would in a normal program.





-E commandline





behaves just like -e, except that it implicitly enables all
optional features (in the main compilation unit). See (feature).





-f







Disable executing $Config{sitelib}/sitecustomize.pl at startup.



Perl can be built so that it by default will try to execute
$Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block).
This is a hook that allows the sysadmin to customize how Perl behaves.
It can for instance be used to add entries to the @INC array to make Perl
find modules in non-standard locations.



Perl actually inserts the following code:



    BEGIN {
        do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
            && do "$Config{sitelib}/sitecustomize.pl";
    }



Since it is an actual do (not a require), sitecustomize.pl
doesn’t need to return a true value. The code is run in package main,
in its own lexical scope. However, if the script dies, $@ will not
be set.



The value of $Config{sitelib} is also determined in C code and not
read from Config.pm, which is not loaded.



The code is executed very early. For example, any changes made to
@INC will show up in the output of ‘perl -V‘. Of course, END
blocks will be likewise executed very late.



To determine at runtime if this capability has been compiled in your
perl, you can check the value of $Config{usesitecustomize}.





-Fpattern





specifies the pattern to split on if -a is also in effect.  The
pattern may be surrounded by //, "", or '', otherwise it will be
put in single quotes. You can’t use literal whitespace in the pattern.





-h





prints a summary of the options.





-i[extension]






specifies that files processed by the <> construct are to be
edited in-place.  It does this by renaming the input file, opening the
output file by the original name, and selecting that output file as the
default for print() statements.  The extension, if supplied, is used to
modify the name of the old file to make a backup copy, following these
rules:



If no extension is supplied, no backup is made and the current file is
overwritten.



If the extension doesn’t contain a *, then it is appended to the
end of the current filename as a suffix.  If the extension does
contain one or more * characters, then each * is replaced
with the current filename.  In Perl terms, you could think of this
as:



    ($backup = $extension) =~ s/\*/$file_name/g;



This allows you to add a prefix to the backup file, instead of (or in
addition to) a suffix:



 $ perl -pi'orig_*' -e 's/bar/baz/' fileA  # backup to
                                           # 'orig_fileA'



Or even to place backup copies of the original files into another
directory (provided the directory already exists):



 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA  # backup to
                                               # 'old/fileA.orig'



These sets of one-liners are equivalent:



 $ perl -pi -e 's/bar/baz/' fileA          # overwrite current file
 $ perl -pi'*' -e 's/bar/baz/' fileA       # overwrite current file

 $ perl -pi'.orig' -e 's/bar/baz/' fileA   # backup to 'fileA.orig'
 $ perl -pi'*.orig' -e 's/bar/baz/' fileA  # backup to 'fileA.orig'



From the shell, saying



    $ perl -p -i.orig -e "s/foo/bar/; ... "



is the same as using the program:



    #!/usr/bin/perl -pi.orig
    s/foo/bar/;



which is equivalent to



    #!/usr/bin/perl
    $extension = '.orig';
    LINE: while (<>) {
        if ($ARGV ne $oldargv) {
            if ($extension !~ /\*/) {
                $backup = $ARGV . $extension;
            }
            else {
                ($backup = $extension) =~ s/\*/$ARGV/g;
            }
            rename($ARGV, $backup);
            open(ARGVOUT, ">$ARGV");
            select(ARGVOUT);
            $oldargv = $ARGV;
        }
        s/foo/bar/;
    }
    continue {
        print;  # this prints to original filename
    }
    select(STDOUT);



except that the -i form doesn’t need to compare $ARGV to $oldargv to
know when the filename has changed.  It does, however, use ARGVOUT for
the selected filehandle.  Note that STDOUT is restored as the default
output filehandle after the loop.



As shown above, Perl creates the backup file whether or not any output
is actually changed.  So this is just a fancy way to copy files:



    $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
or
    $ perl -p -i'.orig' -e 1 file1 file2 file3...



You can use eof without parentheses to locate the end of each input
file, in case you want to append to each file, or reset line numbering
(see example in (perlfunc)eof).



If, for a given file, Perl is unable to create the backup file as
specified in the extension then it will skip that file and continue on
with the next one (if it exists).



For a discussion of issues surrounding file permissions and -i,
see (perlfaq5)Why does Perl let me delete read-only files?  Why does -i clobber protected files?  Isn't this a bug in Perl?.



You cannot use -i to create directories or to strip extensions from
files.



Perl does not expand ~ in filenames, which is good, since some
folks use it for their backup files:



    $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...



Note that because -i renames or deletes the original file before
creating a new file of the same name, Unix-style soft and hard links will
not be preserved.



Finally, the -i switch does not impede execution when no
files are given on the command line.  In this case, no backup is made
(the original file cannot, of course, be determined) and processing
proceeds from STDIN to STDOUT as might be expected.





-Idirectory






Directories specified by -I are prepended to the search path for
modules (@INC).





-l[octnum]







enables automatic line-ending processing.  It has two separate
effects.  First, it automatically chomps $/ (the input record
separator) when used with -n or -p.  Second, it assigns $\
(the output record separator) to have the value of octnum so
that any print statements will have that separator added back on.
If octnum is omitted, sets $\ to the current value of
$/.  For instance, to trim lines to 80 columns:



    perl -lpe 'substr($_, 80) = ""'



Note that the assignment $\ = $/ is done when the switch is processed,
so the input record separator can be different than the output record
separator if the -l switch is followed by a -0 switch:



    gnufind / -print0 | perl -ln0e 'print "found $_" if -p'



This sets $\ to newline and then sets $/ to the null character.





-m[-]module








-M[-]module






-M[-]’module ...’






-[mM][-]module=arg[,arg]...




-mmodule executes use module (); before executing your
program.



-Mmodule executes use module ; before executing your
program.  You can use quotes to add extra code after the module name,
e.g., '-MMODULE qw(foo bar)'.



If the first character after the -M or -m is a dash (-)
then the ’use’ is replaced with ’no’.



A little builtin syntactic sugar means you can also say
-mMODULE=foo,bar or -MMODULE=foo,bar as a shortcut for
’-MMODULE qw(foo bar)’.  This avoids the need to use quotes when
importing symbols.  The actual code generated by -MMODULE=foo,bar is
use module split(/,/,q{foo,bar}).  Note that the = form
removes the distinction between -m and -M.



A consequence of this is that -MMODULE=number never does a version check,
unless MODULE::import() itself is set up to do a version check, which
could happen for example if MODULE inherits from (Exporter).





-n





causes Perl to assume the following loop around your program, which
makes it iterate over filename arguments somewhat like sed -n or
awk:



  LINE:
    while (<>) {
        ...             # your program goes here
    }



Note that the lines are not printed by default.  See -p to have
lines printed.  If a file named by an argument cannot be opened for
some reason, Perl warns you about it and moves on to the next file.



Also note that <> passes command line arguments to
(perlfunc)open, which doesn’t necessarily interpret them as file names.
See  (perlop) for possible security implications.



Here is an efficient way to delete all files that haven’t been modified for
at least a week:



    find . -mtime +7 -print | perl -nle unlink



This is faster than using the -exec switch of find because you don’t
have to start a process on every filename found.  It does suffer from
the bug of mishandling newlines in pathnames, which you can fix if
you follow the example under -0.



BEGIN and END blocks may be used to capture control before or after
the implicit program loop, just as in awk.





-p





causes Perl to assume the following loop around your program, which
makes it iterate over filename arguments somewhat like sed:



  LINE:
    while (<>) {
        ...             # your program goes here
    } continue {
        print or die "-p destination: $!\n";
    }



If a file named by an argument cannot be opened for some reason, Perl
warns you about it, and moves on to the next file.  Note that the
lines are printed automatically.  An error occurring during printing is
treated as fatal.  To suppress printing use the -n switch.  A -p
overrides a -n switch.



BEGIN and END blocks may be used to capture control before or after
the implicit loop, just as in awk.





-s





enables rudimentary switch parsing for switches on the command
line after the program name but before any filename arguments (or before
an argument of –).  Any switch found there is removed from @ARGV and sets the
corresponding variable in the Perl program.  The following program
prints "1" if the program is invoked with a -xyz switch, and "abc"
if it is invoked with -xyz=abc.



    #!/usr/bin/perl -s
    if ($xyz) { print "$xyz\n" }



Do note that a switch like –help creates the variable ${-help}, which is not compliant
with use strict "refs".  Also, when using this option on a script with
warnings enabled you may get a lot of spurious "used only once" warnings.





-S





makes Perl use the PATH environment variable to search for the
program unless the name of the program contains path separators.



On some platforms, this also makes Perl append suffixes to the
filename while searching for it.  For example, on Win32 platforms,
the ".bat" and ".cmd" suffixes are appended if a lookup for the
original name fails, and if the name does not already end in one
of those suffixes.  If your Perl was compiled with DEBUGGING turned
on, using the -Dp switch to Perl shows how the search progresses.



Typically this is used to emulate #! startup on platforms that don’t
support #!.  It’s also convenient when debugging a script that uses #!,
and is thus normally found by the shell’s $PATH search mechanism.



This example works on many platforms that have a shell compatible with
Bourne shell:



    #!/usr/bin/perl
    eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
            if $running_under_some_shell;



The system ignores the first line and feeds the program to /bin/sh,
which proceeds to try to execute the Perl program as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the Perl interpreter.  On some systems $0 doesn’t always
contain the full pathname, so the -S tells Perl to search for the
program if necessary.  After Perl locates the program, it parses the
lines and ignores them because the variable $running_under_some_shell
is never true.  If the program will be interpreted by csh, you will need
to replace ${1+"$@"} with $*, even though that doesn’t understand
embedded spaces (and such) in the argument list.  To start up sh rather
than csh, some systems may have to replace the #! line with a line
containing just a colon, which will be politely ignored by Perl.  Other
systems can’t control that, and need a totally devious construct that
will work under any of csh, sh, or Perl, such as the following:



        eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
        & eval 'exec /usr/bin/perl -wS $0 $argv:q'
                if $running_under_some_shell;



If the filename supplied contains directory separators (and so is an
absolute or relative pathname), and if that file is not found,
platforms that append file extensions will do so and try to look
for the file with those extensions added, one by one.



On DOS-like platforms, if the program does not contain directory
separators, it will first be searched for in the current directory
before being searched for on the PATH.  On Unix platforms, the
program will be searched for strictly on the PATH.





-t





Like -T, but taint checks will issue warnings rather than fatal
errors.  These warnings can now be controlled normally with no warnings
qw(taint).



Note: This is not a substitute for -T! This is meant to be
used only as a temporary development aid while securing legacy code:
for real production code and for new secure code written from scratch,
always use the real -T.





-T





turns on "taint" so you can test them.  Ordinarily
these checks are done only when running setuid or setgid.  It’s a
good idea to turn them on explicitly for programs that run on behalf
of someone else whom you might not necessarily trust, such as CGI
programs or any internet servers you might write in Perl.  See
perlsec NAME for details.  For security reasons, this option must be
seen by Perl quite early; usually this means it must appear early
on the command line or in the #! line for systems which support
that construct.





-u





This switch causes Perl to dump core after compiling your
program.  You can then in theory take this core dump and turn it
into an executable file by using the undump program (not supplied).
This speeds startup at the expense of some disk space (which you
can minimize by stripping the executable).  (Still, a "hello world"
executable comes out to about 200K on my machine.)  If you want to
execute a portion of your program before dumping, use the dump()
operator instead.  Note: availability of undump is platform
specific and may not be available for a specific port of Perl.





-U





allows Perl to do unsafe operations.  Currently the only "unsafe"
operations are attempting to unlink directories while running as superuser
and running setuid programs with fatal taint checks turned into warnings.
Note that warnings must be enabled along with this option to actually
generate the taint-check warnings.





-v





prints the version and patchlevel of your perl executable.





-V





prints summary of the major perl configuration values and the current
values of @INC.





-V:configvar




Prints to STDOUT the value of the named configuration variable(s),
with multiples when your configvar argument looks like a regex (has
non-letters).  For example:



    $ perl -V:libc
        libc='/lib/libc-2.2.4.so';
    $ perl -V:lib.
        libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
        libc='/lib/libc-2.2.4.so';
    $ perl -V:lib.*
        libpth='/usr/local/lib /lib /usr/lib';
        libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
        lib_ext='.a';
        libc='/lib/libc-2.2.4.so';
        libperl='libperl.a';
        ....



Additionally, extra colons can be used to control formatting.  A
trailing colon suppresses the linefeed and terminator ";", allowing
you to embed queries into shell commands.  (mnemonic: PATH separator
":".)



    $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
    compression-vars:  zcat='' zip='zip'  are here !



A leading colon removes the "name=" part of the response, this allows
you to map to the name you need.  (mnemonic: empty label)



    $ echo "goodvfork="`./perl -Ilib -V::usevfork`
    goodvfork=false;



Leading and trailing colons can be used together if you need
positional parameter values without the names.  Note that in the case
below, the PERL_API params are returned in alphabetical order.



    $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
    building_on 'linux' '5' '1' '9' now





-w





prints warnings about dubious constructs, such as variable names
mentioned only once and scalar variables used
before being set; redefined subroutines; references to undefined
filehandles; filehandles opened read-only that you are attempting
to write on; values used as a number that don’t look like numbers;
using an array as though it were a scalar; if your subroutines
recurse more than 100 deep; and innumerable other things.



This switch really just enables the global $^W variable; normally,
the lexically scoped use warnings pragma is preferred. You
can disable or promote into fatal errors specific warnings using
__WARN__ hooks, as described in perlvar NAME and (perlfunc)warn.
See also perldiag NAME and perltrap NAME.  A fine-grained warning
facility is also available if you want to manipulate entire classes
of warnings; see (warnings) or (perllexwarn).





-W





Enables all warnings regardless of no warnings or $^W.
See (perllexwarn).





-X





Disables all warnings regardless of use warnings or $^W.
See (perllexwarn).





-x







-xdirectory




tells Perl that the program is embedded in a larger chunk of unrelated
text, such as in a mail message.  Leading garbage will be
discarded until the first line that starts with #! and contains the
string "perl".  Any meaningful switches on that line will be applied.



All references to line numbers by the program (warnings, errors, ...)
will treat the #! line as the first line.
Thus a warning on the 2nd line of the program, which is on the 100th
line in the file will be reported as line 2, not as line 100.
This can be overridden by using the #line directive.
(See (perlsyn)Plain Old Comments (Not!))



If a directory name is specified, Perl will switch to that directory
before running the program.  The -x switch controls only the
disposal of leading garbage.  The program must be terminated with
__END__ if there is trailing garbage to be ignored;  the program
can process any or all of the trailing garbage via the DATA filehandle
if desired.



The directory, if specified, must appear immediately following the -x
with no intervening whitespace.














Previous: perlrun DESCRIPTION, Up: perlrun   [Contents]





68.4 ENVIRONMENT






HOME





Used if chdir has no argument.





LOGDIR





Used if chdir has no argument and HOME is not set.





PATH





Used in executing subprocesses, and in finding the program if -S is
used.





PERL5LIB





A list of directories in which to look for Perl library
files before looking in the standard library and the current
directory.  Any architecture-specific directories under the specified
locations are automatically included if they exist, with this lookup
done at interpreter startup time.



If PERL5LIB is not defined, PERLLIB is used.  Directories are separated
(like in PATH) by a colon on Unixish platforms and by a semicolon on
Windows (the proper path separator being given by the command perl
-V:path_sep).



When running taint checks, either because the program was running setuid or
setgid, or the -T or -t switch was specified, neither PERL5LIB nor
PERLLIB is consulted. The program should instead say:



    use lib "/my/directory";





PERL5OPT





Command-line options (switches).  Switches in this variable are treated
as if they were on every Perl command line.  Only the -[CDIMUdmtwW]
switches are allowed.  When running taint checks (either because the
program was running setuid or setgid, or because the -T or -t
switch was used), this variable is ignored.  If PERL5OPT begins with
- T, tainting will be enabled and subsequent options ignored.  If
PERL5OPT begins with -t, tainting will be enabled, a writable dot
removed from @INC, and subsequent options honored.





PERLIO





A space (or colon) separated list of PerlIO layers. If perl is built
to use PerlIO system for IO (the default) these layers affect Perl’s IO.



It is conventional to start layer names with a colon (for example, :perlio) to
emphasize their similarity to variable "attributes". But the code that parses
layer specification strings,  which is also used to decode the PERLIO
environment variable, treats the colon as a separator.



An unset or empty PERLIO is equivalent to the default set of layers for
your platform; for example, :unix:perlio on Unix-like systems
and :unix:crlf on Windows and other DOS-like systems.



The list becomes the default for all Perl’s IO. Consequently only built-in
layers can appear in this list, as external layers (such as :encoding()) need
IO in  order to load them!. See (open)"open pragma" for how to add external
encodings as defaults.



Layers it makes sense to include in the PERLIO environment
variable are briefly summarized below. For more details see (PerlIO).





:bytes





A pseudolayer that turns the :utf8 flag off for the layer below;
unlikely to be useful on its own in the global PERLIO environment variable.
You perhaps were thinking of :crlf:bytes or :perlio:bytes.





:crlf





A layer which does CRLF to "\n" translation distinguishing "text" and
"binary" files in the manner of MS-DOS and similar operating systems.
(It currently does not mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.)





:mmap





A layer that implements "reading" of files by using mmap(2) to
make an entire file appear in the process’s address space, and then
using that as PerlIO’s "buffer".





:perlio





This is a re-implementation of stdio-like buffering written as a
PerlIO layer.  As such it will call whatever layer is below it for
its operations, typically :unix.





:pop





An experimental pseudolayer that removes the topmost layer.
Use with the same care as is reserved for nitroglycerine.





:raw





A pseudolayer that manipulates other layers.  Applying the :raw
layer is equivalent to calling binmode($fh).  It makes the stream
pass each byte as-is without translation.  In particular, both CRLF
translation and intuiting :utf8 from the locale are disabled.



Unlike in earlier versions of Perl, :raw is not
just the inverse of :crlf: other layers which would affect the
binary nature of the stream are also removed or disabled.





:stdio





This layer provides a PerlIO interface by wrapping system’s ANSI C "stdio"
library calls. The layer provides both buffering and IO.
Note that the :stdio layer does not do CRLF translation even if that
is the platform’s normal behaviour. You will need a :crlf layer above it
to do that.





:unix





Low-level layer that calls read, write, lseek, etc.





:utf8





A pseudolayer that enables a flag in the layer below to tell Perl
that output should be in utf8 and that input should be regarded as
already in valid utf8 form. WARNING: It does not check for validity and as such
should be handled with extreme caution for input, because security violations
can occur with non-shortest UTF-8 encodings, etc. Generally :encoding(utf8) is
the best option when reading UTF-8 encoded data.





:win32





On Win32 platforms this experimental layer uses native "handle" IO
rather than a Unix-like numeric file descriptor layer. Known to be
buggy in this release (5.14).








The default set of layers should give acceptable results on all platforms



For Unix platforms that will be the equivalent of "unix perlio" or "stdio".
Configure is set up to prefer the "stdio" implementation if the system’s library
provides for fast access to the buffer; otherwise, it uses the "unix perlio"
implementation.



On Win32 the default in this release (5.14) is "unix crlf". Win32’s "stdio"
has a number of bugs/mis-features for Perl IO which are somewhat depending
on the version and vendor of the C compiler. Using our own crlf layer as
the buffer avoids those issues and makes things more uniform.  The crlf
layer provides CRLF conversion as well as buffering.



This release (5.14) uses unix as the bottom layer on Win32, and so still
uses the C compiler’s numeric file descriptor routines. There is an
experimental native win32 layer, which is expected to be enhanced and
should eventually become the default under Win32.



The PERLIO environment variable is completely ignored when Perl
is run in taint mode.





PERLIO_DEBUG





If set to the name of a file or device, certain operations of PerlIO
subsystem will be logged to that file, which is opened in append mode
Typical uses are in Unix:



   % env PERLIO_DEBUG=/dev/tty perl script ...



and under Win32, the approximately equivalent:



   > set PERLIO_DEBUG=CON
   perl script ...



This functionality is disabled for setuid scripts and for scripts run
with -T.





PERLLIB





A list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
If PERL5LIB is defined, PERLLIB is not used.



The PERLLIB environment variable is completely ignored when Perl
is run in taint mode.





PERL5DB





The command used to load the debugger code.  The default is:



        BEGIN { require "perl5db.pl" }



The PERL5DB environment variable is only used when Perl is started with
a bare -d switch.





PERL5DB_THREADED





If set to a true value, indicates to the debugger that the code being
debugged uses threads.





PERL5SHELL (specific to the Win32 port)





On Win32 ports only, may be set to an alternative shell that Perl must use
internally for executing "backtick" commands or system().  Default is
cmd.exe /x/d/c on WindowsNT and command.com /c on Windows95.  The
value is considered space-separated.  Precede any character that
needs to be protected, like a space or backslash, with another backslash.



Note that Perl doesn’t use COMSPEC for this purpose because
COMSPEC has a high degree of variability among users, leading to
portability concerns.  Besides, Perl can use a shell that may not be
fit for interactive use, and setting COMSPEC to such a shell may
interfere with the proper functioning of other programs (which usually
look in COMSPEC to find a shell fit for interactive use).



Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
when running external commands.  It is recommended that
you explicitly set (or delete) $ENV{PERL5SHELL} when running
in taint mode under Windows.





PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)





Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
Perl normally searches for an IFS-compatible LSP because this is required
for its emulation of Windows sockets as real filehandles.  However, this may
cause problems if you have a firewall such as McAfee Guardian, which requires
that all applications use its LSP but which is not IFS-compatible, because clearly
Perl will normally avoid using such an LSP.



Setting this environment variable to 1 means that Perl will simply use the
first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
happy–and in that particular case Perl still works too because McAfee
Guardian’s LSP actually plays other games which allow applications
requiring IFS compatibility to work.





PERL_DEBUG_MSTATS





Relevant only if Perl is compiled with the malloc included with the Perl
distribution; that is, if perl -V:d_mymalloc is "define".



If set, this dumps out memory statistics after execution.  If set
to an integer greater than one, also dumps out memory statistics
after compilation.





PERL_DESTRUCT_LEVEL





Relevant only if your Perl executable was built with -DDEBUGGING,
this controls the behaviour of global destruction of objects and other
references.  See perlhacktips PERL_DESTRUCT_LEVEL for more information.





PERL_DL_NONLAZY





Set to "1" to have Perl resolve all undefined symbols when it loads
a dynamic library.  The default behaviour is to resolve symbols when
they are used.  Setting this variable is useful during testing of
extensions, as it ensures that you get an error on misspelled function
names even if the test suite doesn’t call them.





PERL_ENCODING





If using the use encoding pragma without an explicit encoding name, the
PERL_ENCODING environment variable is consulted for an encoding name.





PERL_HASH_SEED





(Since Perl 5.8.1.)  Used to randomize Perl’s internal hash function.
To emulate the pre-5.8.1 behaviour, set to an integer; "0" means
exactly the same order as in 5.8.0.  "Pre-5.8.1" means, among other
things, that hash keys will always have the same ordering between
different runs of Perl.



Most hashes by default return elements in the same order as in Perl 5.8.0.
On a hash by hash basis, if pathological data is detected during a hash
key insertion, then that hash will switch to an alternative random hash
seed.



The default behaviour is to randomize unless the PERL_HASH_SEED is set.
If Perl has been compiled with -DUSE_HASH_SEED_EXPLICIT, the default
behaviour is not to randomize unless the PERL_HASH_SEED is set.



If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
the pseudorandom seed supplied by the operating system and libraries.



PLEASE NOTE: The hash seed is sensitive information. Hashes are
randomized to protect against local and remote attacks against Perl
code. By manually setting a seed, this protection may be partially or
completely lost.



See perlsec Algorithmic Complexity Attacks and
PERL_HASH_SEED_DEBUG for more information.





PERL_HASH_SEED_DEBUG





(Since Perl 5.8.1.)  Set to "1" to display (to STDERR) the value of
the hash seed at the beginning of execution.  This, combined with
PERL_HASH_SEED is intended to aid in debugging nondeterministic
behaviour caused by hash randomization.



Note that the hash seed is sensitive information: by knowing it, one
can craft a denial-of-service attack against Perl code, even remotely;
see perlsec Algorithmic Complexity Attacks for more information.
Do not disclose the hash seed to people who don’t need to know it.
See also hash_seed() in (Hash-Util).





PERL_MEM_LOG





If your Perl was configured with -Accflags=-DPERL_MEM_LOG, setting
the environment variable PERL_MEM_LOG enables logging debug
messages. The value has the form <number>[m][s][t], where
number is the file descriptor number you want to write to (2 is
default), and the combination of letters specifies that you want
information about (m)emory and/or (s)v, optionally with
(t)imestamps. For example, PERL_MEM_LOG=1mst logs all
information to stdout. You can write to other opened file descriptors
in a variety of ways:



  $ 3>foo3 PERL_MEM_LOG=3m perl ...





PERL_ROOT (specific to the VMS port)





A translation-concealed rooted logical name that contains Perl and the
logical device for the @INC path on VMS only.  Other logical names that
affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and
SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in
perlvms NAME and in README.vms in the Perl source distribution.





PERL_SIGNALS





Available in Perls 5.8.1 and later.  If set to "unsafe", the pre-Perl-5.8.0
signal behaviour (which is immediate but unsafe) is restored.  If set
to safe, then safe (but deferred) signals are used.  See
perlipc Deferred Signals (Safe Signals).





PERL_UNICODE





Equivalent to the -C command-line switch.  Note that this is not
a boolean variable. Setting this to "1" is not the right way to
"enable Unicode" (whatever that would mean).  You can use "0" to
"disable Unicode", though (or alternatively unset PERL_UNICODE in
your shell before starting Perl).  See the description of the -C
switch for more information.





SYS$LOGIN (specific to the VMS port)





Used if chdir has no argument and HOME and LOGDIR are not set.








Perl also has environment variables that control how Perl handles data
specific to particular natural languages; see perllocale NAME.



Perl and its various modules and components, including its test frameworks,
may sometimes make use of certain other environment variables.  Some of
these are specific to a particular platform.  Please consult the
appropriate module documentation and any documentation for your platform
(like (perlsolaris), (perllinux), (perlmacosx), (perlwin32), etc) for
variables peculiar to those specific situations.



Perl makes all environment variables available to the program being
executed, and passes these along to any child processes it starts.
However, programs running setuid would do well to execute the following
lines before doing anything else, just to keep people honest:



    $ENV{PATH}  = "/bin:/usr/bin";    # or whatever you need
    $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
    delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};









Next: , Previous: perlrun, Up: Top   [Contents]





69 perlsec





• perlsec NAME:  



• perlsec DESCRIPTION:  



• perlsec SECURITY VULNERABILITY CONTACT INFORMATION:  



• perlsec SECURITY MECHANISMS AND CONCERNS:  



• perlsec SEE ALSO:  












Next: , Up: perlsec   [Contents]





69.1 NAME



perlsec - Perl security









Next: , Previous: perlsec NAME, Up: perlsec   [Contents]





69.2 DESCRIPTION



Perl is designed to make it easy to program securely even when running
with extra privileges, like setuid or setgid programs.  Unlike most
command line shells, which are based on multiple substitution passes on
each line of the script, Perl uses a more conventional evaluation scheme
with fewer hidden snags.  Additionally, because the language has more
builtin functionality, it can rely less upon external (and possibly
untrustworthy) programs to accomplish its purposes.









Next: , Previous: perlsec DESCRIPTION, Up: perlsec   [Contents]





69.3 SECURITY VULNERABILITY CONTACT INFORMATION



If you believe you have found a security vulnerability in Perl, please email
perl5-security-report@perl.org with details.  This points to a closed
subscription, unarchived mailing list.  Please only use this address for
security issues in the Perl core, not for modules independently distributed on
CPAN.









Next: , Previous: perlsec SECURITY VULNERABILITY CONTACT INFORMATION, Up: perlsec   [Contents]





69.4 SECURITY MECHANISMS AND CONCERNS





• perlsec Taint mode:  



• perlsec Laundering and Detecting Tainted Data:  



• perlsec Switches On the "#!" Line:  



• perlsec Taint mode and @INC:  



• perlsec Cleaning Up Your Path:  



• perlsec Security Bugs:  



• perlsec Protecting Your Programs:  



• perlsec Unicode:  



• perlsec Algorithmic Complexity Attacks:  












Next: , Up: perlsec SECURITY MECHANISMS AND CONCERNS   [Contents]





69.4.1 Taint mode



Perl automatically enables a set of special security checks, called taint
mode, when it detects its program running with differing real and effective
user or group IDs.  The setuid bit in Unix permissions is mode 04000, the
setgid bit mode 02000; either or both may be set.  You can also enable taint
mode explicitly by using the -T command line flag. This flag is
strongly suggested for server programs and any program run on behalf of
someone else, such as a CGI script. Once taint mode is on, it’s on for
the remainder of your script.



While in this mode, Perl takes special precautions called taint
checks to prevent both obvious and subtle traps.  Some of these checks
are reasonably simple, such as verifying that path directories aren’t
writable by others; careful programmers have always used checks like
these.  Other checks, however, are best supported by the language itself,
and it is these checks especially that contribute to making a set-id Perl
program more secure than the corresponding C program.



You may not use data derived from outside your program to affect
something else outside your program–at least, not by accident.  All
command line arguments, environment variables, locale information (see
perllocale NAME), results of certain system calls (readdir(),
readlink(), the variable of shmread(), the messages returned by
msgrcv(), the password, gcos and shell fields returned by the
getpwxxx() calls), and all file input are marked as "tainted".
Tainted data may not be used directly or indirectly in any command
that invokes a sub-shell, nor in any command that modifies files,
directories, or processes, with the following exceptions:





For efficiency reasons, Perl takes a conservative view of
whether data is tainted.  If an expression contains tainted data,
any subexpression may be considered tainted, even if the value
of the subexpression is not itself affected by the tainted data.



Because taintedness is associated with each scalar value, some
elements of an array or hash can be tainted and others not.
The keys of a hash are never tainted.



For example:



    $arg = shift;               # $arg is tainted
    $hid = $arg, 'bar';         # $hid is also tainted
    $line = <>;                 # Tainted
    $line = <STDIN>;            # Also tainted
    open FOO, "/home/me/bar" or die $!;
    $line = <FOO>;              # Still tainted
    $path = $ENV{'PATH'};       # Tainted, but see below
    $data = 'abc';              # Not tainted

    system "echo $arg";         # Insecure
    system "/bin/echo", $arg;   # Considered insecure
                                # (Perl doesn't know about /bin/echo)
    system "echo $hid";         # Insecure
    system "echo $data";        # Insecure until PATH set

    $path = $ENV{'PATH'};       # $path now tainted

    $ENV{'PATH'} = '/bin:/usr/bin';
    delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

    $path = $ENV{'PATH'};       # $path now NOT tainted
    system "echo $data";        # Is secure now!

    open(FOO, "< $arg");        # OK - read-only file
    open(FOO, "> $arg");        # Not OK - trying to write

    open(FOO,"echo $arg|");     # Not OK
    open(FOO,"-|")
        or exec 'echo', $arg;   # Also not OK

    $shout = `echo $arg`;       # Insecure, $shout now tainted

    unlink $data, $arg;         # Insecure
    umask $arg;                 # Insecure

    exec "echo $arg";           # Insecure
    exec "echo", $arg;          # Insecure
    exec "sh", '-c', $arg;      # Very insecure!

    @files = <*.c>;             # insecure (uses readdir() or similar)
    @files = glob('*.c');       # insecure (uses readdir() or similar)

    # In Perl releases older than 5.6.0 the <*.c> and glob('*.c') would
    # have used an external program to do the filename expansion; but in
    # either case the result is tainted since the list of filenames comes
    # from outside of the program.

    $bad = ($arg, 23);          # $bad will be tainted
    $arg, `true`;               # Insecure (although it isn't really)



If you try to do something insecure, you will get a fatal error saying
something like "Insecure dependency" or "Insecure $ENV{PATH}".



The exception to the principle of "one tainted value taints the whole
expression" is with the ternary conditional operator ?:.  Since code
with a ternary conditional



    $result = $tainted_value ? "Untainted" : "Also untainted";



is effectively



    if ( $tainted_value ) {
        $result = "Untainted";
    } else {
        $result = "Also untainted";
    }



it doesn’t make sense for $result to be tainted.









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69.4.2 Laundering and Detecting Tainted Data



To test whether a variable contains tainted data, and whose use would
thus trigger an "Insecure dependency" message, you can use the
tainted() function of the Scalar::Util module, available in your
nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
Or you may be able to use the following is_tainted() function.



    sub is_tainted {
        local $@;   # Don't pollute caller's value.
        return ! eval { eval("#" . substr(join("", @_), 0, 0)); 1 };
    }



This function makes use of the fact that the presence of tainted data
anywhere within an expression renders the entire expression tainted.  It
would be inefficient for every operator to test every argument for
taintedness.  Instead, the slightly more efficient and conservative
approach is used that if any tainted value has been accessed within the
same expression, the whole expression is considered tainted.



But testing for taintedness gets you only so far.  Sometimes you have just
to clear your data’s taintedness.  Values may be untainted by using them
as keys in a hash; otherwise the only way to bypass the tainting
mechanism is by referencing subpatterns from a regular expression match.
Perl presumes that if you reference a substring using $1, $2, etc., that
you knew what you were doing when you wrote the pattern.  That means using
a bit of thought–don’t just blindly untaint anything, or you defeat the
entire mechanism.  It’s better to verify that the variable has only good
characters (for certain values of "good") rather than checking whether it
has any bad characters.  That’s because it’s far too easy to miss bad
characters that you never thought of.



Here’s a test to make sure that the data contains nothing but "word"
characters (alphabetics, numerics, and underscores), a hyphen, an at sign,
or a dot.



    if ($data =~ /^([-\@\w.]+)$/) {
        $data = $1;                     # $data now untainted
    } else {
        die "Bad data in '$data'";      # log this somewhere
    }



This is fairly secure because /\w+/ doesn’t normally match shell
metacharacters, nor are dot, dash, or at going to mean something special
to the shell.  Use of /.+/ would have been insecure in theory because
it lets everything through, but Perl doesn’t check for that.  The lesson
is that when untainting, you must be exceedingly careful with your patterns.
Laundering data using regular expression is the only mechanism for
untainting dirty data, unless you use the strategy detailed below to fork
a child of lesser privilege.



The example does not untaint $data if use locale is in effect,
because the characters matched by \w are determined by the locale.
Perl considers that locale definitions are untrustworthy because they
contain data from outside the program.  If you are writing a
locale-aware program, and want to launder data with a regular expression
containing \w, put no locale ahead of the expression in the same
block.  See perllocale SECURITY for further discussion and examples.









Next: , Previous: perlsec Laundering and Detecting Tainted Data, Up: perlsec SECURITY MECHANISMS AND CONCERNS   [Contents]





69.4.3 Switches On the "#!" Line



When you make a script executable, in order to make it usable as a
command, the system will pass switches to perl from the script’s #!
line.  Perl checks that any command line switches given to a setuid
(or setgid) script actually match the ones set on the #! line.  Some
Unix and Unix-like environments impose a one-switch limit on the #!
line, so you may need to use something like -wU instead of -w -U
under such systems.  (This issue should arise only in Unix or
Unix-like environments that support #! and setuid or setgid scripts.)









Next: , Previous: perlsec Switches On the "#!" Line, Up: perlsec SECURITY MECHANISMS AND CONCERNS   [Contents]





69.4.4 Taint mode and @INC



When the taint mode (-T) is in effect, the "." directory is removed
from @INC, and the environment variables PERL5LIB and PERLLIB
are ignored by Perl. You can still adjust @INC from outside the
program by using the -I command line option as explained in
perlrun NAME. The two environment variables are ignored because
they are obscured, and a user running a program could be unaware that
they are set, whereas the -I option is clearly visible and
therefore permitted.



Another way to modify @INC without modifying the program, is to use
the lib pragma, e.g.:



  perl -Mlib=/foo program



The benefit of using -Mlib=/foo over -I/foo, is that the former
will automagically remove any duplicated directories, while the later
will not.



Note that if a tainted string is added to @INC, the following
problem will be reported:



  Insecure dependency in require while running with -T switch









Next: , Previous: perlsec Taint mode and @INC, Up: perlsec SECURITY MECHANISMS AND CONCERNS   [Contents]





69.4.5 Cleaning Up Your Path



For "Insecure $ENV{PATH}" messages, you need to set $ENV{'PATH'} to
a known value, and each directory in the path must be absolute and
non-writable by others than its owner and group.  You may be surprised to
get this message even if the pathname to your executable is fully
qualified.  This is not generated because you didn’t supply a full path
to the program; instead, it’s generated because you never set your PATH
environment variable, or you didn’t set it to something that was safe.
Because Perl can’t guarantee that the executable in question isn’t itself
going to turn around and execute some other program that is dependent on
your PATH, it makes sure you set the PATH.



The PATH isn’t the only environment variable which can cause problems.
Because some shells may use the variables IFS, CDPATH, ENV, and
BASH_ENV, Perl checks that those are either empty or untainted when
starting subprocesses. You may wish to add something like this to your
setid and taint-checking scripts.



    delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};   # Make %ENV safer



It’s also possible to get into trouble with other operations that don’t
care whether they use tainted values.  Make judicious use of the file
tests in dealing with any user-supplied filenames.  When possible, do
opens and such after properly dropping any special user (or group!)
privileges. Perl doesn’t prevent you from opening tainted filenames for reading,
so be careful what you print out.  The tainting mechanism is intended to
prevent stupid mistakes, not to remove the need for thought.



Perl does not call the shell to expand wild cards when you pass system
and exec explicit parameter lists instead of strings with possible shell
wildcards in them.  Unfortunately, the open, glob, and
backtick functions provide no such alternate calling convention, so more
subterfuge will be required.



Perl provides a reasonably safe way to open a file or pipe from a setuid
or setgid program: just create a child process with reduced privilege who
does the dirty work for you.  First, fork a child using the special
open syntax that connects the parent and child by a pipe.  Now the
child resets its ID set and any other per-process attributes, like
environment variables, umasks, current working directories, back to the
originals or known safe values.  Then the child process, which no longer
has any special permissions, does the open or other system call.
Finally, the child passes the data it managed to access back to the
parent.  Because the file or pipe was opened in the child while running
under less privilege than the parent, it’s not apt to be tricked into
doing something it shouldn’t.



Here’s a way to do backticks reasonably safely.  Notice how the exec is
not called with a string that the shell could expand.  This is by far the
best way to call something that might be subjected to shell escapes: just
never call the shell at all.  



        use English '-no_match_vars';
        die "Can't fork: $!" unless defined($pid = open(KID, "-|"));
        if ($pid) {           # parent
            while (<KID>) {
                # do something
            }
            close KID;
        } else {
            my @temp     = ($EUID, $EGID);
            my $orig_uid = $UID;
            my $orig_gid = $GID;
            $EUID = $UID;
            $EGID = $GID;
            # Drop privileges
            $UID  = $orig_uid;
            $GID  = $orig_gid;
            # Make sure privs are really gone
            ($EUID, $EGID) = @temp;
            die "Can't drop privileges"
                unless $UID == $EUID  && $GID eq $EGID;
            $ENV{PATH} = "/bin:/usr/bin"; # Minimal PATH.
            # Consider sanitizing the environment even more.
            exec 'myprog', 'arg1', 'arg2'
                or die "can't exec myprog: $!";
        }



A similar strategy would work for wildcard expansion via glob, although
you can use readdir instead.



Taint checking is most useful when although you trust yourself not to have
written a program to give away the farm, you don’t necessarily trust those
who end up using it not to try to trick it into doing something bad.  This
is the kind of security checking that’s useful for set-id programs and
programs launched on someone else’s behalf, like CGI programs.



This is quite different, however, from not even trusting the writer of the
code not to try to do something evil.  That’s the kind of trust needed
when someone hands you a program you’ve never seen before and says, "Here,
run this."  For that kind of safety, you might want to check out the Safe
module, included standard in the Perl distribution.  This module allows the
programmer to set up special compartments in which all system operations
are trapped and namespace access is carefully controlled.  Safe should
not be considered bullet-proof, though: it will not prevent the foreign
code to set up infinite loops, allocate gigabytes of memory, or even
abusing perl bugs to make the host interpreter crash or behave in
unpredictable ways. In any case it’s better avoided completely if you’re
really concerned about security.









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69.4.6 Security Bugs



Beyond the obvious problems that stem from giving special privileges to
systems as flexible as scripts, on many versions of Unix, set-id scripts
are inherently insecure right from the start.  The problem is a race
condition in the kernel.  Between the time the kernel opens the file to
see which interpreter to run and when the (now-set-id) interpreter turns
around and reopens the file to interpret it, the file in question may have
changed, especially if you have symbolic links on your system.



Fortunately, sometimes this kernel "feature" can be disabled.
Unfortunately, there are two ways to disable it.  The system can simply
outlaw scripts with any set-id bit set, which doesn’t help much.
Alternately, it can simply ignore the set-id bits on scripts.



However, if the kernel set-id script feature isn’t disabled, Perl will
complain loudly that your set-id script is insecure.  You’ll need to
either disable the kernel set-id script feature, or put a C wrapper around
the script.  A C wrapper is just a compiled program that does nothing
except call your Perl program.   Compiled programs are not subject to the
kernel bug that plagues set-id scripts.  Here’s a simple wrapper, written
in C:



    #define REAL_PATH "/path/to/script"
    main(ac, av)
        char **av;
    {
        execv(REAL_PATH, av);
    }



Compile this wrapper into a binary executable and then make it rather
than your script setuid or setgid.



In recent years, vendors have begun to supply systems free of this
inherent security bug.  On such systems, when the kernel passes the name
of the set-id script to open to the interpreter, rather than using a
pathname subject to meddling, it instead passes /dev/fd/3.  This is a
special file already opened on the script, so that there can be no race
condition for evil scripts to exploit.  On these systems, Perl should be
compiled with -DSETUID_SCRIPTS_ARE_SECURE_NOW.  The Configure
program that builds Perl tries to figure this out for itself, so you
should never have to specify this yourself.  Most modern releases of
SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.









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69.4.7 Protecting Your Programs



There are a number of ways to hide the source to your Perl programs,
with varying levels of "security".



First of all, however, you can’t take away read permission, because
the source code has to be readable in order to be compiled and
interpreted.  (That doesn’t mean that a CGI script’s source is
readable by people on the web, though.)  So you have to leave the
permissions at the socially friendly 0755 level.  This lets 
people on your local system only see your source.



Some people mistakenly regard this as a security problem.  If your program does
insecure things, and relies on people not knowing how to exploit those
insecurities, it is not secure.  It is often possible for someone to
determine the insecure things and exploit them without viewing the
source.  Security through obscurity, the name for hiding your bugs
instead of fixing them, is little security indeed.



You can try using encryption via source filters (Filter::* from CPAN,
or Filter::Util::Call and Filter::Simple since Perl 5.8).
But crackers might be able to decrypt it.  You can try using the byte
code compiler and interpreter described below, but crackers might be
able to de-compile it.  You can try using the native-code compiler
described below, but crackers might be able to disassemble it.  These
pose varying degrees of difficulty to people wanting to get at your
code, but none can definitively conceal it (this is true of every
language, not just Perl).



If you’re concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive license will give you
legal security.  License your software and pepper it with threatening
statements like "This is unpublished proprietary software of XYZ Corp.
Your access to it does not give you permission to use it blah blah
blah."  You should see a lawyer to be sure your license’s wording will
stand up in court.









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69.4.8 Unicode



Unicode is a new and complex technology and one may easily overlook
certain security pitfalls.  See perluniintro NAME for an overview and
perlunicode NAME for details, and perlunicode Security Implications of Unicode for security implications in particular.









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69.4.9 Algorithmic Complexity Attacks



Certain internal algorithms used in the implementation of Perl can
be attacked by choosing the input carefully to consume large amounts
of either time or space or both.  This can lead into the so-called
Denial of Service (DoS) attacks.





See http://www.cs.rice.edu/~scrosby/hash/ for more information,
and any computer science textbook on algorithmic complexity.









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69.5 SEE ALSO



perlrun NAME for its description of cleaning up environment variables.









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70 perlsource





• perlsource NAME:  



• perlsource DESCRIPTION:  



• perlsource FINDING YOUR WAY AROUND:  












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70.1 NAME



perlsource - A guide to the Perl source tree









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70.2 DESCRIPTION



This document describes the layout of the Perl source tree. If you’re
hacking on the Perl core, this will help you find what you’re looking
for.









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70.3 FINDING YOUR WAY AROUND



The Perl source tree is big. Here’s some of the thing you’ll find in
it:





• perlsource C code:  



• perlsource Core modules:  



• perlsource Tests:  



• perlsource Documentation:  



• perlsource Hacking tools and documentation:  



• perlsource Build system:  



• perlsource AUTHORS:  



• perlsource MANIFEST:  












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70.3.1 C code



The C source code and header files mostly live in the root of the
source tree. There are a few platform-specific directories which
contain C code. In addition, some of the modules shipped with Perl
include C or XS code.



See perlinterp NAME for more details on the files that make up the Perl
interpreter, as well as details on how it works.









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70.3.2 Core modules



Modules shipped as part of the Perl core live in four subdirectories.
Two of these directories contain modules that live in the core, and two
contain modules that can also be released separately on CPAN. Modules
which can be released on cpan are known as "dual-life" modules.





For some dual-life modules, it has not yet been determined if the CPAN
version or the blead source is canonical. Until that is done, those
modules should be in cpan/.









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70.3.3 Tests



The Perl core has an extensive test suite. If you add new tests (or new
modules with tests), you may need to update the t/TEST file so that
the tests are run.











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70.3.4 Documentation



All of the core documentation intended for end users lives in pod/.
Individual modules in lib/, ext/, dist/, and cpan/ usually
have their own documentation, either in the Module.pm file or an
accompanying Module.pod file.



Finally, documentation intended for core Perl developers lives in the
Porting/ directory.









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70.3.5 Hacking tools and documentation



The Porting directory contains a grab bag of code and documentation
intended to help porters work on Perl. Some of the highlights include:











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70.3.6 Build system



The Perl build system starts with the Configure script in the root
directory.



Platform-specific pieces of the build system also live in
platform-specific directories like win32/, vms/, etc.



The Configure script is ultimately responsible for generating a
Makefile.



The build system that Perl uses is called metaconfig. This system is
maintained separately from the Perl core.



The metaconfig system has its own git repository. Please see its README
file in http://perl5.git.perl.org/metaconfig.git/ for more details.



The Cross directory contains various files related to
cross-compiling Perl. See Cross/README for more details.









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70.3.7 AUTHORS



This file lists everyone who’s contributed to Perl. If you submit a
patch, you should add your name to this file as part of the patch.









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70.3.8 MANIFEST



The MANIFEST file in the root of the source tree contains a list of
every file in the Perl core, as well as a brief description of each
file.



You can get an overview of all the files with this command:



  % perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST









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71 perlstyle





• perlstyle NAME:  



• perlstyle DESCRIPTION:  












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71.1 NAME



perlstyle - Perl style guide









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71.2 DESCRIPTION



Each programmer will, of course, have his or her own preferences in
regards to formatting, but there are some general guidelines that will
make your programs easier to read, understand, and maintain.



The most important thing is to run your programs under the -w
flag at all times.  You may turn it off explicitly for particular
portions of code via the no warnings pragma or the $^W variable
if you must.  You should also always run under use strict or know the
reason why not.  The use sigtrap and even use diagnostics pragmas
may also prove useful.



Regarding aesthetics of code lay out, about the only thing Larry
cares strongly about is that the closing curly bracket of
a multi-line BLOCK should line up with the keyword that started the construct.
Beyond that, he has other preferences that aren’t so strong:





Larry has his reasons for each of these things, but he doesn’t claim that
everyone else’s mind works the same as his does.



Here are some other more substantive style issues to think about:











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72 NAME subroutine function





• NAME subroutine function NAME:  



• NAME subroutine function SYNOPSIS:  



• NAME subroutine function DESCRIPTION:  



• NAME subroutine function SEE ALSO:  












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72.1 NAME





perlsub - Perl subroutines









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72.2 SYNOPSIS





To declare subroutines:




    sub NAME;                     # A "forward" declaration.
    sub NAME(PROTO);              #  ditto, but with prototypes
    sub NAME : ATTRS;             #  with attributes
    sub NAME(PROTO) : ATTRS;      #  with attributes and prototypes

    sub NAME BLOCK                # A declaration and a definition.
    sub NAME(PROTO) BLOCK         #  ditto, but with prototypes
    sub NAME : ATTRS BLOCK        #  with attributes
    sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes




To define an anonymous subroutine at runtime:




    $subref = sub BLOCK;                 # no proto
    $subref = sub (PROTO) BLOCK;         # with proto
    $subref = sub : ATTRS BLOCK;         # with attributes
    $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes




To import subroutines:




    use MODULE qw(NAME1 NAME2 NAME3);





To call subroutines:




    NAME(LIST);    # & is optional with parentheses.
    NAME LIST;     # Parentheses optional if predeclared/imported.
    &NAME(LIST);   # Circumvent prototypes.
    &NAME;         # Makes current @_ visible to called subroutine.









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72.3 DESCRIPTION



Like many languages, Perl provides for user-defined subroutines.
These may be located anywhere in the main program, loaded in from
other files via the do, require, or use keywords, or
generated on the fly using eval or anonymous subroutines.
You can even call a function indirectly using a variable containing
its name or a CODE reference.





The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
all functions likewise return to their caller one single flat list of
scalars.  Any arrays or hashes in these call and return lists will
collapse, losing their identities–but you may always use
pass-by-reference instead to avoid this.  Both call and return lists may
contain as many or as few scalar elements as you’d like.  (Often a
function without an explicit return statement is called a subroutine, but
there’s really no difference from Perl’s perspective.)







Any arguments passed in show up in the array @_.  Therefore, if
you called a function with two arguments, those would be stored in
$_[0] and $_[1].  The array @_ is a local array, but its
elements are aliases for the actual scalar parameters.  In particular,
if an element $_[0] is updated, the corresponding argument is
updated (or an error occurs if it is not updatable).  If an argument
is an array or hash element which did not exist when the function
was called, that element is created only when (and if) it is modified
or a reference to it is taken.  (Some earlier versions of Perl
created the element whether or not the element was assigned to.)
Assigning to the whole array @_ removes that aliasing, and does
not update any arguments.




A return statement may be used to exit a subroutine, optionally
specifying the returned value, which will be evaluated in the
appropriate context (list, scalar, or void) depending on the context of
the subroutine call.  If you specify no return value, the subroutine
returns an empty list in list context, the undefined value in scalar
context, or nothing in void context.  If you return one or more
aggregates (arrays and hashes), these will be flattened together into
one large indistinguishable list.






If no return is found and if the last statement is an expression, its
value is returned. If the last statement is a loop control structure
like a foreach or a while, the returned value is unspecified. The
empty sub returns the empty list.






Perl does not have named formal parameters.  In practice all you
do is assign to a my() list of these.  Variables that aren’t
declared to be private are global variables.  For gory details
on creating private variables, see Private Variables via my()
and Temporary Values via local().  To create protected
environments for a set of functions in a separate package (and
probably a separate file), see perlmod Packages.




Example:



    sub max {
        my $max = shift(@_);
        foreach $foo (@_) {
            $max = $foo if $max < $foo;
        }
        return $max;
    }
    $bestday = max($mon,$tue,$wed,$thu,$fri);



Example:



    # get a line, combining continuation lines
    #  that start with whitespace

    sub get_line {
        $thisline = $lookahead;  # global variables!
        LINE: while (defined($lookahead = <STDIN>)) {
            if ($lookahead =~ /^[ \t]/) {
                $thisline .= $lookahead;
            }
            else {
                last LINE;
            }
        }
        return $thisline;
    }

    $lookahead = <STDIN>;       # get first line
    while (defined($line = get_line())) {
        ...
    }



Assigning to a list of private variables to name your arguments:



    sub maybeset {
        my($key, $value) = @_;
        $Foo{$key} = $value unless $Foo{$key};
    }





Because the assignment copies the values, this also has the effect
of turning call-by-reference into call-by-value.  Otherwise a
function is free to do in-place modifications of @_ and change
its caller’s values.




    upcase_in($v1, $v2);  # this changes $v1 and $v2
    sub upcase_in {
        for (@_) { tr/a-z/A-Z/ }
    }





You aren’t allowed to modify constants in this way, of course.  If an
argument were actually literal and you tried to change it, you’d take a
(presumably fatal) exception.   For example, this won’t work:




    upcase_in("frederick");



It would be much safer if the upcase_in() function
were written to return a copy of its parameters instead
of changing them in place:



    ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
    sub upcase {
        return unless defined wantarray;  # void context, do nothing
        my @parms = @_;
        for (@parms) { tr/a-z/A-Z/ }
        return wantarray ? @parms : $parms[0];
    }



Notice how this (unprototyped) function doesn’t care whether it was
passed real scalars or arrays.  Perl sees all arguments as one big,
long, flat parameter list in @_.  This is one area where
Perl’s simple argument-passing style shines.  The upcase()
function would work perfectly well without changing the upcase()
definition even if we fed it things like this:



    @newlist   = upcase(@list1, @list2);
    @newlist   = upcase( split /:/, $var );



Do not, however, be tempted to do this:



    (@a, @b)   = upcase(@list1, @list2);



Like the flattened incoming parameter list, the return list is also
flattened on return.  So all you have managed to do here is stored
everything in @a and made @b empty.  See 
Pass by Reference for alternatives.




A subroutine may be called using an explicit & prefix.  The
& is optional in modern Perl, as are parentheses if the
subroutine has been predeclared.  The & is not optional
when just naming the subroutine, such as when it’s used as
an argument to defined() or undef().  Nor is it optional when you
want to do an indirect subroutine call with a subroutine name or
reference using the &$subref() or &{$subref}() constructs,
although the $subref->() notation solves that problem.
See (perlref) for more about all that.





Subroutines may be called recursively.  If a subroutine is called
using the & form, the argument list is optional, and if omitted,
no @_ array is set up for the subroutine: the @_ array at the
time of the call is visible to subroutine instead.  This is an
efficiency mechanism that new users may wish to avoid.




    &foo(1,2,3);        # pass three arguments
    foo(1,2,3);         # the same

    foo();              # pass a null list
    &foo();             # the same

    &foo;               # foo() get current args, like foo(@_) !!
    foo;                # like foo() IFF sub foo predeclared, else "foo"




Not only does the & form make the argument list optional, it also
disables any prototype checking on arguments you do provide.  This
is partly for historical reasons, and partly for having a convenient way
to cheat if you know what you’re doing.  See Prototypes below.




Since Perl 5.16.0, the __SUB__ token is available under use feature
'current_sub' and use 5.16.0.  It will evaluate to a reference to the
currently-running sub, which allows for recursive calls without knowing
your subroutine’s name.



    use 5.16.0;
    my $factorial = sub {
      my ($x) = @_;
      return 1 if $x == 1;
      return($x * __SUB__->( $x - 1 ) );
    };



Subroutines whose names are in all upper case are reserved to the Perl
core, as are modules whose names are in all lower case.  A subroutine in
all capitals is a loosely-held convention meaning it will be called
indirectly by the run-time system itself, usually due to a triggered event.
Subroutines that do special, pre-defined things include AUTOLOAD, CLONE,
DESTROY plus all functions mentioned in (perltie) and (PerlIO-via).



The BEGIN, UNITCHECK, CHECK, INIT and END subroutines
are not so much subroutines as named special code blocks, of which you
can have more than one in a package, and which you can not call
explicitly.  See perlmod BEGIN, UNITCHECK, CHECK, INIT and END





• NAME subroutine function Private Variables via my():  



• NAME subroutine function Persistent Private Variables:  



• NAME subroutine function Temporary Values via local():  



• NAME subroutine function Lvalue subroutines:  



• NAME subroutine function Passing Symbol Table Entries (typeglobs):  



• NAME subroutine function When to Still Use local():  



• NAME subroutine function Pass by Reference:  



• NAME subroutine function Prototypes:  



• NAME subroutine function Constant Functions:  



• NAME subroutine function Overriding Built-in Functions:  



• NAME subroutine function Autoloading:  



• NAME subroutine function Subroutine Attributes:  












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72.3.1 Private Variables via my()










Synopsis:



    my $foo;            # declare $foo lexically local
    my (@wid, %get);    # declare list of variables local
    my $foo = "flurp";  # declare $foo lexical, and init it
    my @oof = @bar;     # declare @oof lexical, and init it
    my $x : Foo = $y;   # similar, with an attribute applied



WARNING: The use of attribute lists on my declarations is still
evolving.  The current semantics and interface are subject to change.
See (attributes) and (Attribute-Handlers).



The my operator declares the listed variables to be lexically
confined to the enclosing block, conditional (if/unless/elsif/else),
loop (for/foreach/while/until/continue), subroutine, eval,
or do/require/use’d file.  If more than one value is listed, the
list must be placed in parentheses.  All listed elements must be
legal lvalues.  Only alphanumeric identifiers may be lexically
scoped–magical built-ins like $/ must currently be localized
with local instead.




Unlike dynamic variables created by the local operator, lexical
variables declared with my are totally hidden from the outside
world, including any called subroutines.  This is true if it’s the
same subroutine called from itself or elsewhere–every call gets
its own copy.




This doesn’t mean that a my variable declared in a statically
enclosing lexical scope would be invisible.  Only dynamic scopes
are cut off.   For example, the bumpx() function below has access
to the lexical $x variable because both the my and the sub
occurred at the same scope, presumably file scope.



    my $x = 10;
    sub bumpx { $x++ } 




An eval(), however, can see lexical variables of the scope it is
being evaluated in, so long as the names aren’t hidden by declarations within
the eval() itself.  See (perlref).




The parameter list to my() may be assigned to if desired, which allows you
to initialize your variables.  (If no initializer is given for a
particular variable, it is created with the undefined value.)  Commonly
this is used to name input parameters to a subroutine.  Examples:



    $arg = "fred";        # "global" variable
    $n = cube_root(27);
    print "$arg thinks the root is $n\n";
 fred thinks the root is 3

    sub cube_root {
        my $arg = shift;  # name doesn't matter
        $arg **= 1/3;
        return $arg;
    }



The my is simply a modifier on something you might assign to.  So when
you do assign to variables in its argument list, my doesn’t
change whether those variables are viewed as a scalar or an array.  So



    my ($foo) = <STDIN>;                # WRONG?
    my @FOO = <STDIN>;



both supply a list context to the right-hand side, while



    my $foo = <STDIN>;



supplies a scalar context.  But the following declares only one variable:



    my $foo, $bar = 1;                  # WRONG



That has the same effect as



    my $foo;
    $bar = 1;



The declared variable is not introduced (is not visible) until after
the current statement.  Thus,



    my $x = $x;



can be used to initialize a new $x with the value of the old $x, and
the expression



    my $x = 123 and $x == 123



is false unless the old $x happened to have the value 123.



Lexical scopes of control structures are not bounded precisely by the
braces that delimit their controlled blocks; control expressions are
part of that scope, too.  Thus in the loop



    while (my $line = <>) {
        $line = lc $line;
    } continue {
        print $line;
    }



the scope of $line extends from its declaration throughout the rest of
the loop construct (including the continue clause), but not beyond
it.  Similarly, in the conditional



    if ((my $answer = <STDIN>) =~ /^yes$/i) {
        user_agrees();
    } elsif ($answer =~ /^no$/i) {
        user_disagrees();
    } else {
        chomp $answer;
        die "'$answer' is neither 'yes' nor 'no'";
    }



the scope of $answer extends from its declaration through the rest
of that conditional, including any elsif and else clauses, 
but not beyond it.  See (perlsyn)Simple Statements for information
on the scope of variables in statements with modifiers.





The foreach loop defaults to scoping its index variable dynamically
in the manner of local.  However, if the index variable is
prefixed with the keyword my, or if there is already a lexical
by that name in scope, then a new lexical is created instead.  Thus
in the loop




    for my $i (1, 2, 3) {
        some_function();
    }





the scope of $i extends to the end of the loop, but not beyond it,
rendering the value of $i inaccessible within some_function().




Some users may wish to encourage the use of lexically scoped variables.
As an aid to catching implicit uses to package variables,
which are always global, if you say



    use strict 'vars';



then any variable mentioned from there to the end of the enclosing
block must either refer to a lexical variable, be predeclared via
our or use vars, or else must be fully qualified with the package name.
A compilation error results otherwise.  An inner block may countermand
this with no strict 'vars'.



A my has both a compile-time and a run-time effect.  At compile
time, the compiler takes notice of it.  The principal usefulness
of this is to quiet use strict 'vars', but it is also essential
for generation of closures as detailed in (perlref).  Actual
initialization is delayed until run time, though, so it gets executed
at the appropriate time, such as each time through a loop, for
example.



Variables declared with my are not part of any package and are therefore
never fully qualified with the package name.  In particular, you’re not
allowed to try to make a package variable (or other global) lexical:



    my $pack::var;      # ERROR!  Illegal syntax



In fact, a dynamic variable (also known as package or global variables)
are still accessible using the fully qualified :: notation even while a
lexical of the same name is also visible:



    package main;
    local $x = 10;
    my    $x = 20;
    print "$x and $::x\n";



That will print out 20 and 10.



You may declare my variables at the outermost scope of a file
to hide any such identifiers from the world outside that file.  This
is similar in spirit to C’s static variables when they are used at
the file level.  To do this with a subroutine requires the use of
a closure (an anonymous function that accesses enclosing lexicals).
If you want to create a private subroutine that cannot be called
from outside that block, it can declare a lexical variable containing
an anonymous sub reference:



    my $secret_version = '1.001-beta';
    my $secret_sub = sub { print $secret_version };
    &$secret_sub();



As long as the reference is never returned by any function within the
module, no outside module can see the subroutine, because its name is not in
any package’s symbol table.  Remember that it’s not REALLY called
$some_pack::secret_version or anything; it’s just $secret_version,
unqualified and unqualifiable.



This does not work with object methods, however; all object methods
have to be in the symbol table of some package to be found.  See
(perlref)Function Templates for something of a work-around to
this.









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72.3.2 Persistent Private Variables









There are two ways to build persistent private variables in Perl 5.10.
First, you can simply use the state feature. Or, you can use closures,
if you want to stay compatible with releases older than 5.10.





• NAME subroutine function Persistent variables via state():  



• NAME subroutine function Persistent variables with closures:  












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72.3.2.1 Persistent variables via state()



Beginning with Perl 5.9.4, you can declare variables with the state
keyword in place of my.  For that to work, though, you must have
enabled that feature beforehand, either by using the feature pragma, or
by using -E on one-liners (see (feature)).  Beginning with Perl 5.16,
the CORE::state form does not require the
feature pragma.



For example, the following code maintains a private counter, incremented
each time the gimme_another() function is called:



    use feature 'state';
    sub gimme_another { state $x; return ++$x }



Also, since $x is lexical, it can’t be reached or modified by any Perl
code outside.



When combined with variable declaration, simple scalar assignment to state
variables (as in state $x = 42) is executed only the first time.  When such
statements are evaluated subsequent times, the assignment is ignored.  The
behavior of this sort of assignment to non-scalar variables is undefined.









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72.3.2.2 Persistent variables with closures



Just because a lexical variable is lexically (also called statically)
scoped to its enclosing block, eval, or do FILE, this doesn’t mean that
within a function it works like a C static.  It normally works more
like a C auto, but with implicit garbage collection.  



Unlike local variables in C or C++, Perl’s lexical variables don’t
necessarily get recycled just because their scope has exited.
If something more permanent is still aware of the lexical, it will
stick around.  So long as something else references a lexical, that
lexical won’t be freed–which is as it should be.  You wouldn’t want
memory being free until you were done using it, or kept around once you
were done.  Automatic garbage collection takes care of this for you.



This means that you can pass back or save away references to lexical
variables, whereas to return a pointer to a C auto is a grave error.
It also gives us a way to simulate C’s function statics.  Here’s a
mechanism for giving a function private variables with both lexical
scoping and a static lifetime.  If you do want to create something like
C’s static variables, just enclose the whole function in an extra block,
and put the static variable outside the function but in the block.



    {
        my $secret_val = 0;
        sub gimme_another {
            return ++$secret_val;
        }
    }
    # $secret_val now becomes unreachable by the outside
    # world, but retains its value between calls to gimme_another



If this function is being sourced in from a separate file
via require or use, then this is probably just fine.  If it’s
all in the main program, you’ll need to arrange for the my
to be executed early, either by putting the whole block above
your main program, or more likely, placing merely a BEGIN
code block around it to make sure it gets executed before your program
starts to run:



    BEGIN {
        my $secret_val = 0;
        sub gimme_another {
            return ++$secret_val;
        }
    }



See perlmod BEGIN, UNITCHECK, CHECK, INIT and END about the
special triggered code blocks, BEGIN, UNITCHECK, CHECK,
INIT and END.



If declared at the outermost scope (the file scope), then lexicals
work somewhat like C’s file statics.  They are available to all
functions in that same file declared below them, but are inaccessible
from outside that file.  This strategy is sometimes used in modules
to create private variables that the whole module can see.









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72.3.3 Temporary Values via local()








WARNING: In general, you should be using my instead of local, because
it’s faster and safer.  Exceptions to this include the global punctuation
variables, global filehandles and formats, and direct manipulation of the
Perl symbol table itself.  local is mostly used when the current value
of a variable must be visible to called subroutines.



Synopsis:



    # localization of values

    local $foo;                 # make $foo dynamically local
    local (@wid, %get);         # make list of variables local
    local $foo = "flurp";       # make $foo dynamic, and init it
    local @oof = @bar;          # make @oof dynamic, and init it

    local $hash{key} = "val";   # sets a local value for this hash entry
    delete local $hash{key};    # delete this entry for the current block
    local ($cond ? $v1 : $v2);  # several types of lvalues support
                                # localization

    # localization of symbols

    local *FH;                  # localize $FH, @FH, %FH, &FH  ...
    local *merlyn = *randal;    # now $merlyn is really $randal, plus
                                #     @merlyn is really @randal, etc
    local *merlyn = 'randal';   # SAME THING: promote 'randal' to *randal
    local *merlyn = \$randal;   # just alias $merlyn, not @merlyn etc



A local modifies its listed variables to be "local" to the
enclosing block, eval, or do FILE–and to any subroutine
called from within that block.  A local just gives temporary
values to global (meaning package) variables.  It does not create
a local variable.  This is known as dynamic scoping.  Lexical scoping
is done with my, which works more like C’s auto declarations.



Some types of lvalues can be localized as well: hash and array elements
and slices, conditionals (provided that their result is always
localizable), and symbolic references.  As for simple variables, this
creates new, dynamically scoped values.



If more than one variable or expression is given to local, they must be
placed in parentheses.  This operator works
by saving the current values of those variables in its argument list on a
hidden stack and restoring them upon exiting the block, subroutine, or
eval.  This means that called subroutines can also reference the local
variable, but not the global one.  The argument list may be assigned to if
desired, which allows you to initialize your local variables.  (If no
initializer is given for a particular variable, it is created with an
undefined value.)



Because local is a run-time operator, it gets executed each time
through a loop.  Consequently, it’s more efficient to localize your
variables outside the loop.





• NAME subroutine function Grammatical note on local():  



• NAME subroutine function Localization of special variables:  



• NAME subroutine function Localization of globs:  



• NAME subroutine function Localization of elements of composite types:  



• NAME subroutine function Localized deletion of elements of composite types:  












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72.3.3.1 Grammatical note on local()




A local is simply a modifier on an lvalue expression.  When you assign to
a localized variable, the local doesn’t change whether its list is viewed
as a scalar or an array.  So



    local($foo) = <STDIN>;
    local @FOO = <STDIN>;



both supply a list context to the right-hand side, while



    local $foo = <STDIN>;



supplies a scalar context.









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72.3.3.2 Localization of special variables




If you localize a special variable, you’ll be giving a new value to it,
but its magic won’t go away.  That means that all side-effects related
to this magic still work with the localized value.



This feature allows code like this to work :



    # Read the whole contents of FILE in $slurp
    { local $/ = undef; $slurp = <FILE>; }



Note, however, that this restricts localization of some values ; for
example, the following statement dies, as of perl 5.9.0, with an error
Modification of a read-only value attempted, because the $1 variable is
magical and read-only :



    local $1 = 2;



One exception is the default scalar variable: starting with perl 5.14
local($_) will always strip all magic from $_, to make it possible
to safely reuse $_ in a subroutine.




WARNING: Localization of tied arrays and hashes does not currently
work as described.
This will be fixed in a future release of Perl; in the meantime, avoid
code that relies on any particular behaviour of localising tied arrays
or hashes (localising individual elements is still okay).
See (perl58delta)Localising Tied Arrays and Hashes Is Broken for more
details.










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72.3.3.3 Localization of globs





The construct



    local *name;



creates a whole new symbol table entry for the glob name in the
current package.  That means that all variables in its glob slot ($name,
@name, %name, &name, and the name filehandle) are dynamically reset.



This implies, among other things, that any magic eventually carried by
those variables is locally lost.  In other words, saying local */
will not have any effect on the internal value of the input record
separator.









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72.3.3.4 Localization of elements of composite types






It’s also worth taking a moment to explain what happens when you
localize a member of a composite type (i.e. an array or hash element).
In this case, the element is localized by name. This means that
when the scope of the local() ends, the saved value will be
restored to the hash element whose key was named in the local(), or
the array element whose index was named in the local().  If that
element was deleted while the local() was in effect (e.g. by a
delete() from a hash or a shift() of an array), it will spring
back into existence, possibly extending an array and filling in the
skipped elements with undef.  For instance, if you say



    %hash = ( 'This' => 'is', 'a' => 'test' );
    @ary  = ( 0..5 );
    {
         local($ary[5]) = 6;
         local($hash{'a'}) = 'drill';
         while (my $e = pop(@ary)) {
             print "$e . . .\n";
             last unless $e > 3;
         }
         if (@ary) {
             $hash{'only a'} = 'test';
             delete $hash{'a'};
         }
    }
    print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
    print "The array has ",scalar(@ary)," elements: ",
          join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";



Perl will print



    6 . . .
    4 . . .
    3 . . .
    This is a test only a test.
    The array has 6 elements: 0, 1, 2, undef, undef, 5



The behavior of local() on non-existent members of composite
types is subject to change in future.









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72.3.3.5 Localized deletion of elements of composite types







You can use the delete local $array[$idx] and delete local $hash{key}
constructs to delete a composite type entry for the current block and restore
it when it ends. They return the array/hash value before the localization,
which means that they are respectively equivalent to



    do {
        my $val = $array[$idx];
        local  $array[$idx];
        delete $array[$idx];
        $val
    }



and



    do {
        my $val = $hash{key};
        local  $hash{key};
        delete $hash{key};
        $val
    }



except that for those the local is scoped to the do block. Slices are
also accepted.



    my %hash = (
     a => [ 7, 8, 9 ],
     b => 1,
    )

    {
     my $a = delete local $hash{a};
     # $a is [ 7, 8, 9 ]
     # %hash is (b => 1)

     {
      my @nums = delete local @$a[0, 2]
      # @nums is (7, 9)
      # $a is [ undef, 8 ]

      $a[0] = 999; # will be erased when the scope ends
     }
     # $a is back to [ 7, 8, 9 ]

    }
    # %hash is back to its original state









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72.3.4 Lvalue subroutines





WARNING: Lvalue subroutines are still experimental and the
implementation may change in future versions of Perl.



It is possible to return a modifiable value from a subroutine.
To do this, you have to declare the subroutine to return an lvalue.



    my $val;
    sub canmod : lvalue {
        $val;  # or:  return $val;
    }
    sub nomod {
        $val;
    }

    canmod() = 5;   # assigns to $val
    nomod()  = 5;   # ERROR



The scalar/list context for the subroutine and for the right-hand
side of assignment is determined as if the subroutine call is replaced
by a scalar. For example, consider:



    data(2,3) = get_data(3,4);



Both subroutines here are called in a scalar context, while in:



    (data(2,3)) = get_data(3,4);



and in:



    (data(2),data(3)) = get_data(3,4);



all the subroutines are called in a list context.





Lvalue subroutines are EXPERIMENTAL




They appear to be convenient, but there is at least one reason to be
circumspect.



They violate encapsulation.  A normal mutator can check the supplied
argument before setting the attribute it is protecting, an lvalue
subroutine never gets that chance.  Consider;



    my $some_array_ref = [];    # protected by mutators ??

    sub set_arr {               # normal mutator
        my $val = shift;
        die("expected array, you supplied ", ref $val)
           unless ref $val eq 'ARRAY';
        $some_array_ref = $val;
    }
    sub set_arr_lv : lvalue {   # lvalue mutator
        $some_array_ref;
    }

    # set_arr_lv cannot stop this !
    set_arr_lv() = { a => 1 };














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72.3.5 Passing Symbol Table Entries (typeglobs)





WARNING: The mechanism described in this section was originally
the only way to simulate pass-by-reference in older versions of
Perl.  While it still works fine in modern versions, the new reference
mechanism is generally easier to work with.  See below.



Sometimes you don’t want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy.  In perl you can
refer to all objects of a particular name by prefixing the name
with a star: *foo.  This is often known as a "typeglob", because the
star on the front can be thought of as a wildcard match for all the
funny prefix characters on variables and subroutines and such.



When evaluated, the typeglob produces a scalar value that represents
all the objects of that name, including any filehandle, format, or
subroutine.  When assigned to, it causes the name mentioned to refer to
whatever * value was assigned to it.  Example:



    sub doubleary {
        local(*someary) = @_;
        foreach $elem (@someary) {
            $elem *= 2;
        }
    }
    doubleary(*foo);
    doubleary(*bar);



Scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to $_[0] etc.  You can modify all the elements of an array by passing
all the elements as scalars, but you have to use the * mechanism (or
the equivalent reference mechanism) to push, pop, or change the size of
an array.  It will certainly be faster to pass the typeglob (or reference).



Even if you don’t want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, because normally the LIST
mechanism will merge all the array values so that you can’t extract out
the individual arrays.  For more on typeglobs, see
perldata Typeglobs and Filehandles.









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72.3.6 When to Still Use local()





Despite the existence of my, there are still three places where the
local operator still shines.  In fact, in these three places, you
must use local instead of my.



    

  1.  You need to give a global variable a temporary value, especially $_.


    The global variables, like @ARGV or the punctuation variables, must be 
localized with local().  This block reads in /etc/motd, and splits
it up into chunks separated by lines of equal signs, which are placed
in @Fields.

    

        {
        local @ARGV = ("/etc/motd");
        local $/ = undef;
        local $_ = <>;  
        @Fields = split /^\s*=+\s*$/;
    } 

    

    It particular, it’s important to localize $_ in any routine that assigns
to it.  Look out for implicit assignments in while conditionals.

    

  2.  You need to create a local file or directory handle or a local function.


    A function that needs a filehandle of its own must use
local() on a complete typeglob.   This can be used to create new symbol
table entries:

    

        sub ioqueue {
        local  (*READER, *WRITER);    # not my!
        pipe    (READER,  WRITER)     or die "pipe: $!";
        return (*READER, *WRITER);
    }
    ($head, $tail) = ioqueue();

    

    See the Symbol module for a way to create anonymous symbol table
entries.

    

    Because assignment of a reference to a typeglob creates an alias, this
can be used to create what is effectively a local function, or at least,
a local alias.

    

        {
        local *grow = \&shrink; # only until this block exits
        grow();                 # really calls shrink()
        move();                 # if move() grow()s, it shrink()s too
    }
    grow();                     # get the real grow() again

    

    See (perlref)Function Templates for more about manipulating
functions by name in this way.

    

  3.  You want to temporarily change just one element of an array or hash.


    You can localize just one element of an aggregate.  Usually this
is done on dynamics:

    

        {
        local $SIG{INT} = 'IGNORE';
        funct();                            # uninterruptible
    } 
    # interruptibility automatically restored here

    

    But it also works on lexically declared aggregates.  Prior to 5.005,
this operation could on occasion misbehave.

    










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72.3.7 Pass by Reference






If you want to pass more than one array or hash into a function–or
return them from it–and have them maintain their integrity, then
you’re going to have to use an explicit pass-by-reference.  Before you
do that, you need to understand references as detailed in (perlref).
This section may not make much sense to you otherwise.



Here are a few simple examples.  First, let’s pass in several arrays
to a function and have it pop all of then, returning a new list
of all their former last elements:



    @tailings = popmany ( \@a, \@b, \@c, \@d );

    sub popmany {
        my $aref;
        my @retlist = ();
        foreach $aref ( @_ ) {
            push @retlist, pop @$aref;
        }
        return @retlist;
    }



Here’s how you might write a function that returns a
list of keys occurring in all the hashes passed to it:



    @common = inter( \%foo, \%bar, \%joe );
    sub inter {
        my ($k, $href, %seen); # locals
        foreach $href (@_) {
            while ( $k = each %$href ) {
                $seen{$k}++;
            }
        }
        return grep { $seen{$_} == @_ } keys %seen;
    }



So far, we’re using just the normal list return mechanism.
What happens if you want to pass or return a hash?  Well,
if you’re using only one of them, or you don’t mind them
concatenating, then the normal calling convention is ok, although
a little expensive.



Where people get into trouble is here:



    (@a, @b) = func(@c, @d);
or
    (%a, %b) = func(%c, %d);



That syntax simply won’t work.  It sets just @a or %a and
clears the @b or %b.  Plus the function didn’t get passed
into two separate arrays or hashes: it got one long list in @_,
as always.



If you can arrange for everyone to deal with this through references, it’s
cleaner code, although not so nice to look at.  Here’s a function that
takes two array references as arguments, returning the two array elements
in order of how many elements they have in them:



    ($aref, $bref) = func(\@c, \@d);
    print "@$aref has more than @$bref\n";
    sub func {
        my ($cref, $dref) = @_;
        if (@$cref > @$dref) {
            return ($cref, $dref);
        } else {
            return ($dref, $cref);
        }
    }



It turns out that you can actually do this also:



    (*a, *b) = func(\@c, \@d);
    print "@a has more than @b\n";
    sub func {
        local (*c, *d) = @_;
        if (@c > @d) {
            return (\@c, \@d);
        } else {
            return (\@d, \@c);
        }
    }



Here we’re using the typeglobs to do symbol table aliasing.  It’s
a tad subtle, though, and also won’t work if you’re using my
variables, because only globals (even in disguise as locals)
are in the symbol table.



If you’re passing around filehandles, you could usually just use the bare
typeglob, like *STDOUT, but typeglobs references work, too.
For example:



    splutter(\*STDOUT);
    sub splutter {
        my $fh = shift;
        print $fh "her um well a hmmm\n";
    }

    $rec = get_rec(\*STDIN);
    sub get_rec {
        my $fh = shift;
        return scalar <$fh>;
    }



If you’re planning on generating new filehandles, you could do this.
Notice to pass back just the bare *FH, not its reference.



    sub openit {
        my $path = shift;
        local *FH;
        return open (FH, $path) ? *FH : undef;
    }









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72.3.8 Prototypes





Perl supports a very limited kind of compile-time argument checking
using function prototyping.  If you declare



    sub mypush (+@)



then mypush() takes arguments exactly like push() does.  The
function declaration must be visible at compile time.  The prototype
affects only interpretation of new-style calls to the function,
where new-style is defined as not using the & character.  In
other words, if you call it like a built-in function, then it behaves
like a built-in function.  If you call it like an old-fashioned
subroutine, then it behaves like an old-fashioned subroutine.  It
naturally falls out from this rule that prototypes have no influence
on subroutine references like \&foo or on indirect subroutine
calls like &{$subref} or $subref->().



Method calls are not influenced by prototypes either, because the
function to be called is indeterminate at compile time, since
the exact code called depends on inheritance.



Because the intent of this feature is primarily to let you define
subroutines that work like built-in functions, here are prototypes
for some other functions that parse almost exactly like the
corresponding built-in.



    Declared as                 Called as

    sub mylink ($$)          mylink $old, $new
    sub myvec ($$$)          myvec $var, $offset, 1
    sub myindex ($$;$)       myindex &getstring, "substr"
    sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) - $off, $off
    sub myreverse (@)        myreverse $a, $b, $c
    sub myjoin ($@)          myjoin ":", $a, $b, $c
    sub mypop (+)            mypop @array
    sub mysplice (+$$@)      mysplice @array, 0, 2, @pushme
    sub mykeys (+)           mykeys %{$hashref}
    sub myopen (*;$)         myopen HANDLE, $name
    sub mypipe (**)          mypipe READHANDLE, WRITEHANDLE
    sub mygrep (&@)          mygrep { /foo/ } $a, $b, $c
    sub myrand (;$)          myrand 42
    sub mytime ()            mytime



Any backslashed prototype character represents an actual argument
that must start with that character (optionally preceded by my,
our or local), with the exception of $, which will
accept any scalar lvalue expression, such as $foo = 7 or
my_function()->[0]. The value passed as part of @_ will be a
reference to the actual argument given in the subroutine call,
obtained by applying \ to that argument.



You can use the \[] backslash group notation to specify more than one
allowed argument type. For example:



    sub myref (\[$@%&*])



will allow calling myref() as



    myref $var
    myref @array
    myref %hash
    myref &sub
    myref *glob



and the first argument of myref() will be a reference to
a scalar, an array, a hash, a code, or a glob.



Unbackslashed prototype characters have special meanings.  Any
unbackslashed @ or % eats all remaining arguments, and forces
list context.  An argument represented by $ forces scalar context.  An
& requires an anonymous subroutine, which, if passed as the first
argument, does not require the sub keyword or a subsequent comma.



A * allows the subroutine to accept a bareword, constant, scalar expression,
typeglob, or a reference to a typeglob in that slot.  The value will be
available to the subroutine either as a simple scalar, or (in the latter
two cases) as a reference to the typeglob.  If you wish to always convert
such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
follows:



    use Symbol 'qualify_to_ref';

    sub foo (*) {
        my $fh = qualify_to_ref(shift, caller);
        ...
    }



The + prototype is a special alternative to $ that will act like
\[@%] when given a literal array or hash variable, but will otherwise
force scalar context on the argument.  This is useful for functions which
should accept either a literal array or an array reference as the argument:



    sub mypush (+@) {
        my $aref = shift;
        die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
        push @$aref, @_;
    }



When using the + prototype, your function must check that the argument
is of an acceptable type.



A semicolon (;) separates mandatory arguments from optional arguments.
It is redundant before @ or %, which gobble up everything else.



As the last character of a prototype, or just before a semicolon, a @
or a %, you can use _ in place of $: if this argument is not
provided, $_ will be used instead.



Note how the last three examples in the table above are treated
specially by the parser.  mygrep() is parsed as a true list
operator, myrand() is parsed as a true unary operator with unary
precedence the same as rand(), and mytime() is truly without
arguments, just like time().  That is, if you say



    mytime +2;



you’ll get mytime() + 2, not mytime(2), which is how it would be parsed
without a prototype.  If you want to force a unary function to have the
same precedence as a list operator, add ; to the end of the prototype:



    sub mygetprotobynumber($;);
    mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)




The interesting thing about & is that you can generate new syntax with it,
provided it’s in the initial position:




    sub try (&@) {
        my($try,$catch) = @_;
        eval { &$try };
        if ($@) {
            local $_ = $@;
            &$catch;
        }
    }
    sub catch (&) { $_[0] }

    try {
        die "phooey";
    } catch {
        /phooey/ and print "unphooey\n";
    };



That prints "unphooey".  (Yes, there are still unresolved
issues having to do with visibility of @_.  I’m ignoring that
question for the moment.  (But note that if we make @_ lexically
scoped, those anonymous subroutines can act like closures... (Gee,
is this sounding a little Lispish?  (Never mind.))))




And here’s a reimplementation of the Perl grep operator:




    sub mygrep (&@) {
        my $code = shift;
        my @result;
        foreach $_ (@_) {
            push(@result, $_) if &$code;
        }
        @result;
    }



Some folks would prefer full alphanumeric prototypes.  Alphanumerics have
been intentionally left out of prototypes for the express purpose of
someday in the future adding named, formal parameters.  The current
mechanism’s main goal is to let module writers provide better diagnostics
for module users.  Larry feels the notation quite understandable to Perl
programmers, and that it will not intrude greatly upon the meat of the
module, nor make it harder to read.  The line noise is visually
encapsulated into a small pill that’s easy to swallow.



If you try to use an alphanumeric sequence in a prototype you will
generate an optional warning - "Illegal character in prototype...".
Unfortunately earlier versions of Perl allowed the prototype to be
used as long as its prefix was a valid prototype.  The warning may be
upgraded to a fatal error in a future version of Perl once the
majority of offending code is fixed.



It’s probably best to prototype new functions, not retrofit prototyping
into older ones.  That’s because you must be especially careful about
silent impositions of differing list versus scalar contexts.  For example,
if you decide that a function should take just one parameter, like this:



    sub func ($) {
        my $n = shift;
        print "you gave me $n\n";
    }



and someone has been calling it with an array or expression
returning a list:



    func(@foo);
    func( split /:/ );



Then you’ve just supplied an automatic scalar in front of their
argument, which can be more than a bit surprising.  The old @foo
which used to hold one thing doesn’t get passed in.  Instead,
func() now gets passed in a 1; that is, the number of elements
in @foo.  And the split gets called in scalar context so it
starts scribbling on your @_ parameter list.  Ouch!



This is all very powerful, of course, and should be used only in moderation
to make the world a better place.









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72.3.9 Constant Functions




Functions with a prototype of () are potential candidates for
inlining.  If the result after optimization and constant folding
is either a constant or a lexically-scoped scalar which has no other
references, then it will be used in place of function calls made
without &.  Calls made using & are never inlined.  (See
constant.pm for an easy way to declare most constants.)



The following functions would all be inlined:



    sub pi ()           { 3.14159 }             # Not exact, but close.
    sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets,
                                                # and it's inlined, too!
    sub ST_DEV ()       { 0 }
    sub ST_INO ()       { 1 }

    sub FLAG_FOO ()     { 1 << 8 }
    sub FLAG_BAR ()     { 1 << 9 }
    sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

    sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

    sub N () { int(OPT_BAZ) / 3 }

    sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }



Be aware that these will not be inlined; as they contain inner scopes,
the constant folding doesn’t reduce them to a single constant:



    sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }

    sub baz_val () {
        if (OPT_BAZ) {
            return 23;
        }
        else {
            return 42;
        }
    }



If you redefine a subroutine that was eligible for inlining, you’ll get
a warning by default.  (You can use this warning to tell whether or not a
particular subroutine is considered constant.)  The warning is
considered severe enough not to be affected by the -w
switch (or its absence) because previously compiled
invocations of the function will still be using the old value of the
function.  If you need to be able to redefine the subroutine, you need to
ensure that it isn’t inlined, either by dropping the () prototype
(which changes calling semantics, so beware) or by thwarting the
inlining mechanism in some other way, such as



    sub not_inlined () {
        23 if $];
    }









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72.3.10 Overriding Built-in Functions







Many built-in functions may be overridden, though this should be tried
only occasionally and for good reason.  Typically this might be
done by a package attempting to emulate missing built-in functionality
on a non-Unix system.



Overriding may be done only by importing the name from a module at
compile time–ordinary predeclaration isn’t good enough.  However, the
use subs pragma lets you, in effect, predeclare subs
via the import syntax, and these names may then override built-in ones:



    use subs 'chdir', 'chroot', 'chmod', 'chown';
    chdir $somewhere;
    sub chdir { ... }



To unambiguously refer to the built-in form, precede the
built-in name with the special package qualifier CORE::.  For example,
saying CORE::open() always refers to the built-in open(), even
if the current package has imported some other subroutine called
&open() from elsewhere.  Even though it looks like a regular
function call, it isn’t: the CORE:: prefix in that case is part of Perl’s
syntax, and works for any keyword, regardless of what is in the CORE
package.  Taking a reference to it, that is, \&CORE::open, only works
for some keywords.  See (CORE).



Library modules should not in general export built-in names like open
or chdir as part of their default @EXPORT list, because these may
sneak into someone else’s namespace and change the semantics unexpectedly.
Instead, if the module adds that name to @EXPORT_OK, then it’s
possible for a user to import the name explicitly, but not implicitly.
That is, they could say



    use Module 'open';



and it would import the open override.  But if they said



    use Module;



they would get the default imports without overrides.



The foregoing mechanism for overriding built-in is restricted, quite
deliberately, to the package that requests the import.  There is a second
method that is sometimes applicable when you wish to override a built-in
everywhere, without regard to namespace boundaries.  This is achieved by
importing a sub into the special namespace CORE::GLOBAL::.  Here is an
example that quite brazenly replaces the glob operator with something
that understands regular expressions.



    package REGlob;
    require Exporter;
    @ISA = 'Exporter';
    @EXPORT_OK = 'glob';

    sub import {
        my $pkg = shift;
        return unless @_;
        my $sym = shift;
        my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
        $pkg->export($where, $sym, @_);
    }

    sub glob {
        my $pat = shift;
        my @got;
        if (opendir my $d, '.') { 
            @got = grep /$pat/, readdir $d; 
            closedir $d;   
        }
        return @got;
    }
    1;



And here’s how it could be (ab)used:



    #use REGlob 'GLOBAL_glob';      # override glob() in ALL namespaces
    package Foo;
    use REGlob 'glob';              # override glob() in Foo:: only
    print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules



The initial comment shows a contrived, even dangerous example.
By overriding glob globally, you would be forcing the new (and
subversive) behavior for the glob operator for every namespace,
without the complete cognizance or cooperation of the modules that own
those namespaces.  Naturally, this should be done with extreme caution–if
it must be done at all.



The REGlob example above does not implement all the support needed to
cleanly override perl’s glob operator.  The built-in glob has
different behaviors depending on whether it appears in a scalar or list
context, but our REGlob doesn’t.  Indeed, many perl built-in have such
context sensitive behaviors, and these must be adequately supported by
a properly written override.  For a fully functional example of overriding
glob, study the implementation of File::DosGlob in the standard
library.



When you override a built-in, your replacement should be consistent (if
possible) with the built-in native syntax.  You can achieve this by using
a suitable prototype.  To get the prototype of an overridable built-in,
use the prototype function with an argument of "CORE::builtin_name"
(see (perlfunc)prototype).



Note however that some built-ins can’t have their syntax expressed by a
prototype (such as system or chomp).  If you override them you won’t
be able to fully mimic their original syntax.



The built-ins do, require and glob can also be overridden, but due
to special magic, their original syntax is preserved, and you don’t have
to define a prototype for their replacements.  (You can’t override the
do BLOCK syntax, though).



require has special additional dark magic: if you invoke your
require replacement as require Foo::Bar, it will actually receive
the argument "Foo/Bar.pm" in @_.  See (perlfunc)require.



And, as you’ll have noticed from the previous example, if you override
glob, the <*> glob operator is overridden as well.



In a similar fashion, overriding the readline function also overrides
the equivalent I/O operator <FILEHANDLE>. Also, overriding
readpipe also overrides the operators `` and qx//.



Finally, some built-ins (e.g. exists or grep) can’t be overridden.









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72.3.11 Autoloading





If you call a subroutine that is undefined, you would ordinarily
get an immediate, fatal error complaining that the subroutine doesn’t
exist.  (Likewise for subroutines being used as methods, when the
method doesn’t exist in any base class of the class’s package.)
However, if an AUTOLOAD subroutine is defined in the package or
packages used to locate the original subroutine, then that
AUTOLOAD subroutine is called with the arguments that would have
been passed to the original subroutine.  The fully qualified name
of the original subroutine magically appears in the global $AUTOLOAD
variable of the same package as the AUTOLOAD routine.  The name
is not passed as an ordinary argument because, er, well, just
because, that’s why.  (As an exception, a method call to a nonexistent
import or unimport method is just skipped instead.  Also, if
the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
subroutine name.  See perlguts Autoloading with XSUBs for details.)



Many AUTOLOAD routines load in a definition for the requested
subroutine using eval(), then execute that subroutine using a special
form of goto() that erases the stack frame of the AUTOLOAD routine
without a trace.  (See the source to the standard module documented
in (AutoLoader), for example.)  But an AUTOLOAD routine can
also just emulate the routine and never define it.   For example,
let’s pretend that a function that wasn’t defined should just invoke
system with those arguments.  All you’d do is:



    sub AUTOLOAD {
        my $program = $AUTOLOAD;
        $program =~ s/.*:://;
        system($program, @_);
    }
    date();
    who('am', 'i');
    ls('-l');



In fact, if you predeclare functions you want to call that way, you don’t
even need parentheses:



    use subs qw(date who ls);
    date;
    who "am", "i";
    ls '-l';



A more complete example of this is the Shell module on CPAN, which
can treat undefined subroutine calls as calls to external programs.



Mechanisms are available to help modules writers split their modules
into autoloadable files.  See the standard AutoLoader module
described in (AutoLoader) and in (AutoSplit), the standard
SelfLoader modules in (SelfLoader), and the document on adding C
functions to Perl code in (perlxs).









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72.3.12 Subroutine Attributes






A subroutine declaration or definition may have a list of attributes
associated with it.  If such an attribute list is present, it is
broken up at space or colon boundaries and treated as though a
use attributes had been seen.  See (attributes) for details
about what attributes are currently supported.
Unlike the limitation with the obsolescent use attrs, the
sub : ATTRLIST syntax works to associate the attributes with
a pre-declaration, and not just with a subroutine definition.



The attributes must be valid as simple identifier names (without any
punctuation other than the ’_’ character).  They may have a parameter
list appended, which is only checked for whether its parentheses (’(’,’)’)
nest properly.



Examples of valid syntax (even though the attributes are unknown):



    sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
    sub plugh () : Ugly('\(") :Bad;
    sub xyzzy : _5x5 { ... }



Examples of invalid syntax:



    sub fnord : switch(10,foo(); # ()-string not balanced
    sub snoid : Ugly('(');        # ()-string not balanced
    sub xyzzy : 5x5;              # "5x5" not a valid identifier
    sub plugh : Y2::north;        # "Y2::north" not a simple identifier
    sub snurt : foo + bar;        # "+" not a colon or space



The attribute list is passed as a list of constant strings to the code
which associates them with the subroutine.  In particular, the second example
of valid syntax above currently looks like this in terms of how it’s
parsed and invoked:



    use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';



For further details on attribute lists and their manipulation,
see (attributes) and (Attribute-Handlers).









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72.4 SEE ALSO



See (perlref)Function Templates for more about references and closures.
See (perlxs) if you’d like to learn about calling C subroutines from Perl.  
See perlembed NAME if you’d like to learn about calling Perl subroutines from C.  
See perlmod NAME to learn about bundling up your functions in separate files.
See (perlmodlib) to learn what library modules come standard on your system.
See perlootut NAME to learn how to make object method calls.









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73 NAME syntax





• NAME syntax NAME:  



• NAME syntax DESCRIPTION:  












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73.1 NAME




perlsyn - Perl syntax









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73.2 DESCRIPTION



A Perl program consists of a sequence of declarations and statements
which run from the top to the bottom.  Loops, subroutines, and other
control structures allow you to jump around within the code.



Perl is a free-form language: you can format and indent it however
you like.  Whitespace serves mostly to separate tokens, unlike
languages like Python where it is an important part of the syntax,
or Fortran where it is immaterial.



Many of Perl’s syntactic elements are optional.  Rather than
requiring you to put parentheses around every function call and
declare every variable, you can often leave such explicit elements off
and Perl will figure out what you meant.  This is known as Do What I
Mean, abbreviated DWIM.  It allows programmers to be lazy and to
code in a style with which they are comfortable.



Perl borrows syntax and concepts from many languages: awk, sed, C,
Bourne Shell, Smalltalk, Lisp and even English.  Other
languages have borrowed syntax from Perl, particularly its regular
expression extensions.  So if you have programmed in another language
you will see familiar pieces in Perl.  They often work the same, but
see perltrap NAME for information about how they differ.





• NAME syntax Declarations:  



• NAME syntax Comments:  



• NAME syntax Simple Statements:  



• NAME syntax Truth and Falsehood:  



• NAME syntax Statement Modifiers:  



• NAME syntax Compound Statements:  



• NAME syntax Loop Control:  



• NAME syntax For Loops:  



• NAME syntax Foreach Loops:  



• NAME syntax Basic BLOCKs:  



• NAME syntax Switch Statements:  



• NAME syntax Goto:  



• NAME syntax The Ellipsis Statement:  



• NAME syntax PODs: Embedded Documentation:  



• NAME syntax Plain Old Comments (Not!):  



• NAME syntax Experimental Details on given and when:  












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73.2.1 Declarations







The only things you need to declare in Perl are report formats and
subroutines (and sometimes not even subroutines).  A scalar variable holds
the undefined value (undef) until it has been assigned a defined
value, which is anything other than undef.  When used as a number,
undef is treated as 0; when used as a string, it is treated as
the empty string, ""; and when used as a reference that isn’t being
assigned to, it is treated as an error.  If you enable warnings,
you’ll be notified of an uninitialized value whenever you treat
undef as a string or a number.  Well, usually.  Boolean contexts,
such as:



    if ($a) {}



are exempt from warnings (because they care about truth rather than
definedness).  Operators such as ++, --, +=,
-=, and .=, that operate on undefined variables such as:



    undef $a;
    $a++;



are also always exempt from such warnings.



A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements: declarations all
take effect at compile time.  All declarations are typically put at
the beginning or the end of the script.  However, if you’re using
lexically-scoped private variables created with my(),
state(), or our(), you’ll have to make sure
your format or subroutine definition is within the same block scope
as the my if you expect to be able to access those private variables.




Declaring a subroutine allows a subroutine name to be used as if it were a
list operator from that point forward in the program.  You can declare a
subroutine without defining it by saying sub name, thus:




    sub myname;
    $me = myname $0             or die "can't get myname";



A bare declaration like that declares the function to be a list operator,
not a unary operator, so you have to be careful to use parentheses (or
or instead of ||.)  The || operator binds too tightly to use after
list operators; it becomes part of the last element.  You can always use
parentheses around the list operators arguments to turn the list operator
back into something that behaves more like a function call.  Alternatively,
you can use the prototype ($) to turn the subroutine into a unary
operator:



  sub myname ($);
  $me = myname $0             || die "can't get myname";



That now parses as you’d expect, but you still ought to get in the habit of
using parentheses in that situation.  For more on prototypes, see
(perlsub)



Subroutines declarations can also be loaded up with the require statement
or both loaded and imported into your namespace with a use statement.
See perlmod NAME for details on this.



A statement sequence may contain declarations of lexically-scoped
variables, but apart from declaring a variable name, the declaration acts
like an ordinary statement, and is elaborated within the sequence of
statements as if it were an ordinary statement.  That means it actually
has both compile-time and run-time effects.









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73.2.2 Comments





Text from a "#" character until the end of the line is a comment,
and is ignored.  Exceptions include "#" inside a string or regular
expression.









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73.2.3 Simple Statements







The only kind of simple statement is an expression evaluated for its
side-effects.  Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional.  But put the semicolon in anyway if the
block takes up more than one line, because you may eventually add
another line.  Note that there are operators like eval {}, sub {}, and
do {} that look like compound statements, but aren’t–they’re just
TERMs in an expression–and thus need an explicit termination when used
as the last item in a statement.









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73.2.4 Truth and Falsehood











The number 0, the strings '0' and "", the empty list (), and
undef are all false in a boolean context.  All other values are true.
Negation of a true value by ! or not returns a special false value.
When evaluated as a string it is treated as "", but as a number, it
is treated as 0.  Most Perl operators
that return true or false behave this way.









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73.2.5 Statement Modifiers












Any simple statement may optionally be followed by a SINGLE modifier,
just before the terminating semicolon (or block ending).  The possible
modifiers are:



    if EXPR
    unless EXPR
    while EXPR
    until EXPR
    for LIST
    foreach LIST
    when EXPR



The EXPR following the modifier is referred to as the "condition".
Its truth or falsehood determines how the modifier will behave.



if executes the statement once if and only if the condition is
true.  unless is the opposite, it executes the statement unless
the condition is true (that is, if the condition is false).



    print "Basset hounds got long ears" if length $ear >= 10;
    go_outside() and play() unless $is_raining;



The for(each) modifier is an iterator: it executes the statement once
for each item in the LIST (with $_ aliased to each item in turn).



    print "Hello $_!\n" for qw(world Dolly nurse);



while repeats the statement while the condition is true.
until does the opposite, it repeats the statement until the
condition is true (or while the condition is false):



    # Both of these count from 0 to 10.
    print $i++ while $i <= 10;
    print $j++ until $j >  10;



The while and until modifiers have the usual "while loop"
semantics (conditional evaluated first), except when applied to a
do-BLOCK (or to the Perl4 do-SUBROUTINE statement), in
which case the block executes once before the conditional is
evaluated.



This is so that you can write loops like:



    do {
        $line = <STDIN>;
        ...
    } until !defined($line) || $line eq ".\n"






See (perlfunc)do.  Note also that the loop control statements described
later will NOT work in this construct, because modifiers don’t take
loop labels.  Sorry.  You can always put another block inside of it
(for next) or around it (for last) to do that sort of thing.
For next, just double the braces:




    do {{
        next if $x == $y;
        # do something here
    }} until $x++ > $z;




For last, you have to be more elaborate:




    LOOP: { 
            do {
                last if $x = $y**2;
                # do something here
            } while $x++ <= $z;
    }




NOTE: The behaviour of a my, state, or
our modified with a statement modifier conditional
or loop construct (for example, my $x if ...) is
undefined.  The value of the my variable may be undef, any
previously assigned value, or possibly anything else.  Don’t rely on
it.  Future versions of perl might do something different from the
version of perl you try it out on.  Here be dragons.




The when modifier is an experimental feature that first appeared in Perl
5.14.  To use it, you should include a use v5.14 declaration.
(Technically, it requires only the switch feature, but that aspect of it
was not available before 5.14.)  Operative only from within a foreach
loop or a given block, it executes the statement only if the smartmatch
$_ ~~ EXPR is true.  If the statement executes, it is followed by
a next from inside a foreach and break from inside a given.



Under the current implementation, the foreach loop can be
anywhere within the when modifier’s dynamic scope, but must be
within the given block’s lexical scope.  This restricted may
be relaxed in a future release.  See Switch Statements below.









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73.2.6 Compound Statements


















In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case
of a required file, or the program as a whole), and sometimes a block
is delimited by the extent of a string (in the case of an eval).



But generally, a block is delimited by curly brackets, also known as braces.
We will call this syntactic construct a BLOCK.



The following compound statements may be used to control flow:



    if (EXPR) BLOCK
    if (EXPR) BLOCK else BLOCK
    if (EXPR) BLOCK elsif (EXPR) BLOCK ...
    if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

    unless (EXPR) BLOCK
    unless (EXPR) BLOCK else BLOCK
    unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
    unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

    given (EXPR) BLOCK

    LABEL while (EXPR) BLOCK
    LABEL while (EXPR) BLOCK continue BLOCK

    LABEL until (EXPR) BLOCK
    LABEL until (EXPR) BLOCK continue BLOCK

    LABEL for (EXPR; EXPR; EXPR) BLOCK
    LABEL for VAR (LIST) BLOCK
    LABEL for VAR (LIST) BLOCK continue BLOCK

    LABEL foreach (EXPR; EXPR; EXPR) BLOCK
    LABEL foreach VAR (LIST) BLOCK
    LABEL foreach VAR (LIST) BLOCK continue BLOCK

    LABEL BLOCK
    LABEL BLOCK continue BLOCK

    PHASE BLOCK



The experimental given statement is not automatically enabled; see 
Switch Statements below for how to do so, and the attendant caveats.



Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
not statements.  This means that the curly brackets are required–no
dangling statements allowed.  If you want to write conditionals without
curly brackets, there are several other ways to do it.  The following
all do the same thing:



    if (!open(FOO)) { die "Can't open $FOO: $!" }
    die "Can't open $FOO: $!" unless open(FOO);
    open(FOO)  || die "Can't open $FOO: $!";
    open(FOO) ? () : die "Can't open $FOO: $!";
                        # a bit exotic, that last one



The if statement is straightforward.  Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
if an else goes with.  If you use unless in place of if,
the sense of the test is reversed.  Like if, unless can be followed
by else.  unless can even be followed by one or more elsif
statements, though you may want to think twice before using that particular
language construct, as everyone reading your code will have to think at least
twice before they can understand what’s going on.



The while statement executes the block as long as the expression is
true.
The until statement executes the block as long as the expression is
false.
The LABEL is optional, and if present, consists of an identifier followed
by a colon.  The LABEL identifies the loop for the loop control
statements next, last, and redo.
If the LABEL is omitted, the loop control statement
refers to the innermost enclosing loop.  This may include dynamically
looking back your call-stack at run time to find the LABEL.  Such
desperate behavior triggers a warning if you use the use warnings
pragma or the -w flag.



If there is a continue BLOCK, it is always executed just before the
conditional is about to be evaluated again.  Thus it can be used to
increment a loop variable, even when the loop has been continued via
the next statement.



When a block is preceding by a compilation phase keyword such as BEGIN,
END, INIT, CHECK, or UNITCHECK, then the block will run only
during the corresponding phase of execution.  See perlmod NAME for more details.



Extension modules can also hook into the Perl parser to define new
kinds of compound statements.  These are introduced by a keyword which
the extension recognizes, and the syntax following the keyword is
defined entirely by the extension.  If you are an implementor, see
(perlapi)PL_keyword_plugin for the mechanism.  If you are using such
a module, see the module’s documentation for details of the syntax that
it defines.









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73.2.7 Loop Control









The next command starts the next iteration of the loop:



    LINE: while (<STDIN>) {
        next LINE if /^#/;      # discard comments
        ...
    }



The last command immediately exits the loop in question.  The
continue block, if any, is not executed:



    LINE: while (<STDIN>) {
        last LINE if /^$/;      # exit when done with header
        ...
    }



The redo command restarts the loop block without evaluating the
conditional again.  The continue block, if any, is not executed.
This command is normally used by programs that want to lie to themselves
about what was just input.



For example, when processing a file like /etc/termcap.
If your input lines might end in backslashes to indicate continuation, you
want to skip ahead and get the next record.



    while (<>) {
        chomp;
        if (s/\\$//) {
            $_ .= <>;
            redo unless eof();
        }
        # now process $_
    }



which is Perl shorthand for the more explicitly written version:



    LINE: while (defined($line = <ARGV>)) {
        chomp($line);
        if ($line =~ s/\\$//) {
            $line .= <ARGV>;
            redo LINE unless eof(); # not eof(ARGV)!
        }
        # now process $line
    }



Note that if there were a continue block on the above code, it would
get executed only on lines discarded by the regex (since redo skips the
continue block).  A continue block is often used to reset line counters
or m?pat? one-time matches:



    # inspired by :1,$g/fred/s//WILMA/
    while (<>) {
        m?(fred)?    && s//WILMA $1 WILMA/;
        m?(barney)?  && s//BETTY $1 BETTY/;
        m?(homer)?   && s//MARGE $1 MARGE/;
    } continue {
        print "$ARGV $.: $_";
        close ARGV  if eof;             # reset $.
        reset       if eof;             # reset ?pat?
    }



If the word while is replaced by the word until, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.



Loop control statements don’t work in an if or unless, since
they aren’t loops.  You can double the braces to make them such, though.



    if (/pattern/) {{
        last if /fred/;
        next if /barney/; # same effect as "last",
                          # but doesn't document as well
        # do something here
    }}



This is caused by the fact that a block by itself acts as a loop that
executes once, see Basic BLOCKs.



The form while/if BLOCK BLOCK, available in Perl 4, is no longer
available.   Replace any occurrence of if BLOCK by if (do BLOCK).









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73.2.8 For Loops





Perl’s C-style for loop works like the corresponding while loop;
that means that this:



    for ($i = 1; $i < 10; $i++) {
        ...
    }



is the same as this:



    $i = 1;
    while ($i < 10) {
        ...
    } continue {
        $i++;
    }




There is one minor difference: if variables are declared with my
in the initialization section of the for, the lexical scope of
those variables is exactly the for loop (the body of the loop
and the control sections).







Besides the normal array index looping, for can lend itself
to many other interesting applications.  Here’s one that avoids the
problem you get into if you explicitly test for end-of-file on
an interactive file descriptor causing your program to appear to
hang.




    $on_a_tty = -t STDIN && -t STDOUT;
    sub prompt { print "yes? " if $on_a_tty }
    for ( prompt(); <STDIN>; prompt() ) {
        # do something
    }





Using readline (or the operator form, <EXPR>) as the
conditional of a for loop is shorthand for the following.  This
behaviour is the same as a while loop conditional.




    for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
        # do something
    }









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73.2.9 Foreach Loops







The foreach loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn.  If the variable
is preceded with the keyword my, then it is lexically scoped, and
is therefore visible only within the loop.  Otherwise, the variable is
implicitly local to the loop and regains its former value upon exiting
the loop.  If the variable was previously declared with my, it uses
that variable instead of the global one, but it’s still localized to
the loop.  This implicit localization occurs only in a foreach
loop.





The foreach keyword is actually a synonym for the for keyword, so
you can use either.  If VAR is omitted, $_ is set to each value.





If any element of LIST is an lvalue, you can modify it by modifying
VAR inside the loop.  Conversely, if any element of LIST is NOT an
lvalue, any attempt to modify that element will fail.  In other words,
the foreach loop index variable is an implicit alias for each item
in the list that you’re looping over.





If any part of LIST is an array, foreach will get very confused if
you add or remove elements within the loop body, for example with
splice.   So don’t do that.




foreach probably won’t do what you expect if VAR is a tied or other
special variable.   Don’t do that either.



Examples:



    for (@ary) { s/foo/bar/ }

    for my $elem (@elements) {
        $elem *= 2;
    }

    for $count (reverse(1..10), "BOOM") {
        print $count, "\n";
        sleep(1);
    }

    for (1..15) { print "Merry Christmas\n"; }

    foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
        print "Item: $item\n";
    }



Here’s how a C programmer might code up a particular algorithm in Perl:



    for (my $i = 0; $i < @ary1; $i++) {
        for (my $j = 0; $j < @ary2; $j++) {
            if ($ary1[$i] > $ary2[$j]) {
                last; # can't go to outer :-(
            }
            $ary1[$i] += $ary2[$j];
        }
        # this is where that last takes me
    }



Whereas here’s how a Perl programmer more comfortable with the idiom might
do it:



    OUTER: for my $wid (@ary1) {
    INNER:   for my $jet (@ary2) {
                next OUTER if $wid > $jet;
                $wid += $jet;
             }
          }



See how much easier this is?  It’s cleaner, safer, and faster.  It’s
cleaner because it’s less noisy.  It’s safer because if code gets added
between the inner and outer loops later on, the new code won’t be
accidentally executed.  The next explicitly iterates the other loop
rather than merely terminating the inner one.  And it’s faster because
Perl executes a foreach statement more rapidly than it would the
equivalent for loop.









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73.2.10 Basic BLOCKs




A BLOCK by itself (labeled or not) is semantically equivalent to a
loop that executes once.  Thus you can use any of the loop control
statements in it to leave or restart the block.  (Note that this is
NOT true in eval{}, sub{}, or contrary to popular belief
do{} blocks, which do NOT count as loops.)  The continue
block is optional.



The BLOCK construct can be used to emulate case structures.



    SWITCH: {
        if (/^abc/) { $abc = 1; last SWITCH; }
        if (/^def/) { $def = 1; last SWITCH; }
        if (/^xyz/) { $xyz = 1; last SWITCH; }
        $nothing = 1;
    }



You’ll also find that foreach loop used to create a topicalizer
and a switch:



    SWITCH:
    for ($var) {
        if (/^abc/) { $abc = 1; last SWITCH; }
        if (/^def/) { $def = 1; last SWITCH; }
        if (/^xyz/) { $xyz = 1; last SWITCH; }
        $nothing = 1;
    }



Such constructs are quite frequently used, both because older versions of
Perl had no official switch statement, and also because the new version
described immediately below remains experimental and can sometimes be confusing.









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73.2.11 Switch Statements










Starting from Perl 5.10.1 (well, 5.10.0, but it didn’t work
right), you can say



    use feature "switch";



to enable an experimental switch feature.  This is loosely based on an
old version of a Perl 6 proposal, but it no longer resembles the Perl 6
construct.   You also get the switch feature whenever you declare that your
code prefers to run under a version of Perl that is 5.10 or later.  For
example:



    use v5.14;



Under the "switch" feature, Perl gains the experimental keywords
given, when, default, continue, and break.
Starting from Perl 5.16, one can prefix the switch
keywords with CORE:: to access the feature without a use feature
statement.  The keywords given and
when are analogous to switch and
case in other languages, so the code in the previous section could be
rewritten as



    use v5.10.1;
    for ($var) {
        when (/^abc/) { $abc = 1 }
        when (/^def/) { $def = 1 }
        when (/^xyz/) { $xyz = 1 }
        default       { $nothing = 1 }
    }



The foreach is the non-experimental way to set a topicalizer.
If you wish to use the highly experimental given, that could be
written like this:



    use v5.10.1;
    given ($var) {
        when (/^abc/) { $abc = 1 }
        when (/^def/) { $def = 1 }
        when (/^xyz/) { $xyz = 1 }
        default       { $nothing = 1 }
    }



As of 5.14, that can also be written this way:



    use v5.14;
    for ($var) {
        $abc = 1 when /^abc/;
        $def = 1 when /^def/;
        $xyz = 1 when /^xyz/;
        default { $nothing = 1 }
    }



Or if you don’t care to play it safe, like this:



    use v5.14;
    given ($var) {
        $abc = 1 when /^abc/;
        $def = 1 when /^def/;
        $xyz = 1 when /^xyz/;
        default { $nothing = 1 }
    }



The arguments to given and when are in scalar context,
and given assigns the $_ variable its topic value.



Exactly what the EXPR argument to when does is hard to describe
precisely, but in general, it tries to guess what you want done.  Sometimes
it is interpreted as $_ ~~ EXPR, and sometimes it does not.  It
also behaves differently when lexically enclosed by a given block than
it does when dynamically enclosed by a foreach loop.  The rules are far
too difficult to understand to be described here.  See Experimental Details
on given and when later on.



Due to an unfortunate bug in how given was implemented between Perl 5.10
and 5.14, under those implementations the version of $_ governed by
given is merely a lexically scoped copy of the original, not a
dynamically scoped alias to the original, as it would be if it were a
foreach or under both the original and the current Perl 6 language
specification.  This bug is expected to be addressed in a future release of
Perl.  For forwards compatibility, if you really want a lexical $_,
specify that explicitly:



    given(my $_ = EXPR) { ... }



In the meanwhile, stick to foreach for your topicalizer and
you will be less unhappy.









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73.2.12 Goto




Although not for the faint of heart, Perl does support a goto
statement.  There are three forms: goto-LABEL, goto-EXPR, and
goto-&NAME.  A loop’s LABEL is not actually a valid target for
a goto; it’s just the name of the loop.



The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there.  It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop.  It
also can’t be used to go into a construct that is optimized away.  It
can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it’s usually better to use some other
construct such as last or die.  The author of Perl has never felt the
need to use this form of goto (in Perl, that is–C is another matter).



The goto-EXPR form expects a label name, whose scope will be resolved
dynamically.  This allows for computed gotos per FORTRAN, but isn’t
necessarily recommended if you’re optimizing for maintainability:



    goto(("FOO", "BAR", "GLARCH")[$i]);



The goto-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine.  This is used by
AUTOLOAD() subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_ in the current subroutine are
propagated to the other subroutine.)  After the goto, not even caller()
will be able to tell that this routine was called first.



In almost all cases like this, it’s usually a far, far better idea to use the
structured control flow mechanisms of next, last, or redo instead of
resorting to a goto.  For certain applications, the catch and throw pair of
eval{} and die() for exception processing can also be a prudent approach.









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73.2.13 The Ellipsis Statement














Beginning in Perl 5.12, Perl accepts an ellipsis, "...", as a
placeholder for code that you haven’t implemented yet.  This form of
ellipsis, the unimplemented statement, should not be confused with the
binary flip-flop ... operator.  One is a statement and the other an
operator.  (Perl doesn’t usually confuse them because usually Perl can tell
whether it wants an operator or a statement, but see below for exceptions.)



When Perl 5.12 or later encounters an ellipsis statement, it parses this
without error, but if and when you should actually try to execute it, Perl
throws an exception with the text Unimplemented:



    use v5.12;
    sub unimplemented { ... }
    eval { unimplemented() };
    if ($@ =~ /^Unimplemented at /) {
        say "I found an ellipsis!";
    }



You can only use the elliptical statement to stand in for a
complete statement.  These examples of how the ellipsis works:



    use v5.12;
    { ... }
    sub foo { ... }
    ...;
    eval { ... };
    sub somemeth {
        my $self = shift;
        ...;
    }
    $x = do {
        my $n;
        ...;
        say "Hurrah!";
        $n;
    };



The elliptical statement cannot stand in for an expression that
is part of a larger statement, since the ... is also the three-dot
version of the flip-flop operator (see (perlop)Range Operators).



These examples of attempts to use an ellipsis are syntax errors:



    use v5.12;

    print ...;
    open(my $fh, ">", "/dev/passwd") or ...;
    if ($condition && ... ) { say "Howdy" };



There are some cases where Perl can’t immediately tell the difference
between an expression and a statement.  For instance, the syntax for a
block and an anonymous hash reference constructor look the same unless
there’s something in the braces to give Perl a hint.  The ellipsis is a
syntax error if Perl doesn’t guess that the { ... } is a block.  In that
case, it doesn’t think the ... is an ellipsis because it’s expecting an
expression instead of a statement:



    @transformed = map { ... } @input;  # syntax error



You can use a ; inside your block to denote that the { ...  } is a
block and not a hash reference constructor.  Now the ellipsis works:



    @transformed = map {; ... } @input; # ; disambiguates

    @transformed = map { ...; } @input; # ; disambiguates



Note: Some folks colloquially refer to this bit of punctuation as a
"yada-yada" or "triple-dot", but its true name
is actually an ellipsis.  Perl does not yet
accept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an alias for
..., but someday it may.









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73.2.14 PODs: Embedded Documentation





Perl has a mechanism for intermixing documentation with source code.
While it’s expecting the beginning of a new statement, if the compiler
encounters a line that begins with an equal sign and a word, like this



    =head1 Here There Be Pods!



Then that text and all remaining text up through and including a line
beginning with =cut will be ignored.  The format of the intervening
text is described in (perlpod).



This allows you to intermix your source code
and your documentation text freely, as in



    =item snazzle($)

    The snazzle() function will behave in the most spectacular
    form that you can possibly imagine, not even excepting
    cybernetic pyrotechnics.

    =cut back to the compiler, nuff of this pod stuff!

    sub snazzle($) {
        my $thingie = shift;
        .........
    }



Note that pod translators should look at only paragraphs beginning
with a pod directive (it makes parsing easier), whereas the compiler
actually knows to look for pod escapes even in the middle of a
paragraph.  This means that the following secret stuff will be
ignored by both the compiler and the translators.



    $a=3;
    =secret stuff
     warn "Neither POD nor CODE!?"
    =cut back
    print "got $a\n";



You probably shouldn’t rely upon the warn() being podded out forever.
Not all pod translators are well-behaved in this regard, and perhaps
the compiler will become pickier.



One may also use pod directives to quickly comment out a section
of code.









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73.2.15 Plain Old Comments (Not!)








Perl can process line directives, much like the C preprocessor.  Using
this, one can control Perl’s idea of filenames and line numbers in
error or warning messages (especially for strings that are processed
with eval()).  The syntax for this mechanism is almost the same as for
most C preprocessors: it matches the regular expression



    # example: '# line 42 "new_filename.plx"'
    /^\#   \s*
      line \s+ (\d+)   \s*
      (?:\s("?)([^"]+)\g2)? \s*
     $/x



with $1 being the line number for the next line, and $3 being
the optional filename (specified with or without quotes).  Note that
no whitespace may precede the #, unlike modern C preprocessors.



There is a fairly obvious gotcha included with the line directive:
Debuggers and profilers will only show the last source line to appear
at a particular line number in a given file.  Care should be taken not
to cause line number collisions in code you’d like to debug later.



Here are some examples that you should be able to type into your command
shell:



    % perl
    # line 200 "bzzzt"
    # the '#' on the previous line must be the first char on line
    die 'foo';
    __END__
    foo at bzzzt line 201.

    % perl
    # line 200 "bzzzt"
    eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
    __END__
    foo at - line 2001.

    % perl
    eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
    __END__
    foo at foo bar line 200.

    % perl
    # line 345 "goop"
    eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
    print $@;
    __END__
    foo at goop line 345.









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73.2.16 Experimental Details on given and when



As previously mentioned, the "switch" feature is considered highly
experimental; it is subject to change with little notice.  In particular,
both given and when have tricky behaviours that are expected to
change to become less tricky in the future.  Do not rely upon their
current (mis)implementations.



Here is a longer example of given:



    use feature ":5.10";
    given ($foo) {
        when (undef) {
            say '$foo is undefined';
        }
        when ("foo") {
            say '$foo is the string "foo"';
        }
        when ([1,3,5,7,9]) {
            say '$foo is an odd digit';
            continue; # Fall through
        }
        when ($_ < 100) {
            say '$foo is numerically less than 100';
        }
        when (\&complicated_check) {
            say 'a complicated check for $foo is true';
        }
        default {
            die q(I don't know what to do with $foo);
        }
    }



As currently implemented, given(EXPR) assign the value of EXPR to
merely a lexically scoped copy (!) of $_, not a dynamically
scoped alias the way foreach does.  That makes it similar to



        do { my $_ = EXPR; ... }



except that the block is automatically broken out of by a
successful when or an explicit break.  Because it is only a
copy, and because it is only lexically scoped, not dynamically
scoped, you cannot do the things with it that you are used to in
a foreach loop.  In particular, you probably cannot use
arbitrary function calls.  Best stick to foreach for that.



Most of the power comes from the implicit smartmatching that can
sometimes apply.  Most of the time, when(EXPR) is treated as an
implicit smartmatch of $_, that is, $_ ~~ EXPR.  (See
(perlop)Smartmatch Operator for more information on smartmatching.)
But when EXPR is one of the 10 exceptional cases (or things like them)
listed below, it is used directly as a boolean.



    

  1.  A user-defined subroutine call or a method invocation.


  2.  A regular expression match in the form of /REGEX/, $foo =~ /REGEX/,
or $foo =~ EXPR.  Also, a negated regular expression match in
the form !/REGEX/, $foo !~ /REGEX/, or $foo !~ EXPR.


  3.  A smart match that uses an explicit ~~ operator, such as EXPR ~~ EXPR.


  4.  A boolean comparison operator such as $_ < 10 or $x eq "abc" The
relational operators that this applies to are the six numeric comparisons
(<, >, <=, >=, ==, and !=), and
the six string comparisons (lt, gt, le, ge, eq, and ne).


    NOTE: You will often have to use $c ~~ $_ because
the default case uses $_ ~~ $c , which is frequently
the opposite of what you want.

    

  5.  At least the three builtin functions defined(...), exists(...), and
eof(...).  We might someday add more of these later if we think of them.


  6.  A negated expression, whether !(EXPR) or not(EXPR), or a logical
exclusive-or, (EXPR1) xor (EXPR2).  The bitwise versions (~ and ^)
are not included.


  7.  A filetest operator, with exactly 4 exceptions: -s, -M, -A, and
-C, as these return numerical values, not boolean ones.  The -z
filetest operator is not included in the exception list.


  8.  The .. and ... flip-flop operators.  Note that the ... flip-flop
operator is completely different from the ... elliptical statement
just described.





In those 8 cases above, the value of EXPR is used directly as a boolean, so
no smartmatching is done.  You may think of when as a smartsmartmatch.



Furthermore, Perl inspects the operands of logical operators to
decide whether to use smartmatching for each one by applying the
above test to the operands:





9.




If EXPR is EXPR1 && EXPR2 or EXPR1 and EXPR2, the test is applied
recursively to both EXPR1 and EXPR2.
Only if both operands also pass the
test, recursively, will the expression be treated as boolean.  Otherwise,
smartmatching is used.





10.




If EXPR is EXPR1 || EXPR2, EXPR1 // EXPR2, or EXPR1 or EXPR2, the
test is applied recursively to EXPR1 only (which might itself be a
higher-precedence AND operator, for example, and thus subject to the
previous rule), not to EXPR2.  If EXPR1 is to use smartmatching, then EXPR2
also does so, no matter what EXPR2 contains.  But if EXPR2 does not get to
use smartmatching, then the second argument will not be either.  This is
quite different from the && case just described, so be careful.








These rules are complicated, but the goal is for them to do what you want
(even if you don’t quite understand why they are doing it).  For example:



    when (/^\d+$/ && $_ < 75) { ... }



will be treated as a boolean match because the rules say both
a regex match and an explicit test on $_ will be treated
as boolean.



Also:



    when ([qw(foo bar)] && /baz/) { ... }



will use smartmatching because only one of the operands is a boolean:
the other uses smartmatching, and that wins.



Further:



    when ([qw(foo bar)] || /^baz/) { ... }



will use smart matching (only the first operand is considered), whereas



    when (/^baz/ || [qw(foo bar)]) { ... }



will test only the regex, which causes both operands to be
treated as boolean.  Watch out for this one, then, because an
arrayref is always a true value, which makes it effectively
redundant.  Not a good idea.



Tautologous boolean operators are still going to be optimized
away.  Don’t be tempted to write



    when ("foo" or "bar") { ... }



This will optimize down to "foo", so "bar" will never be considered (even
though the rules say to use a smartmatch
on "foo").  For an alternation like
this, an array ref will work, because this will instigate smartmatching:



    when ([qw(foo bar)] { ... }



This is somewhat equivalent to the C-style switch statement’s fallthrough
functionality (not to be confused with Perl’s fallthrough
functionality–see below), wherein the same block is used for several
case statements.



Another useful shortcut is that, if you use a literal array or hash as the
argument to given, it is turned into a reference.  So given(@foo) is
the same as given(\@foo), for example.



default behaves exactly like when(1 == 1), which is
to say that it always matches.





• NAME syntax Breaking out:  



• NAME syntax Fall-through:  



• NAME syntax Return value:  



• NAME syntax Switching in a loop:  



• NAME syntax Differences from Perl 6:  












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73.2.16.1 Breaking out



You can use the break keyword to break out of the enclosing
given block.  Every when block is implicitly ended with
a break.









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73.2.16.2 Fall-through



You can use the continue keyword to fall through from one
case to the next:



    given($foo) {
        when (/x/) { say '$foo contains an x'; continue }
        when (/y/) { say '$foo contains a y'            }
        default    { say '$foo does not contain a y'    }
    }









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73.2.16.3 Return value



When a given statement is also a valid expression (for example,
when it’s the last statement of a block), it evaluates to:





In both last cases, the last expression is evaluated in the context that
was applied to the given block.



Note that, unlike if and unless, failed when statements always
evaluate to an empty list.



    my $price = do {
        given ($item) {
            when (["pear", "apple"]) { 1 }
            break when "vote";      # My vote cannot be bought
            1e10  when /Mona Lisa/;
            "unknown";
        }
    };



Currently, given blocks can’t always
be used as proper expressions.  This
may be addressed in a future version of Perl.









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73.2.16.4 Switching in a loop



Instead of using given(), you can use a foreach() loop.
For example, here’s one way to count how many times a particular
string occurs in an array:



    use v5.10.1;
    my $count = 0;
    for (@array) {
        when ("foo") { ++$count }
    }
    print "\@array contains $count copies of 'foo'\n";



Or in a more recent version:



    use v5.14;
    my $count = 0;
    for (@array) {
        ++$count when "foo";
    }
    print "\@array contains $count copies of 'foo'\n";



At the end of all when blocks, there is an implicit next.
You can override that with an explicit last if you’re
interested in only the first match alone.



This doesn’t work if you explicitly specify a loop variable, as
in for $item (@array).  You have to use the default variable $_.









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73.2.16.5 Differences from Perl 6



The Perl 5 smartmatch and given/when constructs are not compatible
with their Perl 6 analogues.  The most visible difference and least
important difference is that, in Perl 5, parentheses are required around
the argument to given() and when() (except when this last one is used
as a statement modifier).  Parentheses in Perl 6 are always optional in a
control construct such as if(), while(), or when(); they can’t be
made optional in Perl 5 without a great deal of potential confusion,
because Perl 5 would parse the expression



    given $foo {
        ...
    }



as though the argument to given were an element of the hash
%foo, interpreting the braces as hash-element syntax.



However, their are many, many other differences.  For example,
this works in Perl 5:



    use v5.12;
    my @primary = ("red", "blue", "green");

    if (@primary ~~ "red") {
        say "primary smartmatches red";
    }

    if ("red" ~~ @primary) {
        say "red smartmatches primary";
    }

    say "that's all, folks!";



But it doesn’t work at all in Perl 6.  Instead, you should
use the (parallelizable) any operator instead:



   if any(@primary) eq "red" {
       say "primary smartmatches red";
   }

   if "red" eq any(@primary) {
       say "red smartmatches primary";
   }



The table of smartmatches in (perlop)Smartmatch Operator is not
identical to that proposed by the Perl 6 specification, mainly due to
differences between Perl 6’s and Perl 5’s data models, but also because
the Perl 6 spec has changed since Perl 5 rushed into early adoption.



In Perl 6, when() will always do an implicit smartmatch with its
argument, while in Perl 5 it is convenient (albeit potentially confusing) to
suppress this implicit smartmatch in various rather loosely-defined
situations, as roughly outlined above.  (The difference is largely because
Perl 5 does not have, even internally, a boolean type.)









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74 perlthrtut





• perlthrtut NAME:  



• perlthrtut DESCRIPTION:  



• perlthrtut What Is A Thread Anyway?:  



• perlthrtut Threaded Program Models:  



• perlthrtut What kind of threads are Perl threads?:  



• perlthrtut Thread-Safe Modules:  



• perlthrtut Thread Basics:  



• perlthrtut Threads And Data:  



• perlthrtut Synchronization and control:  



• perlthrtut General Thread Utility Routines:  



• perlthrtut A Complete Example:  



• perlthrtut Different implementations of threads:  



• perlthrtut Performance considerations:  



• perlthrtut Process-scope Changes:  



• perlthrtut Thread-Safety of System Libraries:  



• perlthrtut Conclusion:  



• perlthrtut SEE ALSO:  



• perlthrtut Bibliography:  



• perlthrtut Acknowledgements:  



• perlthrtut AUTHOR:  



• perlthrtut Copyrights:  












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74.1 NAME



perlthrtut - Tutorial on threads in Perl









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74.2 DESCRIPTION



This tutorial describes the use of Perl interpreter threads (sometimes
referred to as ithreads) that was first introduced in Perl 5.6.0.  In this
model, each thread runs in its own Perl interpreter, and any data sharing
between threads must be explicit.  The user-level interface for ithreads
uses the (threads) class.



NOTE: There was another older Perl threading flavor called the 5.005 model
that used the (threads) class.  This old model was known to have problems, is
deprecated, and was removed for release 5.10.  You are
strongly encouraged to migrate any existing 5.005 threads code to the new
model as soon as possible.



You can see which (or neither) threading flavour you have by
running perl -V and looking at the Platform section.
If you have useithreads=define you have ithreads, if you
have use5005threads=define you have 5.005 threads.
If you have neither, you don’t have any thread support built in.
If you have both, you are in trouble.



The (threads) and (threads-shared) modules are included in the core Perl
distribution.  Additionally, they are maintained as a separate modules on
CPAN, so you can check there for any updates.









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74.3 What Is A Thread Anyway?



A thread is a flow of control through a program with a single
execution point.



Sounds an awful lot like a process, doesn’t it? Well, it should.
Threads are one of the pieces of a process.  Every process has at least
one thread and, up until now, every process running Perl had only one
thread.  With 5.8, though, you can create extra threads.  We’re going
to show you how, when, and why.









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74.4 Threaded Program Models



There are three basic ways that you can structure a threaded
program.  Which model you choose depends on what you need your program
to do.  For many non-trivial threaded programs, you’ll need to choose
different models for different pieces of your program.





• perlthrtut Boss/Worker:  



• perlthrtut Work Crew:  



• perlthrtut Pipeline:  












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74.4.1 Boss/Worker



The boss/worker model usually has one boss thread and one or more
worker threads.  The boss thread gathers or generates tasks that need
to be done, then parcels those tasks out to the appropriate worker
thread.



This model is common in GUI and server programs, where a main thread
waits for some event and then passes that event to the appropriate
worker threads for processing.  Once the event has been passed on, the
boss thread goes back to waiting for another event.



The boss thread does relatively little work.  While tasks aren’t
necessarily performed faster than with any other method, it tends to
have the best user-response times.









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74.4.2 Work Crew



In the work crew model, several threads are created that do
essentially the same thing to different pieces of data.  It closely
mirrors classical parallel processing and vector processors, where a
large array of processors do the exact same thing to many pieces of
data.



This model is particularly useful if the system running the program
will distribute multiple threads across different processors.  It can
also be useful in ray tracing or rendering engines, where the
individual threads can pass on interim results to give the user visual
feedback.









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74.4.3 Pipeline



The pipeline model divides up a task into a series of steps, and
passes the results of one step on to the thread processing the
next.  Each thread does one thing to each piece of data and passes the
results to the next thread in line.



This model makes the most sense if you have multiple processors so two
or more threads will be executing in parallel, though it can often
make sense in other contexts as well.  It tends to keep the individual
tasks small and simple, as well as allowing some parts of the pipeline
to block (on I/O or system calls, for example) while other parts keep
going.  If you’re running different parts of the pipeline on different
processors you may also take advantage of the caches on each
processor.



This model is also handy for a form of recursive programming where,
rather than having a subroutine call itself, it instead creates
another thread.  Prime and Fibonacci generators both map well to this
form of the pipeline model. (A version of a prime number generator is
presented later on.)









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74.5 What kind of threads are Perl threads?



If you have experience with other thread implementations, you might
find that things aren’t quite what you expect.  It’s very important to
remember when dealing with Perl threads that Perl Threads Are Not X
Threads for all values of X.  They aren’t POSIX threads, or
DecThreads, or Java’s Green threads, or Win32 threads.  There are
similarities, and the broad concepts are the same, but if you start
looking for implementation details you’re going to be either
disappointed or confused.  Possibly both.



This is not to say that Perl threads are completely different from
everything that’s ever come before. They’re not.  Perl’s threading
model owes a lot to other thread models, especially POSIX.  Just as
Perl is not C, though, Perl threads are not POSIX threads.  So if you
find yourself looking for mutexes, or thread priorities, it’s time to
step back a bit and think about what you want to do and how Perl can
do it.



However, it is important to remember that Perl threads cannot magically
do things unless your operating system’s threads allow it. So if your
system blocks the entire process on sleep(), Perl usually will, as well.



Perl Threads Are Different.









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74.6 Thread-Safe Modules



The addition of threads has changed Perl’s internals
substantially. There are implications for people who write
modules with XS code or external libraries. However, since Perl data is
not shared among threads by default, Perl modules stand a high chance of
being thread-safe or can be made thread-safe easily.  Modules that are not
tagged as thread-safe should be tested or code reviewed before being used
in production code.



Not all modules that you might use are thread-safe, and you should
always assume a module is unsafe unless the documentation says
otherwise.  This includes modules that are distributed as part of the
core.  Threads are a relatively new feature, and even some of the standard
modules aren’t thread-safe.



Even if a module is thread-safe, it doesn’t mean that the module is optimized
to work well with threads. A module could possibly be rewritten to utilize
the new features in threaded Perl to increase performance in a threaded
environment.



If you’re using a module that’s not thread-safe for some reason, you
can protect yourself by using it from one, and only one thread at all.
If you need multiple threads to access such a module, you can use semaphores and
lots of programming discipline to control access to it.  Semaphores
are covered in Basic semaphores.



See also Thread-Safety of System Libraries.









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74.7 Thread Basics



The (threads) module provides the basic functions you need to write
threaded programs.  In the following sections, we’ll cover the basics,
showing you what you need to do to create a threaded program.   After
that, we’ll go over some of the features of the (threads) module that
make threaded programming easier.





• perlthrtut Basic Thread Support:  



• perlthrtut A Note about the Examples:  



• perlthrtut Creating Threads:  



• perlthrtut Waiting For A Thread To Exit:  



• perlthrtut Ignoring A Thread:  



• perlthrtut Process and Thread Termination:  












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74.7.1 Basic Thread Support



Thread support is a Perl compile-time option. It’s something that’s
turned on or off when Perl is built at your site, rather than when
your programs are compiled. If your Perl wasn’t compiled with thread
support enabled, then any attempt to use threads will fail.



Your programs can use the Config module to check whether threads are
enabled. If your program can’t run without them, you can say something
like:



    use Config;
    $Config{useithreads} or die('Recompile Perl with threads to run this program.');



A possibly-threaded program using a possibly-threaded module might
have code like this:



    use Config;
    use MyMod;

    BEGIN {
        if ($Config{useithreads}) {
            # We have threads
            require MyMod_threaded;
            import MyMod_threaded;
        } else {
            require MyMod_unthreaded;
            import MyMod_unthreaded;
        }
    }



Since code that runs both with and without threads is usually pretty
messy, it’s best to isolate the thread-specific code in its own
module.  In our example above, that’s what MyMod_threaded is, and it’s
only imported if we’re running on a threaded Perl.









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74.7.2 A Note about the Examples



In a real situation, care should be taken that all threads are finished
executing before the program exits.  That care has not been taken in these
examples in the interest of simplicity.  Running these examples as is will
produce error messages, usually caused by the fact that there are still
threads running when the program exits.  You should not be alarmed by this.









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74.7.3 Creating Threads



The (threads) module provides the tools you need to create new
threads.  Like any other module, you need to tell Perl that you want to use
it; use threads; imports all the pieces you need to create basic
threads.



The simplest, most straightforward way to create a thread is with create():



    use threads;

    my $thr = threads->create(\&sub1);

    sub sub1 {
        print("In the thread\n");
    }



The create() method takes a reference to a subroutine and creates a new
thread that starts executing in the referenced subroutine.  Control
then passes both to the subroutine and the caller.



If you need to, your program can pass parameters to the subroutine as
part of the thread startup.  Just include the list of parameters as
part of the threads->create() call, like this:



    use threads;

    my $Param3 = 'foo';
    my $thr1 = threads->create(\&sub1, 'Param 1', 'Param 2', $Param3);
    my @ParamList = (42, 'Hello', 3.14);
    my $thr2 = threads->create(\&sub1, @ParamList);
    my $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3));

    sub sub1 {
        my @InboundParameters = @_;
        print("In the thread\n");
        print('Got parameters >', join('<>', @InboundParameters), "<\n");
    }



The last example illustrates another feature of threads.  You can spawn
off several threads using the same subroutine.  Each thread executes
the same subroutine, but in a separate thread with a separate
environment and potentially separate arguments.



new() is a synonym for create().









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74.7.4 Waiting For A Thread To Exit



Since threads are also subroutines, they can return values.  To wait
for a thread to exit and extract any values it might return, you can
use the join() method:



    use threads;

    my ($thr) = threads->create(\&sub1);

    my @ReturnData = $thr->join();
    print('Thread returned ', join(', ', @ReturnData), "\n");

    sub sub1 { return ('Fifty-six', 'foo', 2); }



In the example above, the join() method returns as soon as the thread
ends.  In addition to waiting for a thread to finish and gathering up
any values that the thread might have returned, join() also performs
any OS cleanup necessary for the thread.  That cleanup might be
important, especially for long-running programs that spawn lots of
threads.  If you don’t want the return values and don’t want to wait
for the thread to finish, you should call the detach() method
instead, as described next.



NOTE: In the example above, the thread returns a list, thus necessitating
that the thread creation call be made in list context (i.e., my ($thr)).
See (threads)$thr->join() and (threads)THREAD CONTEXT for more
details on thread context and return values.









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74.7.5 Ignoring A Thread



join() does three things: it waits for a thread to exit, cleans up
after it, and returns any data the thread may have produced.  But what
if you’re not interested in the thread’s return values, and you don’t
really care when the thread finishes? All you want is for the thread
to get cleaned up after when it’s done.



In this case, you use the detach() method.  Once a thread is detached,
it’ll run until it’s finished; then Perl will clean up after it
automatically.



    use threads;

    my $thr = threads->create(\&sub1);   # Spawn the thread

    $thr->detach();   # Now we officially don't care any more

    sleep(15);        # Let thread run for awhile

    sub sub1 {
        $a = 0;
        while (1) {
            $a++;
            print("\$a is $a\n");
            sleep(1);
        }
    }



Once a thread is detached, it may not be joined, and any return data
that it might have produced (if it was done and waiting for a join) is
lost.



detach() can also be called as a class method to allow a thread to
detach itself:



    use threads;

    my $thr = threads->create(\&sub1);

    sub sub1 {
        threads->detach();
        # Do more work
    }









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74.7.6 Process and Thread Termination



With threads one must be careful to make sure they all have a chance to
run to completion, assuming that is what you want.



An action that terminates a process will terminate all running
threads.  die() and exit() have this property,
and perl does an exit when the main thread exits,
perhaps implicitly by falling off the end of your code,
even if that’s not what you want.



As an example of this case, this code prints the message
"Perl exited with active threads: 2 running and unjoined":



    use threads;
    my $thr1 = threads->new(\&thrsub, "test1");
    my $thr2 = threads->new(\&thrsub, "test2");
    sub thrsub {
       my ($message) = @_;
       sleep 1;
       print "thread $message\n";
    }



But when the following lines are added at the end:



    $thr1->join();
    $thr2->join();



it prints two lines of output, a perhaps more useful outcome.









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74.8 Threads And Data



Now that we’ve covered the basics of threads, it’s time for our next
topic: Data.  Threading introduces a couple of complications to data
access that non-threaded programs never need to worry about.





• perlthrtut Shared And Unshared Data:  



• perlthrtut Thread Pitfalls: Races:  












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74.8.1 Shared And Unshared Data



The biggest difference between Perl ithreads and the old 5.005 style
threading, or for that matter, to most other threading systems out there,
is that by default, no data is shared. When a new Perl thread is created,
all the data associated with the current thread is copied to the new
thread, and is subsequently private to that new thread!
This is similar in feel to what happens when a Unix process forks,
except that in this case, the data is just copied to a different part of
memory within the same process rather than a real fork taking place.



To make use of threading, however, one usually wants the threads to share
at least some data between themselves. This is done with the
(threads-shared) module and the :shared attribute:



    use threads;
    use threads::shared;

    my $foo :shared = 1;
    my $bar = 1;
    threads->create(sub { $foo++; $bar++; })->join();

    print("$foo\n");  # Prints 2 since $foo is shared
    print("$bar\n");  # Prints 1 since $bar is not shared



In the case of a shared array, all the array’s elements are shared, and for
a shared hash, all the keys and values are shared. This places
restrictions on what may be assigned to shared array and hash elements: only
simple values or references to shared variables are allowed - this is
so that a private variable can’t accidentally become shared. A bad
assignment will cause the thread to die. For example:



    use threads;
    use threads::shared;

    my $var          = 1;
    my $svar :shared = 2;
    my %hash :shared;

    ... create some threads ...

    $hash{a} = 1;       # All threads see exists($hash{a}) and $hash{a} == 1
    $hash{a} = $var;    # okay - copy-by-value: same effect as previous
    $hash{a} = $svar;   # okay - copy-by-value: same effect as previous
    $hash{a} = \$svar;  # okay - a reference to a shared variable
    $hash{a} = \$var;   # This will die
    delete($hash{a});   # okay - all threads will see !exists($hash{a})



Note that a shared variable guarantees that if two or more threads try to
modify it at the same time, the internal state of the variable will not
become corrupted. However, there are no guarantees beyond this, as
explained in the next section.









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74.8.2 Thread Pitfalls: Races



While threads bring a new set of useful tools, they also bring a
number of pitfalls.  One pitfall is the race condition:



    use threads;
    use threads::shared;

    my $a :shared = 1;
    my $thr1 = threads->create(\&sub1);
    my $thr2 = threads->create(\&sub2);

    $thr1->join();
    $thr2->join();
    print("$a\n");

    sub sub1 { my $foo = $a; $a = $foo + 1; }
    sub sub2 { my $bar = $a; $a = $bar + 1; }



What do you think $a will be? The answer, unfortunately, is it
depends. Both sub1() and sub2() access the global variable $a, once
to read and once to write.  Depending on factors ranging from your
thread implementation’s scheduling algorithm to the phase of the moon,
$a can be 2 or 3.



Race conditions are caused by unsynchronized access to shared
data.  Without explicit synchronization, there’s no way to be sure that
nothing has happened to the shared data between the time you access it
and the time you update it.  Even this simple code fragment has the
possibility of error:



    use threads;
    my $a :shared = 2;
    my $b :shared;
    my $c :shared;
    my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
    my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
    $thr1->join();
    $thr2->join();



Two threads both access $a.  Each thread can potentially be interrupted
at any point, or be executed in any order.  At the end, $a could be 3
or 4, and both $b and $c could be 2 or 3.



Even $a += 5 or $a++ are not guaranteed to be atomic.



Whenever your program accesses data or resources that can be accessed
by other threads, you must take steps to coordinate access or risk
data inconsistency and race conditions. Note that Perl will protect its
internals from your race conditions, but it won’t protect you from you.









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74.9 Synchronization and control



Perl provides a number of mechanisms to coordinate the interactions
between themselves and their data, to avoid race conditions and the like.
Some of these are designed to resemble the common techniques used in thread
libraries such as pthreads; others are Perl-specific. Often, the
standard techniques are clumsy and difficult to get right (such as
condition waits). Where possible, it is usually easier to use Perlish
techniques such as queues, which remove some of the hard work involved.





• perlthrtut Controlling access: lock():  



• perlthrtut A Thread Pitfall: Deadlocks:  



• perlthrtut Queues: Passing Data Around:  



• perlthrtut Semaphores: Synchronizing Data Access:  



• perlthrtut Basic semaphores:  



• perlthrtut Advanced Semaphores:  



• perlthrtut Waiting for a Condition:  



• perlthrtut Giving up control:  












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74.9.1 Controlling access: lock()



The lock() function takes a shared variable and puts a lock on it.
No other thread may lock the variable until the variable is unlocked
by the thread holding the lock. Unlocking happens automatically
when the locking thread exits the block that contains the call to the
lock() function.  Using lock() is straightforward: This example has
several threads doing some calculations in parallel, and occasionally
updating a running total:



    use threads;
    use threads::shared;

    my $total :shared = 0;

    sub calc {
        while (1) {
            my $result;
            # (... do some calculations and set $result ...)
            {
                lock($total);  # Block until we obtain the lock
                $total += $result;
            } # Lock implicitly released at end of scope
            last if $result == 0;
        }
    }

    my $thr1 = threads->create(\&calc);
    my $thr2 = threads->create(\&calc);
    my $thr3 = threads->create(\&calc);
    $thr1->join();
    $thr2->join();
    $thr3->join();
    print("total=$total\n");



lock() blocks the thread until the variable being locked is
available.  When lock() returns, your thread can be sure that no other
thread can lock that variable until the block containing the
lock exits.



It’s important to note that locks don’t prevent access to the variable
in question, only lock attempts.  This is in keeping with Perl’s
longstanding tradition of courteous programming, and the advisory file
locking that flock() gives you.



You may lock arrays and hashes as well as scalars.  Locking an array,
though, will not block subsequent locks on array elements, just lock
attempts on the array itself.



Locks are recursive, which means it’s okay for a thread to
lock a variable more than once.  The lock will last until the outermost
lock() on the variable goes out of scope. For example:



    my $x :shared;
    doit();

    sub doit {
        {
            {
                lock($x); # Wait for lock
                lock($x); # NOOP - we already have the lock
                {
                    lock($x); # NOOP
                    {
                        lock($x); # NOOP
                        lockit_some_more();
                    }
                }
            } # *** Implicit unlock here ***
        }
    }

    sub lockit_some_more {
        lock($x); # NOOP
    } # Nothing happens here



Note that there is no unlock() function - the only way to unlock a
variable is to allow it to go out of scope.



A lock can either be used to guard the data contained within the variable
being locked, or it can be used to guard something else, like a section
of code. In this latter case, the variable in question does not hold any
useful data, and exists only for the purpose of being locked. In this
respect, the variable behaves like the mutexes and basic semaphores of
traditional thread libraries.









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74.9.2 A Thread Pitfall: Deadlocks



Locks are a handy tool to synchronize access to data, and using them
properly is the key to safe shared data.  Unfortunately, locks aren’t
without their dangers, especially when multiple locks are involved.
Consider the following code:



    use threads;

    my $a :shared = 4;
    my $b :shared = 'foo';
    my $thr1 = threads->create(sub {
        lock($a);
        sleep(20);
        lock($b);
    });
    my $thr2 = threads->create(sub {
        lock($b);
        sleep(20);
        lock($a);
    });



This program will probably hang until you kill it.  The only way it
won’t hang is if one of the two threads acquires both locks
first.  A guaranteed-to-hang version is more complicated, but the
principle is the same.



The first thread will grab a lock on $a, then, after a pause during which
the second thread has probably had time to do some work, try to grab a
lock on $b.  Meanwhile, the second thread grabs a lock on $b, then later
tries to grab a lock on $a.  The second lock attempt for both threads will
block, each waiting for the other to release its lock.



This condition is called a deadlock, and it occurs whenever two or
more threads are trying to get locks on resources that the others
own.  Each thread will block, waiting for the other to release a lock
on a resource.  That never happens, though, since the thread with the
resource is itself waiting for a lock to be released.



There are a number of ways to handle this sort of problem.  The best
way is to always have all threads acquire locks in the exact same
order.  If, for example, you lock variables $a, $b, and $c, always lock
$a before $b, and $b before $c.  It’s also best to hold on to locks for
as short a period of time to minimize the risks of deadlock.



The other synchronization primitives described below can suffer from
similar problems.









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74.9.3 Queues: Passing Data Around



A queue is a special thread-safe object that lets you put data in one
end and take it out the other without having to worry about
synchronization issues.  They’re pretty straightforward, and look like
this:



    use threads;
    use Thread::Queue;

    my $DataQueue = Thread::Queue->new();
    my $thr = threads->create(sub {
        while (my $DataElement = $DataQueue->dequeue()) {
            print("Popped $DataElement off the queue\n");
        }
    });

    $DataQueue->enqueue(12);
    $DataQueue->enqueue("A", "B", "C");
    sleep(10);
    $DataQueue->enqueue(undef);
    $thr->join();



You create the queue with Thread::Queue->new().  Then you can
add lists of scalars onto the end with enqueue(), and pop scalars off
the front of it with dequeue().  A queue has no fixed size, and can grow
as needed to hold everything pushed on to it.



If a queue is empty, dequeue() blocks until another thread enqueues
something.  This makes queues ideal for event loops and other
communications between threads.









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74.9.4 Semaphores: Synchronizing Data Access



Semaphores are a kind of generic locking mechanism. In their most basic
form, they behave very much like lockable scalars, except that they
can’t hold data, and that they must be explicitly unlocked. In their
advanced form, they act like a kind of counter, and can allow multiple
threads to have the lock at any one time.









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74.9.5 Basic semaphores



Semaphores have two methods, down() and up(): down() decrements the resource
count, while up() increments it. Calls to down() will block if the
semaphore’s current count would decrement below zero.  This program
gives a quick demonstration:



    use threads;
    use Thread::Semaphore;

    my $semaphore = Thread::Semaphore->new();
    my $GlobalVariable :shared = 0;

    $thr1 = threads->create(\&sample_sub, 1);
    $thr2 = threads->create(\&sample_sub, 2);
    $thr3 = threads->create(\&sample_sub, 3);

    sub sample_sub {
        my $SubNumber = shift(@_);
        my $TryCount = 10;
        my $LocalCopy;
        sleep(1);
        while ($TryCount--) {
            $semaphore->down();
            $LocalCopy = $GlobalVariable;
            print("$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n");
            sleep(2);
            $LocalCopy++;
            $GlobalVariable = $LocalCopy;
            $semaphore->up();
        }
    }

    $thr1->join();
    $thr2->join();
    $thr3->join();



The three invocations of the subroutine all operate in sync.  The
semaphore, though, makes sure that only one thread is accessing the
global variable at once.









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74.9.6 Advanced Semaphores



By default, semaphores behave like locks, letting only one thread
down() them at a time.  However, there are other uses for semaphores.



Each semaphore has a counter attached to it. By default, semaphores are
created with the counter set to one, down() decrements the counter by
one, and up() increments by one. However, we can override any or all
of these defaults simply by passing in different values:



    use threads;
    use Thread::Semaphore;

    my $semaphore = Thread::Semaphore->new(5);
                    # Creates a semaphore with the counter set to five

    my $thr1 = threads->create(\&sub1);
    my $thr2 = threads->create(\&sub1);

    sub sub1 {
        $semaphore->down(5); # Decrements the counter by five
        # Do stuff here
        $semaphore->up(5); # Increment the counter by five
    }

    $thr1->detach();
    $thr2->detach();



If down() attempts to decrement the counter below zero, it blocks until
the counter is large enough.  Note that while a semaphore can be created
with a starting count of zero, any up() or down() always changes the
counter by at least one, and so $semaphore->down(0) is the same as
$semaphore->down(1).



The question, of course, is why would you do something like this? Why
create a semaphore with a starting count that’s not one, or why
decrement or increment it by more than one? The answer is resource
availability.  Many resources that you want to manage access for can be
safely used by more than one thread at once.



For example, let’s take a GUI driven program.  It has a semaphore that
it uses to synchronize access to the display, so only one thread is
ever drawing at once.  Handy, but of course you don’t want any thread
to start drawing until things are properly set up.  In this case, you
can create a semaphore with a counter set to zero, and up it when
things are ready for drawing.



Semaphores with counters greater than one are also useful for
establishing quotas.  Say, for example, that you have a number of
threads that can do I/O at once.  You don’t want all the threads
reading or writing at once though, since that can potentially swamp
your I/O channels, or deplete your process’s quota of filehandles.  You
can use a semaphore initialized to the number of concurrent I/O
requests (or open files) that you want at any one time, and have your
threads quietly block and unblock themselves.



Larger increments or decrements are handy in those cases where a
thread needs to check out or return a number of resources at once.









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74.9.7 Waiting for a Condition



The functions cond_wait() and cond_signal()
can be used in conjunction with locks to notify
co-operating threads that a resource has become available. They are
very similar in use to the functions found in pthreads. However
for most purposes, queues are simpler to use and more intuitive. See
(threads-shared) for more details.









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74.9.8 Giving up control



There are times when you may find it useful to have a thread
explicitly give up the CPU to another thread.  You may be doing something
processor-intensive and want to make sure that the user-interface thread
gets called frequently.  Regardless, there are times that you might want
a thread to give up the processor.



Perl’s threading package provides the yield() function that does
this. yield() is pretty straightforward, and works like this:



    use threads;

    sub loop {
        my $thread = shift;
        my $foo = 50;
        while($foo--) { print("In thread $thread\n"); }
        threads->yield();
        $foo = 50;
        while($foo--) { print("In thread $thread\n"); }
    }

    my $thr1 = threads->create(\&loop, 'first');
    my $thr2 = threads->create(\&loop, 'second');
    my $thr3 = threads->create(\&loop, 'third');



It is important to remember that yield() is only a hint to give up the CPU,
it depends on your hardware, OS and threading libraries what actually happens.
On many operating systems, yield() is a no-op.  Therefore it is important
to note that one should not build the scheduling of the threads around
yield() calls. It might work on your platform but it won’t work on another
platform.









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74.10 General Thread Utility Routines



We’ve covered the workhorse parts of Perl’s threading package, and
with these tools you should be well on your way to writing threaded
code and packages.  There are a few useful little pieces that didn’t
really fit in anyplace else.





• perlthrtut What Thread Am I In?:  



• perlthrtut Thread IDs:  



• perlthrtut Are These Threads The Same?:  



• perlthrtut What Threads Are Running?:  












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74.10.1 What Thread Am I In?



The threads->self() class method provides your program with a way to
get an object representing the thread it’s currently in.  You can use this
object in the same way as the ones returned from thread creation.









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74.10.2 Thread IDs



tid() is a thread object method that returns the thread ID of the
thread the object represents.  Thread IDs are integers, with the main
thread in a program being 0.  Currently Perl assigns a unique TID to
every thread ever created in your program, assigning the first thread
to be created a TID of 1, and increasing the TID by 1 for each new
thread that’s created.  When used as a class method, threads->tid()
can be used by a thread to get its own TID.









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74.10.3 Are These Threads The Same?



The equal() method takes two thread objects and returns true
if the objects represent the same thread, and false if they don’t.



Thread objects also have an overloaded == comparison so that you can do
comparison on them as you would with normal objects.









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74.10.4 What Threads Are Running?



threads->list() returns a list of thread objects, one for each thread
that’s currently running and not detached.  Handy for a number of things,
including cleaning up at the end of your program (from the main Perl thread,
of course):



    # Loop through all the threads
    foreach my $thr (threads->list()) {
        $thr->join();
    }



If some threads have not finished running when the main Perl thread
ends, Perl will warn you about it and die, since it is impossible for Perl
to clean up itself while other threads are running.



NOTE:  The main Perl thread (thread 0) is in a detached state, and so
does not appear in the list returned by threads->list().









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74.11 A Complete Example



Confused yet? It’s time for an example program to show some of the
things we’ve covered.  This program finds prime numbers using threads.



     1 #!/usr/bin/perl
     2 # prime-pthread, courtesy of Tom Christiansen
     3
     4 use strict;
     5 use warnings;
     6
     7 use threads;
     8 use Thread::Queue;
     9
    10 sub check_num {
    11     my ($upstream, $cur_prime) = @_;
    12     my $kid;
    13     my $downstream = Thread::Queue->new();
    14     while (my $num = $upstream->dequeue()) {
    15         next unless ($num % $cur_prime);
    16         if ($kid) {
    17             $downstream->enqueue($num);
    18         } else {
    19             print("Found prime: $num\n");
    20             $kid = threads->create(\&check_num, $downstream, $num);
    21             if (! $kid) {
    22                 warn("Sorry.  Ran out of threads.\n");
    23                 last;
    24             }
    25         }
    26     }
    27     if ($kid) {
    28         $downstream->enqueue(undef);
    29         $kid->join();
    30     }
    31 }
    32
    33 my $stream = Thread::Queue->new(3..1000, undef);
    34 check_num($stream, 2);



This program uses the pipeline model to generate prime numbers.  Each
thread in the pipeline has an input queue that feeds numbers to be
checked, a prime number that it’s responsible for, and an output queue
into which it funnels numbers that have failed the check.  If the thread
has a number that’s failed its check and there’s no child thread, then
the thread must have found a new prime number.  In that case, a new
child thread is created for that prime and stuck on the end of the
pipeline.



This probably sounds a bit more confusing than it really is, so let’s
go through this program piece by piece and see what it does.  (For
those of you who might be trying to remember exactly what a prime
number is, it’s a number that’s only evenly divisible by itself and 1.)



The bulk of the work is done by the check_num() subroutine, which
takes a reference to its input queue and a prime number that it’s
responsible for.  After pulling in the input queue and the prime that
the subroutine is checking (line 11), we create a new queue (line 13)
and reserve a scalar for the thread that we’re likely to create later
(line 12).



The while loop from line 14 to line 26 grabs a scalar off the input
queue and checks against the prime this thread is responsible
for.  Line 15 checks to see if there’s a remainder when we divide the
number to be checked by our prime.  If there is one, the number
must not be evenly divisible by our prime, so we need to either pass
it on to the next thread if we’ve created one (line 17) or create a
new thread if we haven’t.



The new thread creation is line 20.  We pass on to it a reference to
the queue we’ve created, and the prime number we’ve found.  In lines 21
through 24, we check to make sure that our new thread got created, and
if not, we stop checking any remaining numbers in the queue.



Finally, once the loop terminates (because we got a 0 or undef in the
queue, which serves as a note to terminate), we pass on the notice to our
child, and wait for it to exit if we’ve created a child (lines 27 and
30).



Meanwhile, back in the main thread, we first create a queue (line 33) and
queue up all the numbers from 3 to 1000 for checking, plus a termination
notice.  Then all we have to do to get the ball rolling is pass the queue
and the first prime to the check_num() subroutine (line 34).



That’s how it works.  It’s pretty simple; as with many Perl programs,
the explanation is much longer than the program.









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74.12 Different implementations of threads



Some background on thread implementations from the operating system
viewpoint.  There are three basic categories of threads: user-mode threads,
kernel threads, and multiprocessor kernel threads.



User-mode threads are threads that live entirely within a program and
its libraries.  In this model, the OS knows nothing about threads.  As
far as it’s concerned, your process is just a process.



This is the easiest way to implement threads, and the way most OSes
start.  The big disadvantage is that, since the OS knows nothing about
threads, if one thread blocks they all do.  Typical blocking activities
include most system calls, most I/O, and things like sleep().



Kernel threads are the next step in thread evolution.  The OS knows
about kernel threads, and makes allowances for them.  The main
difference between a kernel thread and a user-mode thread is
blocking.  With kernel threads, things that block a single thread don’t
block other threads.  This is not the case with user-mode threads,
where the kernel blocks at the process level and not the thread level.



This is a big step forward, and can give a threaded program quite a
performance boost over non-threaded programs.  Threads that block
performing I/O, for example, won’t block threads that are doing other
things.  Each process still has only one thread running at once,
though, regardless of how many CPUs a system might have.



Since kernel threading can interrupt a thread at any time, they will
uncover some of the implicit locking assumptions you may make in your
program.  For example, something as simple as $a = $a + 2 can behave
unpredictably with kernel threads if $a is visible to other
threads, as another thread may have changed $a between the time it
was fetched on the right hand side and the time the new value is
stored.



Multiprocessor kernel threads are the final step in thread
support.  With multiprocessor kernel threads on a machine with multiple
CPUs, the OS may schedule two or more threads to run simultaneously on
different CPUs.



This can give a serious performance boost to your threaded program,
since more than one thread will be executing at the same time.  As a
tradeoff, though, any of those nagging synchronization issues that
might not have shown with basic kernel threads will appear with a
vengeance.



In addition to the different levels of OS involvement in threads,
different OSes (and different thread implementations for a particular
OS) allocate CPU cycles to threads in different ways.



Cooperative multitasking systems have running threads give up control
if one of two things happen.  If a thread calls a yield function, it
gives up control.  It also gives up control if the thread does
something that would cause it to block, such as perform I/O.  In a
cooperative multitasking implementation, one thread can starve all the
others for CPU time if it so chooses.



Preemptive multitasking systems interrupt threads at regular intervals
while the system decides which thread should run next.  In a preemptive
multitasking system, one thread usually won’t monopolize the CPU.



On some systems, there can be cooperative and preemptive threads
running simultaneously. (Threads running with realtime priorities
often behave cooperatively, for example, while threads running at
normal priorities behave preemptively.)



Most modern operating systems support preemptive multitasking nowadays.









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74.13 Performance considerations



The main thing to bear in mind when comparing Perl’s ithreads to other threading
models is the fact that for each new thread created, a complete copy of
all the variables and data of the parent thread has to be taken. Thus,
thread creation can be quite expensive, both in terms of memory usage and
time spent in creation. The ideal way to reduce these costs is to have a
relatively short number of long-lived threads, all created fairly early
on (before the base thread has accumulated too much data). Of course, this
may not always be possible, so compromises have to be made. However, after
a thread has been created, its performance and extra memory usage should
be little different than ordinary code.



Also note that under the current implementation, shared variables
use a little more memory and are a little slower than ordinary variables.









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74.14 Process-scope Changes



Note that while threads themselves are separate execution threads and
Perl data is thread-private unless explicitly shared, the threads can
affect process-scope state, affecting all the threads.



The most common example of this is changing the current working
directory using chdir().  One thread calls chdir(), and the working
directory of all the threads changes.



Even more drastic example of a process-scope change is chroot():
the root directory of all the threads changes, and no thread can
undo it (as opposed to chdir()).



Further examples of process-scope changes include umask() and
changing uids and gids.



Thinking of mixing fork() and threads?  Please lie down and wait
until the feeling passes.  Be aware that the semantics of fork() vary
between platforms.  For example, some Unix systems copy all the current
threads into the child process, while others only copy the thread that
called fork(). You have been warned!



Similarly, mixing signals and threads may be problematic.
Implementations are platform-dependent, and even the POSIX
semantics may not be what you expect (and Perl doesn’t even
give you the full POSIX API).  For example, there is no way to
guarantee that a signal sent to a multi-threaded Perl application
will get intercepted by any particular thread.  (However, a recently
added feature does provide the capability to send signals between
threads.  See (threads)THREAD SIGNALLING for more details.)









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74.15 Thread-Safety of System Libraries



Whether various library calls are thread-safe is outside the control
of Perl.  Calls often suffering from not being thread-safe include:
localtime(), gmtime(),  functions fetching user, group and
network information (such as getgrent(), gethostent(),
getnetent() and so on), readdir(), rand(), and srand(). In
general, calls that depend on some global external state.



If the system Perl is compiled in has thread-safe variants of such
calls, they will be used.  Beyond that, Perl is at the mercy of
the thread-safety or -unsafety of the calls.  Please consult your
C library call documentation.



On some platforms the thread-safe library interfaces may fail if the
result buffer is too small (for example the user group databases may
be rather large, and the reentrant interfaces may have to carry around
a full snapshot of those databases).  Perl will start with a small
buffer, but keep retrying and growing the result buffer
until the result fits.  If this limitless growing sounds bad for
security or memory consumption reasons you can recompile Perl with
PERL_REENTRANT_MAXSIZE defined to the maximum number of bytes you will
allow.









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74.16 Conclusion



A complete thread tutorial could fill a book (and has, many times),
but with what we’ve covered in this introduction, you should be well
on your way to becoming a threaded Perl expert.









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74.17 SEE ALSO



Annotated POD for (threads):
http://annocpan.org/?mode=search&field=Module&name=threads



Latest version of (threads) on CPAN:
http://search.cpan.org/search?module=threads



Annotated POD for (threads-shared):
http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared



Latest version of (threads-shared) on CPAN:
http://search.cpan.org/search?module=threads%3A%3Ashared



Perl threads mailing list:
http://lists.perl.org/list/ithreads.html









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74.18 Bibliography



Here’s a short bibliography courtesy of Jürgen Christoffel:





• perlthrtut Introductory Texts:  



• perlthrtut OS-Related References:  



• perlthrtut Other References:  












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74.18.1 Introductory Texts



Birrell, Andrew D. An Introduction to Programming with
Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report
#35 online as
ftp://ftp.dec.com/pub/DEC/SRC/research-reports/SRC-035.pdf
(highly recommended)



Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
Guide to Concurrency, Communication, and
Multithreading. Prentice-Hall, 1996.



Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
introduction to threads).



Nelson, Greg (editor). Systems Programming with Modula-3.  Prentice
Hall, 1991, ISBN 0-13-590464-1.



Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
Pthreads Programming. O’Reilly & Associates, 1996, ISBN 156592-115-1
(covers POSIX threads).









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74.18.2 OS-Related References



Boykin, Joseph, David Kirschen, Alan Langerman, and Susan
LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN
0-201-52739-1.



Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
1995, ISBN 0-13-219908-4 (great textbook).



Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4









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74.18.3 Other References



Arnold, Ken and James Gosling. The Java Programming Language, 2nd
ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.



comp.programming.threads FAQ,
http://www.serpentine.com/~bos/threads-faq/



Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
Collection on Virtually Shared Memory Architectures" in Memory
Management: Proc. of the International Workshop IWMM 92, St. Malo,
France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer,
1992, ISBN 3540-55940-X (real-life thread applications).



Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
http://www.perl.com/pub/a/2002/06/11/threads.html









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74.19 Acknowledgements



Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua
Pritikin, and Alan Burlison, for their help in reality-checking and
polishing this article.  Big thanks to Tom Christiansen for his rewrite
of the prime number generator.









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74.20 AUTHOR



Dan Sugalski <dan@sidhe.org<gt>



Slightly modified by Arthur Bergman to fit the new thread model/module.



Reworked slightly by Jörg Walter <jwalt@cpan.org<gt> to be more concise
about thread-safety of Perl code.



Rearranged slightly by Elizabeth Mattijsen <liz@dijkmat.nl<gt> to put
less emphasis on yield().









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74.21 Copyrights



The original version of this article originally appeared in The Perl
Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy
of Jon Orwant and The Perl Journal.  This document may be distributed
under the same terms as Perl itself.









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75 NAME tie





• NAME tie NAME:  



• NAME tie SYNOPSIS:  



• NAME tie DESCRIPTION:  



• NAME tie SEE ALSO:  



• NAME tie BUGS:  



• NAME tie AUTHOR:  












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75.1 NAME




perltie - how to hide an object class in a simple variable









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75.2 SYNOPSIS



 tie VARIABLE, CLASSNAME, LIST

 $object = tied VARIABLE

 untie VARIABLE









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75.3 DESCRIPTION



Prior to release 5.0 of Perl, a programmer could use dbmopen()
to connect an on-disk database in the standard Unix dbm(3x)
format magically to a %HASH in their program.  However, their Perl was either
built with one particular dbm library or another, but not both, and
you couldn’t extend this mechanism to other packages or types of variables.



Now you can.



The tie() function binds a variable to a class (package) that will provide
the implementation for access methods for that variable.  Once this magic
has been performed, accessing a tied variable automatically triggers
method calls in the proper class.  The complexity of the class is
hidden behind magic methods calls.  The method names are in ALL CAPS,
which is a convention that Perl uses to indicate that they’re called
implicitly rather than explicitly–just like the BEGIN() and END()
functions.



In the tie() call, VARIABLE is the name of the variable to be
enchanted.  CLASSNAME is the name of a class implementing objects of
the correct type.  Any additional arguments in the LIST are passed to
the appropriate constructor method for that class–meaning TIESCALAR(),
TIEARRAY(), TIEHASH(), or TIEHANDLE().  (Typically these are arguments
such as might be passed to the dbminit() function of C.) The object
returned by the "new" method is also returned by the tie() function,
which would be useful if you wanted to access other methods in
CLASSNAME. (You don’t actually have to return a reference to a right
"type" (e.g., HASH or CLASSNAME) so long as it’s a properly blessed
object.)  You can also retrieve a reference to the underlying object
using the tied() function.



Unlike dbmopen(), the tie() function will not use or require a module
for you–you need to do that explicitly yourself.





• NAME tie Tying Scalars:  



• NAME tie Tying Arrays:  



• NAME tie Tying Hashes:  



• NAME tie Tying FileHandles:  



• NAME tie UNTIE this 4:  



• NAME tie The untie Gotcha:  












Next: , Up: NAME tie DESCRIPTION   [Contents]





75.3.1 Tying Scalars




A class implementing a tied scalar should define the following methods:
TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.



Let’s look at each in turn, using as an example a tie class for
scalars that allows the user to do something like:



    tie $his_speed, 'Nice', getppid();
    tie $my_speed,  'Nice', $$;



And now whenever either of those variables is accessed, its current
system priority is retrieved and returned.  If those variables are set,
then the process’s priority is changed!



We’ll use Jarkko Hietaniemi <jhi@iki.fi>’s BSD::Resource class (not
included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
from your system, as well as the getpriority() and setpriority() system
calls.  Here’s the preamble of the class.



    package Nice;
    use Carp;
    use BSD::Resource;
    use strict;
    $Nice::DEBUG = 0 unless defined $Nice::DEBUG;





TIESCALAR classname, LIST





This is the constructor for the class.  That means it is
expected to return a blessed reference to a new scalar
(probably anonymous) that it’s creating.  For example:



    sub TIESCALAR {
        my $class = shift;
        my $pid = shift || $$; # 0 means me

        if ($pid !~ /^\d+$/) {
            carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
            return undef;
        }

        unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
            carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
            return undef;
        }

        return bless \$pid, $class;
    }



This tie class has chosen to return an error rather than raising an
exception if its constructor should fail.  While this is how dbmopen() works,
other classes may well not wish to be so forgiving.  It checks the global
variable $^W to see whether to emit a bit of noise anyway.





FETCH this





This method will be triggered every time the tied variable is accessed
(read).  It takes no arguments beyond its self reference, which is the
object representing the scalar we’re dealing with.  Because in this case
we’re using just a SCALAR ref for the tied scalar object, a simple $$self
allows the method to get at the real value stored there.  In our example
below, that real value is the process ID to which we’ve tied our variable.



    sub FETCH {
        my $self = shift;
        confess "wrong type" unless ref $self;
        croak "usage error" if @_;
        my $nicety;
        local($!) = 0;
        $nicety = getpriority(PRIO_PROCESS, $$self);
        if ($!) { croak "getpriority failed: $!" }
        return $nicety;
    }



This time we’ve decided to blow up (raise an exception) if the renice
fails–there’s no place for us to return an error otherwise, and it’s
probably the right thing to do.





STORE this, value





This method will be triggered every time the tied variable is set
(assigned).  Beyond its self reference, it also expects one (and only one)
argument: the new value the user is trying to assign. Don’t worry about
returning a value from STORE; the semantic of assignment returning the
assigned value is implemented with FETCH.



    sub STORE {
        my $self = shift;
        confess "wrong type" unless ref $self;
        my $new_nicety = shift;
        croak "usage error" if @_;

        if ($new_nicety < PRIO_MIN) {
            carp sprintf
              "WARNING: priority %d less than minimum system priority %d",
                  $new_nicety, PRIO_MIN if $^W;
            $new_nicety = PRIO_MIN;
        }

        if ($new_nicety > PRIO_MAX) {
            carp sprintf
              "WARNING: priority %d greater than maximum system priority %d",
                  $new_nicety, PRIO_MAX if $^W;
            $new_nicety = PRIO_MAX;
        }

        unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
            confess "setpriority failed: $!";
        }
    }





UNTIE this





This method will be triggered when the untie occurs. This can be useful
if the class needs to know when no further calls will be made. (Except DESTROY
of course.) See The untie Gotcha below for more details.





DESTROY this





This method will be triggered when the tied variable needs to be destructed.
As with other object classes, such a method is seldom necessary, because Perl
deallocates its moribund object’s memory for you automatically–this isn’t
C++, you know.  We’ll use a DESTROY method here for debugging purposes only.



    sub DESTROY {
        my $self = shift;
        confess "wrong type" unless ref $self;
        carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
    }








That’s about all there is to it.  Actually, it’s more than all there
is to it, because we’ve done a few nice things here for the sake
of completeness, robustness, and general aesthetics.  Simpler
TIESCALAR classes are certainly possible.









Next: , Previous: NAME tie Tying Scalars, Up: NAME tie DESCRIPTION   [Contents]





75.3.2 Tying Arrays




A class implementing a tied ordinary array should define the following
methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.



FETCHSIZE and STORESIZE are used to provide $#array and
equivalent scalar(@array) access.



The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
required if the perl operator with the corresponding (but lowercase) name
is to operate on the tied array. The Tie::Array class can be used as a
base class to implement the first five of these in terms of the basic
methods above.  The default implementations of DELETE and EXISTS in
Tie::Array simply croak.



In addition EXTEND will be called when perl would have pre-extended
allocation in a real array.



For this discussion, we’ll implement an array whose elements are a fixed
size at creation.  If you try to create an element larger than the fixed
size, you’ll take an exception.  For example:



    use FixedElem_Array;
    tie @array, 'FixedElem_Array', 3;
    $array[0] = 'cat';  # ok.
    $array[1] = 'dogs'; # exception, length('dogs') > 3.



The preamble code for the class is as follows:



    package FixedElem_Array;
    use Carp;
    use strict;





TIEARRAY classname, LIST





This is the constructor for the class.  That means it is expected to
return a blessed reference through which the new array (probably an
anonymous ARRAY ref) will be accessed.



In our example, just to show you that you don’t really have to return an
ARRAY reference, we’ll choose a HASH reference to represent our object.
A HASH works out well as a generic record type: the {ELEMSIZE} field will
store the maximum element size allowed, and the {ARRAY} field will hold the
true ARRAY ref.  If someone outside the class tries to dereference the
object returned (doubtless thinking it an ARRAY ref), they’ll blow up.
This just goes to show you that you should respect an object’s privacy.



    sub TIEARRAY {
      my $class    = shift;
      my $elemsize = shift;
      if ( @_ || $elemsize =~ /\D/ ) {
        croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
      }
      return bless {
        ELEMSIZE => $elemsize,
        ARRAY    => [],
      }, $class;
    }





FETCH this, index





This method will be triggered every time an individual element the tied array
is accessed (read).  It takes one argument beyond its self reference: the
index whose value we’re trying to fetch.



    sub FETCH {
      my $self  = shift;
      my $index = shift;
      return $self->{ARRAY}->[$index];
    }



If a negative array index is used to read from an array, the index
will be translated to a positive one internally by calling FETCHSIZE
before being passed to FETCH.  You may disable this feature by
assigning a true value to the variable $NEGATIVE_INDICES in the
tied array class.



As you may have noticed, the name of the FETCH method (et al.) is the same
for all accesses, even though the constructors differ in names (TIESCALAR
vs TIEARRAY).  While in theory you could have the same class servicing
several tied types, in practice this becomes cumbersome, and it’s easiest
to keep them at simply one tie type per class.





STORE this, index, value





This method will be triggered every time an element in the tied array is set
(written).  It takes two arguments beyond its self reference: the index at
which we’re trying to store something and the value we’re trying to put
there.



In our example, undef is really $self->{ELEMSIZE} number of
spaces so we have a little more work to do here:



    sub STORE {
      my $self = shift;
      my( $index, $value ) = @_;
      if ( length $value > $self->{ELEMSIZE} ) {
        croak "length of $value is greater than $self->{ELEMSIZE}";
      }
      # fill in the blanks
      $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
      # right justify to keep element size for smaller elements
      $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
    }



Negative indexes are treated the same as with FETCH.





FETCHSIZE this





Returns the total number of items in the tied array associated with
object this. (Equivalent to scalar(@array)).  For example:



    sub FETCHSIZE {
      my $self = shift;
      return scalar @{$self->{ARRAY}};
    }





STORESIZE this, count





Sets the total number of items in the tied array associated with
object this to be count. If this makes the array larger then
class’s mapping of undef should be returned for new positions.
If the array becomes smaller then entries beyond count should be
deleted. 



In our example, ’undef’ is really an element containing
$self->{ELEMSIZE} number of spaces.  Observe:



    sub STORESIZE {
      my $self  = shift;
      my $count = shift;
      if ( $count > $self->FETCHSIZE() ) {
        foreach ( $count - $self->FETCHSIZE() .. $count ) {
          $self->STORE( $_, '' );
        }
      } elsif ( $count < $self->FETCHSIZE() ) {
        foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
          $self->POP();
        }
      }
    }





EXTEND this, count





Informative call that array is likely to grow to have count entries.
Can be used to optimize allocation. This method need do nothing.



In our example, we want to make sure there are no blank (undef)
entries, so EXTEND will make use of STORESIZE to fill elements
as needed:



    sub EXTEND {   
      my $self  = shift;
      my $count = shift;
      $self->STORESIZE( $count );
    }





EXISTS this, key





Verify that the element at index key exists in the tied array this.



In our example, we will determine that if an element consists of
$self->{ELEMSIZE} spaces only, it does not exist:



    sub EXISTS {
      my $self  = shift;
      my $index = shift;
      return 0 if ! defined $self->{ARRAY}->[$index] ||
                  $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
      return 1;
    }





DELETE this, key





Delete the element at index key from the tied array this.



In our example, a deleted item is $self->{ELEMSIZE} spaces:



    sub DELETE {
      my $self  = shift;
      my $index = shift;
      return $self->STORE( $index, '' );
    }





CLEAR this





Clear (remove, delete, ...) all values from the tied array associated with
object this.  For example:



    sub CLEAR {
      my $self = shift;
      return $self->{ARRAY} = [];
    }





PUSH this, LIST





Append elements of LIST to the array.  For example:



    sub PUSH {  
      my $self = shift;
      my @list = @_;
      my $last = $self->FETCHSIZE();
      $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
      return $self->FETCHSIZE();
    }   





POP this





Remove last element of the array and return it.  For example:



    sub POP {
      my $self = shift;
      return pop @{$self->{ARRAY}};
    }





SHIFT this





Remove the first element of the array (shifting other elements down)
and return it.  For example:



    sub SHIFT {
      my $self = shift;
      return shift @{$self->{ARRAY}};
    }





UNSHIFT this, LIST





Insert LIST elements at the beginning of the array, moving existing elements
up to make room.  For example:



    sub UNSHIFT {
      my $self = shift;
      my @list = @_;
      my $size = scalar( @list );
      # make room for our list
      @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
       = @{$self->{ARRAY}};
      $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
    }





SPLICE this, offset, length, LIST





Perform the equivalent of splice on the array. 



offset is optional and defaults to zero, negative values count back 
from the end of the array. 



length is optional and defaults to rest of the array.



LIST may be empty.



Returns a list of the original length elements at offset.



In our example, we’ll use a little shortcut if there is a LIST:



    sub SPLICE {
      my $self   = shift;
      my $offset = shift || 0;
      my $length = shift || $self->FETCHSIZE() - $offset;
      my @list   = (); 
      if ( @_ ) {
        tie @list, __PACKAGE__, $self->{ELEMSIZE};
        @list   = @_;
      }
      return splice @{$self->{ARRAY}}, $offset, $length, @list;
    }





UNTIE this





Will be called when untie happens. (See The untie Gotcha below.)





DESTROY this





This method will be triggered when the tied variable needs to be destructed.
As with the scalar tie class, this is almost never needed in a
language that does its own garbage collection, so this time we’ll
just leave it out.














Next: , Previous: NAME tie Tying Arrays, Up: NAME tie DESCRIPTION   [Contents]





75.3.3 Tying Hashes




Hashes were the first Perl data type to be tied (see dbmopen()).  A class
implementing a tied hash should define the following methods: TIEHASH is
the constructor.  FETCH and STORE access the key and value pairs.  EXISTS
reports whether a key is present in the hash, and DELETE deletes one.
CLEAR empties the hash by deleting all the key and value pairs.  FIRSTKEY
and NEXTKEY implement the keys() and each() functions to iterate over all
the keys. SCALAR is triggered when the tied hash is evaluated in scalar 
context. UNTIE is called when untie happens, and DESTROY is called when
the tied variable is garbage collected.



If this seems like a lot, then feel free to inherit from merely the
standard Tie::StdHash module for most of your methods, redefining only the
interesting ones.  See (Tie-Hash) for details.



Remember that Perl distinguishes between a key not existing in the hash,
and the key existing in the hash but having a corresponding value of
undef.  The two possibilities can be tested with the exists() and
defined() functions.



Here’s an example of a somewhat interesting tied hash class:  it gives you
a hash representing a particular user’s dot files.  You index into the hash
with the name of the file (minus the dot) and you get back that dot file’s
contents.  For example:



    use DotFiles;
    tie %dot, 'DotFiles';
    if ( $dot{profile} =~ /MANPATH/ ||
         $dot{login}   =~ /MANPATH/ ||
         $dot{cshrc}   =~ /MANPATH/    )
    {
        print "you seem to set your MANPATH\n";
    }



Or here’s another sample of using our tied class:



    tie %him, 'DotFiles', 'daemon';
    foreach $f ( keys %him ) {
        printf "daemon dot file %s is size %d\n",
            $f, length $him{$f};
    }



In our tied hash DotFiles example, we use a regular
hash for the object containing several important
fields, of which only the {LIST} field will be what the
user thinks of as the real hash.





USER




whose dot files this object represents





HOME




where those dot files live





CLOBBER




whether we should try to change or remove those dot files





LIST




the hash of dot file names and content mappings








Here’s the start of Dotfiles.pm:



    package DotFiles;
    use Carp;
    sub whowasi { (caller(1))[3] . '()' }
    my $DEBUG = 0;
    sub debug { $DEBUG = @_ ? shift : 1 }



For our example, we want to be able to emit debugging info to help in tracing
during development.  We keep also one convenience function around
internally to help print out warnings; whowasi() returns the function name
that calls it.



Here are the methods for the DotFiles tied hash.





TIEHASH classname, LIST





This is the constructor for the class.  That means it is expected to
return a blessed reference through which the new object (probably but not
necessarily an anonymous hash) will be accessed.



Here’s the constructor:



    sub TIEHASH {
        my $self = shift;
        my $user = shift || $>;
        my $dotdir = shift || '';
        croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
        $user = getpwuid($user) if $user =~ /^\d+$/;
        my $dir = (getpwnam($user))[7]
                || croak "@{[&whowasi]}: no user $user";
        $dir .= "/$dotdir" if $dotdir;

        my $node = {
            USER    => $user,
            HOME    => $dir,
            LIST    => {},
            CLOBBER => 0,
        };

        opendir(DIR, $dir)
                || croak "@{[&whowasi]}: can't opendir $dir: $!";
        foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
            $dot =~ s/^\.//;
            $node->{LIST}{$dot} = undef;
        }
        closedir DIR;
        return bless $node, $self;
    }



It’s probably worth mentioning that if you’re going to filetest the
return values out of a readdir, you’d better prepend the directory
in question.  Otherwise, because we didn’t chdir() there, it would
have been testing the wrong file.





FETCH this, key





This method will be triggered every time an element in the tied hash is
accessed (read).  It takes one argument beyond its self reference: the key
whose value we’re trying to fetch.



Here’s the fetch for our DotFiles example.



    sub FETCH {
        carp &whowasi if $DEBUG;
        my $self = shift;
        my $dot = shift;
        my $dir = $self->{HOME};
        my $file = "$dir/.$dot";

        unless (exists $self->{LIST}->{$dot} || -f $file) {
            carp "@{[&whowasi]}: no $dot file" if $DEBUG;
            return undef;
        }

        if (defined $self->{LIST}->{$dot}) {
            return $self->{LIST}->{$dot};
        } else {
            return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
        }
    }



It was easy to write by having it call the Unix cat(1) command, but it
would probably be more portable to open the file manually (and somewhat
more efficient).  Of course, because dot files are a Unixy concept, we’re
not that concerned.





STORE this, key, value





This method will be triggered every time an element in the tied hash is set
(written).  It takes two arguments beyond its self reference: the index at
which we’re trying to store something, and the value we’re trying to put
there.



Here in our DotFiles example, we’ll be careful not to let
them try to overwrite the file unless they’ve called the clobber()
method on the original object reference returned by tie().



    sub STORE {
        carp &whowasi if $DEBUG;
        my $self = shift;
        my $dot = shift;
        my $value = shift;
        my $file = $self->{HOME} . "/.$dot";
        my $user = $self->{USER};

        croak "@{[&whowasi]}: $file not clobberable"
            unless $self->{CLOBBER};

        open(my $f, '>', $file) || croak "can't open $file: $!";
        print $f $value;
        close($f);
    }



If they wanted to clobber something, they might say:



    $ob = tie %daemon_dots, 'daemon';
    $ob->clobber(1);
    $daemon_dots{signature} = "A true daemon\n";



Another way to lay hands on a reference to the underlying object is to
use the tied() function, so they might alternately have set clobber
using:



    tie %daemon_dots, 'daemon';
    tied(%daemon_dots)->clobber(1);



The clobber method is simply:



    sub clobber {
        my $self = shift;
        $self->{CLOBBER} = @_ ? shift : 1;
    }





DELETE this, key





This method is triggered when we remove an element from the hash,
typically by using the delete() function.  Again, we’ll
be careful to check whether they really want to clobber files.



    sub DELETE   {
        carp &whowasi if $DEBUG;

        my $self = shift;
        my $dot = shift;
        my $file = $self->{HOME} . "/.$dot";
        croak "@{[&whowasi]}: won't remove file $file"
            unless $self->{CLOBBER};
        delete $self->{LIST}->{$dot};
        my $success = unlink($file);
        carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
        $success;
    }



The value returned by DELETE becomes the return value of the call
to delete().  If you want to emulate the normal behavior of delete(),
you should return whatever FETCH would have returned for this key.
In this example, we have chosen instead to return a value which tells
the caller whether the file was successfully deleted.





CLEAR this





This method is triggered when the whole hash is to be cleared, usually by
assigning the empty list to it.



In our example, that would remove all the user’s dot files!  It’s such a
dangerous thing that they’ll have to set CLOBBER to something higher than
1 to make it happen.



    sub CLEAR    {
        carp &whowasi if $DEBUG;
        my $self = shift;
        croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
            unless $self->{CLOBBER} > 1;
        my $dot;
        foreach $dot ( keys %{$self->{LIST}}) {
            $self->DELETE($dot);
        }
    }





EXISTS this, key





This method is triggered when the user uses the exists() function
on a particular hash.  In our example, we’ll look at the {LIST}
hash element for this:



    sub EXISTS   {
        carp &whowasi if $DEBUG;
        my $self = shift;
        my $dot = shift;
        return exists $self->{LIST}->{$dot};
    }





FIRSTKEY this





This method will be triggered when the user is going
to iterate through the hash, such as via a keys() or each()
call.



    sub FIRSTKEY {
        carp &whowasi if $DEBUG;
        my $self = shift;
        my $a = keys %{$self->{LIST}};          # reset each() iterator
        each %{$self->{LIST}}
    }





NEXTKEY this, lastkey





This method gets triggered during a keys() or each() iteration.  It has a
second argument which is the last key that had been accessed.  This is
useful if you’re carrying about ordering or calling the iterator from more
than one sequence, or not really storing things in a hash anywhere.



For our example, we’re using a real hash so we’ll do just the simple
thing, but we’ll have to go through the LIST field indirectly.



    sub NEXTKEY  {
        carp &whowasi if $DEBUG;
        my $self = shift;
        return each %{ $self->{LIST} }
    }





SCALAR this





This is called when the hash is evaluated in scalar context. In order
to mimic the behaviour of untied hashes, this method should return a
false value when the tied hash is considered empty. If this method does
not exist, perl will make some educated guesses and return true when
the hash is inside an iteration. If this isn’t the case, FIRSTKEY is
called, and the result will be a false value if FIRSTKEY returns the empty
list, true otherwise.



However, you should not blindly rely on perl always doing the right 
thing. Particularly, perl will mistakenly return true when you clear the 
hash by repeatedly calling DELETE until it is empty. You are therefore 
advised to supply your own SCALAR method when you want to be absolutely 
sure that your hash behaves nicely in scalar context.



In our example we can just call scalar on the underlying hash
referenced by $self->{LIST}:



    sub SCALAR {
        carp &whowasi if $DEBUG;
        my $self = shift;
        return scalar %{ $self->{LIST} }
    }





UNTIE this





This is called when untie occurs.  See The untie Gotcha below.





DESTROY this





This method is triggered when a tied hash is about to go out of
scope.  You don’t really need it unless you’re trying to add debugging
or have auxiliary state to clean up.  Here’s a very simple function:



    sub DESTROY  {
        carp &whowasi if $DEBUG;
    }








Note that functions such as keys() and values() may return huge lists
when used on large objects, like DBM files.  You may prefer to use the
each() function to iterate over such.  Example:



    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);









Next: , Previous: NAME tie Tying Hashes, Up: NAME tie DESCRIPTION   [Contents]





75.3.4 Tying FileHandles




This is partially implemented now.



A class implementing a tied filehandle should define the following
methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
READ, and possibly CLOSE, UNTIE and DESTROY.  The class can also provide: BINMODE,
OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
used on the handle.



When STDERR is tied, its PRINT method will be called to issue warnings
and error messages.  This feature is temporarily disabled during the call, 
which means you can use warn() inside PRINT without starting a recursive
loop.  And just like __WARN__ and __DIE__ handlers, STDERR’s PRINT
method may be called to report parser errors, so the caveats mentioned under 
perlvar %SIG apply.



All of this is especially useful when perl is embedded in some other 
program, where output to STDOUT and STDERR may have to be redirected 
in some special way.  See nvi and the Apache module for examples.



When tying a handle, the first argument to tie should begin with an
asterisk.  So, if you are tying STDOUT, use *STDOUT.  If you have
assigned it to a scalar variable, say $handle, use *$handle.
tie $handle ties the scalar variable $handle, not the handle inside
it.



In our example we’re going to create a shouting handle.



    package Shout;





TIEHANDLE classname, LIST





This is the constructor for the class.  That means it is expected to
return a blessed reference of some sort. The reference can be used to
hold some internal information.



    sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }





WRITE this, LIST





This method will be called when the handle is written to via the
syswrite function.



    sub WRITE {
        $r = shift;
        my($buf,$len,$offset) = @_;
        print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
    }





PRINT this, LIST





This method will be triggered every time the tied handle is printed to
with the print() or say() functions.  Beyond its self reference
it also expects the list that was passed to the print function.



    sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }



say() acts just like print() except $\ will be localized to \n so
you need do nothing special to handle say() in PRINT().





PRINTF this, LIST





This method will be triggered every time the tied handle is printed to
with the printf() function.
Beyond its self reference it also expects the format and list that was
passed to the printf function.



    sub PRINTF {
        shift;
        my $fmt = shift;
        print sprintf($fmt, @_);
    }





READ this, LIST





This method will be called when the handle is read from via the read
or sysread functions.



    sub READ {
        my $self = shift;
        my $bufref = \$_[0];
        my(undef,$len,$offset) = @_;
        print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
        # add to $$bufref, set $len to number of characters read
        $len;
    }





READLINE this





This method is called when the handle is read via <HANDLE>
or readline HANDLE.



As per (perlfunc)readline, in scalar context it should return
the next line, or undef for no more data.  In list context it should
return all remaining lines, or an empty list for no more data.  The strings
returned should include the input record separator $/ (see perlvar NAME),
unless it is undef (which means "slurp" mode).



    sub READLINE {
      my $r = shift;
      if (wantarray) {
        return ("all remaining\n",
                "lines up\n",
                "to eof\n");
      } else {
        return "READLINE called " . ++$$r . " times\n";
      }
    }





GETC this





This method will be called when the getc function is called.



    sub GETC { print "Don't GETC, Get Perl"; return "a"; }





EOF this





This method will be called when the eof function is called.



Starting with Perl 5.12, an additional integer parameter will be passed.  It
will be zero if eof is called without parameter; 1 if eof is given
a filehandle as a parameter, e.g. eof(FH); and 2 in the very special
case that the tied filehandle is ARGV and eof is called with an empty
parameter list, e.g. eof().



    sub EOF { not length $stringbuf }





CLOSE this





This method will be called when the handle is closed via the close
function.



    sub CLOSE { print "CLOSE called.\n" }





UNTIE this





As with the other types of ties, this method will be called when untie happens.
It may be appropriate to "auto CLOSE" when this occurs.  See
The untie Gotcha below.





DESTROY this





As with the other types of ties, this method will be called when the
tied handle is about to be destroyed. This is useful for debugging and
possibly cleaning up.



    sub DESTROY { print "</shout>\n" }








Here’s how to use our little example:



    tie(*FOO,'Shout');
    print FOO "hello\n";
    $a = 4; $b = 6;
    print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
    print <FOO>;









Next: , Previous: NAME tie Tying FileHandles, Up: NAME tie DESCRIPTION   [Contents]





75.3.5 UNTIE this




You can define for all tie types an UNTIE method that will be called
at untie().  See The untie Gotcha below.









Previous: NAME tie UNTIE this 4, Up: NAME tie DESCRIPTION   [Contents]





75.3.6 The untie Gotcha




If you intend making use of the object returned from either tie() or
tied(), and if the tie’s target class defines a destructor, there is a
subtle gotcha you must guard against.



As setup, consider this (admittedly rather contrived) example of a
tie; all it does is use a file to keep a log of the values assigned to
a scalar.



    package Remember;

    use strict;
    use warnings;
    use IO::File;

    sub TIESCALAR {
        my $class = shift;
        my $filename = shift;
        my $handle = IO::File->new( "> $filename" )
                         or die "Cannot open $filename: $!\n";

        print $handle "The Start\n";
        bless {FH => $handle, Value => 0}, $class;
    }

    sub FETCH {
        my $self = shift;
        return $self->{Value};
    }

    sub STORE {
        my $self = shift;
        my $value = shift;
        my $handle = $self->{FH};
        print $handle "$value\n";
        $self->{Value} = $value;
    }

    sub DESTROY {
        my $self = shift;
        my $handle = $self->{FH};
        print $handle "The End\n";
        close $handle;
    }

    1;



Here is an example that makes use of this tie:



    use strict;
    use Remember;

    my $fred;
    tie $fred, 'Remember', 'myfile.txt';
    $fred = 1;
    $fred = 4;
    $fred = 5;
    untie $fred;
    system "cat myfile.txt";



This is the output when it is executed:



    The Start
    1
    4
    5
    The End



So far so good.  Those of you who have been paying attention will have
spotted that the tied object hasn’t been used so far.  So lets add an
extra method to the Remember class to allow comments to be included in
the file; say, something like this:



    sub comment {
        my $self = shift;
        my $text = shift;
        my $handle = $self->{FH};
        print $handle $text, "\n";
    }



And here is the previous example modified to use the comment method
(which requires the tied object):



    use strict;
    use Remember;

    my ($fred, $x);
    $x = tie $fred, 'Remember', 'myfile.txt';
    $fred = 1;
    $fred = 4;
    comment $x "changing...";
    $fred = 5;
    untie $fred;
    system "cat myfile.txt";



When this code is executed there is no output.  Here’s why:



When a variable is tied, it is associated with the object which is the
return value of the TIESCALAR, TIEARRAY, or TIEHASH function.  This
object normally has only one reference, namely, the implicit reference
from the tied variable.  When untie() is called, that reference is
destroyed.  Then, as in the first example above, the object’s
destructor (DESTROY) is called, which is normal for objects that have
no more valid references; and thus the file is closed.



In the second example, however, we have stored another reference to
the tied object in $x.  That means that when untie() gets called
there will still be a valid reference to the object in existence, so
the destructor is not called at that time, and thus the file is not
closed.  The reason there is no output is because the file buffers
have not been flushed to disk.



Now that you know what the problem is, what can you do to avoid it?
Prior to the introduction of the optional UNTIE method the only way
was the good old -w flag. Which will spot any instances where you call
untie() and there are still valid references to the tied object.  If
the second script above this near the top use warnings 'untie'
or was run with the -w flag, Perl prints this
warning message:



    untie attempted while 1 inner references still exist



To get the script to work properly and silence the warning make sure
there are no valid references to the tied object before untie() is
called:



    undef $x;
    untie $fred;



Now that UNTIE exists the class designer can decide which parts of the
class functionality are really associated with untie and which with
the object being destroyed. What makes sense for a given class depends
on whether the inner references are being kept so that non-tie-related
methods can be called on the object. But in most cases it probably makes
sense to move the functionality that would have been in DESTROY to the UNTIE
method.



If the UNTIE method exists then the warning above does not occur. Instead the
UNTIE method is passed the count of "extra" references and can issue its own
warning if appropriate. e.g. to replicate the no UNTIE case this method can
be used:



    sub UNTIE
    {
     my ($obj,$count) = @_;
     carp "untie attempted while $count inner references still exist" if $count;
    }









Next: , Previous: NAME tie DESCRIPTION, Up: NAME tie   [Contents]





75.4 SEE ALSO



See (DB_File) or (Config) for some interesting tie() implementations.
A good starting point for many tie() implementations is with one of the
modules (Tie-Scalar), (Tie-Array), (Tie-Hash), or (Tie-Handle).









Next: , Previous: NAME tie SEE ALSO, Up: NAME tie   [Contents]





75.5 BUGS



The bucket usage information provided by scalar(%hash) is not
available.  What this means is that using %tied_hash in boolean
context doesn’t work right (currently this always tests false,
regardless of whether the hash is empty or hash elements).



Localizing tied arrays or hashes does not work.  After exiting the
scope the arrays or the hashes are not restored.



Counting the number of entries in a hash via scalar(keys(%hash))
or scalar(values(%hash)) is inefficient since it needs to iterate
through all the entries with FIRSTKEY/NEXTKEY.



Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
tie methods for slice operations.



You cannot easily tie a multilevel data structure (such as a hash of
hashes) to a dbm file.  The first problem is that all but GDBM and
Berkeley DB have size limitations, but beyond that, you also have problems
with how references are to be represented on disk.  One
module that does attempt to address this need is DBM::Deep.  Check your
nearest CPAN site as described in (perlmodlib) for source code.  Note
that despite its name, DBM::Deep does not use dbm.  Another earlier attempt
at solving the problem is MLDBM, which is also available on the CPAN, but
which has some fairly serious limitations.



Tied filehandles are still incomplete.  sysopen(), truncate(),
flock(), fcntl(), stat() and -X can’t currently be trapped.









Previous: NAME tie BUGS, Up: NAME tie   [Contents]





75.6 AUTHOR



Tom Christiansen



TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and Doug MacEachern <dougm@osf.org>



UNTIE by Nick Ing-Simmons <nick@ing-simmons.net>



SCALAR by Tassilo von Parseval <tassilo.von.parseval@rwth-aachen.de>



Tying Arrays by Casey West <casey@geeknest.com>









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76 perltodo





• perltodo NAME:  



• perltodo DESCRIPTION:  












Next: , Up: perltodo   [Contents]





76.1 NAME



perltodo - Perl TO-DO List









Previous: perltodo NAME, Up: perltodo   [Contents]





76.2 DESCRIPTION



We no longer install the Perl 5 to-do list as a manpage, as installing
snapshot that becomes increasingly out of date isn’t that useful to anyone.
The current Perl 5 to-do list is maintained in the git repository, and can
be viewed at http://perl5.git.perl.org/perl.git/blob/HEAD:/Porting/todo.pod









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77 perltooc





• perltooc NAME:  



• perltooc DESCRIPTION:  












Next: , Up: perltooc   [Contents]





77.1 NAME



perltooc - This document has been deleted









Previous: perltooc NAME, Up: perltooc   [Contents]





77.2 DESCRIPTION



For information on OO programming with Perl, please see perlootut NAME
and (perlobj).









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78 perltoot





• perltoot NAME:  



• perltoot DESCRIPTION:  












Next: , Up: perltoot   [Contents]





78.1 NAME



perltoot - This document has been deleted









Previous: perltoot NAME, Up: perltoot   [Contents]





78.2 DESCRIPTION



For information on OO programming with Perl, please see perlootut NAME
and (perlobj).









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79 perltrap





• perltrap NAME:  



• perltrap DESCRIPTION:  












Next: , Up: perltrap   [Contents]





79.1 NAME



perltrap - Perl traps for the unwary









Previous: perltrap NAME, Up: perltrap   [Contents]





79.2 DESCRIPTION



The biggest trap of all is forgetting to use warnings or use the -w
switch; see (perllexwarn) and perlrun NAME. The second biggest trap is not
making your entire program runnable under use strict.  The third biggest
trap is not reading the list of changes in this version of Perl; see
(perldelta).





• perltrap Awk Traps:  



• perltrap C/C++ Traps:  



• perltrap Sed Traps:  



• perltrap Shell Traps:  



• perltrap Perl Traps:  



• perltrap Perl4 to Perl5 Traps:  



• perltrap Discontinuance, Deprecation, and BugFix traps 1:  



• perltrap Parsing Traps 1:  



• perltrap Numerical Traps 1:  



• perltrap General data type traps 1:  



• perltrap Context Traps - scalar, list contexts 1:  



• perltrap Precedence Traps 1:  



• perltrap General Regular Expression Traps using s///, etc. 1:  



• perltrap Subroutine, Signal, Sorting Traps 1:  



• perltrap OS Traps 1:  



• perltrap Interpolation Traps:  



• perltrap DBM Traps 1:  



• perltrap Unclassified Traps 1:  












Next: , Up: perltrap DESCRIPTION   [Contents]





79.2.1 Awk Traps



Accustomed awk users should take special note of the following:











Next: , Previous: perltrap Awk Traps, Up: perltrap DESCRIPTION   [Contents]





79.2.2 C/C++ Traps



Cerebral C and C++ programmers should take note of the following:











Next: , Previous: perltrap C/C++ Traps, Up: perltrap DESCRIPTION   [Contents]





79.2.3 Sed Traps



Seasoned sed programmers should take note of the following:











Next: , Previous: perltrap Sed Traps, Up: perltrap DESCRIPTION   [Contents]





79.2.4 Shell Traps



Sharp shell programmers should take note of the following:











Next: , Previous: perltrap Shell Traps, Up: perltrap DESCRIPTION   [Contents]





79.2.5 Perl Traps



Practicing Perl Programmers should take note of the following:











Next: , Previous: perltrap Perl Traps, Up: perltrap DESCRIPTION   [Contents]





79.2.6 Perl4 to Perl5 Traps



Practicing Perl4 Programmers should take note of the following
Perl4-to-Perl5 specific traps.



They’re crudely ordered according to the following list:





Discontinuance, Deprecation, and BugFix traps




Anything that’s been fixed as a perl4 bug, removed as a perl4 feature
or deprecated as a perl4 feature with the intent to encourage usage of
some other perl5 feature.





Parsing Traps




Traps that appear to stem from the new parser.





Numerical Traps




Traps having to do with numerical or mathematical operators.





General data type traps




Traps involving perl standard data types.





Context Traps - scalar, list contexts




Traps related to context within lists, scalar statements/declarations.





Precedence Traps




Traps related to the precedence of parsing, evaluation, and execution of
code.





General Regular Expression Traps using s///, etc.




Traps related to the use of pattern matching.





Subroutine, Signal, Sorting Traps




Traps related to the use of signals and signal handlers, general subroutines,
and sorting, along with sorting subroutines.





OS Traps




OS-specific traps.





DBM Traps




Traps specific to the use of dbmopen(), and specific dbm implementations.





Unclassified Traps




Everything else.








If you find an example of a conversion trap that is not listed here,
please submit it to <perlbug@perl.org> for inclusion.
Also note that at least some of these can be caught with the
use warnings pragma or the -w switch.









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79.2.7 Discontinuance, Deprecation, and BugFix traps



Anything that has been discontinued, deprecated, or fixed as
a bug from perl4.











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79.2.8 Parsing Traps



Perl4-to-Perl5 traps from having to do with parsing.











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79.2.9 Numerical Traps



Perl4-to-Perl5 traps having to do with numerical operators,
operands, or output from same.











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79.2.10 General data type traps



Perl4-to-Perl5 traps involving most data-types, and their usage
within certain expressions and/or context.











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79.2.11 Context Traps - scalar, list contexts











Next: , Previous: perltrap Context Traps - scalar, list contexts 1, Up: perltrap DESCRIPTION   [Contents]





79.2.12 Precedence Traps



Perl4-to-Perl5 traps involving precedence order.



Perl 4 has almost the same precedence rules as Perl 5 for the operators
that they both have.  Perl 4 however, seems to have had some
inconsistencies that made the behavior differ from what was documented.











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79.2.13 General Regular Expression Traps using s///, etc.



All types of RE traps.











Next: , Previous: perltrap General Regular Expression Traps using s///, etc. 1, Up: perltrap DESCRIPTION   [Contents]





79.2.14 Subroutine, Signal, Sorting Traps



The general group of Perl4-to-Perl5 traps having to do with
Signals, Sorting, and their related subroutines, as well as
general subroutine traps.  Includes some OS-Specific traps.











Next: , Previous: perltrap Subroutine, Signal, Sorting Traps 1, Up: perltrap DESCRIPTION   [Contents]





79.2.15 OS Traps











Next: , Previous: perltrap OS Traps 1, Up: perltrap DESCRIPTION   [Contents]





79.2.16 Interpolation Traps



Perl4-to-Perl5 traps having to do with how things get interpolated
within certain expressions, statements, contexts, or whatever.











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79.2.17 DBM Traps



General DBM traps.











Previous: perltrap DBM Traps 1, Up: perltrap DESCRIPTION   [Contents]





79.2.18 Unclassified Traps



Everything else.





As always, if any of these are ever officially declared as bugs,
they’ll be fixed and removed.









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80 perlunicode





• perlunicode NAME:  



• perlunicode DESCRIPTION:  



• perlunicode BUGS:  



• perlunicode SEE ALSO:  












Next: , Up: perlunicode   [Contents]





80.1 NAME



perlunicode - Unicode support in Perl









Next: , Previous: perlunicode NAME, Up: perlunicode   [Contents]





80.2 DESCRIPTION





• perlunicode Important Caveats:  



• perlunicode Byte and Character Semantics:  



• perlunicode Effects of Character Semantics:  



• perlunicode Unicode Character Properties:  



• perlunicode User-Defined Character Properties:  



• perlunicode User-Defined Case Mappings (for serious hackers only):  



• perlunicode Character Encodings for Input and Output:  



• perlunicode Unicode Regular Expression Support Level:  



• perlunicode Unicode Encodings:  



• perlunicode Non-character code points:  



• perlunicode Beyond Unicode code points:  



• perlunicode Security Implications of Unicode:  



• perlunicode Unicode in Perl on EBCDIC:  



• perlunicode Locales:  



• perlunicode When Unicode Does Not Happen:  



• perlunicode The "Unicode Bug":  



• perlunicode Forcing Unicode in Perl (Or Unforcing Unicode in Perl):  



• perlunicode Using Unicode in XS:  



• perlunicode Hacking Perl to work on earlier Unicode versions (for very serious hackers only):  












Next: , Up: perlunicode DESCRIPTION   [Contents]





80.2.1 Important Caveats



Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.



People who want to learn to use Unicode in Perl, should probably read
the Perl Unicode tutorial, perlunitut and
perluniintro NAME, before reading
this reference document.



Also, the use of Unicode may present security issues that aren’t obvious.
Read Unicode Security Considerations.





Safest if you "use feature ’unicode_strings’"




In order to preserve backward compatibility, Perl does not turn
on full internal Unicode support unless the pragma
use feature 'unicode_strings' is specified.  (This is automatically
selected if you use use 5.012 or higher.)  Failure to do this can
trigger unexpected surprises.  See The "Unicode Bug" below.



This pragma doesn’t affect I/O, and there are still several places
where Unicode isn’t fully supported, such as in filenames.





Input and Output Layers




Perl knows when a filehandle uses Perl’s internal Unicode encodings
(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
the ":encoding(utf8)" layer.  Other encodings can be converted to Perl’s
encoding on input or from Perl’s encoding on output by use of the
":encoding(...)"  layer.  See (open).



To indicate that Perl source itself is in UTF-8, use use utf8;.





use utf8 still needed to enable UTF-8/UTF-EBCDIC in scripts




As a compatibility measure, the use utf8 pragma must be explicitly
included to enable recognition of UTF-8 in the Perl scripts themselves
(in string or regular expression literals, or in identifier names) on
ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
machines.  These are the only times when an explicit use utf8
is needed.  See (utf8).





BOM-marked scripts and UTF-16 scripts autodetected




If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
endianness, Perl will correctly read in the script as Unicode.
(BOMless UTF-8 cannot be effectively recognized or differentiated from
ISO 8859-1 or other eight-bit encodings.)





use encoding needed to upgrade non-Latin-1 byte strings




By default, there is a fundamental asymmetry in Perl’s Unicode model:
implicit upgrading from byte strings to Unicode strings assumes that
they were encoded in ISO 8859-1 (Latin-1), but Unicode strings are
downgraded with UTF-8 encoding.  This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.



See Byte and Character Semantics for more details.














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80.2.2 Byte and Character Semantics



Beginning with version 5.6, Perl uses logically-wide characters to
represent strings internally.



Starting in Perl 5.14, Perl-level operations work with
characters rather than bytes within the scope of a
(feature)use feature 'unicode_strings' (or equivalently
use 5.012 or higher).  (This is not true if bytes have been
explicitly requested by (bytes)use bytes, nor necessarily true
for interactions with the platform’s operating system.)



For earlier Perls, and when unicode_strings is not in effect, Perl
provides a fairly safe environment that can handle both types of
semantics in programs.  For operations where Perl can unambiguously
decide that the input data are characters, Perl switches to character
semantics.  For operations where this determination cannot be made
without additional information from the user, Perl decides in favor of
compatibility and chooses to use byte semantics.



When use locale (but not use locale ':not_characters') is in
effect, Perl uses the semantics associated with the current locale.
(use locale overrides use feature 'unicode_strings' in the same scope;
while use locale ':not_characters' effectively also selects
use feature 'unicode_strings' in its scope; see perllocale NAME.)
Otherwise, Perl uses the platform’s native
byte semantics for characters whose code points are less than 256, and
Unicode semantics for those greater than 255.  On EBCDIC platforms, this
is almost seamless, as the EBCDIC code pages that Perl handles are
equivalent to Unicode’s first 256 code points.  (The exception is that
EBCDIC regular expression case-insensitive matching rules are not as
as robust as Unicode’s.)   But on ASCII platforms, Perl uses US-ASCII
(or Basic Latin in Unicode terminology) byte semantics, meaning that characters
whose ordinal numbers are in the range 128 - 255 are undefined except for their
ordinal numbers.  This means that none have case (upper and lower), nor are any
a member of character classes, like [:alpha:] or \w.  (But all do belong
to the \W class or the Perl regular expression extension [:^alpha:].)



This behavior preserves compatibility with earlier versions of Perl,
which allowed byte semantics in Perl operations only if
none of the program’s inputs were marked as being a source of Unicode
character data.  Such data may come from filehandles, from calls to
external programs, from information provided by the system (such as %ENV),
or from literals and constants in the source text.



The utf8 pragma is primarily a compatibility device that enables
recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
Note that this pragma is only required while Perl defaults to byte
semantics; when character semantics become the default, this pragma
may become a no-op.  See (utf8).



If strings operating under byte semantics and strings with Unicode
character data are concatenated, the new string will have
character semantics.  This can cause surprises: See BUGS, below.
You can choose to be warned when this happens.  See (encoding-warnings).



Under character semantics, many operations that formerly operated on
bytes now operate on characters. A character in Perl is
logically just a number ranging from 0 to 2**31 or so. Larger
characters may encode into longer sequences of bytes internally, but
this internal detail is mostly hidden for Perl code.
See perluniintro NAME for more.









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80.2.3 Effects of Character Semantics



Character semantics have the following effects:













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80.2.4 Unicode Character Properties



(The only time that Perl considers a sequence of individual code
points as a single logical character is in the \X construct, already
mentioned above.   Therefore "character" in this discussion means a single
Unicode code point.)



Very nearly all Unicode character properties are accessible through
regular expressions by using the \p{} "matches property" construct
and the \P{} "doesn’t match property" for its negation.



For instance, \p{Uppercase} matches any single character with the Unicode
"Uppercase" property, while \p{L} matches any character with a
General_Category of "L" (letter) property.  Brackets are not
required for single letter property names, so \p{L} is equivalent to \pL.



More formally, \p{Uppercase} matches any single character whose Unicode
Uppercase property value is True, and \P{Uppercase} matches any character
whose Uppercase property value is False, and they could have been written as
\p{Uppercase=True} and \p{Uppercase=False}, respectively.



This formality is needed when properties are not binary; that is, if they can
take on more values than just True and False.  For example, the Bidi_Class (see
Bidirectional Character Types below), can take on several different
values, such as Left, Right, Whitespace, and others.  To match these, one needs
to specify both the property name (Bidi_Class), AND the value being
matched against
(Left, Right, etc.).  This is done, as in the examples above, by having the
two components separated by an equal sign (or interchangeably, a colon), like
\p{Bidi_Class: Left}.



All Unicode-defined character properties may be written in these compound forms
of \p{property=value} or \p{property:value}, but Perl provides some
additional properties that are written only in the single form, as well as
single-form short-cuts for all binary properties and certain others described
below, in which you may omit the property name and the equals or colon
separator.



Most Unicode character properties have at least two synonyms (or aliases if you
prefer): a short one that is easier to type and a longer one that is more
descriptive and hence easier to understand.  Thus the "L" and "Letter" properties
above are equivalent and can be used interchangeably.  Likewise,
"Upper" is a synonym for "Uppercase", and we could have written
\p{Uppercase} equivalently as \p{Upper}.  Also, there are typically
various synonyms for the values the property can be.   For binary properties,
"True" has 3 synonyms: "T", "Yes", and "Y"; and "False has correspondingly "F",
"No", and "N".  But be careful.  A short form of a value for one property may
not mean the same thing as the same short form for another.  Thus, for the
General_Category property, "L" means "Letter", but for the Bidi_Class property,
"L" means "Left".  A complete list of properties and synonyms is in
(perluniprops).



Upper/lower case differences in property names and values are irrelevant;
thus \p{Upper} means the same thing as \p{upper} or even \p{UpPeR}.
Similarly, you can add or subtract underscores anywhere in the middle of a
word, so that these are also equivalent to \p{U_p_p_e_r}.  And white space
is irrelevant adjacent to non-word characters, such as the braces and the equals
or colon separators, so \p{   Upper  } and \p{ Upper_case : Y } are
equivalent to these as well.  In fact, white space and even
hyphens can usually be added or deleted anywhere.  So even \p{ Up-per case = Yes} is
equivalent.  All this is called "loose-matching" by Unicode.  The few places
where stricter matching is used is in the middle of numbers, and in the Perl
extension properties that begin or end with an underscore.  Stricter matching
cares about white space (except adjacent to non-word characters),
hyphens, and non-interior underscores.



You can also use negation in both \p{} and \P{} by introducing a caret
(^) between the first brace and the property name: \p{^Tamil} is
equal to \P{Tamil}.



Almost all properties are immune to case-insensitive matching.  That is,
adding a /i regular expression modifier does not change what they
match.  There are two sets that are affected.
The first set is
Uppercase_Letter,
Lowercase_Letter,
and Titlecase_Letter,
all of which match Cased_Letter under /i matching.
And the second set is
Uppercase,
Lowercase,
and Titlecase,
all of which match Cased under /i matching.
This set also includes its subsets PosixUpper and PosixLower both
of which under /i matching match PosixAlpha.
(The difference between these sets is that some things, such as Roman
numerals, come in both upper and lower case so they are Cased, but aren’t considered
letters, so they aren’t Cased_Letters.)



The result is undefined if you try to match a non-Unicode code point
(that is, one above 0x10FFFF) against a Unicode property.  Currently, a
warning is raised, and the match will fail.  In some cases, this is
counterintuitive, as both these fail:



 chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails.
 chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Fails!





• perlunicode General_Category:  



• perlunicode Bidirectional Character Types:  



• perlunicode Scripts:  



• perlunicode Use of "Is" Prefix:  



• perlunicode Blocks:  



• perlunicode Other Properties:  












Next: , Up: perlunicode Unicode Character Properties   [Contents]





80.2.4.1 General_Category



Every Unicode character is assigned a general category, which is the "most
usual categorization of a character" (from
http://www.unicode.org/reports/tr44).



The compound way of writing these is like \p{General_Category=Number}
(short, \p{gc:n}).  But Perl furnishes shortcuts in which everything up
through the equal or colon separator is omitted.  So you can instead just write
\pN.



Here are the short and long forms of the General Category properties:



    Short       Long

    L           Letter
    LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
    Lu          Uppercase_Letter
    Ll          Lowercase_Letter
    Lt          Titlecase_Letter
    Lm          Modifier_Letter
    Lo          Other_Letter

    M           Mark
    Mn          Nonspacing_Mark
    Mc          Spacing_Mark
    Me          Enclosing_Mark

    N           Number
    Nd          Decimal_Number (also Digit)
    Nl          Letter_Number
    No          Other_Number

    P           Punctuation (also Punct)
    Pc          Connector_Punctuation
    Pd          Dash_Punctuation
    Ps          Open_Punctuation
    Pe          Close_Punctuation
    Pi          Initial_Punctuation
                (may behave like Ps or Pe depending on usage)
    Pf          Final_Punctuation
                (may behave like Ps or Pe depending on usage)
    Po          Other_Punctuation

    S           Symbol
    Sm          Math_Symbol
    Sc          Currency_Symbol
    Sk          Modifier_Symbol
    So          Other_Symbol

    Z           Separator
    Zs          Space_Separator
    Zl          Line_Separator
    Zp          Paragraph_Separator

    C           Other
    Cc          Control (also Cntrl)
    Cf          Format
    Cs          Surrogate
    Co          Private_Use
    Cn          Unassigned



Single-letter properties match all characters in any of the
two-letter sub-properties starting with the same letter.
LC and L& are special: both are aliases for the set consisting of everything matched by Ll, Lu, and Lt.









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80.2.4.2 Bidirectional Character Types



Because scripts differ in their directionality (Hebrew and Arabic are
written right to left, for example) Unicode supplies these properties in
the Bidi_Class class:



    Property    Meaning

    L           Left-to-Right
    LRE         Left-to-Right Embedding
    LRO         Left-to-Right Override
    R           Right-to-Left
    AL          Arabic Letter
    RLE         Right-to-Left Embedding
    RLO         Right-to-Left Override
    PDF         Pop Directional Format
    EN          European Number
    ES          European Separator
    ET          European Terminator
    AN          Arabic Number
    CS          Common Separator
    NSM         Non-Spacing Mark
    BN          Boundary Neutral
    B           Paragraph Separator
    S           Segment Separator
    WS          Whitespace
    ON          Other Neutrals



This property is always written in the compound form.
For example, \p{Bidi_Class:R} matches characters that are normally
written right to left.









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80.2.4.3 Scripts



The world’s languages are written in many different scripts.  This sentence
(unless you’re reading it in translation) is written in Latin, while Russian is
written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
Hiragana or Katakana.  There are many more.



The Unicode Script and Script_Extensions properties give what script a
given character is in.  Either property can be specified with the
compound form like
\p{Script=Hebrew} (short: \p{sc=hebr}), or
\p{Script_Extensions=Javanese} (short: \p{scx=java}).
In addition, Perl furnishes shortcuts for all
Script property names.  You can omit everything up through the equals
(or colon), and simply write \p{Latin} or \P{Cyrillic}.
(This is not true for Script_Extensions, which is required to be
written in the compound form.)



The difference between these two properties involves characters that are
used in multiple scripts.  For example the digits ’0’ through ’9’ are
used in many parts of the world.  These are placed in a script named
Common.  Other characters are used in just a few scripts.  For
example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
scripts, Katakana and Hiragana, but nowhere else.  The Script
property places all characters that are used in multiple scripts in the
Common script, while the Script_Extensions property places those
that are used in only a few scripts into each of those scripts; while
still using Common for those used in many scripts.  Thus both these
match:



 "0" =~ /\p{sc=Common}/     # Matches
 "0" =~ /\p{scx=Common}/    # Matches



and only the first of these match:



 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common}  # Matches
 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match



And only the last two of these match:



 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana}  # No match
 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana}  # No match
 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches



Script_Extensions is thus an improved Script, in which there are
fewer characters in the Common script, and correspondingly more in
other scripts.  It is new in Unicode version 6.0, and its data are likely
to change significantly in later releases, as things get sorted out.



(Actually, besides Common, the Inherited script, contains
characters that are used in multiple scripts.  These are modifier
characters which modify other characters, and inherit the script value
of the controlling character.  Some of these are used in many scripts,
and so go into Inherited in both Script and Script_Extensions.
Others are used in just a few scripts, so are in Inherited in
Script, but not in Script_Extensions.)



It is worth stressing that there are several different sets of digits in
Unicode that are equivalent to 0-9 and are matchable by \d in a
regular expression.  If they are used in a single language only, they
are in that language’s Script and Script_Extension.  If they are
used in more than one script, they will be in sc=Common, but only
if they are used in many scripts should they be in scx=Common.



A complete list of scripts and their shortcuts is in (perluniprops).









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80.2.4.4 Use of "Is" Prefix



For backward compatibility (with Perl 5.6), all properties mentioned
so far may have Is or Is_ prepended to their name, so \P{Is_Lu}, for
example, is equal to \P{Lu}, and \p{IsScript:Arabic} is equal to
\p{Arabic}.









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80.2.4.5 Blocks



In addition to scripts, Unicode also defines blocks of
characters.  The difference between scripts and blocks is that the
concept of scripts is closer to natural languages, while the concept
of blocks is more of an artificial grouping based on groups of Unicode
characters with consecutive ordinal values. For example, the "Basic Latin"
block is all characters whose ordinals are between 0 and 127, inclusive; in
other words, the ASCII characters.  The "Latin" script contains some letters
from this as well as several other blocks, like "Latin-1 Supplement",
"Latin Extended-A", etc., but it does not contain all the characters from
those blocks. It does not, for example, contain the digits 0-9, because
those digits are shared across many scripts, and hence are in the
Common script.



For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
http://www.unicode.org/reports/tr24



The Script or Script_Extensions properties are likely to be the
ones you want to use when processing
natural language; the Block property may occasionally be useful in working
with the nuts and bolts of Unicode.



Block names are matched in the compound form, like \p{Block: Arrows} or
\p{Blk=Hebrew}.  Unlike most other properties, only a few block names have a
Unicode-defined short name.  But Perl does provide a (slight) shortcut:  You
can say, for example \p{In_Arrows} or \p{In_Hebrew}.  For backwards
compatibility, the In prefix may be omitted if there is no naming conflict
with a script or any other property, and you can even use an Is prefix
instead in those cases.  But it is not a good idea to do this, for a couple
reasons:



    

  1.  It is confusing.  There are many naming conflicts, and you may forget some.
For example, \p{Hebrew} means the script Hebrew, and NOT the block
Hebrew.  But would you remember that 6 months from now?


  2.  It is unstable.  A new version of Unicode may pre-empt the current meaning by
creating a property with the same name.  There was a time in very early Unicode
releases when \p{Hebrew} would have matched the block Hebrew; now it
doesn’t.





Some people prefer to always use \p{Block: foo} and \p{Script: bar}
instead of the shortcuts, whether for clarity, because they can’t remember the
difference between ’In’ and ’Is’ anyway, or they aren’t confident that those who
eventually will read their code will know that difference.



A complete list of blocks and their shortcuts is in (perluniprops).









Previous: perlunicode Blocks, Up: perlunicode Unicode Character Properties   [Contents]





80.2.4.6 Other Properties



There are many more properties than the very basic ones described here.
A complete list is in (perluniprops).



Unicode defines all its properties in the compound form, so all single-form
properties are Perl extensions.  Most of these are just synonyms for the
Unicode ones, but some are genuine extensions, including several that are in
the compound form.  And quite a few of these are actually recommended by Unicode
(in http://www.unicode.org/reports/tr18).



This section gives some details on all extensions that aren’t just
synonyms for compound-form Unicode properties
(for those properties, you’ll have to refer to the
Unicode Standard.





\p{All}




This matches any of the 1_114_112 Unicode code points.  It is a synonym for
\p{Any}.





\p{Alnum}




This matches any \p{Alphabetic} or \p{Decimal_Number} character.





\p{Any}




This matches any of the 1_114_112 Unicode code points.  It is a synonym for
\p{All}.





\p{ASCII}




This matches any of the 128 characters in the US-ASCII character set,
which is a subset of Unicode.





\p{Assigned}




This matches any assigned code point; that is, any code point whose general
category is not Unassigned (or equivalently, not Cn).





\p{Blank}




This is the same as \h and \p{HorizSpace}:  A character that changes the
spacing horizontally.





\p{Decomposition_Type: Non_Canonical}    (Short: \p{Dt=NonCanon})




Matches a character that has a non-canonical decomposition.



To understand the use of this rarely used property=value combination, it is
necessary to know some basics about decomposition.
Consider a character, say H.  It could appear with various marks around it,
such as an acute accent, or a circumflex, or various hooks, circles, arrows,
etc., above, below, to one side or the other, etc.  There are many
possibilities among the world’s languages.  The number of combinations is
astronomical, and if there were a character for each combination, it would
soon exhaust Unicode’s more than a million possible characters.  So Unicode
took a different approach: there is a character for the base H, and a
character for each of the possible marks, and these can be variously combined
to get a final logical character.  So a logical character–what appears to be a
single character–can be a sequence of more than one individual characters.
This is called an "extended grapheme cluster";  Perl furnishes the \X
regular expression construct to match such sequences.



But Unicode’s intent is to unify the existing character set standards and
practices, and several pre-existing standards have single characters that
mean the same thing as some of these combinations.  An example is ISO-8859-1,
which has quite a few of these in the Latin-1 range, an example being "LATIN
CAPITAL LETTER E WITH ACUTE".  Because this character was in this pre-existing
standard, Unicode added it to its repertoire.  But this character is considered
by Unicode to be equivalent to the sequence consisting of the character
"LATIN CAPITAL LETTER E" followed by the character "COMBINING ACUTE ACCENT".



"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and
its equivalence with the sequence is called canonical equivalence.  All
pre-composed characters are said to have a decomposition (into the equivalent
sequence), and the decomposition type is also called canonical.



However, many more characters have a different type of decomposition, a
"compatible" or "non-canonical" decomposition.  The sequences that form these
decompositions are not considered canonically equivalent to the pre-composed
character.  An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE".
It is somewhat like a regular digit 1, but not exactly; its decomposition
into the digit 1 is called a "compatible" decomposition, specifically a
"super" decomposition.  There are several such compatibility
decompositions (see http://www.unicode.org/reports/tr44), including one
called "compat", which means some miscellaneous type of decomposition
that doesn’t fit into the decomposition categories that Unicode has chosen.



Note that most Unicode characters don’t have a decomposition, so their
decomposition type is "None".



For your convenience, Perl has added the Non_Canonical decomposition
type to mean any of the several compatibility decompositions.





\p{Graph}




Matches any character that is graphic.  Theoretically, this means a character
that on a printer would cause ink to be used.





\p{HorizSpace}




This is the same as \h and \p{Blank}:  a character that changes the
spacing horizontally.





\p{In=*}




This is a synonym for \p{Present_In=*}





\p{PerlSpace}




This is the same as \s, restricted to ASCII, namely [ \f\n\r\t].



Mnemonic: Perl’s (original) space





\p{PerlWord}




This is the same as \w, restricted to ASCII, namely [A-Za-z0-9_]



Mnemonic: Perl’s (original) word.





\p{Posix...}




There are several of these, which are equivalents using the \p
notation for Posix classes and are described in
(perlrecharclass)POSIX Character Classes.





\p{Present_In: *}    (Short: \p{In=*})




This property is used when you need to know in what Unicode version(s) a
character is.



The "*" above stands for some two digit Unicode version number, such as
1.1 or 4.0; or the "*" can also be Unassigned.  This property will
match the code points whose final disposition has been settled as of the
Unicode release given by the version number; \p{Present_In: Unassigned}
will match those code points whose meaning has yet to be assigned.



For example, U+0041 "LATIN CAPITAL LETTER A" was present in the very first
Unicode release available, which is 1.1, so this property is true for all
valid "*" versions.  On the other hand, U+1EFF was not assigned until version
5.1 when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that
would match it are 5.1, 5.2, and later.



Unicode furnishes the Age property from which this is derived.  The problem
with Age is that a strict interpretation of it (which Perl takes) has it
matching the precise release a code point’s meaning is introduced in.  Thus
U+0041 would match only 1.1; and U+1EFF only 5.1.  This is not usually what
you want.



Some non-Perl implementations of the Age property may change its meaning to be
the same as the Perl Present_In property; just be aware of that.



Another confusion with both these properties is that the definition is not
that the code point has been assigned, but that the meaning of the code point
has been determined.  This is because 66 code points will always be
unassigned, and so the Age for them is the Unicode version in which the decision
to make them so was made.  For example, U+FDD0 is to be permanently
unassigned to a character, and the decision to do that was made in version 3.1,
so \p{Age=3.1} matches this character, as also does \p{Present_In: 3.1} and up.





\p{Print}




This matches any character that is graphical or blank, except controls.





\p{SpacePerl}




This is the same as \s, including beyond ASCII.



Mnemonic: Space, as modified by Perl.  (It doesn’t include the vertical tab
which both the Posix standard and Unicode consider white space.)





\p{Title} and  \p{Titlecase}




Under case-sensitive matching, these both match the same code points as
\p{General Category=Titlecase_Letter} (\p{gc=lt}).  The difference
is that under /i caseless matching, these match the same as
\p{Cased}, whereas \p{gc=lt} matches \p{Cased_Letter).





\p{VertSpace}




This is the same as \v:  A character that changes the spacing vertically.





\p{Word}




This is the same as \w, including over 100_000 characters beyond ASCII.





\p{XPosix...}




There are several of these, which are the standard Posix classes
extended to the full Unicode range.  They are described in
(perlrecharclass)POSIX Character Classes.














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80.2.5 User-Defined Character Properties



You can define your own binary character properties by defining subroutines
whose names begin with "In" or "Is".  The subroutines can be defined in any
package.  The user-defined properties can be used in the regular expression
\p and \P constructs; if you are using a user-defined property from a
package other than the one you are in, you must specify its package in the
\p or \P construct.



    # assuming property Is_Foreign defined in Lang::
    package main;  # property package name required
    if ($txt =~ /\p{Lang::IsForeign}+/) { ... }

    package Lang;  # property package name not required
    if ($txt =~ /\p{IsForeign}+/) { ... }



Note that the effect is compile-time and immutable once defined.
However, the subroutines are passed a single parameter, which is 0 if
case-sensitive matching is in effect and non-zero if caseless matching
is in effect.  The subroutine may return different values depending on
the value of the flag, and one set of values will immutably be in effect
for all case-sensitive matches, and the other set for all case-insensitive
matches.



Note that if the regular expression is tainted, then Perl will die rather
than calling the subroutine, where the name of the subroutine is
determined by the tainted data.



The subroutines must return a specially-formatted string, with one
or more newline-separated lines.  Each line must be one of the following:





For example, to define a property that covers both the Japanese
syllabaries (hiragana and katakana), you can define



    sub InKana {
        return <<END;
    3040\t309F
    30A0\t30FF
    END
    }



Imagine that the here-doc end marker is at the beginning of the line.
Now you can use \p{InKana} and \P{InKana}.



You could also have used the existing block property names:



    sub InKana {
        return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    END
    }



Suppose you wanted to match only the allocated characters,
not the raw block ranges: in other words, you want to remove
the non-characters:



    sub InKana {
        return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    -utf8::IsCn
    END
    }



The negation is useful for defining (surprise!) negated classes.



    sub InNotKana {
        return <<'END';
    !utf8::InHiragana
    -utf8::InKatakana
    +utf8::IsCn
    END
    }



This will match all non-Unicode code points, since every one of them is
not in Kana.  You can use intersection to exclude these, if desired, as
this modified example shows:



    sub InNotKana {
        return <<'END';
    !utf8::InHiragana
    -utf8::InKatakana
    +utf8::IsCn
    &utf8::Any
    END
    }



&utf8::Any must be the last line in the definition.



Intersection is used generally for getting the common characters matched
by two (or more) classes.  It’s important to remember not to use "&" for
the first set; that would be intersecting with nothing, resulting in an
empty set.



(Note that official Unicode properties differ from these in that they
automatically exclude non-Unicode code points and a warning is raised if
a match is attempted on one of those.)









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80.2.6 User-Defined Case Mappings (for serious hackers only)



This feature has been removed as of Perl 5.16.
The CPAN module (Unicode-Casing) provides better functionality without
the drawbacks that this feature had.  If you are using a Perl earlier
than 5.16, this feature was most fully documented in the 5.14 version of
this pod:
http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29









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80.2.7 Character Encodings for Input and Output



See (Encode).









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80.2.8 Unicode Regular Expression Support Level



The following list of Unicode supported features for regular expressions describes
all features currently directly supported by core Perl.  The references to "Level N"
and the section numbers refer to the Unicode Technical Standard #18,
"Unicode Regular Expressions", version 13, from August 2008.











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80.2.9 Unicode Encodings



Unicode characters are assigned to code points, which are abstract
numbers.  To use these numbers, various encodings are needed.











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80.2.10 Non-character code points



66 code points are set aside in Unicode as "non-character code points".
These all have the Unassigned (Cn) General Category, and they never will
be assigned.  These are never supposed to be in legal Unicode input
streams, so that code can use them as sentinels that can be mixed in
with character data, and they always will be distinguishable from that data.
To keep them out of Perl input streams, strict UTF-8 should be
specified, such as by using the layer :encoding('UTF-8').  The
non-character code points are the 32 between U+FDD0 and U+FDEF, and the
34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF.
Some people are under the mistaken impression that these are "illegal",
but that is not true.  An application or cooperating set of applications
can legally use them at will internally; but these code points are
"illegal for open interchange".  Therefore, Perl will not accept these
from input streams unless lax rules are being used, and will warn
(using the warning category "nonchar", which is a sub-category of "utf8") if
an attempt is made to output them.









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80.2.11 Beyond Unicode code points



The maximum Unicode code point is U+10FFFF.  But Perl accepts code
points up to the maximum permissible unsigned number available on the
platform.  However, Perl will not accept these from input streams unless
lax rules are being used, and will warn (using the warning category
"non_unicode", which is a sub-category of "utf8") if an attempt is made to
operate on or output them.  For example, uc(0x11_0000) will generate
this warning, returning the input parameter as its result, as the upper
case of every non-Unicode code point is the code point itself.









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80.2.12 Security Implications of Unicode



Read Unicode Security Considerations.
Also, note the following:





As discussed elsewhere, Perl has one foot (two hooves?) planted in
each of two worlds: the old world of bytes and the new world of
characters, upgrading from bytes to characters when necessary.
If your legacy code does not explicitly use Unicode, no automatic
switch-over to characters should happen.  Characters shouldn’t get
downgraded to bytes, either.  It is possible to accidentally mix bytes
and characters, however (see perluniintro NAME), in which case \w in
regular expressions might start behaving differently (unless the /a
modifier is in effect).  Review your code.  Use warnings and the strict pragma.









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80.2.13 Unicode in Perl on EBCDIC



The way Unicode is handled on EBCDIC platforms is still
experimental.  On such platforms, references to UTF-8 encoding in this
document and elsewhere should be read as meaning the UTF-EBCDIC
specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
are specifically discussed. There is no utfebcdic pragma or
":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
the platform’s "natural" 8-bit encoding of Unicode. See perlebcdic NAME
for more discussion of the issues.









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80.2.14 Locales



See perllocale Unicode and UTF-8









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80.2.15 When Unicode Does Not Happen



While Perl does have extensive ways to input and output in Unicode,
and a few other "entry points" like the @ARGV array (which can sometimes be
interpreted as UTF-8), there are still many places where Unicode
(in some encoding or another) could be given as arguments or received as
results, or both, but it is not.



The following are such interfaces.  Also, see The "Unicode Bug".
For all of these interfaces Perl
currently (as of 5.8.3) simply assumes byte strings both as arguments
and results, or UTF-8 strings if the (problematic) encoding pragma has been used.



One reason that Perl does not attempt to resolve the role of Unicode in
these situations is that the answers are highly dependent on the operating
system and the file system(s).  For example, whether filenames can be
in Unicode and in exactly what kind of encoding, is not exactly a
portable concept.  Similarly for qx and system: how well will the
"command-line interface" (and which of them?) handle Unicode?











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80.2.16 The "Unicode Bug"



The term, "Unicode bug" has been applied to an inconsistency
on ASCII platforms with the
Unicode code points in the Latin-1 Supplement block, that
is, between 128 and 255.  Without a locale specified, unlike all other
characters or code points, these characters have very different semantics in
byte semantics versus character semantics, unless
use feature 'unicode_strings' is specified, directly or indirectly.
(It is indirectly specified by a use v5.12 or higher.)



In character semantics these upper-Latin1 characters are interpreted as
Unicode code points, which means
they have the same semantics as Latin-1 (ISO-8859-1).



In byte semantics (without unicode_strings), they are considered to
be unassigned characters, meaning that the only semantics they have is
their ordinal numbers, and that they are
not members of various character classes.  None are considered to match \w
for example, but all match \W.



Perl 5.12.0 added unicode_strings to force character semantics on
these code points in some circumstances, which fixed portions of the
bug; Perl 5.14.0 fixed almost all of it; and Perl 5.16.0 fixed the
remainder (so far as we know, anyway).  The lesson here is to enable
unicode_strings to avoid the headaches described below.



The old, problematic behavior affects these areas:





This behavior can lead to unexpected results in which a string’s semantics
suddenly change if a code point above 255 is appended to or removed from it,
which changes the string’s semantics from byte to character or vice versa.  As
an example, consider the following program and its output:



 $ perl -le'
     no feature 'unicode_strings';
     $s1 = "\xC2";
     $s2 = "\x{2660}";
     for ($s1, $s2, $s1.$s2) {
         print /\w/ || 0;
     }
 '
 0
 0
 1



If there’s no \w in s1 or in s2, why does their concatenation have one?



This anomaly stems from Perl’s attempt to not disturb older programs that
didn’t use Unicode, and hence had no semantics for characters outside of the
ASCII range (except in a locale), along with Perl’s desire to add Unicode
support seamlessly.  The result wasn’t seamless: these characters were
orphaned.



For Perls earlier than those described above, or when a string is passed
to a function outside the subpragma’s scope, a workaround is to always
call utf8::upgrade($string),
or to use the standard module (Encode).   Also, a scalar that has any characters
whose ordinal is above 0x100, or which were specified using either of the
\N{...} notations, will automatically have character semantics.









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80.2.17 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)



Sometimes (see When Unicode Does Not Happen or The "Unicode Bug")
there are situations where you simply need to force a byte
string into UTF-8, or vice versa.  The low-level calls
utf8::upgrade($bytestring) and utf8::downgrade($utf8string[, FAIL_OK]) are
the answers.



Note that utf8::downgrade() can fail if the string contains characters
that don’t fit into a byte.



Calling either function on a string that already is in the desired state is a
no-op.









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80.2.18 Using Unicode in XS



If you want to handle Perl Unicode in XS extensions, you may find the
following C APIs useful.  See also perlguts Unicode Support for an
explanation about Unicode at the XS level, and (perlapi) for the API
details.





For more information, see (perlapi), and utf8.c and utf8.h
in the Perl source code distribution.









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80.2.19 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)



Perl by default comes with the latest supported Unicode version built in, but
you can change to use any earlier one.



Download the files in the desired version of Unicode from the Unicode web
site http://www.unicode.org).  These should replace the existing files in
lib/unicore in the Perl source tree.  Follow the instructions in
README.perl in that directory to change some of their names, and then build
perl (see (INSTALL)).









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80.3 BUGS





• perlunicode Interaction with Locales:  



• perlunicode Problems with characters in the Latin-1 Supplement range:  



• perlunicode Interaction with Extensions:  



• perlunicode Speed:  



• perlunicode Problems on EBCDIC platforms:  



• perlunicode Porting code from perl-5.6.X:  












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80.3.1 Interaction with Locales



See perllocale Unicode and UTF-8









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80.3.2 Problems with characters in the Latin-1 Supplement range



See The "Unicode Bug"









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80.3.3 Interaction with Extensions



When Perl exchanges data with an extension, the extension should be
able to understand the UTF8 flag and act accordingly. If the
extension doesn’t recognize that flag, it’s likely that the extension
will return incorrectly-flagged data.



So if you’re working with Unicode data, consult the documentation of
every module you’re using if there are any issues with Unicode data
exchange. If the documentation does not talk about Unicode at all,
suspect the worst and probably look at the source to learn how the
module is implemented. Modules written completely in Perl shouldn’t
cause problems. Modules that directly or indirectly access code written
in other programming languages are at risk.



For affected functions, the simple strategy to avoid data corruption is
to always make the encoding of the exchanged data explicit. Choose an
encoding that you know the extension can handle. Convert arguments passed
to the extensions to that encoding and convert results back from that
encoding. Write wrapper functions that do the conversions for you, so
you can later change the functions when the extension catches up.



To provide an example, let’s say the popular Foo::Bar::escape_html
function doesn’t deal with Unicode data yet. The wrapper function
would convert the argument to raw UTF-8 and convert the result back to
Perl’s internal representation like so:



    sub my_escape_html ($) {
        my($what) = shift;
        return unless defined $what;
        Encode::decode_utf8(Foo::Bar::escape_html(
                                         Encode::encode_utf8($what)));
    }



Sometimes, when the extension does not convert data but just stores
and retrieves them, you will be able to use the otherwise
dangerous Encode::_utf8_on() function. Let’s say the popular
Foo::Bar extension, written in C, provides a param method that
lets you store and retrieve data according to these prototypes:



    $self->param($name, $value);            # set a scalar
    $value = $self->param($name);           # retrieve a scalar



If it does not yet provide support for any encoding, one could write a
derived class with such a param method:



    sub param {
      my($self,$name,$value) = @_;
      utf8::upgrade($name);     # make sure it is UTF-8 encoded
      if (defined $value) {
        utf8::upgrade($value);  # make sure it is UTF-8 encoded
        return $self->SUPER::param($name,$value);
      } else {
        my $ret = $self->SUPER::param($name);
        Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
        return $ret;
      }
    }



Some extensions provide filters on data entry/exit points, such as
DB_File::filter_store_key and family. Look out for such filters in
the documentation of your extensions, they can make the transition to
Unicode data much easier.









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80.3.4 Speed



Some functions are slower when working on UTF-8 encoded strings than
on byte encoded strings.  All functions that need to hop over
characters such as length(), substr() or index(), or matching regular
expressions can work much faster when the underlying data are
byte-encoded.



In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
a caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations.  In general,
operations with UTF-8 encoded strings are still slower. As an example,
the Unicode properties (character classes) like \p{Nd} are known to
be quite a bit slower (5-20 times) than their simpler counterparts
like \d (then again, there are hundreds of Unicode characters matching Nd
compared with the 10 ASCII characters matching d).









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80.3.5 Problems on EBCDIC platforms



There are several known problems with Perl on EBCDIC platforms.  If you
want to use Perl there, send email to perlbug@perl.org.



In earlier versions, when byte and character data were concatenated,
the new string was sometimes created by
decoding the byte strings as ISO 8859-1 (Latin-1), even if the
old Unicode string used EBCDIC.



If you find any of these, please report them as bugs.









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80.3.6 Porting code from perl-5.6.X



Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the utf8 pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.











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80.4 SEE ALSO



perlunitut NAME, perluniintro NAME, (perluniprops), (Encode), (open), (utf8), (bytes),
perlretut NAME, perlvar ${^UNICODE}
http://www.unicode.org/reports/tr44).









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81 perlunifaq





• perlunifaq NAME:  



• perlunifaq Q and A:  



• perlunifaq INTERNALS:  



• perlunifaq AUTHOR:  



• perlunifaq SEE ALSO:  












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81.1 NAME



perlunifaq - Perl Unicode FAQ









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81.2 Q and A



This is a list of questions and answers about Unicode in Perl, intended to be
read after perlunitut NAME.





• perlunifaq perlunitut isn't really a Unicode tutorial, is it?:  



• perlunifaq What character encodings does Perl support?:  



• perlunifaq Which version of perl should I use?:  



• perlunifaq What about binary data, like images?:  



• perlunifaq When should I decode or encode?:  



• perlunifaq What if I don't decode?:  



• perlunifaq What if I don't encode?:  



• perlunifaq Is there a way to automatically decode or encode?:  



• perlunifaq What if I don't know which encoding was used?:  



• perlunifaq Can I use Unicode in my Perl sources?:  



• perlunifaq Data::Dumper doesn't restore the UTF8 flag; is it broken?:  



• perlunifaq Why do regex character classes sometimes match only in the ASCII range?:  



• perlunifaq Why do some characters not uppercase or lowercase correctly?:  



• perlunifaq How can I determine if a string is a text string or a binary string?:  



• perlunifaq How do I convert from encoding FOO to encoding BAR?:  



• perlunifaq What are decode_utf8 and encode_utf8?:  



• perlunifaq What is a "wide character"?:  












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81.2.1 perlunitut isn’t really a Unicode tutorial, is it?



No, and this isn’t really a Unicode FAQ.



Perl has an abstracted interface for all supported character encodings, so this
is actually a generic Encode tutorial and Encode FAQ. But many people
think that Unicode is special and magical, and I didn’t want to disappoint
them, so I decided to call the document a Unicode tutorial.









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81.2.2 What character encodings does Perl support?



To find out which character encodings your Perl supports, run:



    perl -MEncode -le "print for Encode->encodings(':all')"









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81.2.3 Which version of perl should I use?



Well, if you can, upgrade to the most recent, but certainly 5.8.1 or newer.
The tutorial and FAQ assume the latest release.



You should also check your modules, and upgrade them if necessary. For example,
HTML::Entities requires version >= 1.32 to function correctly, even though the
changelog is silent about this.









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81.2.4 What about binary data, like images?



Well, apart from a bare binmode $fh, you shouldn’t treat them specially.
(The binmode is needed because otherwise Perl may convert line endings on Win32
systems.)



Be careful, though, to never combine text strings with binary strings. If you
need text in a binary stream, encode your text strings first using the
appropriate encoding, then join them with binary strings. See also: "What if I
don’t encode?".









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81.2.5 When should I decode or encode?



Whenever you’re communicating text with anything that is external to your perl
process, like a database, a text file, a socket, or another program. Even if
the thing you’re communicating with is also written in Perl.









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81.2.6 What if I don’t decode?



Whenever your encoded, binary string is used together with a text string, Perl
will assume that your binary string was encoded with ISO-8859-1, also known as
latin-1. If it wasn’t latin-1, then your data is unpleasantly converted. For
example, if it was UTF-8, the individual bytes of multibyte characters are seen
as separate characters, and then again converted to UTF-8. Such double encoding
can be compared to double HTML encoding (&amp;gt;), or double URI encoding
(%253E).



This silent implicit decoding is known as "upgrading". That may sound
positive, but it’s best to avoid it.









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81.2.7 What if I don’t encode?



Your text string will be sent using the bytes in Perl’s internal format. In
some cases, Perl will warn you that you’re doing something wrong, with a
friendly warning:



    Wide character in print at example.pl line 2.



Because the internal format is often UTF-8, these bugs are hard to spot,
because UTF-8 is usually the encoding you wanted! But don’t be lazy, and don’t
use the fact that Perl’s internal format is UTF-8 to your advantage. Encode
explicitly to avoid weird bugs, and to show to maintenance programmers that you
thought this through.









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81.2.8 Is there a way to automatically decode or encode?



If all data that comes from a certain handle is encoded in exactly the same
way, you can tell the PerlIO system to automatically decode everything, with
the encoding layer. If you do this, you can’t accidentally forget to decode
or encode anymore, on things that use the layered handle.



You can provide this layer when opening the file:



  open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
  open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read



Or if you already have an open filehandle:



  binmode $fh, ':encoding(UTF-8)';



Some database drivers for DBI can also automatically encode and decode, but
that is sometimes limited to the UTF-8 encoding.









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81.2.9 What if I don’t know which encoding was used?



Do whatever you can to find out, and if you have to: guess. (Don’t forget to
document your guess with a comment.)



You could open the document in a web browser, and change the character set or
character encoding until you can visually confirm that all characters look the
way they should.



There is no way to reliably detect the encoding automatically, so if people
keep sending you data without charset indication, you may have to educate them.









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81.2.10 Can I use Unicode in my Perl sources?



Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
use utf8 pragma.



    use utf8;



This doesn’t do anything to your input, or to your output. It only influences
the way your sources are read. You can use Unicode in string literals, in
identifiers (but they still have to be "word characters" according to \w),
and even in custom delimiters.









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81.2.11 Data::Dumper doesn’t restore the UTF8 flag; is it broken?



No, Data::Dumper’s Unicode abilities are as they should be. There have been
some complaints that it should restore the UTF8 flag when the data is read
again with eval. However, you should really not look at the flag, and
nothing indicates that Data::Dumper should break this rule.



Here’s what happens: when Perl reads in a string literal, it sticks to 8 bit
encoding as long as it can. (But perhaps originally it was internally encoded
as UTF-8, when you dumped it.) When it has to give that up because other
characters are added to the text string, it silently upgrades the string to
UTF-8. 



If you properly encode your strings for output, none of this is of your
concern, and you can just eval dumped data as always.









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81.2.12 Why do regex character classes sometimes match only in the ASCII range?









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81.2.13 Why do some characters not uppercase or lowercase correctly?



Starting in Perl 5.14 (and partially in Perl 5.12), just put a
use feature 'unicode_strings' near the beginning of your program.
Within its lexical scope you shouldn’t have this problem.  It also is
automatically enabled under use feature ':5.12' or using -E on the
command line for Perl 5.12 or higher.



The rationale for requiring this is to not break older programs that
rely on the way things worked before Unicode came along.  Those older
programs knew only about the ASCII character set, and so may not work
properly for additional characters.  When a string is encoded in UTF-8,
Perl assumes that the program is prepared to deal with Unicode, but when
the string isn’t, Perl assumes that only ASCII (unless it is an EBCDIC
platform) is wanted, and so those characters that are not ASCII
characters aren’t recognized as to what they would be in Unicode.
use feature 'unicode_strings' tells Perl to treat all characters as
Unicode, whether the string is encoded in UTF-8 or not, thus avoiding
the problem.



However, on earlier Perls, or if you pass strings to subroutines outside
the feature’s scope, you can force Unicode semantics by changing the
encoding to UTF-8 by doing utf8::upgrade($string). This can be used
safely on any string, as it checks and does not change strings that have
already been upgraded.



For a more detailed discussion, see (Unicode-Semantics) on CPAN.









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81.2.14 How can I determine if a string is a text string or a binary string?



You can’t. Some use the UTF8 flag for this, but that’s misuse, and makes well
behaved modules like Data::Dumper look bad. The flag is useless for this
purpose, because it’s off when an 8 bit encoding (by default ISO-8859-1) is
used to store the string.



This is something you, the programmer, has to keep track of; sorry. You could
consider adopting a kind of "Hungarian notation" to help with this.









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81.2.15 How do I convert from encoding FOO to encoding BAR?



By first converting the FOO-encoded byte string to a text string, and then the
text string to a BAR-encoded byte string:



    my $text_string = decode('FOO', $foo_string);
    my $bar_string  = encode('BAR', $text_string);



or by skipping the text string part, and going directly from one binary
encoding to the other:



    use Encode qw(from_to);
    from_to($string, 'FOO', 'BAR');  # changes contents of $string



or by letting automatic decoding and encoding do all the work:



    open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
    open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
    print { $barfh } $_ while <$foofh>;









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81.2.16 What are decode_utf8 and encode_utf8?



These are alternate syntaxes for decode('utf8', ...) and encode('utf8',
...).









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81.2.17 What is a "wide character"?



This is a term used both for characters with an ordinal value greater than 127,
characters with an ordinal value greater than 255, or any character occupying
more than one byte, depending on the context.



The Perl warning "Wide character in ..." is caused by a character with an
ordinal value greater than 255. With no specified encoding layer, Perl tries to
fit things in ISO-8859-1 for backward compatibility reasons. When it can’t, it
emits this warning (if warnings are enabled), and outputs UTF-8 encoded data
instead.



To avoid this warning and to avoid having different output encodings in a single
stream, always specify an encoding explicitly, for example with a PerlIO layer:



    binmode STDOUT, ":encoding(UTF-8)";









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81.3 INTERNALS





• perlunifaq What is "the UTF8 flag"?:  



• perlunifaq What about the use bytes pragma?:  



• perlunifaq What about the use encoding pragma?:  



• perlunifaq What is the difference between :encoding and :utf8?:  



• perlunifaq What's the difference between UTF-8 and utf8?:  



• perlunifaq I lost track; what encoding is the internal format really?:  












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81.3.1 What is "the UTF8 flag"?



Please, unless you’re hacking the internals, or debugging weirdness, don’t
think about the UTF8 flag at all. That means that you very probably shouldn’t
use is_utf8, _utf8_on or _utf8_off at all.



The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the
current internal representation is UTF-8. Without the flag, it is assumed to be
ISO-8859-1. Perl converts between these automatically.  (Actually Perl usually
assumes the representation is ASCII; see Why do regex character classes
sometimes match only in the ASCII range? above.)



One of Perl’s internal formats happens to be UTF-8. Unfortunately, Perl can’t
keep a secret, so everyone knows about this. That is the source of much
confusion. It’s better to pretend that the internal format is some unknown
encoding, and that you always have to encode and decode explicitly.









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81.3.2 What about the use bytes pragma?



Don’t use it. It makes no sense to deal with bytes in a text string, and it
makes no sense to deal with characters in a byte string. Do the proper
conversions (by decoding/encoding), and things will work out well: you get
character counts for decoded data, and byte counts for encoded data.



use bytes is usually a failed attempt to do something useful. Just forget
about it.









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81.3.3 What about the use encoding pragma?



Don’t use it. Unfortunately, it assumes that the programmer’s environment and
that of the user will use the same encoding. It will use the same encoding for
the source code and for STDIN and STDOUT. When a program is copied to another
machine, the source code does not change, but the STDIO environment might.



If you need non-ASCII characters in your source code, make it a UTF-8 encoded
file and use utf8.



If you need to set the encoding for STDIN, STDOUT, and STDERR, for example
based on the user’s locale, use open.









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81.3.4 What is the difference between :encoding and :utf8?



Because UTF-8 is one of Perl’s internal formats, you can often just skip the
encoding or decoding step, and manipulate the UTF8 flag directly.



Instead of :encoding(UTF-8), you can simply use :utf8, which skips the
encoding step if the data was already represented as UTF8 internally. This is
widely accepted as good behavior when you’re writing, but it can be dangerous
when reading, because it causes internal inconsistency when you have invalid
byte sequences. Using :utf8 for input can sometimes result in security
breaches, so please use :encoding(UTF-8) instead.



Instead of decode and encode, you could use _utf8_on and _utf8_off,
but this is considered bad style. Especially _utf8_on can be dangerous, for
the same reason that :utf8 can.



There are some shortcuts for oneliners;
see -C in perlrun NAME.









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81.3.5 What’s the difference between UTF-8 and utf8?



UTF-8 is the official standard. utf8 is Perl’s way of being liberal in
what it accepts. If you have to communicate with things that aren’t so liberal,
you may want to consider using UTF-8. If you have to communicate with things
that are too liberal, you may have to use utf8. The full explanation is in
(Encode).



UTF-8 is internally known as utf-8-strict. The tutorial uses UTF-8
consistently, even where utf8 is actually used internally, because the
distinction can be hard to make, and is mostly irrelevant.



For example, utf8 can be used for code points that don’t exist in Unicode, like
9999999, but if you encode that to UTF-8, you get a substitution character (by
default; see (Encode)Handling Malformed Data for more ways of dealing with
this.)



Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it’s not
some other encoding.)









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81.3.6 I lost track; what encoding is the internal format really?



It’s good that you lost track, because you shouldn’t depend on the internal
format being any specific encoding. But since you asked: by default, the
internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
history of the string. On EBCDIC platforms, this may be different even.



Perl knows how it stored the string internally, and will use that knowledge
when you encode. In other words: don’t try to find out what the internal
encoding for a certain string is, but instead just encode it into the encoding
that you want.









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81.4 AUTHOR



Juerd Waalboer <#####@juerd.nl>









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81.5 SEE ALSO



perlunicode NAME, perluniintro NAME, (Encode)









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82 perluniintro





• perluniintro NAME:  



• perluniintro DESCRIPTION:  



• perluniintro UNICODE IN OLDER PERLS:  



• perluniintro SEE ALSO:  



• perluniintro ACKNOWLEDGMENTS:  



• perluniintro AUTHOR, COPYRIGHT, AND LICENSE:  












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82.1 NAME



perluniintro - Perl Unicode introduction









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82.2 DESCRIPTION



This document gives a general idea of Unicode and how to use Unicode
in Perl.  See Further Resources for references to more in-depth
treatments of Unicode.





• perluniintro Unicode:  



• perluniintro Perl's Unicode Support:  



• perluniintro Perl's Unicode Model:  



• perluniintro Unicode and EBCDIC:  



• perluniintro Creating Unicode:  



• perluniintro Handling Unicode:  



• perluniintro Legacy Encodings:  



• perluniintro Unicode I/O:  



• perluniintro Displaying Unicode As Text:  



• perluniintro Special Cases:  



• perluniintro Advanced Topics:  



• perluniintro Miscellaneous:  



• perluniintro Questions With Answers:  



• perluniintro Hexadecimal Notation:  



• perluniintro Further Resources:  












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82.2.1 Unicode



Unicode is a character set standard which plans to codify all of the
writing systems of the world, plus many other symbols.



Unicode and ISO/IEC 10646 are coordinated standards that unify
almost all other modern character set standards,
covering more than 80 writing systems and hundreds of languages,
including all commercially-important modern languages.  All characters
in the largest Chinese, Japanese, and Korean dictionaries are also
encoded. The standards will eventually cover almost all characters in
more than 250 writing systems and thousands of languages.
Unicode 1.0 was released in October 1991, and 6.0 in October 2010.



A Unicode character is an abstract entity.  It is not bound to any
particular integer width, especially not to the C language char.
Unicode is language-neutral and display-neutral: it does not encode the
language of the text, and it does not generally define fonts or other graphical
layout details.  Unicode operates on characters and on text built from
those characters.



Unicode defines characters like LATIN CAPITAL LETTER A or GREEK
SMALL LETTER ALPHA and unique numbers for the characters, in this
case 0x0041 and 0x03B1, respectively.  These unique numbers are called
code points.  A code point is essentially the position of the
character within the set of all possible Unicode characters, and thus in
Perl, the term ordinal is often used interchangeably with it.



The Unicode standard prefers using hexadecimal notation for the code
points.  If numbers like 0x0041 are unfamiliar to you, take a peek
at a later section, Hexadecimal Notation.  The Unicode standard
uses the notation U+0041 LATIN CAPITAL LETTER A, to give the
hexadecimal code point and the normative name of the character.



Unicode also defines various properties for the characters, like
"uppercase" or "lowercase", "decimal digit", or "punctuation";
these properties are independent of the names of the characters.
Furthermore, various operations on the characters like uppercasing,
lowercasing, and collating (sorting) are defined.



A Unicode logical "character" can actually consist of more than one internal
actual "character" or code point.  For Western languages, this is adequately
modelled by a base character (like LATIN CAPITAL LETTER A) followed
by one or more modifiers (like COMBINING ACUTE ACCENT).  This sequence of
base character and modifiers is called a combining character
sequence.  Some non-western languages require more complicated
models, so Unicode created the grapheme cluster concept, which was
later further refined into the extended grapheme cluster.  For
example, a Korean Hangul syllable is considered a single logical
character, but most often consists of three actual
Unicode characters: a leading consonant followed by an interior vowel followed
by a trailing consonant.



Whether to call these extended grapheme clusters "characters" depends on your
point of view. If you are a programmer, you probably would tend towards seeing
each element in the sequences as one unit, or "character".  However from
the user’s point of view, the whole sequence could be seen as one
"character" since that’s probably what it looks like in the context of the
user’s language.  In this document, we take the programmer’s point of
view: one "character" is one Unicode code point.



For some combinations of base character and modifiers, there are
precomposed characters.  There is a single character equivalent, for
example, to the sequence LATIN CAPITAL LETTER A followed by
COMBINING ACUTE ACCENT.  It is called  LATIN CAPITAL LETTER A WITH
ACUTE.  These precomposed characters are, however, only available for
some combinations, and are mainly meant to support round-trip
conversions between Unicode and legacy standards (like ISO 8859).  Using
sequences, as Unicode does, allows for needing fewer basic building blocks
(code points) to express many more potential grapheme clusters.  To
support conversion between equivalent forms, various normalization
forms are also defined.  Thus, LATIN CAPITAL LETTER A WITH ACUTE is
in Normalization Form Composed, (abbreviated NFC), and the sequence
LATIN CAPITAL LETTER A followed by COMBINING ACUTE ACCENT
represents the same character in Normalization Form Decomposed (NFD).



Because of backward compatibility with legacy encodings, the "a unique
number for every character" idea breaks down a bit: instead, there is
"at least one number for every character".  The same character could
be represented differently in several legacy encodings.  The
converse is not also true: some code points do not have an assigned
character.  Firstly, there are unallocated code points within
otherwise used blocks.  Secondly, there are special Unicode control
characters that do not represent true characters.



When Unicode was first conceived, it was thought that all the world’s
characters could be represented using a 16-bit word; that is a maximum of
0x10000 (or 65536) characters from 0x0000 to 0xFFFF would be
needed.  This soon proved to be false, and since Unicode 2.0 (July
1996), Unicode has been defined all the way up to 21 bits (0x10FFFF),
and Unicode 3.1 (March 2001) defined the first characters above 0xFFFF.
The first 0x10000 characters are called the Plane 0, or the
Basic Multilingual Plane (BMP).  With Unicode 3.1, 17 (yes,
seventeen) planes in all were defined–but they are nowhere near full of
defined characters, yet.



When a new language is being encoded, Unicode generally will choose a
block of consecutive unallocated code points for its characters.  So
far, the number of code points in these blocks has always been evenly
divisible by 16.  Extras in a block, not currently needed, are left
unallocated, for future growth.  But there have been occasions when
a later relase needed more code points than the available extras, and a
new block had to allocated somewhere else, not contiguous to the initial
one, to handle the overflow.  Thus, it became apparent early on that
"block" wasn’t an adequate organizing principal, and so the Script
property was created.  (Later an improved script property was added as
well, the Script_Extensions property.)  Those code points that are in
overflow blocks can still
have the same script as the original ones.  The script concept fits more
closely with natural language: there is Latin script, Greek
script, and so on; and there are several artificial scripts, like
Common for characters that are used in multiple scripts, such as
mathematical symbols.  Scripts usually span varied parts of several
blocks.  For more information about scripts, see perlunicode Scripts.
The division into blocks exists, but it is almost completely
accidental–an artifact of how the characters have been and still are
allocated.  (Note that this paragraph has oversimplified things for the
sake of this being an introduction.  Unicode doesn’t really encode
languages, but the writing systems for them–their scripts; and one
script can be used by many languages.  Unicode also encodes things that
aren’t really about languages, such as symbols like BAGGAGE CLAIM.)



The Unicode code points are just abstract numbers.  To input and
output these abstract numbers, the numbers must be encoded or
serialised somehow.  Unicode defines several character encoding
forms, of which UTF-8 is perhaps the most popular.  UTF-8 is a
variable length encoding that encodes Unicode characters as 1 to 6
bytes.  Other encodings
include UTF-16 and UTF-32 and their big- and little-endian variants
(UTF-8 is byte-order independent) The ISO/IEC 10646 defines the UCS-2
and UCS-4 encoding forms.



For more information about encodings–for instance, to learn what
surrogates and byte order marks (BOMs) are–see perlunicode NAME.









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82.2.2 Perl’s Unicode Support



Starting from Perl 5.6.0, Perl has had the capacity to handle Unicode
natively.  Perl 5.8.0, however, is the first recommended release for
serious Unicode work.  The maintenance release 5.6.1 fixed many of the
problems of the initial Unicode implementation, but for example
regular expressions still do not work with Unicode in 5.6.1.
Perl 5.14.0 is the first release where Unicode support is
(almost) seamlessly integrable without some gotchas (the exception being
some differences in (perlfunc)quotemeta, which is fixed
starting in Perl 5.16.0).   To enable this
seamless support, you should use feature 'unicode_strings' (which is
automatically selected if you use 5.012 or higher).  See (feature).
(5.14 also fixes a number of bugs and departures from the Unicode
standard.)



Before Perl 5.8.0, the use of use utf8 was used to declare
that operations in the current block or file would be Unicode-aware.
This model was found to be wrong, or at least clumsy: the "Unicodeness"
is now carried with the data, instead of being attached to the
operations.
Starting with Perl 5.8.0, only one case remains where an explicit use
utf8 is needed: if your Perl script itself is encoded in UTF-8, you can
use UTF-8 in your identifier names, and in string and regular expression
literals, by saying use utf8.  This is not the default because
scripts with legacy 8-bit data in them would break.  See (utf8).









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82.2.3 Perl’s Unicode Model



Perl supports both pre-5.6 strings of eight-bit native bytes, and
strings of Unicode characters.  The general principle is that Perl tries
to keep its data as eight-bit bytes for as long as possible, but as soon
as Unicodeness cannot be avoided, the data is transparently upgraded
to Unicode.  Prior to Perl 5.14, the upgrade was not completely
transparent (see perlunicode The "Unicode Bug"), and for backwards
compatibility, full transparency is not gained unless use feature
'unicode_strings' (see (feature)) or use 5.012 (or higher) is
selected.



Internally, Perl currently uses either whatever the native eight-bit
character set of the platform (for example Latin-1) is, defaulting to
UTF-8, to encode Unicode strings. Specifically, if all code points in
the string are 0xFF or less, Perl uses the native eight-bit
character set.  Otherwise, it uses UTF-8.



A user of Perl does not normally need to know nor care how Perl
happens to encode its internal strings, but it becomes relevant when
outputting Unicode strings to a stream without a PerlIO layer (one with
the "default" encoding).  In such a case, the raw bytes used internally
(the native character set or UTF-8, as appropriate for each string)
will be used, and a "Wide character" warning will be issued if those
strings contain a character beyond 0x00FF.



For example,



      perl -e 'print "\x{DF}\n", "\x{0100}\x{DF}\n"'



produces a fairly useless mixture of native bytes and UTF-8, as well
as a warning:



     Wide character in print at ...



To output UTF-8, use the :encoding or :utf8 output layer.  Prepending



      binmode(STDOUT, ":utf8");



to this sample program ensures that the output is completely UTF-8,
and removes the program’s warning.



You can enable automatic UTF-8-ification of your standard file
handles, default open() layer, and @ARGV by using either
the -C command line switch or the PERL_UNICODE environment
variable, see perlrun NAME for the documentation of the -C switch.



Note that this means that Perl expects other software to work the same
way:
if Perl has been led to believe that STDIN should be UTF-8, but then
STDIN coming in from another command is not UTF-8, Perl will likely
complain about the malformed UTF-8.



All features that combine Unicode and I/O also require using the new
PerlIO feature.  Almost all Perl 5.8 platforms do use PerlIO, though:
you can see whether yours is by running "perl -V" and looking for
useperlio=define.









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82.2.4 Unicode and EBCDIC



Perl 5.8.0 also supports Unicode on EBCDIC platforms.  There,
Unicode support is somewhat more complex to implement since
additional conversions are needed at every step.



Later Perl releases have added code that will not work on EBCDIC platforms, and
no one has complained, so the divergence has continued.  If you want to run
Perl on an EBCDIC platform, send email to perlbug@perl.org



On EBCDIC platforms, the internal Unicode encoding form is UTF-EBCDIC
instead of UTF-8.  The difference is that as UTF-8 is "ASCII-safe" in
that ASCII characters encode to UTF-8 as-is, while UTF-EBCDIC is
"EBCDIC-safe".









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82.2.5 Creating Unicode



To create Unicode characters in literals for code points above 0xFF,
use the \x{...} notation in double-quoted strings:



    my $smiley = "\x{263a}";



Similarly, it can be used in regular expression literals



    $smiley =~ /\x{263a}/;



At run-time you can use chr():



    my $hebrew_alef = chr(0x05d0);



See Further Resources for how to find all these numeric codes.



Naturally, ord() will do the reverse: it turns a character into
a code point.



Note that \x.. (no {} and only two hexadecimal digits), \x{...},
and chr(...) for arguments less than 0x100 (decimal 256)
generate an eight-bit character for backward compatibility with older
Perls.  For arguments of 0x100 or more, Unicode characters are
always produced. If you want to force the production of Unicode
characters regardless of the numeric value, use pack("U", ...)
instead of \x.., \x{...}, or chr().



You can invoke characters
by name in double-quoted strings:



    my $arabic_alef = "\N{ARABIC LETTER ALEF}";



And, as mentioned above, you can also pack() numbers into Unicode
characters:



   my $georgian_an  = pack("U", 0x10a0);



Note that both \x{...} and \N{...} are compile-time string
constants: you cannot use variables in them.  if you want similar
run-time functionality, use chr() and charnames::string_vianame().



If you want to force the result to Unicode characters, use the special
"U0" prefix.  It consumes no arguments but causes the following bytes
to be interpreted as the UTF-8 encoding of Unicode characters:



   my $chars = pack("U0W*", 0x80, 0x42);



Likewise, you can stop such UTF-8 interpretation by using the special
"C0" prefix.









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82.2.6 Handling Unicode



Handling Unicode is for the most part transparent: just use the
strings as usual.  Functions like index(), length(), and
substr() will work on the Unicode characters; regular expressions
will work on the Unicode characters (see perlunicode NAME and perlretut NAME).



Note that Perl considers grapheme clusters to be separate characters, so for
example



 print length("\N{LATIN CAPITAL LETTER A}\N{COMBINING ACUTE ACCENT}"),
       "\n";



will print 2, not 1.  The only exception is that regular expressions
have \X for matching an extended grapheme cluster.  (Thus \X in a
regular expression would match the entire sequence of both the example
characters.)



Life is not quite so transparent, however, when working with legacy
encodings, I/O, and certain special cases:









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82.2.7 Legacy Encodings



When you combine legacy data and Unicode, the legacy data needs
to be upgraded to Unicode.  Normally the legacy data is assumed to be
ISO 8859-1 (or EBCDIC, if applicable).



The Encode module knows about many encodings and has interfaces
for doing conversions between those encodings:



    use Encode 'decode';
    $data = decode("iso-8859-3", $data); # convert from legacy to utf-8









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82.2.8 Unicode I/O



Normally, writing out Unicode data



    print FH $some_string_with_unicode, "\n";



produces raw bytes that Perl happens to use to internally encode the
Unicode string.  Perl’s internal encoding depends on the system as
well as what characters happen to be in the string at the time. If
any of the characters are at code points 0x100 or above, you will get
a warning.  To ensure that the output is explicitly rendered in the
encoding you desire–and to avoid the warning–open the stream with
the desired encoding. Some examples:



    open FH, ">:utf8", "file";

    open FH, ">:encoding(ucs2)",      "file";
    open FH, ">:encoding(UTF-8)",     "file";
    open FH, ">:encoding(shift_jis)", "file";



and on already open streams, use binmode():



    binmode(STDOUT, ":utf8");

    binmode(STDOUT, ":encoding(ucs2)");
    binmode(STDOUT, ":encoding(UTF-8)");
    binmode(STDOUT, ":encoding(shift_jis)");



The matching of encoding names is loose: case does not matter, and
many encodings have several aliases.  Note that the :utf8 layer
must always be specified exactly like that; it is not subject to
the loose matching of encoding names. Also note that currently :utf8 is unsafe for
input, because it accepts the data without validating that it is indeed valid
UTF-8; you should instead use :encoding(utf-8) (with or without a
hyphen).



See (PerlIO) for the :utf8 layer, (PerlIO-encoding) and
(Encode-PerlIO) for the :encoding() layer, and
(Encode-Supported) for many encodings supported by the Encode
module.



Reading in a file that you know happens to be encoded in one of the
Unicode or legacy encodings does not magically turn the data into
Unicode in Perl’s eyes.  To do that, specify the appropriate
layer when opening files



    open(my $fh,'<:encoding(utf8)', 'anything');
    my $line_of_unicode = <$fh>;

    open(my $fh,'<:encoding(Big5)', 'anything');
    my $line_of_unicode = <$fh>;



The I/O layers can also be specified more flexibly with
the open pragma.  See (open), or look at the following example.



    use open ':encoding(utf8)'; # input/output default encoding will be
                                # UTF-8
    open X, ">file";
    print X chr(0x100), "\n";
    close X;
    open Y, "<file";
    printf "%#x\n", ord(<Y>); # this should print 0x100
    close Y;



With the open pragma you can use the :locale layer



    BEGIN { $ENV{LC_ALL} = $ENV{LANG} = 'ru_RU.KOI8-R' }
    # the :locale will probe the locale environment variables like
    # LC_ALL
    use open OUT => ':locale'; # russki parusski
    open(O, ">koi8");
    print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
    close O;
    open(I, "<koi8");
    printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
    close I;



These methods install a transparent filter on the I/O stream that
converts data from the specified encoding when it is read in from the
stream.  The result is always Unicode.



The (open) pragma affects all the open() calls after the pragma by
setting default layers.  If you want to affect only certain
streams, use explicit layers directly in the open() call.



You can switch encodings on an already opened stream by using
binmode(); see (perlfunc)binmode.



The :locale does not currently (as of Perl 5.8.0) work with
open() and binmode(), only with the open pragma.  The
:utf8 and :encoding(...) methods do work with all of open(),
binmode(), and the open pragma.



Similarly, you may use these I/O layers on output streams to
automatically convert Unicode to the specified encoding when it is
written to the stream. For example, the following snippet copies the
contents of the file "text.jis" (encoded as ISO-2022-JP, aka JIS) to
the file "text.utf8", encoded as UTF-8:



    open(my $nihongo, '<:encoding(iso-2022-jp)', 'text.jis');
    open(my $unicode, '>:utf8',                  'text.utf8');
    while (<$nihongo>) { print $unicode $_ }



The naming of encodings, both by the open() and by the open
pragma allows for flexible names: koi8-r and KOI8R will both be
understood.



Common encodings recognized by ISO, MIME, IANA, and various other
standardisation organisations are recognised; for a more detailed
list see (Encode-Supported).



read() reads characters and returns the number of characters.
seek() and tell() operate on byte counts, as do sysread()
and sysseek().



Notice that because of the default behaviour of not doing any
conversion upon input if there is no default layer,
it is easy to mistakenly write code that keeps on expanding a file
by repeatedly encoding the data:



    # BAD CODE WARNING
    open F, "file";
    local $/; ## read in the whole file of 8-bit characters
    $t = <F>;
    close F;
    open F, ">:encoding(utf8)", "file";
    print F $t; ## convert to UTF-8 on output
    close F;



If you run this code twice, the contents of the file will be twice
UTF-8 encoded.  A use open ':encoding(utf8)' would have avoided the
bug, or explicitly opening also the file for input as UTF-8.



NOTE: the :utf8 and :encoding features work only if your
Perl has been built with the new PerlIO feature (which is the default
on most systems).









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82.2.9 Displaying Unicode As Text



Sometimes you might want to display Perl scalars containing Unicode as
simple ASCII (or EBCDIC) text.  The following subroutine converts
its argument so that Unicode characters with code points greater than
255 are displayed as \x{...}, control characters (like \n) are
displayed as \x.., and the rest of the characters as themselves:



 sub nice_string {
     join("",
       map { $_ > 255 ?                  # if wide character...
              sprintf("\\x{%04X}", $_) :  # \x{...}
              chr($_) =~ /[[:cntrl:]]/ ?  # else if control character...
              sprintf("\\x%02X", $_) :    # \x..
              quotemeta(chr($_))          # else quoted or as themselves
         } unpack("W*", $_[0]));           # unpack Unicode characters
   }



For example,



   nice_string("foo\x{100}bar\n")



returns the string



   'foo\x{0100}bar\x0A'



which is ready to be printed.









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82.2.10 Special Cases











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82.2.11 Advanced Topics











Next: , Previous: perluniintro Advanced Topics, Up: perluniintro DESCRIPTION   [Contents]





82.2.12 Miscellaneous











Next: , Previous: perluniintro Miscellaneous, Up: perluniintro DESCRIPTION   [Contents]





82.2.13 Questions With Answers











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82.2.14 Hexadecimal Notation



The Unicode standard prefers using hexadecimal notation because
that more clearly shows the division of Unicode into blocks of 256 characters.
Hexadecimal is also simply shorter than decimal.  You can use decimal
notation, too, but learning to use hexadecimal just makes life easier
with the Unicode standard.  The U+HHHH notation uses hexadecimal,
for example.



The 0x prefix means a hexadecimal number, the digits are 0-9 and
a-f (or A-F, case doesn’t matter).  Each hexadecimal digit represents
four bits, or half a byte.  print 0x..., "\n" will show a
hexadecimal number in decimal, and printf "%x\n", $decimal will
show a decimal number in hexadecimal.  If you have just the
"hex digits" of a hexadecimal number, you can use the hex() function.



    print 0x0009, "\n";    # 9
    print 0x000a, "\n";    # 10
    print 0x000f, "\n";    # 15
    print 0x0010, "\n";    # 16
    print 0x0011, "\n";    # 17
    print 0x0100, "\n";    # 256

    print 0x0041, "\n";    # 65

    printf "%x\n",  65;    # 41
    printf "%#x\n", 65;    # 0x41

    print hex("41"), "\n"; # 65









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82.2.15 Further Resources











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82.3 UNICODE IN OLDER PERLS



If you cannot upgrade your Perl to 5.8.0 or later, you can still
do some Unicode processing by using the modules Unicode::String,
Unicode::Map8, and Unicode::Map, available from CPAN.
If you have the GNU recode installed, you can also use the
Perl front-end Convert::Recode for character conversions.



The following are fast conversions from ISO 8859-1 (Latin-1) bytes
to UTF-8 bytes and back, the code works even with older Perl 5 versions.



    # ISO 8859-1 to UTF-8
    s/([\x80-\xFF])/chr(0xC0|ord($1)>>6).chr(0x80|ord($1)&0x3F)/eg;

    # UTF-8 to ISO 8859-1
    s/([\xC2\xC3])([\x80-\xBF])/chr(ord($1)<<6&0xC0|ord($2)&0x3F)/eg;









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82.4 SEE ALSO



perlunitut NAME, perlunicode NAME, (Encode), (open), (utf8), (bytes),
perlretut NAME, perlrun NAME, (Unicode-Collate), (Unicode-Normalize),
(Unicode-UCD)









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82.5 ACKNOWLEDGMENTS



Thanks to the kind readers of the perl5-porters@perl.org,
perl-unicode@perl.org, linux-utf8@nl.linux.org, and unicore@unicode.org
mailing lists for their valuable feedback.









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82.6 AUTHOR, COPYRIGHT, AND LICENSE



Copyright 2001-2011 Jarkko Hietaniemi <jhi@iki.fi>



This document may be distributed under the same terms as Perl itself.









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83 perlunitut





• perlunitut NAME:  



• perlunitut DESCRIPTION:  



• perlunitut SUMMARY:  



• perlunitut Q and A (or FAQ):  



• perlunitut ACKNOWLEDGEMENTS:  



• perlunitut AUTHOR:  



• perlunitut SEE ALSO:  












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83.1 NAME



perlunitut - Perl Unicode Tutorial









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83.2 DESCRIPTION



The days of just flinging strings around are over. It’s well established that
modern programs need to be capable of communicating funny accented letters, and
things like euro symbols. This means that programmers need new habits. It’s
easy to program Unicode capable software, but it does require discipline to do
it right.



There’s a lot to know about character sets, and text encodings. It’s probably
best to spend a full day learning all this, but the basics can be learned in
minutes. 



These are not the very basics, though. It is assumed that you already
know the difference between bytes and characters, and realise (and accept!)
that there are many different character sets and encodings, and that your
program has to be explicit about them. Recommended reading is "The Absolute
Minimum Every Software Developer Absolutely, Positively Must Know About Unicode
and Character Sets (No Excuses!)" by Joel Spolsky, at
http://joelonsoftware.com/articles/Unicode.html.



This tutorial speaks in rather absolute terms, and provides only a limited view
of the wealth of character string related features that Perl has to offer. For
most projects, this information will probably suffice.





• perlunitut Definitions:  



• perlunitut Your new toolkit:  



• perlunitut I/O flow (the actual 5 minute tutorial):  












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83.2.1 Definitions



It’s important to set a few things straight first. This is the most important
part of this tutorial. This view may conflict with other information that you
may have found on the web, but that’s mostly because many sources are wrong.



You may have to re-read this entire section a few times...





• perlunitut Unicode:  



• perlunitut UTF-8:  



• perlunitut Text strings (character strings):  



• perlunitut Binary strings (byte strings):  



• perlunitut Encoding:  



• perlunitut Decoding:  



• perlunitut Internal format:  












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83.2.1.1 Unicode



Unicode is a character set with room for lots of characters. The ordinal
value of a character is called a code point.   (But in practice, the
distinction between code point and character is blurred, so the terms often
are used interchangeably.)



There are many, many code points, but computers work with bytes, and a byte has
room for only 256 values.  Unicode has many more characters than that,
so you need a method to make these accessible.



Unicode is encoded using several competing encodings, of which UTF-8 is the
most used. In a Unicode encoding, multiple subsequent bytes can be used to
store a single code point, or simply: character.









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83.2.1.2 UTF-8



UTF-8 is a Unicode encoding. Many people think that Unicode and UTF-8 are
the same thing, but they’re not. There are more Unicode encodings, but much of
the world has standardized on UTF-8. 



UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They take
only one byte per character. All other characters are encoded as two or more
(up to six) bytes using a complex scheme. Fortunately, Perl handles this for
us, so we don’t have to worry about this.









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83.2.1.3 Text strings (character strings)



Text strings, or character strings are made of characters. Bytes are
irrelevant here, and so are encodings. Each character is just that: the
character.



On a text string, you would do things like:



    $text =~ s/foo/bar/;
    if ($string =~ /^\d+$/) { ... }
    $text = ucfirst $text;
    my $character_count = length $text;



The value of a character (ord, chr) is the corresponding Unicode code
point.









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83.2.1.4 Binary strings (byte strings)



Binary strings, or byte strings are made of bytes. Here, you don’t have
characters, just bytes. All communication with the outside world (anything
outside of your current Perl process) is done in binary.



On a binary string, you would do things like:



    my (@length_content) = unpack "(V/a)*", $binary;
    $binary =~ s/\x00\x0F/\xFF\xF0/;  # for the brave :)
    print {$fh} $binary;
    my $byte_count = length $binary;









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83.2.1.5 Encoding



Encoding (as a verb) is the conversion from text to binary. To encode,
you have to supply the target encoding, for example iso-8859-1 or UTF-8.
Some encodings, like the iso-8859 ("latin") range, do not support the full
Unicode standard; characters that can’t be represented are lost in the
conversion.









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83.2.1.6 Decoding



Decoding is the conversion from binary to text. To decode, you have to
know what encoding was used during the encoding phase. And most of all, it must
be something decodable. It doesn’t make much sense to decode a PNG image into a
text string.









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83.2.1.7 Internal format



Perl has an internal format, an encoding that it uses to encode text strings
so it can store them in memory. All text strings are in this internal format.
In fact, text strings are never in any other format!



You shouldn’t worry about what this format is, because conversion is
automatically done when you decode or encode.









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83.2.2 Your new toolkit



Add to your standard heading the following line:



    use Encode qw(encode decode);



Or, if you’re lazy, just:



    use Encode;









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83.2.3 I/O flow (the actual 5 minute tutorial)



The typical input/output flow of a program is:



    1. Receive and decode
    2. Process
    3. Encode and output



If your input is binary, and is supposed to remain binary, you shouldn’t decode
it to a text string, of course. But in all other cases, you should decode it.



Decoding can’t happen reliably if you don’t know how the data was encoded. If
you get to choose, it’s a good idea to standardize on UTF-8.



    my $foo   = decode('UTF-8', get 'http://example.com/');
    my $bar   = decode('ISO-8859-1', readline STDIN);
    my $xyzzy = decode('Windows-1251', $cgi->param('foo'));



Processing happens as you knew before. The only difference is that you’re now
using characters instead of bytes. That’s very useful if you use things like
substr, or length.



It’s important to realize that there are no bytes in a text string. Of course,
Perl has its internal encoding to store the string in memory, but ignore that.
If you have to do anything with the number of bytes, it’s probably best to move
that part to step 3, just after you’ve encoded the string. Then you know
exactly how many bytes it will be in the destination string.



The syntax for encoding text strings to binary strings is as simple as decoding:



    $body = encode('UTF-8', $body);



If you needed to know the length of the string in bytes, now’s the perfect time
for that. Because $body is now a byte string, length will report the
number of bytes, instead of the number of characters. The number of
characters is no longer known, because characters only exist in text strings.



    my $byte_count = length $body;



And if the protocol you’re using supports a way of letting the recipient know
which character encoding you used, please help the receiving end by using that
feature! For example, E-mail and HTTP support MIME headers, so you can use the
Content-Type header. They can also have Content-Length to indicate the
number of bytes, which is always a good idea to supply if the number is
known.



    "Content-Type: text/plain; charset=UTF-8",
    "Content-Length: $byte_count"









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83.3 SUMMARY



Decode everything you receive, encode everything you send out. (If it’s text
data.)









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83.4 Q and A (or FAQ)



After reading this document, you ought to read perlunifaq NAME too. 









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83.5 ACKNOWLEDGEMENTS



Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the
Amsterdam Perl Mongers meetings got me interested and determined to find out
how to use character encodings in Perl in ways that don’t break easily.



Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch
Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial.



Thanks to the people who asked about this kind of stuff in several Perl IRC
channels, and have constantly reminded me that a simpler explanation was
needed.



Thanks to the people who reviewed this document for me, before it went public.
They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan
Gray.









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83.6 AUTHOR



Juerd Waalboer <#####@juerd.nl>









Previous: perlunitut AUTHOR, Up: perlunitut   [Contents]





83.7 SEE ALSO



perlunifaq NAME, perlunicode NAME, perluniintro NAME, (Encode)









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84 perlutil





• perlutil NAME:  



• perlutil DESCRIPTION:  



• perlutil LIST OF UTILITIES:  



• perlutil SEE ALSO:  












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84.1 NAME



perlutil - utilities packaged with the Perl distribution









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84.2 DESCRIPTION



Along with the Perl interpreter itself, the Perl distribution installs a
range of utilities on your system. There are also several utilities
which are used by the Perl distribution itself as part of the install
process. This document exists to list all of these utilities, explain
what they are for and provide pointers to each module’s documentation,
if appropriate.









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84.3 LIST OF UTILITIES





• perlutil Documentation:  



• perlutil Converters:  



• perlutil Administration:  



• perlutil Development:  



• perlutil General tools:  



• perlutil Installation:  












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84.3.1 Documentation





(perldoc)perldoc




The main interface to Perl’s documentation is perldoc, although
if you’re reading this, it’s more than likely that you’ve already found
it. perldoc will extract and format the documentation from any file
in the current directory, any Perl module installed on the system, or
any of the standard documentation pages, such as this one. Use 
perldoc <name> to get information on any of the utilities
described in this document.





(pod2man)pod2man and (pod2text)pod2text




If it’s run from a terminal, perldoc will usually call pod2man to
translate POD (Plain Old Documentation - see (perlpod) for an
explanation) into a manpage, and then run man to display it; if
man isn’t available, pod2text will be used instead and the output
piped through your favourite pager.





(pod2html)pod2html and (pod2latex)pod2latex




As well as these two, there are two other converters: pod2html will
produce HTML pages from POD, and pod2latex, which produces LaTeX
files.





(pod2usage)pod2usage




If you just want to know how to use the utilities described here,
pod2usage will just extract the "USAGE" section; some of
the utilities will automatically call pod2usage on themselves when
you call them with -help.





(podselect)podselect




pod2usage is a special case of podselect, a utility to extract
named sections from documents written in POD. For instance, while
utilities have "USAGE" sections, Perl modules usually have "SYNOPSIS"
sections: podselect -s "SYNOPSIS" ... will extract this section for
a given file.





(podchecker)podchecker




If you’re writing your own documentation in POD, the podchecker
utility will look for errors in your markup.





(splain)splain




splain is an interface to perldiag NAME - paste in your error message
to it, and it’ll explain it for you.





roffitall




The roffitall utility is not installed on your system but lives in
the pod/ directory of your Perl source kit; it converts all the
documentation from the distribution to *roff format, and produces a
typeset PostScript or text file of the whole lot.














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84.3.2 Converters



To help you convert legacy programs to Perl, we’ve included three
conversion filters:





(a2p)a2p




a2p converts awk scripts to Perl programs; for example, a2p -F:
on the simple awk script {print $2} will produce a Perl program
based around this code:



    while (<>) {
        ($Fld1,$Fld2) = split(/[:\n]/, $_, -1);
        print $Fld2;
    }





(s2p)s2p and (psed)




Similarly, s2p converts sed scripts to Perl programs. s2p run
on s/foo/bar will produce a Perl program based around this:



    while (<>) {
        chomp;
        s/foo/bar/g;
        print if $printit;
    }



When invoked as psed, it behaves as a sed implementation, written in
Perl.





(find2perl)find2perl




Finally, find2perl translates find commands to Perl equivalents which 
use the (File-Find)File::Find module. As an example, 
find2perl . -user root -perm 4000 -print produces the following callback
subroutine for File::Find:



    sub wanted {
        my ($dev,$ino,$mode,$nlink,$uid,$gid);
        (($dev,$ino,$mode,$nlink,$uid,$gid) = lstat($_)) &&
        $uid == $uid{'root'}) &&
        (($mode & 0777) == 04000);
        print("$name\n");
    }








As well as these filters for converting other languages, the
(pl2pm)pl2pm utility will help you convert old-style Perl 4 libraries to 
new-style Perl5 modules.









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84.3.3 Administration





(config_data)config_data




Query or change configuration of Perl modules that use Module::Build-based
configuration files for features and config data.





(libnetcfg)libnetcfg




To display and change the libnet configuration run the libnetcfg command.





(perlivp)




The perlivp program is set up at Perl source code build time to test
the Perl version it was built under.  It can be used after running make
install (or your platform’s equivalent procedure) to verify that perl
and its libraries have been installed correctly.














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84.3.4 Development



There are a set of utilities which help you in developing Perl programs, 
and in particular, extending Perl with C.





(perlbug)perlbug




perlbug is the recommended way to report bugs in the perl interpreter
itself or any of the standard library modules back to the developers;
please read through the documentation for perlbug thoroughly before
using it to submit a bug report.





(perlbug)perlthanks




This program provides an easy way to send a thank-you message back to the
authors and maintainers of perl. It’s just perlbug installed under
another name.





(h2ph)h2ph




Back before Perl had the XS system for connecting with C libraries,
programmers used to get library constants by reading through the C
header files. You may still see require 'syscall.ph' or similar
around - the .ph file should be created by running h2ph on the
corresponding .h file. See the h2ph documentation for more on how
to convert a whole bunch of header files at once.





(c2ph)c2ph and (pstruct)pstruct




c2ph and pstruct, which are actually the same program but behave
differently depending on how they are called, provide another way of
getting at C with Perl - they’ll convert C structures and union declarations
to Perl code. This is deprecated in favour of h2xs these days.





(h2xs)h2xs




h2xs converts C header files into XS modules, and will try and write
as much glue between C libraries and Perl modules as it can. It’s also
very useful for creating skeletons of pure Perl modules.





(enc2xs)




enc2xs builds a Perl extension for use by Encode from either
Unicode Character Mapping files (.ucm) or Tcl Encoding Files (.enc).
Besides being used internally during the build process of the Encode
module, you can use enc2xs to add your own encoding to perl.
No knowledge of XS is necessary.





(xsubpp)




xsubpp is a compiler to convert Perl XS code into C code.
It is typically run by the makefiles created by (ExtUtils-MakeMaker).



xsubpp will compile XS code into C code by embedding the constructs
necessary to let C functions manipulate Perl values and creates the glue
necessary to let Perl access those functions.





(prove)




prove is a command-line interface to the test-running functionality
of Test::Harness.  It’s an alternative to make test.





(corelist)




A command-line front-end to Module::CoreList, to query what modules
were shipped with given versions of perl.














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84.3.5 General tools



A few general-purpose tools are shipped with perl, mostly because they
came along modules included in the perl distribution.





(piconv)




piconv is a Perl version of iconv, a character encoding converter
widely available for various Unixen today.  This script was primarily a
technology demonstrator for Perl 5.8.0, but you can use piconv in the
place of iconv for virtually any case.





(ptar)




ptar is a tar-like program, written in pure Perl.





(ptardiff)




ptardiff is a small utility that produces a diff between an extracted
archive and an unextracted one. (Note that this utility requires the
Text::Diff module to function properly; this module isn’t distributed
with perl, but is available from the CPAN.)





(ptargrep)




ptargrep is a utility to apply pattern matching to the contents of files 
in a tar archive.





(shasum)




This utility, that comes with the Digest::SHA module, is used to print
or verify SHA checksums.





(zipdetails)




(zipdetails) displays information about the internal record structure of the zip file.
It is not concerned with displaying any details of the compressed data stored in the zip file.














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84.3.6 Installation



These utilities help manage extra Perl modules that don’t come with the perl
distribution.





(cpan)




cpan is a command-line interface to CPAN.pm.  It allows you to install
modules or distributions from CPAN, or just get information about them, and
a lot more.  It is similar to the command line mode of the (CPAN) module,



    perl -MCPAN -e shell





(cpanp)




cpanp is, like cpan, a command-line interface to the CPAN, using
the CPANPLUS module as a back-end. It can be used interactively or
imperatively.





(cpan2dist)




cpan2dist is a tool to create distributions (or packages) from CPAN
modules, then suitable for your package manager of choice. Support for
specific formats are available from CPAN as CPANPLUS::Dist::* modules.





(instmodsh)




A little interface to ExtUtils::Installed to examine installed modules,
validate your packlists and even create a tarball from an installed module.














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84.4 SEE ALSO



(perldoc)perldoc, (pod2man)pod2man, (perlpod),
(pod2html)pod2html, (pod2usage)pod2usage, (podselect)podselect,
(podchecker)podchecker, (splain)splain, perldiag NAME,
roffitall|roffitall, (a2p)a2p, (s2p)s2p, (find2perl)find2perl,
(File-Find)File::Find, (pl2pm)pl2pm, (perlbug)perlbug,
(h2ph)h2ph, (c2ph)c2ph, (h2xs)h2xs, (enc2xs), (xsubpp),
(cpan), (cpanp), (cpan2dist), (instmodsh), (piconv), (prove),
(corelist), (ptar), (ptardiff), (shasum), (zipdetails)









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85 perlvar





• perlvar NAME:  



• perlvar DESCRIPTION:  



• perlvar SPECIAL VARIABLES:  












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85.1 NAME



perlvar - Perl predefined variables









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85.2 DESCRIPTION





• perlvar The Syntax of Variable Names:  












Up: perlvar DESCRIPTION   [Contents]





85.2.1 The Syntax of Variable Names



Variable names in Perl can have several formats.  Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
:: or '.  In this case, the part before the last :: or
' is taken to be a package qualifier; see perlmod NAME.



Perl variable names may also be a sequence of digits or a single
punctuation or control character.  These names are all reserved for
special uses by Perl; for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
match.  Perl has a special syntax for the single-control-character
names: It understands ^X (caret X) to mean the control-X
character.  For example, the notation $^W (dollar-sign caret
W) is the scalar variable whose name is the single character
control-W.  This is better than typing a literal control-W
into your program.



Since Perl 5.6, Perl variable names may be alphanumeric
strings that begin with control characters (or better yet, a caret).
These variables must be written in the form ${^Foo}; the braces
are not optional.  ${^Foo} denotes the scalar variable whose
name is a control-F followed by two o’s.  These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (control-underscore or caret-underscore).  No
control-character name that begins with ^_ will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs.  $^_ itself, however, is reserved.



Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the package
declaration and are always forced to be in package main; they are
also exempt from strict 'vars' errors.  A few other names are also
exempt in these ways:



    ENV      STDIN
    INC      STDOUT
    ARGV     STDERR
    ARGVOUT
    SIG



In particular, the special ${^_XYZ} variables are always taken
to be in package main, regardless of any package declarations
presently in scope.









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85.3 SPECIAL VARIABLES



The following names have special meaning to Perl.  Most punctuation
names have reasonable mnemonics, or analogs in the shells.
Nevertheless, if you wish to use long variable names, you need only say:



    use English;



at the top of your program.  This aliases all the short names to the long
names in the current package.  Some even have medium names, generally
borrowed from awk.  To avoid a performance hit, if you don’t need the
$PREMATCH, $MATCH, or $POSTMATCH it’s best to use the English
module without them:



    use English '-no_match_vars';



Before you continue, note the sort order for variables.  In general, we
first list the variables in case-insensitive, almost-lexigraphical
order (ignoring the { or ^ preceding words, as in ${^UNICODE}
or $^T), although $_ and @_ move up to the top of the pile.
For variables with the same identifier, we list it in order of scalar,
array, hash, and bareword.





• perlvar General Variables:  



• perlvar Variables related to regular expressions:  



• perlvar Variables related to filehandles:  



• perlvar Error Variables:  



• perlvar Variables related to the interpreter state:  



• perlvar Deprecated and removed variables:  












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85.3.1 General Variables





$ARG






$_






The default input and pattern-searching space.  The following pairs are
equivalent:



    while (<>) {...}    # equivalent only in while!
    while (defined($_ = <>)) {...}

    /^Subject:/
    $_ =~ /^Subject:/

    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/

    chomp
    chomp($_)



Here are the places where Perl will assume $_ even if you don’t use it:





As $_ is a global variable, this may lead in some cases to unwanted
side-effects.  As of perl 5.10, you can now use a lexical version of
$_ by declaring it in a file or in a block with my.  Moreover,
declaring our $_ restores the global $_ in the current scope.



Mnemonic: underline is understood in certain operations.





@ARG






@_






Within a subroutine the array @_ contains the parameters passed to
that subroutine.  Inside a subroutine, @_ is the default array for
the array operators push, pop, shift, and unshift.



See (perlsub).





$LIST_SEPARATOR






$"






When an array or an array slice is interpolated into a double-quoted
string or a similar context such as /.../, its elements are
separated by this value.  Default is a space.  For example, this:



    print "The array is: @array\n";



is equivalent to this:



    print "The array is: " . join($", @array) . "\n";



Mnemonic: works in double-quoted context.





$PROCESS_ID






$PID






$$







The process number of the Perl running this script.  Though you can set
this variable, doing so is generally discouraged, although it can be
invaluable for some testing purposes.  It will be reset automatically
across fork() calls.



Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
would emulate POSIX semantics on Linux systems using LinuxThreads, a
partial implementation of POSIX Threads that has since been superseded
by the Native POSIX Thread Library (NPTL).



LinuxThreads is now obsolete on Linux, and and caching getpid()
like this made embedding perl unnecessarily complex (since you’d have
to manually update the value of $$), so now $$ and getppid()
will always return the same values as the underlying C library.



Debian GNU/kFreeBSD systems also used LinuxThreads up until and
including the 6.0 release, but after that moved to FreeBSD thread
semantics, which are POSIX-like.



To see if your system is affected by this discrepancy check if
getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL returns a false
value. NTPL threads preserve the POSIX semantics.



Mnemonic: same as shells.





$PROGRAM_NAME






$0






Contains the name of the program being executed.



On some (but not all) operating systems assigning to $0 modifies
the argument area that the ps program sees.  On some platforms you
may have to use special ps options or a different ps to see the
changes.  Modifying the $0 is more useful as a way of indicating the
current program state than it is for hiding the program you’re
running.



Note that there are platform-specific limitations on the maximum
length of $0.  In the most extreme case it may be limited to the
space occupied by the original $0.



In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by ps.
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).



Note for BSD users: setting $0 does not completely remove "perl"
from the ps(1) output.  For example, setting $0 to "foobar" may
result in "perl: foobar (perl)" (whether both the "perl: " prefix
and the " (perl)" suffix are shown depends on your exact BSD variant
and version).  This is an operating system feature, Perl cannot help it.



In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the $0 and the change becomes visible
to ps(1) (assuming the operating system plays along).  Note that
the view of $0 the other threads have will not change since they
have their own copies of it.



If the program has been given to perl via the switches -e or -E,
$0 will contain the string "-e".



On Linux as of perl 5.14 the legacy process name will be set with
prctl(2), in addition to altering the POSIX name via argv[0] as
perl has done since version 4.000.  Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to $0.  The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.



Mnemonic: same as sh and ksh.





$REAL_GROUP_ID






$GID






$(







The real gid of this process.  If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in.  The first number is the one returned by
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.



However, a value assigned to $( must be a single number used to
set the real gid.  So the value given by $( should not be assigned
back to $( without being forced numeric, such as by adding zero.  Note
that this is different to the effective gid ($)) which does take a
list.



You can change both the real gid and the effective gid at the same
time by using POSIX::setgid().  Changes
to $( require a check to $!
to detect any possible errors after an attempted change.



Mnemonic: parentheses are used to group things.  The real gid is the
group you left, if you’re running setgid.





$EFFECTIVE_GROUP_ID






$EGID






$)







The effective gid of this process.  If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in.  The first number is the one
returned by getegid(), and the subsequent ones by getgroups(),
one of which may be the same as the first number.



Similarly, a value assigned to $) must also be a space-separated
list of numbers.  The first number sets the effective gid, and
the rest (if any) are passed to setgroups().  To get the effect of an
empty list for setgroups(), just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say  $) = "5 5" .



You can change both the effective gid and the real gid at the same
time by using POSIX::setgid() (use only a single numeric argument).
Changes to $) require a check to $! to detect any possible errors
after an attempted change.



$<, $>, $( and $) can be set only on
machines that support the corresponding set[re][ug]id() routine.  $(
and $) can be swapped only on machines supporting setregid().



Mnemonic: parentheses are used to group things.  The effective gid
is the group that’s right for you, if you’re running setgid.





$REAL_USER_ID






$UID






$<







The real uid of this process.  You can change both the real uid and the
effective uid at the same time by using POSIX::setuid().  Since
changes to $< require a system call, check $! after a change
attempt to detect any possible errors.



Mnemonic: it’s the uid you came from, if you’re running setuid.





$EFFECTIVE_USER_ID






$EUID






$>







The effective uid of this process.  For example:



    $< = $>;            # set real to effective uid
    ($<,$>) = ($>,$<);  # swap real and effective uids



You can change both the effective uid and the real uid at the same
time by using POSIX::setuid().  Changes to $> require a check
to $! to detect any possible errors after an attempted change.



$< and $> can be swapped only on machines
supporting setreuid().



Mnemonic: it’s the uid you went to, if you’re running setuid.





$SUBSCRIPT_SEPARATOR






$SUBSEP






$;







The subscript separator for multidimensional array emulation.  If you
refer to a hash element as



    $foo{$a,$b,$c}



it really means



    $foo{join($;, $a, $b, $c)}



But don’t put



    @foo{$a,$b,$c}      # a slice--note the @



which means



    ($foo{$a},$foo{$b},$foo{$c})



Default is "\034", the same as SUBSEP in awk.  If your keys contain
binary data there might not be any safe value for $;.



Consider using "real" multidimensional arrays as described
in perllol NAME.



Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.





$a






$b






Special package variables when using sort(), see (perlfunc)sort.
Because of this specialness $a and $b don’t need to be declared
(using use vars, or our()) even when using the strict 'vars'
pragma.  Don’t lexicalize them with my $a or my $b if you want to
be able to use them in the sort() comparison block or function.





%ENV





The hash %ENV contains your current environment.  Setting a
value in ENV changes the environment for any child processes
you subsequently fork() off.





$SYSTEM_FD_MAX






$^F






The maximum system file descriptor, ordinarily 2.  System file
descriptors are passed to exec()ed processes, while higher file
descriptors are not.  Also, during an
open(), system file descriptors are
preserved even if the open() fails (ordinary file descriptors are
closed before the open() is attempted).  The close-on-exec
status of a file descriptor will be decided according to the value of
$^F when the corresponding file, pipe, or socket was opened, not the
time of the exec().





@F





The array @F contains the fields of each line read in when autosplit
mode is turned on.  See perlrun NAME for the -a switch.  This array
is package-specific, and must be declared or given a full package name
if not in package main when running under strict 'vars'.





@INC





The array @INC contains the list of places that the do EXPR,
require, or use constructs look for their library files.  It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent the current
directory.  ("." will not be appended if taint checks are enabled,
either by -T or by -t.)  If you need to modify this at runtime,
you should use the use lib pragma to get the machine-dependent
library properly loaded also:



    use lib '/mypath/libdir/';
    use SomeMod;



You can also insert hooks into the file inclusion system by putting Perl
code directly into @INC.  Those hooks may be subroutine references,
array references or blessed objects.  See (perlfunc)require for details.





%INC





The hash %INC contains entries for each filename included via the
do, require, or use operators.  The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found.  The require
operator uses this hash to determine whether a particular file has
already been included.



If the file was loaded via a hook (e.g. a subroutine reference, see
(perlfunc)require for a description of these hooks), this hook is
by default inserted into %INC in place of a filename.  Note, however,
that the hook may have set the %INC entry by itself to provide some more
specific info.





$INPLACE_EDIT






$^I






The current value of the inplace-edit extension.  Use undef to disable
inplace editing.



Mnemonic: value of -i switch.





$^M





By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of $^M
as an emergency memory pool after die()ing.  Suppose that your Perl
were compiled with -DPERL_EMERGENCY_SBRK and used Perl’s malloc.
Then



    $^M = 'a' x (1 << 16);



would allocate a 64K buffer for use in an emergency.  See the
INSTALL file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl.  To discourage casual
use of this advanced feature, there is no (English)English long name for
this variable.



This variable was added in Perl 5.004.





$OSNAME






$^O






The name of the operating system under which this copy of Perl was
built, as determined during the configuration process.  For examples
see perlport PLATFORMS.



The value is identical to $Config{'osname'}.  See also (Config)
and the -V command-line switch documented in perlrun NAME.



In Windows platforms, $^O is not very helpful: since it is always
MSWin32, it doesn’t tell the difference between
95/98/ME/NT/2000/XP/CE/.NET.  Use Win32::GetOSName() or
Win32::GetOSVersion() (see (Win32) and perlport NAME) to distinguish
between the variants.



This variable was added in Perl 5.003.





%SIG





The hash %SIG contains signal handlers for signals.  For example:



    sub handler {   # 1st argument is signal name
        my($sig) = @_;
        print "Caught a SIG$sig--shutting down\n";
        close(LOG);
        exit(0);
        }

    $SIG{'INT'}  = \&handler;
    $SIG{'QUIT'} = \&handler;
    ...
    $SIG{'INT'}  = 'DEFAULT';   # restore default action
    $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT



Using a value of 'IGNORE' usually has the effect of ignoring the
signal, except for the CHLD signal.  See perlipc NAME for more about
this special case.



Here are some other examples:



    $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not
                                # recommended)
    $SIG{"PIPE"} = \&Plumber;   # just fine; assume current
                                # Plumber
    $SIG{"PIPE"} = *Plumber;    # somewhat esoteric
    $SIG{"PIPE"} = Plumber();   # oops, what did Plumber()
                                # return??



Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.



If your system has the sigaction() function then signal handlers
are installed using it.  This means you get reliable signal handling.



The default delivery policy of signals changed in Perl 5.8.0 from
immediate (also known as "unsafe") to deferred, also known as "safe
signals".  See perlipc NAME for more information.



Certain internal hooks can be also set using the %SIG hash.  The
routine indicated by $SIG{__WARN__} is called when a warning
message is about to be printed.  The warning message is passed as the
first argument.  The presence of a __WARN__ hook causes the
ordinary printing of warnings to STDERR to be suppressed.  You can
use this to save warnings in a variable, or turn warnings into fatal
errors, like this:



    local $SIG{__WARN__} = sub { die $_[0] };
    eval $proggie;



As the 'IGNORE' hook is not supported by __WARN__, you can
disable warnings using the empty subroutine:



    local $SIG{__WARN__} = sub {};



The routine indicated by $SIG{__DIE__} is called when a fatal
exception is about to be thrown.  The error message is passed as the
first argument.  When a __DIE__ hook routine returns, the exception
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a goto &sub, a loop exit,
or a die().  The __DIE__ handler is explicitly disabled during
the call, so that you can die from a __DIE__ handler.  Similarly
for __WARN__.



Due to an implementation glitch, the $SIG{__DIE__} hook is called
even inside an eval().  Do not use this to rewrite a pending
exception in $@, or as a bizarre substitute for overriding
CORE::GLOBAL::die().  This strange action at a distance may be fixed
in a future release so that $SIG{__DIE__} is only called if your
program is about to exit, as was the original intent.  Any other use is
deprecated.



__DIE__/__WARN__ handlers are very special in one respect: they
may be called to report (probable) errors found by the parser.  In such
a case the parser may be in inconsistent state, so any attempt to
evaluate Perl code from such a handler will probably result in a
segfault.  This means that warnings or errors that result from parsing
Perl should be used with extreme caution, like this:



    require Carp if defined $^S;
    Carp::confess("Something wrong") if defined &Carp::confess;
    die "Something wrong, but could not load Carp to give "
      . "backtrace...\n\t"
      . "To see backtrace try starting Perl with -MCarp switch";



Here the first line will load Carp unless it is the parser who
called the handler.  The second line will print backtrace and die if
Carp was available.  The third line will be executed only if Carp was
not available.



Having to even think about the $^S variable in your exception
handlers is simply wrong.  $SIG{__DIE__} as currently implemented
invites grievous and difficult to track down errors.  Avoid it
and use an END{} or CORE::GLOBAL::die override instead.



See (perlfunc)die, (perlfunc)warn, (perlfunc)eval, and
(warnings) for additional information.





$BASETIME






$^T






The time at which the program began running, in seconds since the
epoch (beginning of 1970).  The values returned by the -M, -A,
and -C filetests are based on this value.





$PERL_VERSION






$^V






The revision, version, and subversion of the Perl interpreter,
represented as a version object.



This variable first appeared in perl 5.6.0; earlier versions of perl
will see an undefined value.  Before perl 5.10.0 $^V was represented
as a v-string.



$^V can be used to determine whether the Perl interpreter executing
a script is in the right range of versions.  For example:



    warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1



To convert $^V into its string representation use sprintf()’s
"%vd" conversion:



    printf "version is v%vd\n", $^V;  # Perl's version



See the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.



See also $] for an older representation of the Perl version.



This variable was added in Perl 5.6.



Mnemonic: use ^V for Version Control.





${^WIN32_SLOPPY_STAT}







If this variable is set to a true value, then stat() on Windows will
not try to open the file.  This means that the link count cannot be
determined and file attributes may be out of date if additional
hardlinks to the file exist.  On the other hand, not opening the file
is considerably faster, especially for files on network drives.



This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy" stat() by
default.  See the documentation for -f in
perlrun for more information about site
customization.



This variable was added in Perl 5.10.





$EXECUTABLE_NAME






$^X






The name used to execute the current copy of Perl, from C’s
argv[0] or (where supported) /proc/self/exe.



Depending on the host operating system, the value of $^X may be
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
perl program file.  Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
is no guarantee that the value of $^X is in PATH.  For VMS, the
value may or may not include a version number.



You usually can use the value of $^X to re-invoke an independent
copy of the same perl that is currently running, e.g.,



    @first_run = `$^X -le "print int rand 100 for 1..100"`;



But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.



It is not safe to use the value of $^X as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command.  To convert the value of $^X to a path name, use the
following statements:



    # Build up a set of file names (not command names).
    use Config;
    my $this_perl = $^X;
    if ($^O ne 'VMS') {
        $this_perl .= $Config{_exe}
          unless $this_perl =~ m/$Config{_exe}$/i;
        }



Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
copy referenced by $^X.  The following statements accomplish
this goal, and produce a pathname that can be invoked as a
command or referenced as a file.



    use Config;
    my $secure_perl_path = $Config{perlpath};
    if ($^O ne 'VMS') {
        $secure_perl_path .= $Config{_exe}
            unless $secure_perl_path =~ m/$Config{_exe}$/i;
        }














Next: , Previous: perlvar General Variables, Up: perlvar SPECIAL VARIABLES   [Contents]





85.3.2 Variables related to regular expressions



Most of the special variables related to regular expressions are side
effects.  Perl sets these variables when it has a successful match, so
you should check the match result before using them.  For instance:



    if( /P(A)TT(ER)N/ ) {
        print "I found $1 and $2\n";
        }



These variables are read-only and dynamically-scoped, unless we note
otherwise.



The dynamic nature of the regular expression variables means that
their value is limited to the block that they are in, as demonstrated
by this bit of code:



    my $outer = 'Wallace and Grommit';
    my $inner = 'Mutt and Jeff';

    my $pattern = qr/(\S+) and (\S+)/;

    sub show_n { print "\$1 is $1; \$2 is $2\n" }

    {
    OUTER:
        show_n() if $outer =~ m/$pattern/;

        INNER: {
            show_n() if $inner =~ m/$pattern/;
            }

        show_n();
    }



The output shows that while in the OUTER block, the values of $1
and $2 are from the match against $outer.  Inside the INNER
block, the values of $1 and $2 are from the match against
$inner, but only until the end of the block (i.e. the dynamic
scope).  After the INNER block completes, the values of $1 and
$2 return to the values for the match against $outer even though
we have not made another match:



    $1 is Wallace; $2 is Grommit
    $1 is Mutt; $2 is Jeff
    $1 is Wallace; $2 is Grommit



Due to an unfortunate accident of Perl’s implementation, use
English imposes a considerable performance penalty on all regular
expression matches in a program because it uses the $`, $&, and
$', regardless of whether they occur in the scope of use
English.  For that reason, saying use English in libraries is
strongly discouraged unless you import it without the match variables:



    use English '-no_match_vars'



The Devel::NYTProf and Devel::FindAmpersand
modules can help you find uses of these
problematic match variables in your code.



Since Perl 5.10, you can use the /p match operator flag and the
${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} variables instead
so you only suffer the performance penalties.





$<digits> ($1, $2, ...)







Contains the subpattern from the corresponding set of capturing
parentheses from the last successful pattern match, not counting patterns
matched in nested blocks that have been exited already.



These variables are read-only and dynamically-scoped.



Mnemonic: like \digits.





$MATCH






$&






The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
BLOCK).



The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  To avoid this
penalty, you can extract the same substring by using @-.  Starting
with Perl 5.10, you can use the /p match flag and the ${^MATCH}
variable to do the same thing for particular match operations.



This variable is read-only and dynamically-scoped.



Mnemonic: like & in some editors.





${^MATCH}





This is similar to $& ($MATCH) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.



This variable was added in Perl 5.10.



This variable is read-only and dynamically-scoped.





$PREMATCH






$‘







The string preceding whatever was matched by the last successful
pattern match, not counting any matches hidden within a BLOCK or eval
enclosed by the current BLOCK.



The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  To avoid this
penalty, you can extract the same substring by using @-.  Starting
with Perl 5.10, you can use the /p match flag and the
${^PREMATCH} variable to do the same thing for particular match
operations.



This variable is read-only and dynamically-scoped.



Mnemonic: ` often precedes a quoted string.





${^PREMATCH}






This is similar to $` ($PREMATCH) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.



This variable was added in Perl 5.10



This variable is read-only and dynamically-scoped.





$POSTMATCH






$’








The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or eval()
enclosed by the current BLOCK).  Example:



    local $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'\n";         # prints abc:def:ghi



The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.
To avoid this penalty, you can extract the same substring by
using @-.  Starting with Perl 5.10, you can use the /p match flag
and the ${^POSTMATCH} variable to do the same thing for particular
match operations.



This variable is read-only and dynamically-scoped.



Mnemonic: ' often follows a quoted string.





${^POSTMATCH}







This is similar to $' ($POSTMATCH) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the /p modifier.



This variable was added in Perl 5.10.



This variable is read-only and dynamically-scoped.





$LAST_PAREN_MATCH






$+






The text matched by the last bracket of the last successful search pattern.
This is useful if you don’t know which one of a set of alternative patterns
matched.  For example:



    /Version: (.*)|Revision: (.*)/ && ($rev = $+);



This variable is read-only and dynamically-scoped.



Mnemonic: be positive and forward looking.





$LAST_SUBMATCH_RESULT






$^N






The text matched by the used group most-recently closed (i.e. the group
with the rightmost closing parenthesis) of the last successful search
pattern.



This is primarily used inside (?{...}) blocks for examining text
recently matched.  For example, to effectively capture text to a variable
(in addition to $1, $2, etc.), replace (...) with



    (?:(...)(?{ $var = $^N }))



By setting and then using $var in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.



This variable was added in Perl 5.8.



Mnemonic: the (possibly) Nested parenthesis that most recently closed.





@LAST_MATCH_END






@+






This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope.  $+[0] is
the offset into the string of the end of the entire match.  This
is the same value as what the pos function returns when called
on the variable that was matched against.  The nth element
of this array holds the offset of the nth submatch, so
$+[1] is the offset past where $1 ends, $+[2] the offset
past where $2 ends, and so on.  You can use $#+ to determine
how many subgroups were in the last successful match.  See the
examples given for the @- variable.



This variable was added in Perl 5.6.





%LAST_PAREN_MATCH






%+






Similar to @+, the %+ hash allows access to the named capture
buffers, should they exist, in the last successful match in the
currently active dynamic scope.



For example, $+{foo} is equivalent to $1 after the following match:



    'foo' =~ /(?<foo>foo)/;



The keys of the %+ hash list only the names of buffers that have
captured (and that are thus associated to defined values).



The underlying behaviour of %+ is provided by the
(Tie-Hash-NamedCapture) module.



Note: %- and %+ are tied views into a common internal hash
associated with the last successful regular expression.  Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.



This variable was added in Perl 5.10.



This variable is read-only and dynamically-scoped.





@LAST_MATCH_START






@-






$-[0] is the offset of the start of the last successful match.
$-[n] is the offset of the start of the substring matched by
n-th subpattern, or undef if the subpattern did not match.



Thus, after a match against $_, $& coincides with substr $_, $-[0],
$+[0] - $-[0].  Similarly, $n coincides with substr $_, $-[n],
$+[n] - $-[n] if $-[n] is defined, and $+ coincides with
substr $_, $-[$#-], $+[$#-] - $-[$#-].  One can use $#- to find the
last matched subgroup in the last successful match.  Contrast with
$#+, the number of subgroups in the regular expression.  Compare
with @+.



This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
$-[0] is the offset into the string of the beginning of the
entire match.  The nth element of this array holds the offset
of the nth submatch, so $-[1] is the offset where $1
begins, $-[2] the offset where $2 begins, and so on.



After a match against some variable $var:





$` is the same as substr($var, 0, $-[0])






$& is the same as substr($var, $-[0], $+[0] - $-[0])






$' is the same as substr($var, $+[0])






$1 is the same as substr($var, $-[1], $+[1] - $-[1])






$2 is the same as substr($var, $-[2], $+[2] - $-[2])






$3 is the same as substr($var, $-[3], $+[3] - $-[3])









This variable was added in Perl 5.6.





%LAST_MATCH_START






%-






Similar to %+, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope.  To
each capture group name found in the regular expression, it associates a
reference to an array containing the list of values captured by all
buffers with that name (should there be several of them), in the order
where they appear.



Here’s an example:



    if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
        foreach my $bufname (sort keys %-) {
            my $ary = $-{$bufname};
            foreach my $idx (0..$#$ary) {
                print "\$-{$bufname}[$idx] : ",
                      (defined($ary->[$idx])
                          ? "'$ary->[$idx]'"
                          : "undef"),
                      "\n";
            }
        }
    }



would print out:



    $-{A}[0] : '1'
    $-{A}[1] : '3'
    $-{B}[0] : '2'
    $-{B}[1] : '4'



The keys of the %- hash correspond to all buffer names found in
the regular expression.



The behaviour of %- is implemented via the
(Tie-Hash-NamedCapture) module.



Note: %- and %+ are tied views into a common internal hash
associated with the last successful regular expression.  Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.



This variable was added in Perl 5.10



This variable is read-only and dynamically-scoped.





$LAST_REGEXP_CODE_RESULT






$^R






The result of evaluation of the last successful (?{ code })
regular expression assertion (see (perlre)).  May be written to.



This variable was added in Perl 5.005.





${^RE_DEBUG_FLAGS}





The current value of the regex debugging flags.  Set to 0 for no debug output
even when the re 'debug' module is loaded.  See (re) for details.



This variable was added in Perl 5.10.





${^RE_TRIE_MAXBUF}





Controls how certain regex optimisations are applied and how much memory they
utilize.  This value by default is 65536 which corresponds to a 512kB
temporary cache.  Set this to a higher value to trade
memory for speed when matching large alternations.  Set
it to a lower value if you want the optimisations to
be as conservative of memory as possible but still occur, and set it to a
negative value to prevent the optimisation and conserve the most memory.
Under normal situations this variable should be of no interest to you.



This variable was added in Perl 5.10.














Next: , Previous: perlvar Variables related to regular expressions, Up: perlvar SPECIAL VARIABLES   [Contents]





85.3.3 Variables related to filehandles



Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the IO::Handle object,
although this is less efficient than using the regular built-in
variables.  (Summary lines below for this contain the word HANDLE.)
First you must say



    use IO::Handle;



after which you may use either



    method HANDLE EXPR



or more safely,



    HANDLE->method(EXPR)



Each method returns the old value of the IO::Handle attribute.  The
methods each take an optional EXPR, which, if supplied, specifies the
new value for the IO::Handle attribute in question.  If not
supplied, most methods do nothing to the current value–except for
autoflush(), which will assume a 1 for you, just to be different.



Because loading in the IO::Handle class is an expensive operation,
you should learn how to use the regular built-in variables.



A few of these variables are considered "read-only".  This means that
if you try to assign to this variable, either directly or indirectly
through a reference, you’ll raise a run-time exception.



You should be very careful when modifying the default values of most
special variables described in this document.  In most cases you want
to localize these variables before changing them, since if you don’t,
the change may affect other modules which rely on the default values
of the special variables that you have changed.  This is one of the
correct ways to read the whole file at once:



    open my $fh, "<", "foo" or die $!;
    local $/; # enable localized slurp mode
    my $content = <$fh>;
    close $fh;



But the following code is quite bad:



    open my $fh, "<", "foo" or die $!;
    undef $/; # enable slurp mode
    my $content = <$fh>;
    close $fh;



since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.



Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible.  So unless you are already
inside some short {} block, you should create one yourself.  For
example:



    my $content = '';
    open my $fh, "<", "foo" or die $!;
    {
        local $/;
        $content = <$fh>;
    }
    close $fh;



Here is an example of how your own code can go broken:



    for ( 1..3 ){
        $\ = "\r\n";
        nasty_break();
        print "$_";
    }

    sub nasty_break {
        $\ = "\f";
        # do something with $_
    }



You probably expect this code to print the equivalent of



    "1\r\n2\r\n3\r\n"



but instead you get:



    "1\f2\f3\f"



Why? Because nasty_break() modifies $\ without localizing it
first.  The value you set in  nasty_break() is still there when you
return.  The fix is to add local() so the value doesn’t leak out of
nasty_break():



    local $\ = "\f";



It’s easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don’t localize
changes to the special variables.





$ARGV





Contains the name of the current file when reading from <>.





@ARGV





The array @ARGV contains the command-line arguments intended for
the script.  $#ARGV is generally the number of arguments minus
one, because $ARGV[0] is the first argument, not the program’s
command name itself.  See $0 for the command name.





ARGV





The special filehandle that iterates over command-line filenames in
@ARGV.  Usually written as the null filehandle in the angle operator
<>.  Note that currently ARGV only has its magical effect
within the <> operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by <>.  In particular,
passing \*ARGV as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in @ARGV.





ARGVOUT





The special filehandle that points to the currently open output file
when doing edit-in-place processing with -i.  Useful when you have
to do a lot of inserting and don’t want to keep modifying $_.  See
perlrun NAME for the -i switch.





Handle->output_field_separator( EXPR )






$OUTPUT_FIELD_SEPARATOR






$OFS






$,







The output field separator for the print operator.  If defined, this
value is printed between each of print’s arguments.  Default is undef.



Mnemonic: what is printed when there is a "," in your print statement.





HANDLE->input_line_number( EXPR )






$INPUT_LINE_NUMBER






$NR






$.








Current line number for the last filehandle accessed.



Each filehandle in Perl counts the number of lines that have been read
from it.  (Depending on the value of $/, Perl’s idea of what
constitutes a line may not match yours.)  When a line is read from a
filehandle (via readline() or <>), or when tell() or
seek() is called on it, $. becomes an alias to the line counter
for that filehandle.



You can adjust the counter by assigning to $., but this will not
actually move the seek pointer.  Localizing $. will not localize
the filehandle’s line count.  Instead, it will localize perl’s notion
of which filehandle $. is currently aliased to.



$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening close().  For more
details, see (perlop)I/O Operators.  Because <> never does
an explicit close, line numbers increase across ARGV files (but see
examples in (perlfunc)eof).



You can also use HANDLE->input_line_number(EXPR) to access the
line counter for a given filehandle without having to worry about
which handle you last accessed.



Mnemonic: many programs use "." to mean the current line number.





HANDLE->input_record_separator( EXPR )






$INPUT_RECORD_SEPARATOR






$RS






$/







The input record separator, newline by default.  This influences Perl’s
idea of what a "line" is.  Works like awk’s RS variable, including
treating empty lines as a terminator if set to the null string (an
empty line cannot contain any spaces or tabs).  You may set it to a
multi-character string to match a multi-character terminator, or to
undef to read through the end of file.  Setting it to "\n\n"
means something slightly different than setting to "", if the file
contains consecutive empty lines.  Setting to "" will treat two or
more consecutive empty lines as a single empty line.  Setting to
"\n\n" will blindly assume that the next input character belongs to
the next paragraph, even if it’s a newline.



    local $/;           # enable "slurp" mode
    local $_ = <FH>;    # whole file now here
    s/\n[ \t]+/ /g;



Remember: the value of $/ is a string, not a regex.  awk has to
be better for something. :-)



Setting $/ to a reference to an integer, scalar containing an
integer, or scalar that’s convertible to an integer will attempt to
read records instead of lines, with the maximum record size being the
referenced integer.  So this:



    local $/ = \32768; # or \"32768", or \$var_containing_32768
    open my $fh, "<", $myfile or die $!;
    local $_ = <$fh>;



will read a record of no more than 32768 bytes from FILE.  If you’re
not reading from a record-oriented file (or your OS doesn’t have
record-oriented files), then you’ll likely get a full chunk of data
with every read.  If a record is larger than the record size you’ve
set, you’ll get the record back in pieces.  Trying to set the record
size to zero or less will cause reading in the (rest of the) whole file.



On VMS only, record reads bypass PerlIO layers and any associated
buffering,so you must not mix record and non-record reads on the
same filehandle.  Record mode mixes with line mode only when the
same buffering layer is in use for both modes.



If you perform a record read on a FILE with an encoding layer such as
:encoding(latin1) or :utf8, you may get an invalid string as a
result, may leave the FILE positioned between characters in the stream
and may not be reading the number of bytes from the underlying file
that you specified.  This behaviour may change without warning in a
future version of perl.



See also perlport Newlines. Also see $..



Mnemonic: / delimits line boundaries when quoting poetry.





Handle->output_record_separator( EXPR )






$OUTPUT_RECORD_SEPARATOR






$ORS






$\







The output record separator for the print operator.  If defined, this
value is printed after the last of print’s arguments.  Default is undef.



Mnemonic: you set $\ instead of adding "\n" at the end of the print.
Also, it’s just like $/, but it’s what you get "back" from Perl.





HANDLE->autoflush( EXPR )






$OUTPUT_AUTOFLUSH






$|








If set to nonzero, forces a flush right away and after every write or
print on the currently selected output channel.  Default is 0
(regardless of whether the channel is really buffered by the system or
not; $| tells you only whether you’ve asked Perl explicitly to
flush after each write).  STDOUT will typically be line buffered if
output is to the terminal and block buffered otherwise.  Setting this
variable is useful primarily when you are outputting to a pipe or
socket, such as when you are running a Perl program under rsh and
want to see the output as it’s happening.  This has no effect on input
buffering.  See (perlfunc)getc for that.  See (perlfunc)select on
how to select the output channel.  See also (IO-Handle).



Mnemonic: when you want your pipes to be piping hot.










• perlvar Variables related to formats:  












Up: perlvar Variables related to filehandles   [Contents]





85.3.3.1 Variables related to formats



The special variables for formats are a subset of those for
filehandles.  See (perlform) for more information about Perl’s
formats.





$ACCUMULATOR






$^A






The current value of the write() accumulator for format() lines.
A format contains formline() calls that put their result into
$^A.  After calling its format, write() prints out the contents
of $^A and empties.  So you never really see the contents of $^A
unless you call formline() yourself and then look at it.  See
(perlform) and (perlfunc)formline PICTURE,LIST.





HANDLE->format_formfeed(EXPR)






$FORMAT_FORMFEED






$^L






What formats output as a form feed.  The default is \f.





HANDLE->format_page_number(EXPR)






$FORMAT_PAGE_NUMBER






$%






The current page number of the currently selected output channel.



Mnemonic: % is page number in nroff.





HANDLE->format_lines_left(EXPR)






$FORMAT_LINES_LEFT






$-






The number of lines left on the page of the currently selected output
channel.



Mnemonic: lines_on_page - lines_printed.





Handle->format_line_break_characters EXPR






$FORMAT_LINE_BREAK_CHARACTERS






$:






The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format.  The default is
" \n-", to break on a space, newline, or a hyphen.



Mnemonic: a "colon" in poetry is a part of a line.





HANDLE->format_lines_per_page(EXPR)






$FORMAT_LINES_PER_PAGE






$=






The current page length (printable lines) of the currently selected
output channel.  The default is 60.



Mnemonic: = has horizontal lines.





HANDLE->format_top_name(EXPR)






$FORMAT_TOP_NAME






$^






The name of the current top-of-page format for the currently selected
output channel.  The default is the name of the filehandle with _TOP
appended.  For example, the default format top name for the STDOUT
filehandle is STDOUT_TOP.



Mnemonic: points to top of page.





HANDLE->format_name(EXPR)






$FORMAT_NAME






$~






The name of the current report format for the currently selected
output channel.  The default format name is the same as the filehandle
name.  For example, the default format name for the STDOUT
filehandle is just STDOUT.



Mnemonic: brother to $^.














Next: , Previous: perlvar Variables related to filehandles, Up: perlvar SPECIAL VARIABLES   [Contents]





85.3.4 Error Variables





The variables $@, $!, $^E, and $? contain information
about different types of error conditions that may appear during
execution of a Perl program.  The variables are shown ordered by
the "distance" between the subsystem which reported the error and
the Perl process.  They correspond to errors detected by the Perl
interpreter, C library, operating system, or an external program,
respectively.



To illustrate the differences between these variables, consider the
following Perl expression, which uses a single-quoted string.  After
execution of this statement, perl may have set all four special error
variables:



    eval q{
        open my $pipe, "/cdrom/install |" or die $!;
        my @res = <$pipe>;
        close $pipe or die "bad pipe: $?, $!";
    };



When perl executes the eval() expression, it translates the
open(), <PIPE>, and close calls in the C run-time library
and thence to the operating system kernel.  perl sets $! to
the C library’s errno if one of these calls fails.



$@ is set if the string to be eval-ed did not compile (this may
happen if open or close were imported with bad prototypes), or
if Perl code executed during evaluation die()d.  In these cases the
value of $@ is the compile error, or the argument to die (which
will interpolate $! and $?).  (See also (Fatal), though.)



Under a few operating systems, $^E may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed."  Systems that
do not support extended error messages leave $^E the same as $!.



Finally, $? may be set to non-0 value if the external program
/cdrom/install fails.  The upper eight bits reflect specific error
conditions encountered by the program (the program’s exit() value).
The lower eight bits reflect mode of failure, like signal death and
core dump information.  See wait(2) for details.  In contrast to
$! and $^E, which are set only if error condition is detected,
the variable $? is set on each wait or pipe close,
overwriting the old value.  This is more like $@, which on every
eval() is always set on failure and cleared on success.



For more details, see the individual descriptions at $@, $!,
$^E, and $?.





${^CHILD_ERROR_NATIVE}





The native status returned by the last pipe close, backtick (``)
command, successful call to wait() or waitpid(), or from the
system() operator.  On POSIX-like systems this value can be decoded
with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
WSTOPSIG and WIFCONTINUED functions provided by the (POSIX) module.



Under VMS this reflects the actual VMS exit status; i.e. it is the
same as $? when the pragma use vmsish 'status' is in effect.



This variable was added in Perl 5.8.9.





$EXTENDED_OS_ERROR






$^E






Error information specific to the current operating system.  At the
moment, this differs from $! under only VMS, OS/2, and Win32 (and
for MacPerl).  On all other platforms, $^E is always just the same
as $!.



Under VMS, $^E provides the VMS status value from the last system
error.  This is more specific information about the last system error
than that provided by $!.  This is particularly important when $!
is set to EVMSERR.



Under OS/2, $^E is set to the error code of the last call to OS/2
API either via CRT, or directly from perl.



Under Win32, $^E always returns the last error information reported
by the Win32 call GetLastError() which describes the last error
from within the Win32 API.  Most Win32-specific code will report errors
via $^E.  ANSI C and Unix-like calls set errno and so most
portable Perl code will report errors via $!.



Caveats mentioned in the description of $! generally apply to
$^E, also.



This variable was added in Perl 5.003.



Mnemonic: Extra error explanation.





$EXCEPTIONS_BEING_CAUGHT






$^S






Current state of the interpreter.



        $^S         State
        ---------   -------------------
        undef       Parsing module/eval
        true (1)    Executing an eval
        false (0)   Otherwise



The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.



This variable was added in Perl 5.004.





$WARNING






$^W






The current value of the warning switch, initially true if -w was
used, false otherwise, but directly modifiable.



See also (warnings).



Mnemonic: related to the -w switch.





${^WARNING_BITS}





The current set of warning checks enabled by the use warnings pragma.
It has the same scoping as the $^H and %^H variables.  The exact
values are considered internal to the (warnings) pragma and may change
between versions of Perl.



This variable was added in Perl 5.6.





$OS_ERROR






$ERRNO






$!







When referenced, $! retrieves the current value
of the C errno integer variable.
If $! is assigned a numerical value, that value is stored in errno.
When referenced as a string, $! yields the system error string
corresponding to errno.



Many system or library calls set errno if they fail,
to indicate the cause of failure.  They usually do not
set errno to zero if they succeed.  This means errno,
hence $!, is meaningful only immediately after a failure:



    if (open my $fh, "<", $filename) {
                # Here $! is meaningless.
                ...
    }
    else {
                # ONLY here is $! meaningful.
                ...
                # Already here $! might be meaningless.
    }
    # Since here we might have either success or failure,
    # $! is meaningless.



Here, meaningless means that $! may be unrelated to the outcome
of the open() operator.  Assignment to $! is similarly ephemeral.
It can be used immediately before invoking the die() operator,
to set the exit value, or to inspect the system error string
corresponding to error n, or to restore $! to a meaningful state.



Mnemonic: What just went bang?





%OS_ERROR






%ERRNO






%!







Each element of %! has a true value only if $! is set to that
value.  For example, $!{ENOENT} is true if and only if the current
value of $! is ENOENT; that is, if the most recent error was "No
such file or directory" (or its moral equivalent: not all operating
systems give that exact error, and certainly not all languages).  To
check if a particular key is meaningful on your system, use exists
$!{the_key}; for a list of legal keys, use keys %!.  See (Errno)
for more information, and also see $!.



This variable was added in Perl 5.005.





$CHILD_ERROR






$?






The status returned by the last pipe close, backtick (``) command,
successful call to wait() or waitpid(), or from the system()
operator.  This is just the 16-bit status word returned by the
traditional Unix wait() system call (or else is made up to look
like it).  Thus, the exit value of the subprocess is really ($? >>
8), and $? & 127 gives which signal, if any, the process died
from, and $? & 128 reports whether there was a core dump.



Additionally, if the h_errno variable is supported in C, its value
is returned via $? if any gethost*() function fails.



If you have installed a signal handler for SIGCHLD, the
value of $? will usually be wrong outside that handler.



Inside an END subroutine $? contains the value that is going to be
given to exit().  You can modify $? in an END subroutine to
change the exit status of your program.  For example:



    END {
        $? = 1 if $? == 255;  # die would make it 255
    }



Under VMS, the pragma use vmsish 'status' makes $? reflect the
actual VMS exit status, instead of the default emulation of POSIX
status; see perlvms $? for details.



Mnemonic: similar to sh and ksh.





$EVAL_ERROR






$@






The Perl syntax error message from the
last eval() operator.  If $@ is
the null string, the last eval() parsed and executed correctly
(although the operations you invoked may have failed in the normal
fashion).



Warning messages are not collected in this variable.  You can, however,
set up a routine to process warnings by setting $SIG{__WARN__} as
described in %SIG.



Mnemonic: Where was the syntax error "at"?














Next: , Previous: perlvar Error Variables, Up: perlvar SPECIAL VARIABLES   [Contents]





85.3.5 Variables related to the interpreter state



These variables provide information about the current interpreter state.





$COMPILING






$^C






The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD at compile
time rather than normal, deferred loading.  Setting
$^C = 1 is similar to calling B::minus_c.



This variable was added in Perl 5.6.





$DEBUGGING






$^D






The current value of the debugging flags.  May be read or set.  Like its
command-line equivalent, you can use numeric or symbolic values, eg
$^D = 10 or $^D = "st".



Mnemonic: value of -D switch.





${^ENCODING}





The object reference to the Encode object that is used to convert
the source code to Unicode.  Thanks to this variable your Perl script
does not have to be written in UTF-8.  Default is undef.  The direct
manipulation of this variable is highly discouraged.



This variable was added in Perl 5.8.2.





${^GLOBAL_PHASE}





The current phase of the perl interpreter.



Possible values are:





CONSTRUCT




The PerlInterpreter* is being constructed via perl_construct.  This
value is mostly there for completeness and for use via the
underlying C variable PL_phase.  It’s not really possible for Perl
code to be executed unless construction of the interpreter is
finished.





START




This is the global compile-time.  That includes, basically, every
BEGIN block executed directly or indirectly from during the
compile-time of the top-level program.



This phase is not called "BEGIN" to avoid confusion with
BEGIN-blocks, as those are executed during compile-time of any
compilation unit, not just the top-level program.  A new, localised
compile-time entered at run-time, for example by constructs as
eval "use SomeModule" are not global interpreter phases, and
therefore aren’t reflected by ${^GLOBAL_PHASE}.





CHECK




Execution of any CHECK blocks.





INIT




Similar to "CHECK", but for INIT-blocks, not CHECK blocks.





RUN




The main run-time, i.e. the execution of PL_main_root.





END




Execution of any END blocks.





DESTRUCT




Global destruction.








Also note that there’s no value for UNITCHECK-blocks.  That’s because
those are run for each compilation unit individually, and therefore is
not a global interpreter phase.



Not every program has to go through each of the possible phases, but
transition from one phase to another can only happen in the order
described in the above list.



An example of all of the phases Perl code can see:



    BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }

    INIT  { print "init-time: ${^GLOBAL_PHASE}\n" }

    CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }

    {
        package Print::Phase;

        sub new {
            my ($class, $time) = @_;
            return bless \$time, $class;
        }

        sub DESTROY {
            my $self = shift;
            print "$$self: ${^GLOBAL_PHASE}\n";
        }
    }

    print "run-time: ${^GLOBAL_PHASE}\n";

    my $runtime = Print::Phase->new(
        "lexical variables are garbage collected before END"
    );

    END   { print "end-time: ${^GLOBAL_PHASE}\n" }

    our $destruct = Print::Phase->new(
        "package variables are garbage collected after END"
    );



This will print out



    compile-time: START
    check-time: CHECK
    init-time: INIT
    run-time: RUN
    lexical variables are garbage collected before END: RUN
    end-time: END
    package variables are garbage collected after END: DESTRUCT



This variable was added in Perl 5.14.0.





$^H





WARNING: This variable is strictly for
internal use only.  Its availability,
behavior, and contents are subject to change without notice.



This variable contains compile-time hints for the Perl interpreter.  At the
end of compilation of a BLOCK the value of this variable is restored to the
value when the interpreter started to compile the BLOCK.



When perl begins to parse any block construct that provides a lexical scope
(e.g., eval body, required file, subroutine body, loop body, or conditional
block), the existing value of $^H is saved, but its value is left unchanged.
When the compilation of the block is completed, it regains the saved value.
Between the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of $^H.



This behavior provides the semantic of lexical scoping, and is used in,
for instance, the use strict pragma.



The contents should be an integer; different bits of it are used for
different pragmatic flags.  Here’s an example:



    sub add_100 { $^H |= 0x100 }

    sub foo {
        BEGIN { add_100() }
        bar->baz($boon);
    }



Consider what happens during execution of the BEGIN block.  At this point
the BEGIN block has already been compiled, but the body of foo() is still
being compiled.  The new value of $^H
will therefore be visible only while
the body of foo() is being compiled.



Substitution of BEGIN { add_100() } block with:



    BEGIN { require strict; strict->import('vars') }



demonstrates how use strict 'vars' is implemented.  Here’s a conditional
version of the same lexical pragma:



    BEGIN {
        require strict; strict->import('vars') if $condition
    }



This variable was added in Perl 5.003.





%^H





The %^H hash provides the same scoping semantic as $^H.  This makes
it useful for implementation of lexically scoped pragmas.  See
perlpragma NAME.



When putting items into %^H, in order to avoid conflicting with other
users of the hash there is a convention regarding which keys to use.
A module should use only keys that begin with the module’s name (the
name of its main package) and a "/" character.  For example, a module
Foo::Bar should use keys such as Foo::Bar/baz.



This variable was added in Perl 5.6.





${^OPEN}





An internal variable used by PerlIO.  A string in two parts, separated
by a \0 byte, the first part describes the input layers, the second
part describes the output layers.



This variable was added in Perl 5.8.0.





$PERLDB






$^P






The internal variable for debugging support.  The meanings of the
various bits are subject to change, but currently indicate:





0x01




Debug subroutine enter/exit.





0x02




Line-by-line debugging.  Causes DB::DB() subroutine to be called for
each statement executed.  Also causes saving source code lines (like
0x400).





0x04




Switch off optimizations.





0x08




Preserve more data for future interactive inspections.





0x10




Keep info about source lines on which a subroutine is defined.





0x20




Start with single-step on.





0x40




Use subroutine address instead of name when reporting.





0x80




Report goto &subroutine as well.





0x100




Provide informative "file" names for evals based on the place they were compiled.





0x200




Provide informative names to anonymous subroutines based on the place they
were compiled.





0x400




Save source code lines into @{"_<$filename"}.








Some bits may be relevant at compile-time only, some at
run-time only.  This is a new mechanism and the details may change.
See also perldebguts NAME.





${^TAINT}





Reflects if taint mode is on or off.  1 for on (the program was run with
-T), 0 for off, -1 when only taint warnings are enabled (i.e. with
-t or -TU).



This variable is read-only.



This variable was added in Perl 5.8.





${^UNICODE}





Reflects certain Unicode settings of Perl.  See perlrun NAME
documentation for the -C switch for more information about
the possible values.



This variable is set during Perl startup and is thereafter read-only.



This variable was added in Perl 5.8.2.





${^UTF8CACHE}





This variable controls the state of the internal UTF-8 offset caching code.
1 for on (the default), 0 for off, -1 to debug the caching code by checking
all its results against linear scans, and panicking on any discrepancy.



This variable was added in Perl 5.8.9.





${^UTF8LOCALE}





This variable indicates whether a UTF-8 locale was detected by perl at
startup.  This information is used by perl when it’s in
adjust-utf8ness-to-locale mode (as when run with the -CL command-line
switch); see perlrun NAME for more info on this.



This variable was added in Perl 5.8.8.














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85.3.6 Deprecated and removed variables



Deprecating a variable announces the intent of the perl maintainers to
eventually remove the variable from the language.  It may still be
available despite its status.  Using a deprecated variable triggers
a warning.



Once a variable is removed, its use triggers an error telling you
the variable is unsupported.



See perldiag NAME for details about error messages.





$OFMT






$#






$# was a variable that could be used to format printed numbers.
After a deprecation cycle, its magic was removed in Perl 5.10 and
using it now triggers a warning: $# is no longer supported.



This is not the sigil you use in front of an array name to get the
last index, like $#array.  That’s still how you get the last index
of an array in Perl.  The two have nothing to do with each other.



Deprecated in Perl 5.



Removed in Perl 5.10.





$*





$* was a variable that you could use to enable multiline matching.
After a deprecation cycle, its magic was removed in Perl 5.10.
Using it now triggers a warning: $* is no longer supported.
You should use the /s and /m regexp modifiers instead.



Deprecated in Perl 5.



Removed in Perl 5.10.





$ARRAY_BASE






$[






This variable stores the index of the first element in an array, and
of the first character in a substring.  The default is 0, but you could
theoretically set it to 1 to make Perl behave more like awk (or Fortran)
when subscripting and when evaluating the index() and substr() functions.



As of release 5 of Perl, assignment to $[ is treated as a compiler
directive, and cannot influence the behavior of any other file.
(That’s why you can only assign compile-time constants to it.)
Its use is highly discouraged.



Prior to Perl 5.10, assignment to $[ could be seen from outer lexical
scopes in the same file, unlike other compile-time directives (such as
(strict)).  Using local() on it would bind its value strictly to a lexical
block.  Now it is always lexically scoped.



As of Perl 5.16, it is implemented by the (arybase) module.  See
(arybase) for more details on its behaviour.



Under use v5.16, or no feature "array_base", $[ no longer has any
effect, and always contains 0.  Assigning 0 to it is permitted, but any
other value will produce an error.



Mnemonic: [ begins subscripts.



Deprecated in Perl 5.12.





$OLD_PERL_VERSION






$]






See $^V for a more modern representation of the Perl version that allows
accurate string comparisons.



The version + patchlevel / 1000 of the Perl interpreter.  This variable
can be used to determine whether the Perl interpreter executing a
script is in the right range of versions:



    warn "No checksumming!\n" if $] < 3.019;



The floating point representation can sometimes lead to inaccurate
numeric comparisons.



See also the documentation of use VERSION and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.



Mnemonic: Is this version of perl in the right bracket?














Previous: perlvar, Up: Top   [Contents]





86 perlvms





• perlvms NAME:  



• perlvms DESCRIPTION:  



• perlvms Installation:  



• perlvms Organization of Perl Images:  



• perlvms File specifications:  



• perlvms PERL5LIB and PERLLIB:  



• perlvms The Perl Forked Debugger:  



• perlvms PERL_VMS_EXCEPTION_DEBUG:  



• perlvms Command line:  



• perlvms Perl functions:  



• perlvms Perl variables:  



• perlvms Standard modules with VMS-specific differences:  



• perlvms Revision date:  



• perlvms AUTHOR:  












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86.1 NAME



perlvms - VMS-specific documentation for Perl









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86.2 DESCRIPTION



Gathered below are notes describing details of Perl 5’s 
behavior on VMS.  They are a supplement to the regular Perl 5 
documentation, so we have focussed on the ways in which Perl 
5 functions differently under VMS than it does under Unix, 
and on the interactions between Perl and the rest of the 
operating system.  We haven’t tried to duplicate complete 
descriptions of Perl features from the main Perl 
documentation, which can be found in the [.pod] 
subdirectory of the Perl distribution.



We hope these notes will save you from confusion and lost 
sleep when writing Perl scripts on VMS.  If you find we’ve 
missed something you think should appear here, please don’t 
hesitate to drop a line to vmsperl@perl.org.









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86.3 Installation



Directions for building and installing Perl 5 can be found in 
the file README.vms in the main source directory of the 
Perl distribution..









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86.4 Organization of Perl Images





• perlvms Core Images:  



• perlvms Perl Extensions:  



• perlvms Installing static extensions:  



• perlvms Installing dynamic extensions:  












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86.4.1 Core Images



During the installation process, three Perl images are produced.
Miniperl.Exe is an executable image which contains all of
the basic functionality of Perl, but cannot take advantage of
Perl extensions.  It is used to generate several files needed
to build the complete Perl and various extensions.  Once you’ve
finished installing Perl, you can delete this image.



Most of the complete Perl resides in the shareable image
PerlShr.Exe, which provides a core to which the Perl executable
image and all Perl extensions are linked.  You should place this
image in Sys$Share, or define the logical name PerlShr to
translate to the full file specification of this image.  It should
be world readable.  (Remember that if a user has execute only access
to PerlShr, VMS will treat it as if it were a privileged shareable
image, and will therefore require all downstream shareable images to be
INSTALLed, etc.)



Finally, Perl.Exe is an executable image containing the main
entry point for Perl, as well as some initialization code.  It
should be placed in a public directory, and made world executable.
In order to run Perl with command line arguments, you should
define a foreign command to invoke this image.









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86.4.2 Perl Extensions



Perl extensions are packages which provide both XS and Perl code
to add new functionality to perl.  (XS is a meta-language which
simplifies writing C code which interacts with Perl, see
(perlxs) for more details.)  The Perl code for an
extension is treated like any other library module - it’s
made available in your script through the appropriate
use or require statement, and usually defines a Perl
package containing the extension.



The portion of the extension provided by the XS code may be
connected to the rest of Perl in either of two ways.  In the
static configuration, the object code for the extension is
linked directly into PerlShr.Exe, and is initialized whenever
Perl is invoked.  In the dynamic configuration, the extension’s
machine code is placed into a separate shareable image, which is
mapped by Perl’s DynaLoader when the extension is used or
required in your script.  This allows you to maintain the
extension as a separate entity, at the cost of keeping track of the
additional shareable image.  Most extensions can be set up as either
static or dynamic.



The source code for an extension usually resides in its own
directory.  At least three files are generally provided:
Extshortname.xs (where Extshortname is the portion of
the extension’s name following the last ::), containing
the XS code, Extshortname.pm, the Perl library module
for the extension, and Makefile.PL, a Perl script which uses
the MakeMaker library modules supplied with Perl to generate
a Descrip.MMS file for the extension.









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86.4.3 Installing static extensions



Since static extensions are incorporated directly into
PerlShr.Exe, you’ll have to rebuild Perl to incorporate a
new extension.  You should edit the main Descrip.MMS or Makefile
you use to build Perl, adding the extension’s name to the ext
macro, and the extension’s object file to the extobj macro.
You’ll also need to build the extension’s object file, either
by adding dependencies to the main Descrip.MMS, or using a
separate Descrip.MMS for the extension.  Then, rebuild
PerlShr.Exe to incorporate the new code.



Finally, you’ll need to copy the extension’s Perl library
module to the [.Extname] subdirectory under one
of the directories in @INC, where Extname is the name
of the extension, with all :: replaced by . (e.g.
the library module for extension Foo::Bar would be copied
to a [.Foo.Bar] subdirectory).









Previous: perlvms Installing static extensions, Up: perlvms Organization of Perl Images   [Contents]





86.4.4 Installing dynamic extensions



In general, the distributed kit for a Perl extension includes
a file named Makefile.PL, which is a Perl program which is used
to create a Descrip.MMS file which can be used to build and
install the files required by the extension.  The kit should be
unpacked into a directory tree not under the main Perl source
directory, and the procedure for building the extension is simply



    $ perl Makefile.PL  ! Create Descrip.MMS
    $ mmk               ! Build necessary files
    $ mmk test          ! Run test code, if supplied
    $ mmk install       ! Install into public Perl tree



N.B. The procedure by which extensions are built and
tested creates several levels (at least 4) under the
directory in which the extension’s source files live.
For this reason if you are running a version of VMS prior
to V7.1 you shouldn’t nest the source directory
too deeply in your directory structure lest you exceed RMS’
maximum of 8 levels of subdirectory in a filespec.  (You
can use rooted logical names to get another 8 levels of
nesting, if you can’t place the files near the top of
the physical directory structure.)



VMS support for this process in the current release of Perl
is sufficient to handle most extensions.  However, it does
not yet recognize extra libraries required to build shareable
images which are part of an extension, so these must be added
to the linker options file for the extension by hand.  For
instance, if the PGPLOT extension to Perl requires the
PGPLOTSHR.EXE shareable image in order to properly link
the Perl extension, then the line PGPLOTSHR/Share must
be added to the linker options file PGPLOT.Opt produced
during the build process for the Perl extension.



By default, the shareable image for an extension is placed in
the [.lib.site_perl.autoArch.Extname] directory of the
installed Perl directory tree (where Arch is VMS_VAX or
VMS_AXP, and Extname is the name of the extension, with
each :: translated to .).  (See the MakeMaker documentation
for more details on installation options for extensions.)
However, it can be manually placed in any of several locations:





If the shareable image isn’t in any of these places, you’ll need
to define a logical name Extshortname, where Extshortname
is the portion of the extension’s name after the last ::, which
translates to the full file specification of the shareable image.









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86.5 File specifications





• perlvms Syntax:  



• perlvms Filename Case:  



• perlvms Symbolic Links:  



• perlvms Wildcard expansion:  



• perlvms Pipes:  












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86.5.1 Syntax



We have tried to make Perl aware of both VMS-style and Unix-style file
specifications wherever possible.  You may use either style, or both,
on the command line and in scripts, but you may not combine the two
styles within a single file specification.  VMS Perl interprets Unix
pathnames in much the same way as the CRTL (e.g. the first component
of an absolute path is read as the device name for the VMS file
specification).  There are a set of functions provided in the
VMS::Filespec package for explicit interconversion between VMS and
Unix syntax; its documentation provides more details.



We’ve tried to minimize the dependence of Perl library
modules on Unix syntax, but you may find that some of these,
as well as some scripts written for Unix systems, will
require that you use Unix syntax, since they will assume that
’/’ is the directory separator, etc.  If you find instances
of this in the Perl distribution itself, please let us know,
so we can try to work around them.



Also when working on Perl programs on VMS, if you need a syntax
in a specific operating system format, then you need either to
check the appropriate DECC$ feature logical, or call a conversion
routine to force it to that format.



The feature logical name DECC$FILENAME_UNIX_REPORT modifies traditional
Perl behavior in the conversion of file specifications from Unix to VMS
format in order to follow the extended character handling rules now
expected by the CRTL.  Specifically, when this feature is in effect, the
./.../ in a Unix path is now translated to [.^.^.^.] instead of
the traditional VMS [...].  To be compatible with what MakeMaker
expects, if a VMS path cannot be translated to a Unix path, it is
passed through unchanged, so unixify("[...]") will return [...].



The handling of extended characters is largely complete in the
VMS-specific C infrastructure of Perl, but more work is still needed to
fully support extended syntax filenames in several core modules.  In
particular, at this writing PathTools has only partial support for
directories containing some extended characters.



There are several ambiguous cases where a conversion routine cannot
determine whether an input filename is in Unix format or in VMS format,
since now both VMS and Unix file specifications may have characters in
them that could be mistaken for syntax delimiters of the other type. So
some pathnames simply cannot be used in a mode that allows either type
of pathname to be present.  Perl will tend to assume that an ambiguous
filename is in Unix format.



Allowing "." as a version delimiter is simply incompatible with
determining whether a pathname is in VMS format or in Unix format with
extended file syntax.  There is no way to know whether "perl-5.8.6" is a
Unix "perl-5.8.6" or a VMS "perl-5.8;6" when passing it to unixify() or
vmsify().



The DECC$FILENAME_UNIX_REPORT logical name controls how Perl interprets
filenames to the extent that Perl uses the CRTL internally for many
purposes, and attempts to follow CRTL conventions for reporting
filenames.  The DECC$FILENAME_UNIX_ONLY feature differs in that it
expects all filenames passed to the C run-time to be already in Unix
format.  This feature is not yet supported in Perl since Perl uses
traditional OpenVMS file specifications internally and in the test
harness, and it is not yet clear whether this mode will be useful or
useable.  The feature logical name DECC$POSIX_COMPLIANT_PATHNAMES is new
with the RMS Symbolic Link SDK and included with OpenVMS v8.3, but is
not yet supported in Perl.









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86.5.2 Filename Case



Perl follows VMS defaults and override settings in preserving (or not
preserving) filename case.  Case is not preserved on ODS-2 formatted
volumes on any architecture.  On ODS-5 volumes, filenames may be case
preserved depending on process and feature settings.  Perl now honors
DECC$EFS_CASE_PRESERVE and DECC$ARGV_PARSE_STYLE on those systems where
the CRTL supports these features.  When these features are not enabled
or the CRTL does not support them, Perl follows the traditional CRTL
behavior of downcasing command-line arguments and returning file
specifications in lower case only.



N. B.  It is very easy to get tripped up using a mixture of other
programs, external utilities, and Perl scripts that are in varying
states of being able to handle case preservation.  For example, a file
created by an older version of an archive utility or a build utility
such as MMK or MMS may generate a filename in all upper case even on an
ODS-5 volume.  If this filename is later retrieved by a Perl script or
module in a case preserving environment, that upper case name may not
match the mixed-case or lower-case exceptions of the Perl code.  Your
best bet is to follow an all-or-nothing approach to case preservation:
either don’t use it at all, or make sure your entire toolchain and
application environment support and use it.



OpenVMS Alpha v7.3-1 and later and all version of OpenVMS I64 support
case sensitivity as a process setting (see SET PROCESS
/CASE_LOOKUP=SENSITIVE). Perl does not currently support case
sensitivity on VMS, but it may in the future, so Perl programs should
use the File::Spec->case_tolerant method to determine the state, and
not the $^O variable.









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86.5.3 Symbolic Links



When built on an ODS-5 volume with symbolic links enabled, Perl by
default supports symbolic links when the requisite support is available
in the filesystem and CRTL (generally 64-bit OpenVMS v8.3 and later). 
There are a number of limitations and caveats to be aware of when
working with symbolic links on VMS.  Most notably, the target of a valid
symbolic link must be expressed as a Unix-style path and it must exist
on a volume visible from your POSIX root (see the SHOW ROOT command
in DCL help).  For further details on symbolic link capabilities and
requirements, see chapter 12 of the CRTL manual that ships with OpenVMS
v8.3 or later.









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86.5.4 Wildcard expansion



File specifications containing wildcards are allowed both on 
the command line and within Perl globs (e.g. <*.c>).  If
the wildcard filespec uses VMS syntax, the resultant 
filespecs will follow VMS syntax; if a Unix-style filespec is 
passed in, Unix-style filespecs will be returned.
Similar to the behavior of wildcard globbing for a Unix shell,
one can escape command line wildcards with double quotation
marks " around a perl program command line argument.  However,
owing to the stripping of " characters carried out by the C
handling of argv you will need to escape a construct such as
this one (in a directory containing the files PERL.C, PERL.EXE,
PERL.H, and PERL.OBJ):



    $ perl -e "print join(' ',@ARGV)" perl.*
    perl.c perl.exe perl.h perl.obj



in the following triple quoted manner:



    $ perl -e "print join(' ',@ARGV)" """perl.*"""
    perl.*



In both the case of unquoted command line arguments or in calls
to glob() VMS wildcard expansion is performed. (csh-style
wildcard expansion is available if you use File::Glob::glob.)
If the wildcard filespec contains a device or directory 
specification, then the resultant filespecs will also contain 
a device and directory; otherwise, device and directory 
information are removed.  VMS-style resultant filespecs will 
contain a full device and directory, while Unix-style 
resultant filespecs will contain only as much of a directory 
path as was present in the input filespec.  For example, if 
your default directory is Perl_Root:[000000], the expansion 
of [.t]*.* will yield filespecs  like 
"perl_root:[t]base.dir", while the expansion of t/*/* will 
yield filespecs like "t/base.dir".  (This is done to match 
the behavior of glob expansion performed by Unix shells.) 



Similarly, the resultant filespec will contain the file version
only if one was present in the input filespec.









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86.5.5 Pipes



Input and output pipes to Perl filehandles are supported; the 
"file name" is passed to lib$spawn() for asynchronous 
execution.  You should be careful to close any pipes you have 
opened in a Perl script, lest you leave any "orphaned" 
subprocesses around when Perl exits. 



You may also use backticks to invoke a DCL subprocess, whose 
output is used as the return value of the expression.  The 
string between the backticks is handled as if it were the
argument to the system operator (see below).  In this case,
Perl will wait for the subprocess to complete before continuing. 



The mailbox (MBX) that perl can create to communicate with a pipe
defaults to a buffer size of 8192 on 64-bit systems, 512 on VAX.  The
default buffer size is adjustable via the logical name PERL_MBX_SIZE
provided that the value falls between 128 and the SYSGEN parameter
MAXBUF inclusive.  For example, to set the mailbox size to 32767 use
$ENV{'PERL_MBX_SIZE'} = 32767; and then open and use pipe constructs. 
An alternative would be to issue the command:



    $ Define PERL_MBX_SIZE 32767



before running your wide record pipe program.  A larger value may
improve performance at the expense of the BYTLM UAF quota.









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86.6 PERL5LIB and PERLLIB



The PERL5LIB and PERLLIB logical names work as documented in perl NAME,
except that the element separator is ’|’ instead of ’:’.  The
directory specifications may use either VMS or Unix syntax.









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86.7 The Perl Forked Debugger



The Perl forked debugger places the debugger commands and output in a
separate X-11 terminal window so that commands and output from multiple
processes are not mixed together.



Perl on VMS supports an emulation of the forked debugger when Perl is
run on a VMS system that has X11 support installed.



To use the forked debugger, you need to have the default display set to an
X-11 Server and some environment variables set that Unix expects.



The forked debugger requires the environment variable TERM to be xterm,
and the environment variable DISPLAY to exist.  xterm must be in
lower case.



  $define TERM "xterm"

  $define DISPLAY "hostname:0.0"



Currently the value of DISPLAY is ignored.  It is recommended that it be set
to be the hostname of the display, the server and screen in Unix notation.  In
the future the value of DISPLAY may be honored by Perl instead of using the
default display.



It may be helpful to always use the forked debugger so that script I/O is
separated from debugger I/O.  You can force the debugger to be forked by
assigning a value to the logical name <PERLDB_PIDS> that is not a process
identification number.



  $define PERLDB_PIDS XXXX









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86.8 PERL_VMS_EXCEPTION_DEBUG



The PERL_VMS_EXCEPTION_DEBUG being defined as "ENABLE" will cause the VMS
debugger to be invoked if a fatal exception that is not otherwise
handled is raised.  The purpose of this is to allow debugging of
internal Perl problems that would cause such a condition.



This allows the programmer to look at the execution stack and variables to
find out the cause of the exception.  As the debugger is being invoked as
the Perl interpreter is about to do a fatal exit, continuing the execution
in debug mode is usually not practical.



Starting Perl in the VMS debugger may change the program execution
profile in a way that such problems are not reproduced.



The kill function can be used to test this functionality from within
a program.



In typical VMS style, only the first letter of the value of this logical
name is actually checked in a case insensitive mode, and it is considered
enabled if it is the value "T","1" or "E".



This logical name must be defined before Perl is started.









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86.9 Command line





• perlvms I/O redirection and backgrounding:  



• perlvms Command line switches:  












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86.9.1 I/O redirection and backgrounding



Perl for VMS supports redirection of input and output on the 
command line, using a subset of Bourne shell syntax:





In addition, output may be piped to a subprocess, using the  
character ’|’.  Anything after this character on the command 
line is passed to a subprocess for execution; the subprocess 
takes the output of Perl as its input.



Finally, if the command line ends with ’&’, the entire 
command is run in the background as an asynchronous 
subprocess.









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86.9.2 Command line switches



The following command line switches behave differently under
VMS than described in perlrun NAME.  Note also that in order
to pass uppercase switches to Perl, you need to enclose
them in double-quotes on the command line, since the CRTL
downcases all unquoted strings.



On newer 64 bit versions of OpenVMS, a process setting now
controls if the quoting is needed to preserve the case of
command line arguments.





-i




If the -i switch is present but no extension for a backup
copy is given, then inplace editing creates a new version of
a file; the existing copy is not deleted.  (Note that if
an extension is given, an existing file is renamed to the backup
file, as is the case under other operating systems, so it does
not remain as a previous version under the original filename.)





-S




If the "-S" or -"S" switch is present and the script
name does not contain a directory, then Perl translates the
logical name DCL$PATH as a searchlist, using each translation
as a directory in which to look for the script.  In addition,
if no file type is specified, Perl looks in each directory
for a file matching the name specified, with a blank type,
a type of .pl, and a type of .com, in that order.





-u




The -u switch causes the VMS debugger to be invoked
after the Perl program is compiled, but before it has
run.  It does not create a core dump file.














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86.10 Perl functions



As of the time this document was last revised, the following 
Perl functions were implemented in the VMS port of Perl 
(functions marked with * are discussed in more detail below):



    file tests*, abs, alarm, atan, backticks*, binmode*, bless,
    caller, chdir, chmod, chown, chomp, chop, chr,
    close, closedir, cos, crypt*, defined, delete, die, do, dump*, 
    each, endgrent, endpwent, eof, eval, exec*, exists, exit, exp, 
    fileno, flock  getc, getgrent*, getgrgid*, getgrnam, getlogin, getppid,
    getpwent*, getpwnam*, getpwuid*, glob, gmtime*, goto,
    grep, hex, ioctl, import, index, int, join, keys, kill*,
    last, lc, lcfirst, lchown*, length, link*, local, localtime, log, lstat, m//,
    map, mkdir, my, next, no, oct, open, opendir, ord, pack,
    pipe, pop, pos, print, printf, push, q//, qq//, qw//,
    qx//*, quotemeta, rand, read, readdir, readlink*, redo, ref, rename,
    require, reset, return, reverse, rewinddir, rindex,
    rmdir, s///, scalar, seek, seekdir, select(internal),
    select (system call)*, setgrent, setpwent, shift, sin, sleep,
    socketpair, sort, splice, split, sprintf, sqrt, srand, stat,
    study, substr, symlink*, sysread, system*, syswrite, tell,
    telldir, tie, time, times*, tr///, uc, ucfirst, umask,
    undef, unlink*, unpack, untie, unshift, use, utime*,
    values, vec, wait, waitpid*, wantarray, warn, write, y///



The following functions were not implemented in the VMS port, 
and calling them produces a fatal error (usually) or 
undefined behavior (rarely, we hope):



    chroot, dbmclose, dbmopen, fork*, getpgrp, getpriority,  
    msgctl, msgget, msgsend, msgrcv, semctl,
    semget, semop, setpgrp, setpriority, shmctl, shmget,
    shmread, shmwrite, syscall



The following functions are available on Perls compiled with Dec C
5.2 or greater and running VMS 7.0 or greater:



    truncate



The following functions are available on Perls built on VMS 7.2 or
greater:



    fcntl (without locking)



The following functions may or may not be implemented, 
depending on what type of socket support you’ve built into 
your copy of Perl:



    accept, bind, connect, getpeername,
    gethostbyname, getnetbyname, getprotobyname,
    getservbyname, gethostbyaddr, getnetbyaddr,
    getprotobynumber, getservbyport, gethostent,
    getnetent, getprotoent, getservent, sethostent,
    setnetent, setprotoent, setservent, endhostent,
    endnetent, endprotoent, endservent, getsockname,
    getsockopt, listen, recv, select(system call)*,
    send, setsockopt, shutdown, socket



The following function is available on Perls built on 64 bit OpenVMS v8.2
with hard links enabled on an ODS-5 formatted build disk.  CRTL support
is in principle available as of OpenVMS v7.3-1, and better configuration
support could detect this.



    link



The following functions are available on Perls built on 64 bit OpenVMS
v8.2 and later.  CRTL support is in principle available as of OpenVMS
v7.3-2, and better configuration support could detect this.



   getgrgid, getgrnam, getpwnam, getpwuid,
   setgrent, ttyname



The following functions are available on Perls built on 64 bit OpenVMS v8.2
and later.  



   statvfs, socketpair





File tests




The tests -b, -B, -c, -C, -d, -e, -f,
-o, -M, -s, -S, -t, -T, and -z work as
advertised.  The return values for -r, -w, and -x
tell you whether you can actually access the file; this may
not reflect the UIC-based file protections.  Since real and
effective UIC don’t differ under VMS, -O, -R, -W,
and -X are equivalent to -o, -r, -w, and -x.
Similarly, several other tests, including -A, -g, -k,
-l, -p, and -u, aren’t particularly meaningful under
VMS, and the values returned by these tests reflect whatever
your CRTL stat() routine does to the equivalent bits in the
st_mode field.  Finally, -d returns true if passed a device
specification without an explicit directory (e.g. DUA1:), as
well as if passed a directory.



There are DECC feature logical names AND ODS-5 volume attributes that
also control what values are returned for the date fields.



Note: Some sites have reported problems when using the file-access
tests (-r, -w, and -x) on files accessed via DEC’s DFS.
Specifically, since DFS does not currently provide access to the
extended file header of files on remote volumes, attempts to
examine the ACL fail, and the file tests will return false,
with $! indicating that the file does not exist.  You can
use stat on these files, since that checks UIC-based protection
only, and then manually check the appropriate bits, as defined by
your C compiler’s stat.h, in the mode value it returns, if you
need an approximation of the file’s protections.





backticks




Backticks create a subprocess, and pass the enclosed string
to it for execution as a DCL command.  Since the subprocess is
created directly via lib$spawn(), any valid DCL command string
may be specified.





binmode FILEHANDLE




The binmode operator will attempt to insure that no translation
of carriage control occurs on input from or output to this filehandle.
Since this involves reopening the file and then restoring its
file position indicator, if this function returns FALSE, the
underlying filehandle may no longer point to an open file, or may
point to a different position in the file than before binmode
was called.



Note that binmode is generally not necessary when using normal
filehandles; it is provided so that you can control I/O to existing
record-structured files when necessary.  You can also use the
vmsfopen function in the VMS::Stdio extension to gain finer
control of I/O to files and devices with different record structures.





crypt PLAINTEXT, USER




The crypt operator uses the sys$hash_password system
service to generate the hashed representation of PLAINTEXT.
If USER is a valid username, the algorithm and salt values
are taken from that user’s UAF record.  If it is not, then
the preferred algorithm and a salt of 0 are used.  The
quadword encrypted value is returned as an 8-character string.



The value returned by crypt may be compared against
the encrypted password from the UAF returned by the getpw*
functions, in order to authenticate users.  If you’re
going to do this, remember that the encrypted password in
the UAF was generated using uppercase username and
password strings; you’ll have to upcase the arguments to
crypt to insure that you’ll get the proper value:



    sub validate_passwd {
        my($user,$passwd) = @_;
        my($pwdhash);
        if ( !($pwdhash = (getpwnam($user))[1]) ||
               $pwdhash ne crypt("\U$passwd","\U$name") ) {
            intruder_alert($name);
        }
        return 1;
    }





die




die will force the native VMS exit status to be an SS$_ABORT code
if neither of the $! or $? status values are ones that would cause
the native status to be interpreted as being what VMS classifies as
SEVERE_ERROR severity for DCL error handling.



When PERL_VMS_POSIX_EXIT is active (see $? below), the native VMS exit
status value will have either one of the $! or $? or $^E or
the Unix value 255 encoded into it in a way that the effective original
value can be decoded by other programs written in C, including Perl
and the GNV package.  As per the normal non-VMS behavior of die if
either $! or $? are non-zero, one of those values will be
encoded into a native VMS status value.  If both of the Unix status
values are 0, and the $^E value is set one of ERROR or SEVERE_ERROR
severity, then the $^E value will be used as the exit code as is.
If none of the above apply, the Unix value of 255 will be encoded into
a native VMS exit status value.



Please note a significant difference in the behavior of die in
the PERL_VMS_POSIX_EXIT mode is that it does not force a VMS
SEVERE_ERROR status on exit.  The Unix exit values of 2 through
255 will be encoded in VMS status values with severity levels of
SUCCESS.  The Unix exit value of 1 will be encoded in a VMS status
value with a severity level of ERROR.  This is to be compatible with
how the VMS C library encodes these values.



The minimum severity level set by die in PERL_VMS_POSIX_EXIT mode
may be changed to be ERROR or higher in the future depending on the 
results of testing and further review.



See $? for a description of the encoding of the Unix value to
produce a native VMS status containing it.





dump




Rather than causing Perl to abort and dump core, the dump
operator invokes the VMS debugger.  If you continue to
execute the Perl program under the debugger, control will
be transferred to the label specified as the argument to
dump, or, if no label was specified, back to the
beginning of the program.  All other state of the program
(e.g. values of variables, open file handles) are not
affected by calling dump.





exec LIST




A call to exec will cause Perl to exit, and to invoke the command
given as an argument to exec via lib$do_command.  If the
argument begins with ’@’ or ’$’ (other than as part of a filespec),
then it is executed as a DCL command.  Otherwise, the first token on
the command line is treated as the filespec of an image to run, and
an attempt is made to invoke it (using .Exe and the process
defaults to expand the filespec) and pass the rest of exec’s
argument to it as parameters.  If the token has no file type, and
matches a file with null type, then an attempt is made to determine
whether the file is an executable image which should be invoked
using MCR or a text file which should be passed to DCL as a
command procedure.





fork




While in principle the fork operator could be implemented via
(and with the same rather severe limitations as) the CRTL vfork()
routine, and while some internal support to do just that is in
place, the implementation has never been completed, making fork
currently unavailable.  A true kernel fork() is expected in a
future version of VMS, and the pseudo-fork based on interpreter
threads may be available in a future version of Perl on VMS (see
perlfork NAME).  In the meantime, use system, backticks, or piped
filehandles to create subprocesses.





getpwent






getpwnam






getpwuid




These operators obtain the information described in (perlfunc),
if you have the privileges necessary to retrieve the named user’s
UAF information via sys$getuai.  If not, then only the $name,
$uid, and $gid items are returned.  The $dir item contains
the login directory in VMS syntax, while the $comment item
contains the login directory in Unix syntax. The $gcos item
contains the owner field from the UAF record.  The $quota
item is not used.





gmtime




The gmtime operator will function properly if you have a
working CRTL gmtime() routine, or if the logical name
SYS$TIMEZONE_DIFFERENTIAL is defined as the number of seconds
which must be added to UTC to yield local time.  (This logical
name is defined automatically if you are running a version of
VMS with built-in UTC support.)  If neither of these cases is
true, a warning message is printed, and undef is returned.





kill




In most cases, kill is implemented via the undocumented system
service $SIGPRC, which has the same calling sequence as $FORCEX, but
throws an exception in the target process rather than forcing it to call
$EXIT.  Generally speaking, kill follows the behavior of the
CRTL’s kill() function, but unlike that function can be called from
within a signal handler.  Also, unlike the kill in some versions of
the CRTL, Perl’s kill checks the validity of the signal passed in and
returns an error rather than attempting to send an unrecognized signal.



Also, negative signal values don’t do anything special under
VMS; they’re just converted to the corresponding positive value.





qx//




See the entry on backticks above.





select (system call)




If Perl was not built with socket support, the system call
version of select is not available at all.  If socket
support is present, then the system call version of
select functions only for file descriptors attached
to sockets.  It will not provide information about regular
files or pipes, since the CRTL select() routine does not
provide this functionality.





stat EXPR




Since VMS keeps track of files according to a different scheme
than Unix, it’s not really possible to represent the file’s ID
in the st_dev and st_ino fields of a struct stat.  Perl
tries its best, though, and the values it uses are pretty unlikely
to be the same for two different files.  We can’t guarantee this,
though, so caveat scriptor.





system LIST




The system operator creates a subprocess, and passes its 
arguments to the subprocess for execution as a DCL command.  
Since the subprocess is created directly via lib$spawn(), any 
valid DCL command string may be specified.  If the string begins with
’@’, it is treated as a DCL command unconditionally.  Otherwise, if
the first token contains a character used as a delimiter in file
specification (e.g. : or ]), an attempt is made to expand it
using  a default type of .Exe and the process defaults, and if
successful, the resulting file is invoked via MCR. This allows you
to invoke an image directly simply by passing the file specification
to system, a common Unixish idiom.  If the token has no file type,
and matches a file with null type, then an attempt is made to
determine whether the file is an executable image which should be
invoked using MCR or a text file which should be passed to DCL
as a command procedure.



If LIST consists of the empty string, system spawns an
interactive DCL subprocess, in the same fashion as typing
SPAWN at the DCL prompt.



Perl waits for the subprocess to complete before continuing
execution in the current process.  As described in (perlfunc),
the return value of system is a fake "status" which follows
POSIX semantics unless the pragma use vmsish 'status' is in
effect; see the description of $? in this document for more 
detail.  





time




The value returned by time is the offset in seconds from
01-JAN-1970 00:00:00 (just like the CRTL’s times() routine), in order
to make life easier for code coming in from the POSIX/Unix world.





times




The array returned by the times operator is divided up 
according to the same rules the CRTL times() routine.  
Therefore, the "system time" elements will always be 0, since 
there is no difference between "user time" and "system" time 
under VMS, and the time accumulated by a subprocess may or may 
not appear separately in the "child time" field, depending on 
whether times() keeps track of subprocesses separately.  Note
especially that the VAXCRTL (at least) keeps track only of
subprocesses spawned using fork() and exec(); it will not
accumulate the times of subprocesses spawned via pipes, system(),
or backticks.





unlink LIST




unlink will delete the highest version of a file only; in
order to delete all versions, you need to say



    1 while unlink LIST;



You may need to make this change to scripts written for a
Unix system which expect that after a call to unlink,
no files with the names passed to unlink will exist.
(Note: This can be changed at compile time; if you
use Config and $Config{'d_unlink_all_versions'} is
define, then unlink will delete all versions of a
file on the first call.)



unlink will delete a file if at all possible, even if it
requires changing file protection (though it won’t try to
change the protection of the parent directory).  You can tell
whether you’ve got explicit delete access to a file by using the
VMS::Filespec::candelete operator.  For instance, in order
to delete only files to which you have delete access, you could
say something like



    sub safe_unlink {
        my($file,$num);
        foreach $file (@_) {
            next unless VMS::Filespec::candelete($file);
            $num += unlink $file;
        }
        $num;
    }



(or you could just use VMS::Stdio::remove, if you’ve installed
the VMS::Stdio extension distributed with Perl). If unlink has to
change the file protection to delete the file, and you interrupt it
in midstream, the file may be left intact, but with a changed ACL
allowing you delete access.



This behavior of unlink is to be compatible with POSIX behavior
and not traditional VMS behavior.





utime LIST




This operator changes only the modification time of the file (VMS 
revision date) on ODS-2 volumes and ODS-5 volumes without access 
dates enabled. On ODS-5 volumes with access dates enabled, the 
true access time is modified.





waitpid PID,FLAGS




If PID is a subprocess started by a piped open() (see (open)), 
waitpid will wait for that subprocess, and return its final status
value in $?.  If PID is a subprocess created in some other way (e.g.
SPAWNed before Perl was invoked), waitpid will simply check once per
second whether the process has completed, and return when it has.  (If
PID specifies a process that isn’t a subprocess of the current process,
and you invoked Perl with the -w switch, a warning will be issued.)



Returns PID on success, -1 on error.  The FLAGS argument is ignored
in all cases.














Next: , Previous: perlvms Perl functions, Up: perlvms   [Contents]





86.11 Perl variables



The following VMS-specific information applies to the indicated
"special" Perl variables, in addition to the general information
in perlvar NAME.  Where there is a conflict, this information
takes precedence.





%ENV




The operation of the %ENV array depends on the translation
of the logical name PERL_ENV_TABLES.  If defined, it should
be a search list, each element of which specifies a location
for %ENV elements.  If you tell Perl to read or set the
element $ENV{name}, then Perl uses the translations of
PERL_ENV_TABLES as follows:





CRTL_ENV




This string tells Perl to consult the CRTL’s internal environ
array of key-value pairs, using name as the key.  In most cases,
this contains only a few keys, but if Perl was invoked via the C
exec[lv]e() function, as is the case for CGI processing by some
HTTP servers, then the environ array may have been populated by
the calling program.





CLISYM_[LOCAL]




A string beginning with CLISYM_tells Perl to consult the CLI’s
symbol tables, using name as the name of the symbol.  When reading
an element of %ENV, the local symbol table is scanned first, followed
by the global symbol table..  The characters following CLISYM_ are
significant when an element of %ENV is set or deleted: if the
complete string is CLISYM_LOCAL, the change is made in the local
symbol table; otherwise the global symbol table is changed.





Any other string




If an element of PERL_ENV_TABLES translates to any other string,
that string is used as the name of a logical name table, which is
consulted using name as the logical name.  The normal search
order of access modes is used.








PERL_ENV_TABLES is translated once when Perl starts up; any changes
you make while Perl is running do not affect the behavior of %ENV.
If PERL_ENV_TABLES is not defined, then Perl defaults to consulting
first the logical name tables specified by LNM$FILE_DEV, and then
the CRTL environ array.



In all operations on %ENV, the key string is treated as if it 
were entirely uppercase, regardless of the case actually 
specified in the Perl expression.



When an element of %ENV is read, the locations to which
PERL_ENV_TABLES points are checked in order, and the value
obtained from the first successful lookup is returned.  If the
name of the %ENV element contains a semi-colon, it and
any characters after it are removed.  These are ignored when
the CRTL environ array or a CLI symbol table is consulted.
However, the name is looked up in a logical name table, the
suffix after the semi-colon is treated as the translation index
to be used for the lookup.   This lets you look up successive values
for search list logical names.  For instance, if you say



   $  Define STORY  once,upon,a,time,there,was
   $  perl -e "for ($i = 0; $i <= 6; $i++) " -
   _$ -e "{ print $ENV{'story;'.$i},' '}"



Perl will print ONCE UPON A TIME THERE WAS, assuming, of course,
that PERL_ENV_TABLES is set up so that the logical name story
is found, rather than a CLI symbol or CRTL environ element with
the same name.



When an element of %ENV is set to a defined string, the
corresponding definition is made in the location to which the
first translation of PERL_ENV_TABLES points.  If this causes a
logical name to be created, it is defined in supervisor mode.
(The same is done if an existing logical name was defined in
executive or kernel mode; an existing user or supervisor mode
logical name is reset to the new value.)  If the value is an empty
string, the logical name’s translation is defined as a single NUL
(ASCII 00) character, since a logical name cannot translate to a
zero-length string.  (This restriction does not apply to CLI symbols
or CRTL environ values; they are set to the empty string.)
An element of the CRTL environ array can be set only if your
copy of Perl knows about the CRTL’s setenv() function.  (This is
present only in some versions of the DECCRTL; check $Config{d_setenv}
to see whether your copy of Perl was built with a CRTL that has this
function.)



When an element of %ENV is set to undef,
the element is looked up as if it were being read, and if it is
found, it is deleted.  (An item "deleted" from the CRTL environ
array is set to the empty string; this can only be done if your
copy of Perl knows about the CRTL setenv() function.)  Using
delete to remove an element from %ENV has a similar effect,
but after the element is deleted, another attempt is made to
look up the element, so an inner-mode logical name or a name in
another location will replace the logical name just deleted.
In either case, only the first value found searching PERL_ENV_TABLES
is altered.  It is not possible at present to define a search list
logical name via %ENV.



The element $ENV{DEFAULT} is special: when read, it returns
Perl’s current default device and directory, and when set, it
resets them, regardless of the definition of PERL_ENV_TABLES.
It cannot be cleared or deleted; attempts to do so are silently
ignored.



Note that if you want to pass on any elements of the
C-local environ array to a subprocess which isn’t
started by fork/exec, or isn’t running a C program, you
can "promote" them to logical names in the current
process, which will then be inherited by all subprocesses,
by saying



    foreach my $key (qw[C-local keys you want promoted]) {
        my $temp = $ENV{$key}; # read from C-local array
        $ENV{$key} = $temp;    # and define as logical name
    }



(You can’t just say $ENV{$key} = $ENV{$key}, since the
Perl optimizer is smart enough to elide the expression.)



Don’t try to clear %ENV by saying %ENV = ();, it will throw
a fatal error.  This is equivalent to doing the following from DCL:



    DELETE/LOGICAL *



You can imagine how bad things would be if, for example, the SYS$MANAGER
or SYS$SYSTEM logical names were deleted.



At present, the first time you iterate over %ENV using
keys, or values,  you will incur a time penalty as all
logical names are read, in order to fully populate %ENV.
Subsequent iterations will not reread logical names, so they
won’t be as slow, but they also won’t reflect any changes
to logical name tables caused by other programs.



You do need to be careful with the logical names representing
process-permanent files, such as SYS$INPUT and SYS$OUTPUT.
The translations for these logical names are prepended with a
two-byte binary value (0x1B 0x00) that needs to be stripped off
if you wantto use it. (In previous versions of Perl it wasn’t
possible to get the values of these logical names, as the null
byte acted as an end-of-string marker)





$!




The string value of $! is that returned by the CRTL’s
strerror() function, so it will include the VMS message for
VMS-specific errors.  The numeric value of $! is the
value of errno, except if errno is EVMSERR, in which
case $! contains the value of vaxc$errno.  Setting $!
always sets errno to the value specified.  If this value is
EVMSERR, it also sets vaxc$errno to 4 (NONAME-F-NOMSG), so
that the string value of $! won’t reflect the VMS error
message from before $! was set.





$^E




This variable provides direct access to VMS status values
in vaxc$errno, which are often more specific than the
generic Unix-style error messages in $!.  Its numeric value
is the value of vaxc$errno, and its string value is the
corresponding VMS message string, as retrieved by sys$getmsg().
Setting $^E sets vaxc$errno to the value specified.



While Perl attempts to keep the vaxc$errno value to be current, if
errno is not EVMSERR, it may not be from the current operation.





$?




The "status value" returned in $? is synthesized from the
actual exit status of the subprocess in a way that approximates
POSIX wait(5) semantics, in order to allow Perl programs to
portably test for successful completion of subprocesses.  The
low order 8 bits of $? are always 0 under VMS, since the
termination status of a process may or may not have been
generated by an exception.



The next 8 bits contain the termination status of the program.



If the child process follows the convention of C programs
compiled with the _POSIX_EXIT macro set, the status value will
contain the actual value of 0 to 255 returned by that program
on a normal exit.



With the _POSIX_EXIT macro set, the Unix exit value of zero is
represented as a VMS native status of 1, and the Unix values
from 2 to 255 are encoded by the equation:



   VMS_status = 0x35a000 + (unix_value * 8) + 1.



And in the special case of Unix value 1 the encoding is:



   VMS_status = 0x35a000 + 8 + 2 + 0x10000000.



For other termination statuses, the severity portion of the
subprocess’s exit status is used: if the severity was success or
informational, these bits are all 0; if the severity was
warning, they contain a value of 1; if the severity was
error or fatal error, they contain the actual severity bits,
which turns out to be a value of 2 for error and 4 for severe_error.
Fatal is another term for the severe_error status.



As a result, $? will always be zero if the subprocess’s exit
status indicated successful completion, and non-zero if a
warning or error occurred or a program compliant with encoding
_POSIX_EXIT values was run and set a status.



How can you tell the difference between a non-zero status that is
the result of a VMS native error status or an encoded Unix status?
You can not unless you look at the ${^CHILD_ERROR_NATIVE} value.
The ${^CHILD_ERROR_NATIVE} value returns the actual VMS status value
and check the severity bits. If the severity bits are equal to 1,
then if the numeric value for $? is between 2 and 255 or 0, then
$? accurately reflects a value passed back from a Unix application.
If $? is 1, and the severity bits indicate a VMS error (2), then
$? is from a Unix application exit value.



In practice, Perl scripts that call programs that return _POSIX_EXIT
type status values will be expecting those values, and programs that
call traditional VMS programs will either be expecting the previous
behavior or just checking for a non-zero status.



And success is always the value 0 in all behaviors.



When the actual VMS termination status of the child is an error,
internally the $! value will be set to the closest Unix errno
value to that error so that Perl scripts that test for error
messages will see the expected Unix style error message instead
of a VMS message.



Conversely, when setting $? in an END block, an attempt is made
to convert the POSIX value into a native status intelligible to
the operating system upon exiting Perl.  What this boils down to
is that setting $? to zero results in the generic success value
SS$_NORMAL, and setting $? to a non-zero value results in the
generic failure status SS$_ABORT.  See also perlport exit.



With the PERL_VMS_POSIX_EXIT logical name defined as "ENABLE",
setting $? will cause the new value to be encoded into $^E
so that either the original parent or child exit status values 
 0 to 255 can be automatically recovered by C programs expecting
_POSIX_EXIT behavior.  If both a parent and a child exit value are
non-zero, then it will be assumed that this is actually a VMS native
status value to be passed through.  The special value of 0xFFFF is
almost a NOOP as it will cause the current native VMS status in the
C library to become the current native Perl VMS status, and is handled
this way as it is known to not be a valid native VMS status value.
It is recommend that only values in the range of normal Unix parent or
child status numbers, 0 to 255 are used.



The pragma use vmsish 'status' makes $? reflect the actual 
VMS exit status instead of the default emulation of POSIX status 
described above.  This pragma also disables the conversion of
non-zero values to SS$_ABORT when setting $? in an END
block (but zero will still be converted to SS$_NORMAL).



Do not use the pragma use vmsish 'status' with PERL_VMS_POSIX_EXIT
enabled, as they are at times requesting conflicting actions and the
consequence of ignoring this advice will be undefined to allow future
improvements in the POSIX exit handling.



In general, with PERL_VMS_POSIX_EXIT enabled, more detailed information
will be available in the exit status for DCL scripts or other native VMS tools,
and will give the expected information for Posix programs.  It has not been
made the default in order to preserve backward compatibility.



N.B. Setting DECC$FILENAME_UNIX_REPORT implicitly enables 
PERL_VMS_POSIX_EXIT.





$|




Setting $| for an I/O stream causes data to be flushed
all the way to disk on each write (i.e. not just to
the underlying RMS buffers for a file).  In other words,
it’s equivalent to calling fflush() and fsync() from C.














Next: , Previous: perlvms Perl variables, Up: perlvms   [Contents]





86.12 Standard modules with VMS-specific differences





• perlvms SDBM_File:  












Up: perlvms Standard modules with VMS-specific differences   [Contents]





86.12.1 SDBM_File



SDBM_File works properly on VMS. It has, however, one minor
difference. The database directory file created has a .sdbm_dir
extension rather than a .dir extension. .dir files are VMS filesystem
directory files, and using them for other purposes could cause unacceptable
problems.









Next: , Previous: perlvms Standard modules with VMS-specific differences, Up: perlvms   [Contents]





86.13 Revision date



Please see the git repository for revision history.









Previous: perlvms Revision date, Up: perlvms   [Contents]





86.14 AUTHOR



Charles Bailey  bailey@cor.newman.upenn.edu
Craig Berry  craigberry@mac.com
Dan Sugalski  dan@sidhe.org
John Malmberg wb8tyw@qsl.net